Biology Module 10

population density

Another characteristic of a population is population density, which is calculated by dividing the population size by the area of land that population inhabits. Population density is an important characteristic because it tells us how many individuals there are per unit of land--in other words, how crowded the population is. Population Density=Population SizeInhabited AreaPopulation Density=Population SizeInhabited Area However, recall that it can be difficult to count every member of a population. Watch the video below to see an example of how challenging it can be to directly measure population size. As of 2014, it was estimated that there are over 318 million residents of the United States. Imagine trying to count every human in the United States at a single moment! All over the country, babies are being born and people are dying. The population is constantly increasing and decreasing in size. If we know the size of our population, we can be better prepared to feed everyone, give everyone access to healthcare, and educate our nation's children. If we don't know how many people we have, it is harder to manage our resources and plan for the future. But counting 318 million people is no easy task. There has to be a better way, but what could it be? Transcript Instead of counting everyone and dividing by the entire inhabited area, we can pick a small section and just count the individuals within it. If we divide the number of individuals by the sampled area, we get population density. For example, let's say we've sampled a rectangular section of a field and counted the number of cotton mice in the section. If the rectangular section is 4 km long and 2 km wide, the area of the sampled section is 4 km x 2 km = 8 km2. If there were 64 mice, the population density would be 64 mice / 8 km2, or 8 mice / km2. Population density can also help us estimate the size of the entire population if it is too hard to count every individual. If we know the entire area of land, we can multiply that area by the density to get an estimate of the total population. If the field from the mouse population we've sampled is actually 100 km2, we can estimate the size of the entire population: 100 km2 x 8 mice/km2 = 800 mice Many populations are too large or too mobile to count, so it is common to sample a small section and calculate population density and then use the land area to estimate population size. However, this process of estimating population size has its limitations. Populations are often not distributed evenly across their habitat area. For example, humans are not distributed evenly across the world as you can see in this image. Instead, humans are highly concentrated in a few areas (e.g., central Europe, India, China), less concentrated in many others (e.g., midwestern United States, southern Russia), and almost completely absent in others (e.g., Antarctica, most of Australia). If we calculate the population density of people in each country, we will get many different numbers. Thus, the accuracy of using population density to estimate population size depends on a well-chosen sampling area. Population density of humans per square kilometer (km2) in 2010 Question Imagine you are trying to estimate the number of humans on Earth. Your sample area is the country of India. Would you overestimate or underestimate the number of humans on Earth? Overestimate . . . by a lot! India is one of the most densely populated regions of the world with approximately 1,000 people per square kilometer. Compare this to the New England region of the United States, which has only 450 people per square kilometer.

niches

A community is all of the species that occupy the same habitat from the smallest bacteria to the largest animal. Each species in a community does not occupy all parts of the habitat at once. Instead, each species occupies a different part. The part of the habitat each species occupies is part of its living pattern within its environment. The role, or living pattern, of a species within its environment is its niche. A species' niche includes what part of the habitat it lives in, what it eats, the resources it uses, when it reproduces, when it is active, and all other interactions with its environment. Raccoons are niche generalists.Pandas are niche specialists. Some species have a broad range of resources they can use and a broad range of living conditions they can tolerate. These species are called generalists. An example of a generalist is the raccoon which is found in many different habitats and can eat almost anything--that's why they're often found raiding dumpsters! In contrast, other species have narrow niches and are called specialists. An example of a specialist is the panda of east Asia, a herbivore that eats only bamboo. Some species have more than one niche depending on which developmental stage they are in. This can happen when a change in diet occurs. For example, a baby mammal only drinks milk from its mother; then after it matures, it eats different food that is particular to its species. Another example is when a species changes its habitat, such as tadpoles living exclusively in water and then living partially on land and partially in water when they grow up to become frogs. As a tadpole becomes a frog, its niche changes from being aquatic to semi-aquatic. Many plant and animal species are able to share the same habitat because they occupy different niches. The plants and animals need different things to survive and interact with their environments in different ways. They do not live in the same places, and their reproductive and social behaviors do not interfere with each other.

Mutalism

A symbiotic relationship is a close, long lasting interaction between two animals. For a particular species, there may be another species with which individuals commonly have a symbiotic relationship. When you find one species, you will find the other close by. There are three common types of symbiotic relationships which are defined by the interactions between the two individuals. One of these is mutualism. Mutualism is a symbiotic relationship in which both individuals benefit from their relationship. In fact, some species that have mutualistic relationships with each other have evolved such that they can no longer survive without each other. Click through the tabs to learn about different types of mutualistic relationships. In trophic mutualism, both species in the relationship get the nutrients or energy that all organisms need to survive. Many trophic mutualistic relationships involve a plant that uses photosynthesis to produce organic carbon-containing molecules like glucose, which another organism can use as food. In turn, that organism (often a microbe) provides other nutrients to the plant. Another example is bacteria living in the gut of termites. Termites are insects (shown in this image) that feed on wood and other plant matter, but they are unable to digest the fiber in the wood themselves. Instead, the fiber is digested by unicellular protists that live in their gut. The digested fiber serves as food both for the bacteria, as well as the termites that house them. In defensive mutualism, one species receives a benefit (such as food or shelter) for protecting the other species from being eaten or attacked by other organisms. Some ants nest inside the large thorns of acacia plants. Both the ants and the plant benefit from this long-term relationship. The ants receive food from the acacia and other insects that are attracted to the plant. Because the ants will defend their nest, they will fight off herbivores that attempt to eat the acacia. Small insects called aphids drain sweet fluid from plants so rapidly that much of it comes out of their other end. Ants live close by and feed on this fluid. This does not harm the aphid, as the fluid is in excess of what the aphid can consume. In return, the aphids benefit by being protected from predators by the ants. In dispersive mutualism, one species receives food from a plant, while the plant benefits by having its pollen or seeds dispersed. Animals called pollinators are attracted to flowering plants by their brightly colored, strongly scented flowers. Pollinators like bees and butterflies feed on the sweet fluid inside the flower or the pollen itself. While it is feeding, it will pick up pollen on its body and spread it to other plants of the same species, fertilizing them. Without pollinators, flowering plants would be much less likely to reproduce. Other plants produce appetizing fruits with their seeds hidden inside. After an animal eats the fruit, the seeds go through the digestive system and are excreted elsewhere. The animal gets the benefit of food, while the plant gets the benefit of having its seeds dispersed so that they can grow into new plants.

Heterotroph

All heterotrophs must consume other organisms that contain organic carbon compounds to provide energy to make ATP. These organisms can be autotrophs (who make their own carbon compounds) or other heterotrophs (who either eat autotrophs or have also eaten other heterotrophs). Heterotrophs are either predators, parasites, or both, depending on the relationships they have with the organisms they eat. A predator generally kills and eats an individual of another species, which is called the prey. (Some "predators" merely graze on their prey, such as cows nibbling at grass or mosquitoes briefly landing on animals to pick up protein, neither of which kill the prey). On the other hand, recall that a parasite does not immediately kill the species it uses as food. The relationship between a parasite and the animal it uses as food (its host) is an example of a symbiotic relationship. These are long-lasting, intimate relationships between individuals of different species. Some animals have mutual relationships where both individuals benefit. Others have commensal relationships where one individual benefits and the other is neither helped nor harmed. A parasite, on the other hand, derives a benefit, but the host is harmed by the symbiotic relationship. The parasite lives on or inside the host for a long time, using it as both food and shelter, possibly for years. A fungus (parasite) living on and getting nutrients from a tree (host). Both predators and parasites use other organisms that contain energy stored in the bonds of their molecules to fuel their own ATP production because they are heterotrophs. However, they differ in the length of their relationship with their food source. They generally also differ in whether or not the food source is kept alive.

energy

All living organisms are made of cells, which require energy in the form of ATP to carry out normal functions. Autotrophs can capture energy from the environment and store it in the chemical bonds of organic carbon compounds, like glucose and sucrose. Then when they need cellular energy (in the form of ATP), they break down the carbon compounds and convert the stored energy into ATP.Autotrophs need a source of energy, like sunlight and various molecular building blocks, to form the components they need to keep their cells working. However, autotrophs do not need to eat other organisms to get energy. Heterotrophs, on the other hand, cannot capture energy from the environment and store it to make ATP. But they still need ATP for their cells to function. Where do they get their energy to make ATP? Watch this video to learn how heterotrophs get energy to keep their cells working. Narrator:Heterotrophs need energy to make ATP, but they cannot capture energy from the environment like autotrophs. Instead, many heterotrophs eat autotrophs. By eating autotrophs, heterotrophs consume the organic carbon compounds that the autotrophs make. They can then use the energy stored in the carbon compounds to make ATP, just like the autotroph would.For example, as plants carry out photosynthesis, they bring in carbon dioxide from the air and bond them together, using energy from sunlight, to form sugars. If we leave the plants alone, they will break down their sugars and use the energy to make ATP.Or we can eat the plants, in which case we get to use their sugars to make our own ATP. When heterotrophs eat autotrophs, they take advantage of autotrophs' ability to capture energy from the environment.Without autotrophs, heterotrophs like ourselves would quickly run out of ATP and energy stores and be unable to acquire any more. Heterotrophs can be described based on what kinds of organisms they eat to get their carbon compounds, as well as their relationships with the things they eat, but they are all consumers in the energy pyramid. Transcript Question Humans are heterotophs that consume autotrophs to obtain energy. Can you name any autotrophs that you've eaten today? Hopefully you've had some fruits and vegetables--even French fries count! All of these come from plants that use photosynthesis to bond together the carbon dioxide that we breathe out. Energy from the sun is stored in these bonds, we eat them, and then use them to make ATP for our own cells.

exponential growth

Based on the birth rate and death rate of a population, we can calculate the growth rate. If you have the per capita growth rate of a population, as well as the size, you can predict how much the population will increase in the next year. If we multiply the growth rate (r) by the population size (N), we can calculate the change in population size (ΔN). We often use the Greek letter Δ, pronounced "delta," to indicate change in a variable. This is expressed in the following equation: ΔN = rN ΔN : change in population sizer : per capita growth rateN : population size For example, we calculated the growth rate of the human population in 2012 to be 0.01116 per capita. How much would a human population of 10,000 increase in the next year if it grew at this rate? ΔN = per capita growth rate x population size = 0.01116 x 10,000 = 111.6 people What would the population size be next year at this growth rate? 10,000 + 111.6 = 10,111.6 people To illustrate this model of population growth, we can also make a graph. If we plot population size (N) against time, we will get a roughly J-shaped curve called an exponential growth curve. In this model, r stays constant. According to our equation, as N increases, so will ΔN. This makes sense! If our population gets bigger and grows at the same rate as before, more individuals will get added as time goes on. When the population begins to grow, it does so slowly. However, as all of the offspring reproduce and their offspring reproduce, the population grows very quickly. This is reflected in the shape of the graph. To better understand exponential growth, walk through the slideshow below, which uses a population of rabbits as an example. For the sake of simplicity, we will use a death rate of zero. You begin with a pair of rabbits: N = 2The pair reproduces, creating two more rabbits. The growth rate is one rabbit per capita. Question Look at the steeply sloped line on the right side of your graph where the population is growing rapidly. Do you think the rabbits will be able to maintain this growth rate forever? No, they won't because every rabbit needs resources to live, such as food and water. Their habitat does not have an infinite amount of food and water, so the population cannot grow infinitely larger.

where is the greatest amount of biodiversity found?

