Biology Chapter 5
What factory determine carrying cappacity
Acting separately or together, limiting factors determine the carrying capacity of an environment of species. A limiting factor is a factor that controls the growth of a population
Why do population growth rate differ among countries
Birthrates, death rates and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly. Scientists have indetified several social and economic factors that affect human population growth.
Density independent limiting factors
Density independent limiting factors affect all populations in similar ways regardless of population size and density. Sometime the effect of so called density independent factors can actually vary with population density. It is sometime hard to say if a limiting factor acts only in a density independent way.
Effects of herbivory
Herbivory can also contribute to changes in population numbers. From a plants perspective herbivores are predators.
Human activity limiting population
In some situations human activity limits population. Fishing can cause a fish population to decrease These populations can recover if we scale back fishing to lower the death rate sufficiently.
Fighting and overcrowding
In some species stress from overcrowding can cause females to neglect kill or even eat their own offspring Stress from overcrowding can lower birthrates, increase death rates and increase emigration. What limit factors do not typically depend on population density? Unusual wildfires such as hurricanes droughts or floods and natural disasters such as wildfires can act as density independent limiting factors.
Parasites and disease limiting population
Parasites and disease causing organisms feed at the expense of their hosts weakening them and often causing disease and death. Parasites and disease are density dependent effects because the denser the host population the more easily parasites can spread from one host to another.
What made population begin to grow
Several factors including improved nutrition sanitation medicine and most healthcare, dramatically reduced death rates. Yet birthrates in most parts of the world remained high.
How is demographic transition going today
So far the Us Japan and EU have completed their demographic transition Pars of south America Africa and asia are passing through stage 2 A large part of the ongoing human population is only happening is 10 counties with India and China in the lead
Explain human exponetial growth
The combination of lower death rates and higher birthrates let to exponential growth. This kind of exponential growth could not continue forever
The effects of predators and prey
The effect of predators on prey and the effects of herbivores on plants are two very important density dependent population controls.
What is logistic growth
logistic growth occurs when a population's growth slows and then stops, following a period of exponential growth. Natural populations don't grow exponentially for long. Sooner or later, something stops exponential growth. What happens? Suppose that a few individuals are introduced into a real-world environment. This graph traces the phases of growth that the population goes through. After a short time, the population begins to grow exponentially. During this phase, resources are unlimited, so individuals grow and reproduce rapidly. Few individuals die, and many offspring are produced, so both the population size and the rate of growth increase more and more rapidly. In real-world populations, exponential growth does not continue for long. At some point, the rate of population growth begins to slow down. The population still grows, but the rate of growth slows down, so the population size increases more slowly. At some point, the rate of population growth drops to zero and the size of the population levels off. Under some conditions, the population will remain at or near this size indefinitely. This curve has an S-shape that represents what is called logistic growth. Logistic growth occurs when a population's growth slows and then stops, following a period of exponential growth. Many familiar plant and animal populations follow a logistic growth curve. Population growth may slow for several reasons. Growth may slow if the population's birthrate decreases or the death rate increases—or if births fall and deaths rise together. In addition, population growth may slow if the rate of immigration decreases, the rate of emigration increases, or both. When the birthrate and the death rate are the same, and when immigration equals emigration, population growth stops. There is a dotted, horizontal line through the region of this graph where population growth levels off. The point at which this dotted line intersects the y-axis represents the carrying capacity. Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support. Once a population reaches the carrying capacity of its environment, a variety of factors act to stabilize it at that size.
Density dependent limiting factors
Density-Dependent limiting factors include competition predation herbivory parasitism disease and stress from overcrowding. Density Dependent limiting factors operate strongly only when population density- the number of organisms per unit area- reaches a certain level. These factors do not affect small scattered populations as much.
Stages of demography
In stage one birthrates and death rates are high for most of history. In stage 2 advances in nutrition sanitation and medicine lead to lower death rates Birthrates remain high for a time so births greatly exceed deaths and the population increases exponentially. During stage 3 as the level of education and living standard rise families have fewer children and the birthrate falls and population growth slows The demographic transition is complete when the birthrate meets the death rate and population growth stops
How do ecologists study populations
Researchers study populations' geographic range, density and distribution, growth rate, and age structure. The stories of hydrilla and cod both involve dramatic changes in the sizes of populations. A population is a group of organisms of a single species that lives in a given area, such as the hydrilla population represented on this map. Researchers study populations' geographic range, density and distribution, growth rate, and age structure. The area inhabited by a population is called its geographic range. A population's range can vary enormously in size, depending on the species. A bacterial population in a rotting pumpkin may have a range smaller than a cubic meter, whereas the population of cod in the western Atlantic covers a range that stretches from Greenland down to North Carolina. Humans have carried hydrilla to so many places that its range now includes every continent except Antarctica, and it is found in many places in the United States. Population density refers to the number of individuals per unit area. Populations of different species often have very different densities, even in the same environment. A population of ducks in a pond may have a low density, while fish and other animals in the same pond community may have higher densities. Distribution refers to how individuals in a population are spaced out across the range of the population—randomly, uniformly, or mostly