Biodiversity, or species richness, is the variety of life. But biodiversity is not evenly distributed across the globe. There are patterns to biodiversity, which vary greatly across Earth and within its regions. There is a well known pattern of biodiversity called latitudinal gradient. Latitudinal gradient explains that biodiversity increases as you move from the Earth's poles toward the equator. Recall that latitude refers to the imaginary lines running east-west on the globe. Watch this video to learn more about this pattern of global biodiversity distribution. Narrator:Our planet is occupied by millions of species, including microbes, fungi, plants, and animals. The distribution of all these organisms is uneven, yet there is a pattern.The most widely recognized pattern of biodiversity is the latitudinal gradient.The latitudinal gradient explains that biodiversity increases as you move from the polar regions towards the tropics.Biodiversity is greater in the tropical regions near the Equator when compared to temperate regions and polar regions.It stands to reason that biodiversity would be greatest in areas with a stable climate. In fact, new species are discovered every day in these tropical regions. Transcript Question How would you describe tropical regions? Tropical regions are wet and warm. They fall along the equator between the Tropic of Cancer and Tropic of Capricorn.

biodiversity hotspots

Brazil's Atlantic Forest Madagascar Indonesia Colombia The majority of biodiversity hotspots are forest areas, mostly in the tropics like Brazil's Atlantic Forest that stretches along the country's coast. This forest contains approximately 20,000 plant species and 1,350 vertebrate species. Additionally, there are millions of insects here that are found nowhere else on Earth. The high biodiversity found in these areas is threatened, mainly from habitat destruction. Humans destroy these natural habits for many reasons, including harvesting the natural resources or clearing habitats for agriculture, mining, and logging. When a natural habitat is changed by these activities, it may no longer be able to provide food, water, and cover to support the species, and as a result, the species that lived in or used the site are displaced or destroyed. Question Why should we care about biodiversity? Biodiversity provides humans with many services. We rely on the millions of species of life on Earth for many reasons, including for food and medicine. The island of Madagascar separated from mainland Africa approximately 66 million years ago. Since then, Madagascar's ecosystems and species, like these ring-tailed lemurs, evolved independently. For that reason, Madagascar's dry deciduous forest and lowland rainforests possess a high ratio of endemism, which is the association of a species with a well-defined geographic area. Indonesia consists of 17,000 islands. Together, the islands contain 10 percent of the world's flowering plants, 12 percent of the world's mammals, and 17 percent of the world's reptiles, amphibians, and birds. Colombia is home to more than 1,900 different species of birds--that's more birds than are found in Europe and North America combined. In addition to 18 percent of the world's bird species, Colombia contains 10 percent of the world's mammal species and 14 percent of the world's amphibian species.

stability

Communities differ in how they respond to disturbances. The tendency of a community to maintain relatively constant conditions over time is referred to as stability. Although stability is an important characteristic of a population, it is difficult to define and difficult to measure. Some of the variables that are measured to quantify stability are the amount of living plant material, population sizes, and species diversity within the community. In a stable community, these variables will be maintained at fairly constant levels. After a disturbance, a stable community is able to either resist change (called resistance) or recover quickly (called resilience). Look through the tabs below to learn how a community may respond to disturbances. Resistance Sensitivity Resilience If a very stable community is exposed to a disturbance, it may not change at all. All of the characteristics examined may stay the same. This quality is referred to as resistance. A community may be resistant to disturbances because it is difficult for other species to invade it. For example, extreme temperatures or amounts of precipitation may allow for a stable community because this limits the kinds of animals that can successfully invade that community. An area in the Arctic, like the one shown here, will be resistant to invasion by most biotic disturbances because very few organisms are adapted to survive the cold climate. If the community contains many predators, smaller animals will get eaten and other predators may encounter competition. If many resources are limited, this may also make a community more resistant to disturbances. The opposite of resistance is sensitivity. A sensitive community is inherently unstable because it is easily affected by disturbances. Communities may be very sensitive to some types of disturbances, but resistant to others. For example, if many populations within a community are susceptible to a particular pest, the community displays sensitivity. Many individuals may get sick or be killed if the community is exposed to that pest. If many crops of the same species are grown in one area and exposed to a disease, the entire crop is wiped out, which is what caused the Irish potato famine in the mid-1800s that killed over a million people. However, that same community may be resistant to abiotic disturbances, such as drastic weather changes, if the species are adapted to handle large, abrupt changes in temperature and precipitation. A more resilient community is able to recover faster from a change caused by a disturbance. Following a disturbance, a resilient community will change and then return to its previous state. How fertile its individuals are, how much they interact, and how water and energy move through the ecosystem can all allow for a stable community. For example, if a forest fire kills many of the organisms in an area, but the survivors are very fertile, the population size can be brought back up to previous numbers quickly. On the other hand, if the survivors are not very fertile, it will take a long time for the populations to be restored to their previous sizes. This type of community is not very resilient. Some disturbances are drastic enough that the community can never return to its previous state regardless of how resilient it is, such as in the featureless land in this image. As more land and resources are being used today, ecologists and developers are trying to be more mindful of the resilience of the communities that they disturb. Ideally no community would be disturbed past the point from which it can recover.

competition

Each species in a community must have its own niche. In other words, each species needs to have its own living pattern that includes what it eats, where it lives, and when it is active. Some aspects of a niche, such as diet, use limited resources. If multiple species try to use the same limited resource, those species will fight over resources. For example, if two species eat the same leaves from the same species of plant, those two species will fight over the plant. When two or more species use the same limited resource, it results in an interaction between the species called competition. Some limited resources that species may compete for are food, water, nesting sites, or access to sunlight. Look through the tabs below to see different outcomes of competition. the winner When two species compete for the same resource, one will often be more successful than the other and use more of the resource. For example, if two plant species, such as sunflowers and grass, are competing for sunlight in the same area, the plant that grows faster will shade the other and get more sunlight. the loser If this resource is necessary for both species, the losing species will decrease in numbers. In this example, the grass is receiving less sunlight than the sunflowers and dying out. elimination Sometimes one species is extremely more successful than the other. In this case, the losing species may be entirely eliminated from the community in a process called competitive exclusion. In this example, the sunflowers have completely shaded the grass, and all the grass has died. sharing if the resource is space, the two species may divide the desired space between the two of them, a phenomenon called resource partitioning. The two species of barnacles have divided up a resource so that each species gets a different section of the habitat. They have partitioned the resource so that they can coexist in the same community without competing. .

benefits of biodiversity

Food and Medicine: Biodiversity is the foundation for our survival and well-being. We use many different plants and animals for food and medicine to keep us alive and healthy. Shelter and Fuel: We use wood from a variety of trees for shelter to protect us from the elements. Historically, we also used wood as fuel to cook our food and keep warm. Crop Pollination: Our agricultural crops are pollinated by many different insects, birds, and bats. Many of our food plants evolved with these animals and continue to depend on them for pollination. Water Purification: Some of our clean water supply comes from water flowing through forests systems. The root systems of trees and other plants help keep soils porous. This, in turn, helps filter impurities from water and prevents them from entering our streams, lakes, and ground water.

how animals live together

Have you ever fought with your siblings for the last piece of pizza? Have you ever been on a school bus or airplane and seen people argue about how to share a limited amount of space? When people try to use the same resources, they are in competition with each other. They may argue or fight until the winner gets what he or she wants, or they may come to an agreement about how to share. Like humans, other animals do the same thing: When individuals from two populations in a community attempt to use the same limited resource, they may fight, or they may form a relationship such that they no longer compete. Watch this video to learn more about how animals can live together by forming relationships. Narrator:Animals will compete for resources that are limited. For example, if two species eat the same fruit from the same trees, they are trying to fill the same dietary niche.They may fight for food, then the winners will get to eat. If the losers want to eat, they will have to find a new food source, or move to a new community.But many species that live in the same community have found ways to live together, and even take advantage of their differences.For example, a clownfish has many predators in the open seas. The sea anemone also has many predators. These two species may work out a deal that benefits both.The very territorial clown fish protect the sea anemone from fish that would otherwise eat it, and the stinging nettles of the anemone protect the clownfish.Instead of competing, species can form relationships that benefit one or both members of the relationship. Transcript Question Individuals that form beneficial relationships rather than fighting will get to eat more and will sustain fewer fighting injuries; thus, they are often more fit. How will these individuals affect the evolution of the species? These individuals will pass on their alleles with greater frequency. If the tendency to work together instead of fight has a genetic component, each generation will have more animals that form relationships instead of fighting; therefore, the two species could evolve to help each other out and maybe even become dependent on each other.

how many humans are there on earth?

Homo sapiens (humans, as we know them today) evolved approximately 400,000 years ago. The global population of humans grew steadily, reaching 1 billion in the year 1800. Although it took about 399,800 years to grow to the first billion, the population grew from 3 billion to 5 billion in only 27 years, from 1970 to 1987! The current global population of humans is roughly 7 billion. Watch the following video to get a better idea of how large 7 billion is. 7 Billion... it is a number that is hard to imagine. Just how big is 7 billion?It's the number of seconds in almost 222 years.7 billion steps would take you around the world 133 times.7 billion M&M's would fill three Olympic size swimming pools.In the 1800's, the population of the earth was estimated to be about 1 billion.Within 80 years the population had doubled, reaching 2 billion.By the mid-1970's it had doubled again, reaching 4 billion.According to the United Nations, the world population hit 7 billion in October of 2013.Every second, another 5 people are born, but only 2 people die.At this rate, it only takes three years for the world population to increase by the equivalent of another United States.Can the world sustain this population growth? Transcript Question The human population has experienced rapid growth since 1800. Can the human population grow indefinitely? No, there are density-dependent limiting factors of living on Earth that prevent any population, even humans, from growing indefinitely. Because each individual needs resources to live (such as food and water) and these resources are limited, population size is also limited.

character displacement

If species compete for niches, this may result in the evolution of differences in particular characteristics. This process is called character displacement. Take for example Darwin's finches that he observed on the Galapagos Islands and wrote about in his famous book about evolution by natural selection, On the Origin of Species. Each of these finch species had a beak that was specialized for the type of nut, seed, or other food item that it ate. All of the finch species on the island were descended from a single ancestral finch species. When individuals competed for the same food source over many years, mutations led to advantageous changes in beak shape and size that made certain individuals better at eating particular foods. Over many years of competition over food, these beak differences became more pronounced, eventually leading to different species, each with its own beak shape and size and own food source. The evolution of different beaks after generations of competition is an example of character displacement. Question Will competition always result in character displacement? No, this only occurs if the advantageous traits are genetic. For example, if the Galapagos finches wore down their beaks from eating certain seeds, these changes would not be genetic. A smaller, worn-down beak would not be inherited by that finch's offspring even if it helped the finch eat smaller seeds.

How does growth rate differ in various parts of the world?

If the human population continues to grow at the same per capita rate, our population will reach almost 9 billion by the year 2050. As we grow, we use up more of Earth's resources such as food, water, and energy. There are already populations in many parts of the world where resources are strained or insufficient to support life. Approximately 80% of the world population lives in developing countries where many people live in villages without roads, electricity, or running water. As countries become more developed, they become more industrialized, which means that there are more large businesses and factories and more people work jobs instead of subsisting off the land. This map shows how urbanization differs between countries. Urbanization is similar to development in that it is a measure of how dense the houses and roads are. When infrastructure, such as living spaces and transportation, are more dense, the area is more developed and more industrialized. Areas in Africa and Latin America are largely yellow because they are undeveloped. In these areas, the availability of food, clean water, and healthcare are limited resources. The land is not as fertile, and there is little infrastructure for transporting and cleaning water for human use and consumption. The unavailability of life-sustaining resources is worsened by the high growth rates in these areas. Although death rates are higher than in more developed countries, birth rates more than compensate. Most of the world's population growth (about 99%) occurs in less-developed countries. For example, in 1996 the growth rate of Nigeria, an undeveloped country in West Africa, was 0.0305 per capita per year. Most of these countries grow at about 0.0204 per capita per year and will double in size in about 30 years. Compare this to the growth rates of developed countries, which are much slower. The expected life span of an individual in a developed country is longer due to better access to nutrition and healthcare and less strenuous lifestyles. Also, in developed countries the death rate is lower. Each of these could lead to an increase in growth rate. As people live longer, they have more chances to reproduce. However, individuals in developed countries have better access to birth control, are more educated about birth control methods, and often are home to cultures where it is common to have only a few children at most. Thus, in developed countries, birth rate is lower than that of less-developed countries, so the growth rate in developed countries is also much lower. For example, the annual per capita growth rate in the United States is only 0.008. The average per capita growth rate in developed countries such as those of North America and Europe is 0.005 per year, which will double these populations in about 137 years. There are differences in birth rates and death rates between developed and less-developed countries that lead to differences in growth rates. Do you understand these relationships? In the following activity, identify whether each concept is associated with developed or less-developed countries.

ecological succession

In different ways, this also can happen to plant communities. A plant community is a group of different plants living together in the same area. The plants are living rather peacefully in a stable community until some type of disturbance upsets it. Disturbances vary from natural causes, such as volcanic eruptions and flooding, to human causes, such as logging or accidental fires. After the disturbance, a predictable process--ecological succession--follows in an orderly manner until the plant community reaches a climax or stable community again. Ecological succession begins with simple plant communities, which are replaced with more complex plant communities over time. The type of succession depends on the disturbance. If the disturbance destroys the soil, for example, the succession involves more stages of plants and takes more time to return to a stable community. If the soil is not destroyed, the succession involves fewer stages of plants and takes less time to reach a climax community. Question What initiates ecological succession? Ecological succession begins with a natural or human disturbance to a stable community.

What can we learn from other animal populations? (reindeer)

In less developed parts of the world where resources are strained and often insufficient to support life, growth rate is high due to high birth rates. Although the global per capita growth rate has decreased in the last few decades, our population is still growing and will eventually run out of resources. We can look at other animal populations, their growth rates, and how they interact with their environment to predict how our growth will affect us and the environment. We may also be able to prepare for our future or take measures to prevent negative effects. For example, in 1911 a population of 25 reindeer were introduced to an island off Alaska where there were no native reindeer. Like the global human population in the last 200 years, this population of reindeer experienced rapid exponential growth. By 1938, the population had grown to over 2,000 animals. The island this population inhabited is only 41 square miles. The population density exploded from less than one individual per square mile to almost 50 individuals per square mile in only 27 years. The reindeer ate the plants on the island faster than they could regrow and ran out of food. Food was a density-dependent limiting factor that prevented the population from growing infinitely. As the plants were cleared away, the reindeer had nothing left to eat and many died. By 1950, only eight reindeer were found. Our population is growing exponentially like the reindeer population on this Alaskan island. In less-developed countries, there are already insufficient resources to support the population size. It is only a matter of time before this is the case worldwide. It is possible that we will eventually exhaust our resources like the reindeer on the island, and our population will crash in size due to massive die-offs. Use the activity below to compare and contrast our population with the example of the reindeer. Sort each descriptor into the correct category.

Commensionalism

In some symbiotic relationships between individuals, both members benefit (mutualism), or one may benefit while harming the other (parasitism). There are also symbiotic relationships in which one member benefits, and the other is neither harmed nor helped. This type of relationship in which one member is basically unaffected is called commensalism. Click through these tabs to learn about the different benefits of commensal relationships. Food Transportation When one species gets food from its relationship with another species without hurting or helping it, this is a type of commensal relationship. For example, scavengers look for food that other animals have left behind, like the hyena in this image. The animal that gets food will benefit from this relationship. The animal that left the food behind is neither positively nor negatively affected. When one species attaches to another for transportation without hurting or helping it, this is another type of commensal relationship. For example, some small insects, called mites, attach to larger insects like beetles or flies, as in this image. This allows the mite to save energy and travel longer distances than it could on its own, and the larger insect is unaffected.

predators

Living organisms can be classified as either autotrophs or heterotrophs, depending on whether they can capture their own energy from the environment. Heterotrophs cannot do this, so they must eat autotrophs to increase the amount of energy in their body. Heterotrophs are either parasites or predators, depending on whether they feed on other organisms for a long time or kill them immediately. A food chain in which the bird eats the snake, who eats the rodent,who eats the praying mantis, who eats the grass (an autotroph). Predators can be classified based on what kinds of organisms they kill and eat as prey. Predators that kill and eat animals are called carnivores, while predators that kill and eat plants and other autotrophs are called herbivores. Many predatory species eat both; they are called omnivores. Most organisms can act as predators or prey. For example, a rabbit will act as a herbivorous predator when it eats grass (its prey), but the rabbit will be the prey when it is eaten by a coyote (the carnivorous predator). Organisms can be ordered by who eats whom to form a chain of predators and prey, called a food chain. Generally, organisms at the bottom of the food chain are herbivores; organisms higher on the food chain eat animals as their prey, and are thus carnivores. At the bottom of every food chain is an autotroph, which is eaten by a herbivore. This is the point at which energy from the environment enters the food chain. If every heterotroph ate another heterotroph, the food chain would run out of energy because only autotrophs can capture energy from the environment. As each organism acts as prey for the next organism in the food chain, only about 10% of this energy is passed upward to the next organism.

Yellowstone National Park

Narrator:From June through November 1988, almost 250 individual wildfires spread out of control in Yellowstone National Park in the United States. It was the largest wildfire in the park's recorded history.The fires affected almost 1.2 million acres of the park. In some areas, entire forests were destroyed. In a matter of days, though, ecological succession began.Fireweed was one of the first plants to reappear. In fact, most of the small plants regrew from existing sprouts about ground and roots and seeds below ground.One year after the fires, a blanket of wildflowers covered the burned areas. Wildflowers continued to grow and dominate these areas for about the next four years.Lodgepole pines are the predominant tree in Yellowstone National Park. Many were destroyed in the fires. However, the lodgepole pine is serotinous, meaning it produces pine cones sealed with a waxy resin. The pine cones remain closed until exposed to temperatures above 113 degrees Fahrenheit.In other words, fire is needed for lodgepoles to reproduce. After a fire had passed through a stand of lodgepole pines, any pine cones opened from the fire and reseeded that area. One year after the fires, the first lodgepole pine seedlings were appearing in the burned areas.Twenty-five years after the fires, the burned areas of Yellowstone National Park are still recovering. You can still see dead trees (called snags) standing among 24-year-old lodgepole pines. Transcript Question Which type of succession, primary or secondary, occurred in Yellowstone National Park after the 1988 wildfires? Secondary succession began in Yellowstone National Park after the 1988 wildfires.

density independent factors

Not all organisms follow a logistic model of growth. Some species, especially many plants and insects, grow exponentially and are primarily limited by environmental factors such as weather. For example, mosquito populations grow exponentially when it is warm outside; then the population shrinks when it is cold. Many species of plants and insects follow this pattern: populations exhibit exponential growth and then are prevented from growing further by some environmental variable. Environmental conditions that limit population size and growth independent of population density are referred to as density-independent factors. They do not contribute to calculations of carrying capacity because they affect populations of any size and density. The impact of density-independent factors does not scale with population size or density. Density-Independent factors can affect any species. Some species, like many insects and plants, are limited primarily by density-independent factors and not density-dependent factors. These species more closely follow the exponential model of growth than a logistic model as shown below. Exponential model of growth. Another example of a density-independent factor is a forest fire that kills a large portion of a population living in the forest. The extent to which the forest fire limits population size and growth is not affected by how dense the population is. It will kill the same number of animals in a large population inhabiting that forest as it will a small population inhabiting that forest. Natural disasters, weather, and other changes in the environment typically limit population size in a density-independent manner. Density-Independent factors limit population size and growth equally, regardless of population size or density. They are environmental conditions that do not contribute to carrying capacity. In the following activity, connect the circle on the left to the density-independent factor.

Age Distribution

One factor that makes direct measures of population size difficult is that populations are dynamic. In other words, they are constantly changing. Individuals die; new individuals are born. Not only do deaths and births affect population size, they also affect other aspects of populations that are relevant, such as how that population evolves and how its individuals behave. For example, if you unknowingly chose to study a population of mammals that are past reproductive age, you might mistakenly conclude that this species has bizarrely secretive mating habits or that they were organized into a social hierarchy in which only an extreme minority of the population was permitted to breed, when in fact you are simply studying very old animals. Thus, it is helpful to know another characteristic of the population: the age distribution. The age distribution of a population is how many individuals there are of each age. We can compare the age distributions of different groups of animals to understand more about each population. For example, one population of a species may have many young animals, while another population of the same species has mostly old animals. We can make some predictions based on these differences. We would expect the younger population to produce offspring but not the older population. We would expect individuals in the older population to die before those in the younger population. Age distribution is often presented as a graph like this one. This graph shows the age distribution of humans in a county in Massachusetts according to 2000 census data. To create this graph, age was split up into ranges (y-axis). The number of individuals in the population in each range were counted and are displayed as percentages (x-axis). This allows us to compare the percentage of people in different age brackets. Males are shown on the left in red and females are on the right in blue. This allows us to compare the age distribution of males vs. females. For example, within males (red on left), we can see that there are more 15-19 year old males than 20-24 year old males. Between sexes, we can see that there is a higher percentage of males from 20 to 24 than females. Do you understand what an age distribution tells us? Answer the following questions to test your understanding.

paracitism

Parasitism < 5 of 9 > One partner benefits, and the other is harmed. In a mutualistic symbiotic relationship, both members of the relationship benefit. The second type of symbiotic relationship is parasitism. In a parasitic relationship, one member (the host) is harmed, while the other member (the parasite) benefits. Usually, the host is larger than the parasite and serves as a food source and home for the parasite. Also, the host may help transmit the parasite's offspring to new hosts. Click the tabs below to read about the two types of parasites defined by where they live in relation to their host. Ectoparasite Endoparasites Ectoparasites live on the outside of the host's body, where they receive some benefit while harming the host. Usually, the parasite will use the host as food, which can lead to discomfort, blood loss, and transmission of diseases to the host. Click the image to see examples of ectoparasites. Lice.Mosquitoes.Leeches. Because parasites are harmful to hosts, animals have evolved defenses to prevent parasitic infection. For example, an animal's skin has many functions, one of which is to prevent parasites from entering the body. At openings in the skin, such as the eyes and mouth, secretions like tears and saliva help prevent parasites from entering. However, if these defenses are insufficient, and parasites do enter the body, cells of the animal's immune system may attack the parasites to further prevent infection. Endoparasites live inside the host's body, where they receive some benefit while harming the host. Like ectoparasites, these organisms usually use the host as food or take food from the host. This can make the host sick either due directly to the parasite's feeding habits or due to transmission of diseases to the host. Tapeworms.Heartworms.Giardia.

Affects population growth

Population size and population density are two important characteristics of a population. Population size is the absolute number of individuals in the population, while density takes in to account the area that the population inhabits. The density of a population is important because whether individuals are crowded or spread out affects their lives and their interactions with each other. For example, being in a crowded population can have both advantages and disadvantages. Having more individuals of your species nearby increases the likelihood of finding a mate and can help protect the group from predators. Because there are more individuals, the genetic diversity of the population will be higher. This will make the population more likely to survive outbreaks of disease. However, living in a smaller population can also be advantageous. With fewer individuals in your group, it will decrease the likelihood of disease outbreak and reduce competition for resources.

Measuring populations

Population size is an important characteristic of a population. Many processes that affect a population differ depending on how large or small it is. There are several ways to determine the size of a population. Small populations can be counted directly by simply going to the habitat area and counting the number of individuals. For small populations, we can often precisely determine population size. However, even for small populations, the ease of directly counting the number of individuals varies. Take a look at each of these animals. What factors affect our ability to measure their population size directly? Click the image to see if you guessed correctly. In addition to these factors, many populations are simply too large for us to count individual by individual. Because of the impracticalities of counting individuals, there are several techniques to estimate population size. Read about each in the table below. population indexWe can measure an index, which is a factor that is directly related to population size. For example, this could be the number of nests (instead of counting birds), the number of tracks on the ground (instead of counting possums), or the fecal droppings on the ground (instead of counting deer). mark-and-recaptureWe can capture animals in their habitat, mark them, and release them. Later, we can go back to the habitat, capture again, and count how many marked animals there are. The proportion of marked to captured animals can be used to estimate the entire population size. This process is referred to as mark-and-recapture. samplingWe can estimate how large the population is by counting only members of the population in a particular area (a process called sampling) and then use this to estimate the size of the entire population over its entire inhabited area.

makeup of a community

Populations are members of a single species that live close together. Many populations of different species often live together and share the same area of land. All of the species that live in the same area and interact are called a community. For example, all the fish, turtles, insects, plants, algae, single-celled protists, and bacteria in a pond make up a community. How many different species make up your community? Watch this video to learn about the organisms that are part of the community you live in. Narrator:Your community consists of you and all the organisms that live in your habitat with you. What kinds of organisms are in your community?Family members living in your house.Plants in your yard and house.Maybe some mold growing in the refrigerator.Rodents, cockroaches, small insects, and other arthropods living in the walls and under the floor.Small insects, like mites, living in your hair.Bacteria that live on your skin and in your gut.So you see, you are far from alone. Your community--in the biological sense--consists of thousands (if not millions) of organisms of dozens of different species all occupying the same area of land.

predator-prey effect

Predators have adaptations that make them more efficient at locating, obtaining, and eating their prey. A predator that is faster or stronger or has teeth well suited for its particular food will obtain more energy and be able to reproduce more, so it will be more fit and will pass on its alleles with greater frequency than a predator that is less well adapted. Prey also have adaptations that increase their fitness by making them better at evading predators. Because predators eat prey, they affect how many prey there are (the population size of the prey). Because predators depend on prey, prey also affect the population size of the predators. When predators eat their prey, the population size of the prey decreases. If predators eat fewer prey, the population size of the prey increases. A habitat has limited resources, such as food, water, and space, that all of the individuals in the population must share. When population size within a given habitat increases, the number of individuals per unit area (population density) also increases. As population density increases, the individuals in the population will compete for resources. If there are too many predators, they will compete over resources, including prey if they are limited. If prey become too abundant, they will also compete for resources, such as food, water, or nesting sites. Populations of both predators and prey are subject to fluctuations in size, density, and competition, and populations of one can influence the other. Look through the tabs below to see how this works. Too Many Prey Too Few Prey Too Many Predators Too Few Predators When there are many prey, predators are well fed and will increase in population size. This is shown in the graph above where an increase in prey population size is followed shortly by an increase in predator population size. Further, prey actually depend on predators to prevent their populations from growing too large. As the density of the population (number of individuals in a given area) increases, the individuals will compete for resources. If there are too many prey, they will compete for resources. The prey may also be driven out of the area into other areas, disrupting those communities. These results of overpopulation can be relieved by predation. If some of the individuals are eaten, the population size (and thus the population density) of the prey will decrease. There will be more resources for each individual. In this way, prey rely on predators to reduce competition for resources because predators will reduce the population size of prey.

Allogenic and Autogenic Factors

Primary and secondary succession both begin with a disturbance to a plant community. The disturbance initiates the process of succession. As the process of either type of succession continues, changes in the vegetation of the area occur over time. The changes in the vegetation that occur during succession are influenced and driven by the biotic and abiotic factors of the community. Read each tab to find out what these are. Allogenic Factors Autogenic Factors Allogenic factors are due to the abiotic components of the ecosystem. Examples of allogenic factors include soil changes due to erosion, sediments collecting in a marsh, climate change, even periodic fires or floods.The allogenic factors influence and drive the growth of the vegetation in a given area. Autogenic factors are due to the plants themselves. Autogenic factors include leaves capturing light (as seen in this image), production of detritus (bodies of dead organisms and fecal material), uptake of water and nutrients, and fixation of nitrogen. These factors can make the community more or less suitable for certain plant species. For example, when large trees mature, they cause shade that blocks out the sun. As a result, forms of vegetation that require lots of sunlight will not be able to grow, but other forms of vegetation that can grow in the shade will invade the area.

secondary succession

Primary succession begins in areas without soil, so soil formation is the most important process in this type of succession. Another type of ecological succession, secondary succession, occurs under different conditions. Secondary succession occurs in areas where soil already exists and once supported vegetation. It begins when that vegetation is destroyed by some disturbance, such as a flood, hurricane, tornado, fire, deforestation, overgrazing, or agricultural crop. Some plants and animals may still exist in the area, yet sometimes it's only plant seeds and roots in the soil. Since soil is already present for secondary succession to begin, there is no need for those pioneer species (algae, fungus, lichens, and moss) that form soil. What plants do you think begin secondary succession? Click through the slides below to check your answer and learn about the stages of secondary succession. A Disturbance Secondary succession begins with some type of disturbance that destroys most of the vegetation in an area. Looking at this photo, you can see the difference in vegetation on both sides of the fence line. The left side has been disturbed by animals overgrazing (eating too much), leaving bare soil and few plants. On the right side of the fence line, you see there has been little, if any, grazing activity. If the animals who are overgrazing in the area are removed, secondary succession will begin. Test your knowledge of secondary succession by placing these stages in an order that best represents the process of secondary succession from beginning to end. Which do you think takes less time, primary or secondary succession? Why? Secondary succession is faster than primary succession because secondary succession begins with soil already formed. After a disturbance that destroys most, if not all, vegetation in an area, grasses and other herbaceous plants grow back first. Herbaceous plants have leaves and non-woody stems that die at the end of a growing season, leaving the roots in tact. This means that a herbaceous plant's roots may still be active in the soil even after a disturbance. After grasses and other herbaceous plants are established for a few years, you will see small shrubs and young trees appearing and growing. This photo shows young pine trees growing tall above the grasses several years after a forest fire in Flathead National Forest, Montana. It may be many years after the initial disturbance, but secondary succession will eventually slow down and reach a climax or stable state. The end result may look like this mature forest. It may look like this for many years--unless another disturbance destroys it.

population growth

Recall that a population is a group of individuals that are able to interbreed because they are the same species and live close to each other. Population size, which is the number of individuals in the population, is not static. Since individuals are born and die, the size of the population changes. Population growth describes the changes in the number of individuals in a population. As populations fluctuate in size and experience population growth, they use resources differently and interact with each other in different ways. Watch this video to see how the growth of one population can affect another. In many parts of the United States, the white-tailed deer has become a nuisance. The populations of their natural predators - such as wolves and mountain lions - have decreased in size. Without predation, populations of white-tailed deer have grown. During mating season, males travel many miles to mate with females they are not related to. In urban areas, deer can cause car accidents and destroy people's yards and gardens. Where deer are particularly abundant, government entities like Parks and Wildlife Departments spend a lot of time, energy, and money trying to control local deer populations while still being humane and maintaining balance in the local ecosystem. Although the deer can be annoying, costly, and even dangerous in urban areas, we do not want to get rid of them entirely. By studying factors that affect population size, we can predict how size will change so that we can attempt to control invasive species like the white-tailed deer in a responsible way. Transcript Question Can the growth of one population affect the growth of another? Yes, all animals, plants, and nonliving components of an ecosystem interact with each other. As you saw in the video, the population size of deer increased when the population size of its predators decreased.

population size

Recall that a population is a group of individuals that live close to each other and are able to interbreed because they are the same species. One way to describe a population is by its size. Population size is the number of individuals in the population. The boundaries of a population may be naturally occurring, or they may be defined by the person studying them, like the number of birds in a particular forest or the number of humans in a county. These individuals may be able to interbreed with others outside the boundaries, but defining the perimeter of the population allows us to categorize individuals based on where they live and compare different habitats or parts of the world. Humans can obviously travel from one country to another, but it is still meaningful to compare the population of one country to another. Size is an important characteristic of a population because large and small populations evolve in different ways. Larger populations are usually more genetically diverse, so they are less likely to go extinct. Smaller populations are more susceptible to disease and will have fewer new mutations in each generation (because there are fewer individuals). If we know the size of the population, we can predict how it will evolve over time. Knowing population size is important, as is identifying changes in population size. Click through the tabs below to see examples of why changes in population size are important. As you read, think about how information about changes in population size can be used. Conservation Bioindicators Planning for the Future We can track how population size increases and decreases to identify species that are at risk of becoming endangered or extinct. If we notice these trends in time, we may be able to intervene with conservation efforts to preserve biodiversity. The California condor, shown here, was on the brink of extinction in the 1980s but was saved by emergency conservation efforts. Some animals are highly sensitive to environmental changes, such as increased pollution or changes in temperature. For example, amphibians like frogs absorb many substances from the water through their skin, so they are particularly susceptible to toxins that are present in the water. Changes in population sizes in these species can be indicators of environmental damage, giving us a chance to try to prevent further damage. If we know how the size of a population is changing in response to other factors, we can predict how it will change in the future and attempt to prepare for those changes. Some models predict that the world population will reach 10 billion by the end of this century. Considering such possibilities shapes agricultural practices, land use, global politics, and innovations in technology.

evolutionary past

Since biodiversity is affected by temperature, precipitation, geography, and disturbances, it is no surprise to see that biodiversity has fluctuated throughout Earth's history. The fossil record shows that biodiversity has fluctuated with differences in climate and major disturbances during various phases of history. Geological evidence also supports that temperate regions were subject to frequent glaciations. In contrast, tropical regions have remained relatively undisturbed for millions of years, which allows for evolution of species and increased biodiversity. There have been five major mass extinctions in Earth's history. These are called "The Big Five," and they led to large and sometimes sudden drops in biodiversity. Furthermore, biologists believe that we are currently in another mass extinction, often referred to as "The Sixth Extinction." Read about these major mass extinctions and their effects on biodiversity in this slideshow. Ordovician-Silurian Mass Extinction The fossil record indicates high biodiversity first occurred during the "Cambrian Explosion" around 542 million years ago. The rapid increase in biodiversity lasted about 20 million years, marking the first appearance of nearly every phylum of multicellular organism, including now extinct trilobites shown in this image. Sixty to seventy percent of all species were killed with the Ordovician-Silurian extinction event (450 to 440 million year ago), thought to have been caused by a combination of glaciation, volcanic activity, and a decrease in carbon dioxide. During the Devonian period (419 to 360 million years ago), the first significant adaptive radiation of terrestrial life occurred and increased biodiversity again. Adaptive radiation is the process in which life forms evolve rapidly into many new forms of life. During the late Devonian period (375 million years ago), the second mass extinction was underway with a series of extinction pulses caused by major environmental changes over 20 million years. Eventually, 70 percent of all species were eliminated. During the Tertiary period (299 to 252 million years ago), biodiversity increased once again, this time with the ancestral groups of amniotes. These were tetrapods that laid eggs on land, such as turtles. The Permian-Triassic extinction event (252 million years ago) was the most catastrophic mass extinction in Earth's history, most likely caused by an impact of a meteor. Up to 96 percent of all marine species and 70 percent of all terrestrial vertebrate species went extinct. This event is the only known mass extinction of insects. So much biodiversity was lost, it took 30 million years for life to recover, well into the Triassic period. During the Triassic period (250 to 200 million years ago), plant and animal life slowly recovered to its former biodiversity, including the amniotes, such as the Cryobatrachus kitchingi pictured here. However, biodiversity decreased once again with the Triassic-Jurassic extinction event (200 million years ago), potentially caused by climate change, an asteroid impact, or massive volcanic eruptions. Regardless of the cause, 70 to 75 percent of all species went extinct. During the Cretaceous period (145 to 66 million years ago), life recovered and biodiversity increased with dinosaurs becoming the dominant terrestrial animal. This period was known as the "age of reptiles." Most likely an asteroid impact began the Cretaceous-Paleogene extinction event (66 million years ago), resulting in the death of 75 percent of all species and marking the end of the dinosaur age. Tertiary is the former term for the first period of the Cenozoic era. Although the term tertiary is still widely used today, this traditional span of time has been divided into the Paleogene and Neogene periods. During the Paleogene period (66 to 23 million years ago) birds and mammals, such as Titanoides shown here, emerged as the dominant terrestrial vertebrates. The fossil record indicates the last few million years has featured the greatest biodiversity in Earth's history. This biodiversity is now in a period of decline that started about 10,000 years ago. The rate of extinction during the last 10,000 years has been between 1,000 and 10,000 times the normal rate of extinction. Most biologists agree that we are in a new mass extinction called the Holocene extinction. Though scientists do not agree on the primary cause of this mass extinction, many agree that humans play a major role in it.

Determining Stability

Stability may be related to two other characteristics of communities. Species evenness is the relative population size of each species in the community. In other words, how common is each species in the community? The other characteristics is species richness, which is the number of different species in a community. It is hypothesized that diversity (species richness) is a factor that allows for a stable community, which is called stability-diversity hypothesis. This may be because in communities that have many species that are about equally common, the species interact more. The species have to share resources more effectively and may depend on one another more for survival. When a disturbance occurs in a community with many connections between species, these connections may diffuse the effect of the disturbance such that the community as a whole is less disrupted. Evidence for the stability-diversity hypothesis includes how vulnerable monocultures are to diseases and pests. Monocultures are large areas where only one plant species is being deliberately grown usually for agricultural purposes, such as vast fields of corn, wheat, or soy. A field of soybean (left) and a field of wheat (right). These areas have extremely low species richness. If the field is exposed to a pest or disease that one plant is susceptible to, that means all of the plants will be susceptible because they are all the same. The community would be extremely sensitive to this pest and thus unable to recover. This community is of low diversity and extremely low stability. However, these areas have not evolved naturally with the pest species around it, so they may not be good models to support the stability-diversity hypothesis. On the other hand, diverse habitats, like tropical rain forests, appear to be fairly resistant to diseases and pests, but these areas contain many species and interactions that are not fully understood. It is possible that there are common outbreaks of diseases or pests of which scientists are unaware. Community X has 10 different species with 5 individuals each for a total of 50 individuals. Community Y has 100 different species with 5 individuals each for a total of 500 individuals. Answer the following questions to compare these two communities. A more rich, interconnected community may be more stable. How could interconnectedness make a community LESS stable? If a key species that many are connected to is disturbed, this will have an impact on many more species than in a less rich, less connected community. If this were the case, a more diverse community would actually be less stable. The relationship between stability and diversity is complicated. Stability and diversity are undoubtedly linked, but there are many factors that contribute to their relationship.

factors that affect biodiversity

Temperature Precipitation Geography Disturbance A warm climate supports high biodiversity. There is a higher mean temperature at the equator compared to that at the poles. Typically, tropical environments are less seasonal and more constant and predictable. Such constant environments often lead to greater species richness. You just learned about some key factors that affect biodiversity. Test your knowledge by connecting each factor with how it affects biodiversity. Warm temperatures increases biodiversity decreases biodiversity Question Why is biodiversity highest in the tropics? Biodiversity is highest in tropical regions because these environments are stable with warm temperatures and predictable precipitation. Predictable precipitation (rainfall) supports high biodiversity. The tropical regions have had wet climates for millions of years, which has allowed for evolution of species diversity. Biodiversity is affected by Earth's geography. Some areas are full of species richness, such as tropical rainforests (shown in this image) and coral reefs, while others, such as deserts, may be devoid of life. Furthermore, scientists estimate terrestrial (land) biodiversity is 25 times greater than aquatic (water) diversity. Biodiversity can be affected by a disturbance, either natural or caused by humans. During Earth's past, natural events, such as asteroid impacts, disrupted or reduced biodiversity. Today, scientists believe human impacts are affecting biodiversity. Habitat destruction is one such human impact, as seen in this image of a farm area cut out of a tropical rainforest in the Ecuadorian Amazon, South America.

habitat

The area in which a population of a species lives is called its habitat. The many populations of species that make up a community all share the same habitat. The habitat is where the community lives; it is the natural environment. A community or population's habitat does not include the living organisms themselves. For example, a community of fish, algae, and other organisms all share a single pond. The pond, including the water, soil, and sunlight, is the habitat for this community.

density dependent factors

The effect of some limiting factors on population size and growth depends on how dense the population is. Population density is the number of individuals per unit of area. population density=population sizeinhabited areapopulation density=population sizeinhabited area Variables that limit population size or growth and depend on population density are called density-dependent factors because their effect on population size and growth depends on how many individuals there are in a given habitat. These limiting factors are usually resources (as opposed to environmental conditions), such as food, water, space, or sunlight (for plants). The availability of these resources for each individual depends on how dense the population is. For example, if a population is small, the amount of food available will probably be sufficient for each individual to have enough. However, as the population gets larger but occupies the same area, each individual will get less and less food. At some point, the population will become dense enough that some individuals will not have enough food to survive, and the population will stop growing. Food is a density-dependent factor: its limiting effect on population size and growth depends on the density of that population. Diseases are also more likely in larger populations, so disease is a density-dependent factor that limits population growth. An individual's chance of surviving due to density-dependent factors is affected by the number of individuals that share its habitat. Logistic model of growth. A logistic model of growth tries to account for the density-dependent factors that limit the size and growth of the population. As you can see in the graph, as the population gets larger (and habitat area stays constant), the slope of the line decreases and eventually levels out. In other words, the rate that the population grows gets steadily smaller until it stops growing and stays at a steady size. At this point, it has reached the carrying capacity of its environment. The rate of population growth steadily decreases because of density-dependent factors. As a population gets larger (and their habitat area stays the same), it is limited more by density-dependent factors. Look through the slides below to see an example. Food Is Abundant Imagine that a population of five squirrels in a forest has plenty of acorns to eat. When the population increases to 10 in the same forest, each squirrel will have half as many acorns to eat. If the population size doubles again to 20, the number of acorns per squirrel reduces by half again. Now each squirrel has only 25% of the food that it had to start with. As the population gets larger but occupies the same area (and thus denser), each individual will have progressively less food. The effect of this limiting factor increases with population density.

population limiting factors

The exponential model of growth is modified to create the more realistic logistic model based on environmental limitations on population size and growth. The factors that limit population size or growth can be resources or environmental conditions and are referred to as limiting factors. These are variables that prevent populations from growing infinitely larger. Limiting factors limit population size to the carrying capacity of the environment that population inhabits. For example, if each individual in a population has only enough food and water to survive, the population cannot grow any larger. Its growth is limited by food and water availability, and the population size has reached the environmental carrying capacity. Scientists try to account for the effects of limiting factors to calculate what the carrying capacity of an environment will be.

exponential growth

The human population has been growing at an extremely fast rate for the last several thousand years. Recall that the growth rate is the speed at which the population size increases, expressed in the number of individuals per unit time. Exponential growth of a population occurs when population size increases at a constant per capita (per individual) rate. For example, if a population of frogs increases by one tadpole per frog every year, it is increasing at a per capita rate of one per year. For every frog, there will be one more tadpole. In other words, each year, the population will double. We can graph exponential growth by plotting population size vs. time, which will give us a J-shaped curve, as seen in this graph. Global population of humans since 1750. This graph shows how the size of the human population has changed from 1750 to present day. As you can see from the J-shaped curve, the global human population is experiencing exponential growth. We are currently in the "rapid exponential growth" phase of the curve.

projected growth

The human population has been increasing in size exponentially due to improvements in agriculture, cleanliness, and medicine. The global per capita growth rate peaked in the 1960s and has been declining ever since. However, the rate is still positive at 0.012 per capita per year. Our population is still getting larger. If we assume that the population maintains this per capita growth rate, we can predict how population size will change in the future, or "project" the growth of the global human population. This is illustrated in the graph by the primary y-axis (the y-axis on the left side of the graph). It is labeled "Annual increments." This axis and the bars on the graph illustrate the increase in population size every ten years. The secondary y-axis (the y-axis on the right side of the graph) and the line were on the graph you were shown earlier in this lesson. Global population of humans since 1750 It is estimated that 180 people are born each minute; that's 260,000 people each day. If we continue to grow at our current per capita rate, it is projected that our population size will reach almost 9 billion people by the year 2050. This projection, based on current global growth rates, is also shown on the graph. By 2150, some scientists predict that our population will reach 9.7 billion and stabilize because our birth and death rates will balance each other out. This could be due to decreases in birth rates, increases in death rates, or both. As countries become more industrialized, their populations are exposed to more toxic substances, some of which both decrease fertility (and thus decrease birth rates) and increase diseases that can be deadly (and thus increase death rates). As we consume the resources present on our planet, we also come closer to exhausting them. Question The global growth rate is positive. Is it possible to have a negative growth rate? Yes, when death rates are higher than birth rates, the growth rate is negative. In other words, the population will decrease in size over time. Growth rate is also affected by migration in or out of the population. Ukraine and Russia are two examples of countries with negative growth rates, largely due to migration out of these countries.

human population history

The human species evolved roughly 400,000 years ago. The global human population grew slowly and steadily until about 10,000 years ago when the population size reached 5 million. Until 10,000 years ago, humans were hunters and gatherers. Their diets consisted of the food they found, either animals or plants, but there was no farming. Around 10,000 years ago, humans became adept at domesticating animals to help with hunting and farming, and the continental ice sheets covering northern Europe and North America receded. This allowed them to learn to farm rather than just relying on hunting and gathering available food during that season. The development of agricultural (food growing) practices increased the food supply for humans and made more food available year-round. This increase in food availability increased life span and birth rates and decreased death rates. Once agricultural practices were developed, the human population grew quickly, reaching 200 million (a 40-fold increase) within only 8,000 years. The development of agriculture led to a rapid increase in population growth rate. By the year 1650, the population had more than doubled to reach 500 million. Look through the slides below to see how the human population has grown since then. In 1800, the Population Reached 1 Billion Around the year 1650, Renaissance scientists and artists like Michelangelo and da Vinci studied anatomy. This, plus the invention of the microscope and an understanding of the toxic effects of human waste improved human hygiene and medicine. This resulted in much lower death rates around the world. The average global birth rate held fairly steady at about 30 births per 1,000 people per year. As a result, the world population reached 1 billion in the year 1800. As you can see in this graph, this roughly marks the beginning of the "rapid exponential growth" phase of human population growth. By the year 1930 (only 130 years later), the world population doubled to 2 billion. Health and hygiene greatly improved in poorer, less developed countries after World War II (WWII). The Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO) were both established in the 1940s. While birth rates in these countries remained high, death rates decreased, resulting in a sharp increase in global growth rate. From 1960 to 1987, the world population of humans went from 3 billion to 5 billion. The global growth rate peaked in the late 1960s at an annual per capita rate of 0.021. In other words, for each person alive, there were 0.021 new people each year. Recall that growth rate is a balance between birth rate and death rate: growth rate = birth rate - death rate Since the late 1960s, the global growth rate has declined steadily due to a decrease in birth rate. However, the population is still increasing in size. By 1999, the population exceeded 6 billion. The current world population of humans is estimated to be 7.2 billion. In only the last 16 years, we have increased by more than 1 billion people--the same amount that it took almost 400,000 years to reach after humans first evolved.

effect of parasites

The interactions between predators and prey can affect the size of the predator and prey populations, as well as populations of other organisms in their community. Parasites in the community can also affect population size by affecting individual fitness and survival. If a population is particularly susceptible to parasites or lives at a high population density such that transmission of parasites between animals is likely, that population size may be greatly reduced due to parasitic infections and subsequent diseases. As seen in this graph, parasite and host population sizes interact in a way that is similar to the interaction between predator and prey population sizes. However, parasites take longer to kill their food than predators do. Thus, the fluctuations of parasite populations take longer to affect fluctuations in host populations than do predator vs. prey. The table below walks through each step of the graph, describing how parasite and host population sizes affect each other. Hosts Increase If there are more host organisms in the population, more parasitic infections can occur. Parasites IncreaseHosts are "food" and shelter to parasites. Thus, when host populations increase in size, there is more food for the parasites. As parasites find the hosts and reproduce, their population size will also increase. Hosts DecreaseMany endoparasites live in an animal's gut, taking nutrients from the animal and causing it to be undernourished over time. Other parasites can transmit diseases to their hosts or cause infections in internal organs, eventually killing the hosts. In these ways, parasites will eventually reduce the population size of the hosts. Parasites DecreaseAs more host animals die, fewer parasites will survive, and so on. The population sizes of the hosts and parasites will fluctuate back and forth, with one following the other.

How do populations change in size?

The size and growth rate are important characteristics of the population. They affect how that population uses resources and how individuals within the population interact with each other. There are also factors that affect the maximum population size and rate of growth. Understanding these factors helps us make sense of how populations change over time, as well as predict how they will change in the future. Watch this video to see how important it is to understand population size and growth rate, as well as the factors that limit these characteristics. Imagine you have discovered a new species of silkworm that produces massive amounts of high quality silk. All you can think of is money... you see the opportunity to get rich by selling silk scarves, and want to calculate how quickly the money will start rolling in. You have to breed the silkworms and figure out how large your population will get if each one successfully breeds. You could use mathematical models to predict how the population will grow. But could the size of the population increase infinitely, making you rich forever? What could limit the size or growth of your population? Transcript Population size and growth can be predicted using mathematical models. If populations grow at a constant per capita rate (in other words, increasing by the same amount per individual), they will grow exponentially. We can predict this growth using the exponential model. The exponential model of growth is displayed in the graph below that shows population size as a function of time. Exponential model of growth. However, populations cannot really grow infinitely larger. Their size is limited by practical issues like the availability of food and water. These limitations on population size and growth are included in the logistic model. The logistic model of growth is displayed in the graph below that shows an environment with a maximum population size it can support (called its carrying capacity). As the size of the population approaches this maximum, growth will slow. Logistic model of growth. Mathematical models are not perfect predictors of how real populations will grow, but they are useful models of population growth that can be used to explain and predict changes in population size. Question Why does growth of a population slow as its size approaches the environmental carrying capacity? Each individual requires resources to live. As the population grows, the resources in the environment must be shared more and more to keep the individuals in a population alive. When resources are stretched thin because there are too many individuals in the population, more individuals will die and fewer will be born. This results in slower growth until the growth rate becomes zero.

primary succession

There are two types of ecological succession, and both begin with a disturbance and progress to a stable environment. Primary succession is one type of ecological succession. It basically begins with nothing and leads to a more diverse climax or stable community. The primary succession process often takes many years. Primary succession follows disturbances like lava flows or severe landslides. It begins on a bare area, such as rock, without vegetation or soil. The development of a bare site is a process called nudation. Click through the slides to learn about the stages of primary succession. Pioneer Plants As the name implies, pioneer plants are the first to arrive and colonize a disturbed or damaged area. These plants are very hardy species, capable of surviving harsh environments devoid of soil or other vegetation. Lichens, as seen in this image, are often the first life forms to appear on a bare area. Lichens form when fungi and algae join together and form a relationship that benefits both. Lichens are very, very important. Not only are they are able to grow on rocks, they break down that rock into soil in a process called pedogenesis (which means "the formation of soil"). Forming soil is the most important process during primary succession. How do lichens do this? Lichens secrete acidic substances that help break down rocks into soil. In addition to algae and fungi that form lichens, mosses may appear. At first, all these pioneer plants dominate and form soil. Pioneer plants are good at showing up and forming soil, yet they are not good at competing with larger plants that appear as soil is formed. Eventually, the pioneer species die and decompose (break down), adding to the very soil they helped form. As soil forms, soil bacteria and fungi appear. These are the most important groups in the decomposition of the dead pioneer plant material. As the soil continues to form and deepen, earthworms and ants also appear. Both organisms alter the soil characteristics by aerating (introducing oxygen) and moving soil particles. Pioneer species, bacteria, fungi, earthworms, and ants all play a part in transforming the once barren area so that larger plants can grow. As the soil layer deepens, grasses, ferns, and wildflowers appear next. They dominate the area for several years before shrubs and trees appear. As the soil continues to form and deepen, it supports more and more plants. In this final succession stage, shrubs and trees are maturing. After many years, the once barren rock is now home to many plants and animals.

Biodiversity

Think about all the people you know. You probably have many friends, and they're probably all different in one way or another. Now extend that to the other animals and the plants in your city, state, and country. That's a lot of diversity in your world. There is a great variety of life on Earth. Scientists call this biodiversity or species richness. The term is the contracted form of biological diversity, which came into common usage in the 1980s. Biodiversity is the total number of different species, including humans, on Earth or in an area. Scientists estimate there are as many as 8.7 million species on Earth, yet only 1.8 million species have been described. The variety of life on Earth occurs at all levels of biological organization. You can find biodiversity at levels such as species, ecosystems, and the biosphere. The biosphere is the sum total of all living things and their environment. It is most likely that you recognize biodiversity at the species level. For example, you can probably identify different plant and animal species in your backyard. In this lesson you will learn about the patterns of biodiversity and what affects it. Question Is biodiversity distributed evenly or unevenly across the biosphere? Biodiversity is distributed unevenly across the biosphere.

Seral Community

This image shows different seral stages (seres) in a forest ecosystem beginning with the pioneer species of plants on the left and ending with the mature trees on the right. The seral stages are called early seral, mid seral, and late seral. A forest stand is a continuous community of trees that has a common set of characteristics. Forest stands develop in several structural stages. A structural stage describes the size and the arrangement of the different trees and tree parts. Stand development begins with a disturbance. This major event causes changes in the structure and composition of a forest; this is called a stand-replacing disturbance. The first structural stage of forest succession begins after this disturbance. Read each row of the table below for a description of forest structural stages. EstablishmentThis stage is also called stand initiation. This is the first stage of forest succession following a stand-replacing disturbance. In this phase a population of vegetation is established. ThinningThis stage is also called stem-exclusion. During this phase, trees grow higher and enlarge their canopy, competing with their neighbors for sunlight and moisture. Eventually the canopies of neighboring trees touch each other and lower the amount of sunlight available to understory plants. TransitionThis stage is also called the understory-reinitiation phase. During this time, overstory trees begin to die and regrowth of understory vegetation occurs. Shifting MosaicThis stage is also called the old-growth stage. By now, three or more tree layers of vegetation have become established. This stage is described as a stable, steady state, or climax. What is the relationship between a sere and a structural stage? Each stage of ecological succession is called a seral community (or a sere). Each sere develops in structural stages.

Adaptation

To be successful--that is, to survive and reproduce more--an animal must obtain nutrients and stay alive. For predators, this means being able to kill and eat prey. For prey, this means avoiding being killed and eaten. Predator and prey species are in a constant evolutionary battle to eat and avoid being eaten. The evolution of one species affects the evolution of the other, and vice versa. This struggle has driven natural selection for a suite of adaptations that make predators ever better at getting prey, and prey better at staying alive. The particular adaptations that make each species successful depend on what it eats or what is trying to eat it. Read through these tabs to learn about adaptations that make predators and prey more successful. Predators Prey The survival of a predator depends on its ability to locate and eat prey. If a predator is more successful at finding and eating prey, it will be favored by natural selection. In other words, a predator that is well fed will be more likely to live longer and reproduce more often than one that is starving. Thus, the well-fed predator will produce more offspring and pass on its alleles more than the predator that is unsuccessful at catching prey. Any alleles associated with being a better predator should increase in frequency in the next generation. This has shaped the evolution of many adaptations that make predators better at locating, obtaining, and feeding on prey. Click each image below to see some examples. The evolution of a predator species relies on the evolution of a prey species, and vice versa. Predators must adapt to be better at locating and obtaining food, while prey must adapt to avoid being food. What are some of the adaptations that make predators and prey more successful? On the other hand, the survival of prey depends on its ability to avoid being located and eaten by its predator. In the same way that being a successful predator is favored by natural selection, so is successfully avoiding predation.

growth rate

To predict changes in population growth for a given population, scientists construct population models. These models are used to predict how a population will grow. A scientific model is a description of an object or occurrence that shares characteristics with that object or occurrence. Models provide a way to explain something that happens in the real word. Models can be material, like a cell model made using candy or a model airplane. A model can also take other forms, such as an equation. Population models are representations of the variables that affect population growth and the relationships between them. Based on a few simple parameters, we can construct mathematical models that predict how a population will grow. Scientists that study population growth have written mathematical equations that describe these variables and the relationships between them. By changing the variables in the model, we can predict how population size and growth rate will change in response. We can also model population growth by graphing different variables from these equations to see a visual representation of how the variables are related. The simplest population models include only two such variables: birth rate and death rate. These rates are the numbers of individuals that are born and die per unit of time respectively. For large populations, it is common to count and report the number of births and deaths out of 1,000 individuals. Let's take the year 2012 for example. For every 1,000 people, there were 19.15 births, so the birth rate could be reported as 19.151,00019.151,000 per year. This simplifies to 0.019 births per person per year (per capita per year). (Per capita means "per person.") We can do the same thing to calculate death rate. If in 2012, there were 7.99 deaths for every 1,000 people, what is the per capita death rate? 7.991,000=0.007997.991,000=0.00799 deaths per capita per year The difference between the number of individuals born and the number of individuals that die in a population is the amount by which the population grows. Thus, the growth rate of the population can be obtained by the following equation: growth rate (r) = birth rate - death rate For example, the per capita growth rate (r) of the human population in 2012 was birth rate - death rate = 0.01915 per capita/year - 0.00799 per capita/year = 0.01116 per capita per year. Note that growth rate does not have to be positive! If the number of deaths in a year outweighs the number of births, growth rate will be negative. If they are exactly equal, the population has a growth rate of zero. In the activity below, match each population scenario to its growth rate. In one year, a population experienced: 75 births and 75 deaths 100 births and 50 deaths 50 births and 100 deaths positive growth rate negative growth rate zero growth rate Question How many children would each monogamous couple have to give birth to for the human population to achieve a growth rate of zero? Two. If each individual has only one offspring, that individual's death will be balanced out by giving birth (in terms of numbers). Thus, a mating pair would need to have exactly two offspring.

Mount St. Helens

When Mount St. Helens erupted in Washington in 1980, 230 square miles of forest was either blown down or scorched by lava and covered by ash. The communities on and around the mountain were almost completely obliterated. However, burrowed deep beneath the surface were pocket gophers, small mammals that dug tunnels in the soil. Many of these mammals survived the eruption, and as they dug more tunnels, the soil was mixed and aerated, allowing plants to grow. Where the gophers emerged on the surface, small mounds of dirt formed, which caught seeds that blew across the barren ground. This helped more plants grow. Also, amphibians used the gopher's tunnels as cool escapes from the hot surface, allowing them to survive the eruption as well. Watch the video below to learn more about how the communities around Mount St. Helens responded to this incredibly destructive event. Narrator:In 1980, Mount St. Helens erupted, causing massive landslides. These left large deposits of minerals.After the ash settled and the ground cooled, groundwater began to seep up from beneath the Earth's surface. Plants slowly grew in these moist areas. Around this shoreline vegetation, insects and eventually amphibians like toads and frogs also thrived.After several years, tree seedlings grew and established dense tree stands, whose seeds were carried to other areas by the wind. Now, this area has become a young, thriving forest, home to beaver, birds, and elk.As the environment stabilizes, species diversity will decrease and the community will reach a sustainable number of species and population sizes. Transcript Question Given enough time, will the community around Mount St. Helens go back to the way it was before the eruption? Probably not. Although many of the same species will be well adapted to this particular environment, it is likely that new species will also happen to move in and take advantage of the uninhabited land. While the community may regrow to be healthy and full of life, it will probably be different from how it was before the eruption.

Symbiosis

When animals live together in a community, they fill different niches. Recall that a niche is a patterns of living, such as what organisms eat and where they live. Species will have different strengths and weaknesses. Animals can take advantage of these differences by forming relationships with animals of other species. When animals develop close relationships with each other that last a long time, this is called symbiosis. Symbiotic relationships typically involve both close physical and chemical interactions between individuals. An example is the bacteria that live in the intestines of many land animals. When the animal eats something, it is digested into smaller molecules, some of which the bacteria use as food. Bacteria may be able to digest molecules that the animal cannot, or they may release products that the animal cannot make itself. In this way, the animal provides shelter and food for the bacteria, and the bacteria aid in digestion. These two species are interacting in both physical and chemical ways. If an individual has a symbiotic relationship that increases its fitness (helps it reproduce more), that individual will pass its alleles on to the next generation more than it would without the relationship. Thus, any alleles associated with forming the relationship will increase in the next generation. In this way, species may evolve to take better advantage of each other. Individuals of the two species will commonly associate with each other, forming symbiotic relationships. Some species have evolved to have such intimate symbiotic relationships that individuals of either species cannot survive without one another--this is an obligate symbiotic relationship. For example, the lichens in this picture (also called moss) live on trees but are actually two organisms: a fungus and a photosynthetic algae. These two species depend on each other for food such that one cannot live without the other. On the other hand, many symbiotic relationships are facultative, meaning that the individuals can have a symbiotic relationship, but they are also capable of living separately. Question Is a pair of mating animals a symbiotic relationship? No, this is not considered symbiosis. Although both members benefit, symbiotic relationships are between members of two different species.

benefits of disturbance

When communities are disturbed, organisms may be removed or killed, or resource availability may decrease. These are usually considered negative effects. However, disturbances can also have benefits. Look through the tabs below to see some potential benefits of disturbances. More Resources Critical for Survival New Opportunities Many disturbances decrease availability of resources, such as water or sunlight, but some disturbances may increase resources, too. For example, as the gophers in the area around Mount St. Helens dug their tunnels, they turned over the soil, increasing the availability of oxygen for plants to grow. A fallen log or destruction of one part of a waterway may divert a river or creek to another part of the habitat, increasing water availability in that area. Some organisms may even depend on specific types of disturbances to survive. For example, the lodgepole pine tree depends on periodic fires to disperse its seeds. The cones, as shown here, contain a tough resin seal that is cracked open by intense heat, allowing the seeds to germinate. A forest fire may devastate the other plant and animal species in a community, as well as the adult pines, but it will also allow new pines to germinate and begin to grow. Disturbances can create opportunities for species that have not previously occupied a habitat to move in and establish themselves. It is hypothesized that the healthiest communities are those that encounter moderate levels of disturbance. If a community is disturbed too often, organisms will be lost, and populations will constantly be trying to adjust to changes in size and resource availability. If a community is never disturbed, individuals may become settled in their habitats and old, dominant animals past reproductive age will be difficult to displace (such as the male orangutan shown here). This leaves some populations stagnant and unable to change or grow significantly. Ownership of territory may make resident species difficult to replace, even by species that are otherwise well suited to the habitat.

disturbances

When natural disasters occur, they can be devastating to the communities of plants, animals, bacteria, and other organisms that occupy the affected habitats. In the field of ecology, events that change communities by removing or destroying organisms or altering resource availability are called disturbances. A disturbance can be abiotic (nonliving), such as the volcanic eruption of Mount St. Helens, or biotic (living). Click each image below to see some examples of abiotic and biotic disturbances that can affect communities.

logistic growth

When populations grow at a constant per capita rate, they grow exponentially. As you saw in the graph of the rabbit population you drew, populations can grow very large in a few generations! However, real populations cannot grow indefinitely. They will grow exponentially for some amount of time, but eventually they will run out of resources--the things they need to survive. These resources could be food, water, space, mates, or other things that each individual needs. The amount of each of these is not infinite; rather, they are limited. The size of a population is limited by the practical daily needs of the individuals in it, which are called logistics. For example, imagine that you buy a pair of fish and set up a fish bowl for them to live in. You feed them the same amount every day and clean the bowl once a week. Each fish has enough food to eat, enough space to live in, and enough oxygen to breathe. If those fish reproduce, they will have to share their food, space, and oxygen with their offspring. Each fish faces logistical concerns, or practical details of sharing the resources in their environment. Due to practical limitations of food, water, and space, a given environment has a maximum population size it can support, which is its carrying capacity (K). If we take an exponential model of population growth and modify it based on carrying capacity, we have a logistic model of growth--one that takes into account the need to obtain resources for life, or "logistics." As you can see in the figure below, we can again plot population size vs. time, but this time we will include carrying capacity in the model. Exponential Growth CurveLogistic Growth Curve Use the slides below to find out how growth rate will change as the population size gets larger. Slow Exponential Growth Just like in an exponential growth curve, the population will initially grow more slowly. Rapid Exponential Growth After a few generations, the population will grow very rapidly. Slow Growth In the logistic model, as the population size nears the carrying capacity (K) of the environment, growth rate will slow because the population will start to run out of resources. More individuals will die and fewer will be born as N approaches K. No Growth Once the population reaches the carrying capacity of the environment, it will no longer grow in size. Death rate will increase and birth rate will decrease until they balance each other out for a net growth rate of zero. Real populations may overshoot the carrying capacity with death rates temporarily outweighing birth rates until K is resumed.

yellowstone wolves

When populations of predators fluctuate in size, the number of their prey also fluctuates, and vice versa. When there are more predators, more prey will be eaten. When there are fewer predators, prey can flourish and increase in population size. A dramatic example of how predators and prey influence each other's population sizes occurred in Yellowstone National Park when gray wolves disappeared. In 1973 Yellowstone National Park was designated by the U.S. Fish and Wildlife Service as a recovery area for gray wolves. Although these wolves had once been numerous, they had been driven to endangerment over the previous 150 years due to habitat loss and intense hunting to ensure the safety of livestock as white settlers traveled west across the U.S. Although a lack of wolves made many people feel safer and possibly allowed for the progression of agriculture, the reduction in population size of this predator had profound impacts on other natural populations. Herbivores that were once prey to the wolves became more numerous. The herbivores overgrazed the plants, reducing their population sizes. They also began eating plants on which beavers depended for shelter, which decreased the beaver populations. Creeks and streams were altered as the plants around them disappeared. By the 1970s, there were no wolves in Yellowstone. Between 1995 and 1997, Yellowstone introduced 41 wild gray wolves, and as of December 2012, there were 79 gray wolves in Yellowstone Park (see graph). Since then, gray wolves have been removed from the endangered species list, and the population sizes of the herbivores and plants that are affected by the wolves are restoring themselves.

realized niches

When two or more species in a community attempt to occupy the same niche, competition can occur. Some species, like generalists, are more flexible about their niche than others. The fundamental niche of a species is the range of conditions that it can potentially tolerate and the range of resources that it can potentially use. Because of competition, the actual range that is used by a species may be limited. The part of the niche that a species actually uses is the realized niche. Because generalists have wide fundamental niches, it is easier for them to have different realized niches and share the same habitat. However, specialists have narrow fundamental niches. If their niches overlap, these species will compete for resources. If their niches do not overlap, they will be able to coexist in the same community. The most famous example of this is Joseph Connell's study of barnacles on the rocks of the Scottish coast in the 1960s. Species A and Species B are two species of barnacles that compete for rocks in shallow waters along the coast of Scotland. Connell observed that Species A was found only high on the rocks, which are exposed to the air when the tide goes out. Species S was found lower on the rocks. It could be that these two species simply prefer different parts of this habitat, but it turns out that this is an example of resource partitioning and overlapping fundamental niches. Use these slides to learn more about Connell's experiment demonstrating resource partitioning. Barnacle SpeciesLive on Different Rocks Connell observed the barnacle Species A high on the rocks and Species B low on the rocks in the same habitat. Species A Has a Wide Fundamental Niche Connell moved a rock covered in Species A to the lower zone and found that Species A could survive equally well on the lower rocks. The fundamental niche of Species A extends from the high rocks to the low rocks.How do we know that Species A has a wide fundamental niche? Although Species A was only naturally found on the high rocks, it was able to survive equally well on the lower rocks. The habitat range that it could tolerate was wider than what it was occupying. Species B Out-Competes Species A After some time, Species B would move onto the rock of Species A and crowd it out. Due to competition between species, the realized niche of Species A is limited to the high rocks. These two species have effectively partitioned the limited resource of space among rocks. Are the fundamental niche and realized niche of Species A the same? No, Species A can live on the high rocks as well as on the low rocks (its fundamental niche). However, due to competition with Species B, it is only found on the high rocks (its realized niche). Species B Has a Smaller Fundamental Niche Connell moved a rock covered in Species B to the higher zone and found that Species B could not tolerate the periods of exposure to air during low tide. Species B has a more specialized niche. Its fundamental and realized niche are the same--low rocks.

Distribution Patterns

Within a given population, individuals may not be spaced evenly throughout the inhabited area. For example, look at the map below of sub-Saharan Africa. The map on the left shows the density of vegetation (how many plants there are in a unit of area). Compare this to the image on the right, which shows the density of humans in the same area. Why do you think the population density of humans differs from area to area? What do you see when you compare the density of vegetation to the density of the human population? Think about the answers and then click Answer to see if you are correct. Humans are concentrated in areas where plants can grow. This is because they can eat the plants, as well as the other animals that feed on these plants. The presence of vegetation also indicates that there is water available in the area. Animals are dependent upon water and food in their habitats, as well as climates that are hospitable to life. The majority of animal life is concentrated in areas with temperate climates and with water and food available year-round. Density of VegetationDensity of Humans As you can see by looking at the map, population density is not constant from place to place. The way individuals of a population are dispersed throughout a given area is called its distribution pattern, which is another characteristic used to describe the population. We can classify distribution patterns into three main types: uniform, random, and clumped. These patterns of distribution often correspond to traits of the population or how that population interacts with its environment. Click each of the images below to learn about the three types of distribution patterns. As you read about each pattern, consider how individuals and their environment interact and how this affects their distribution.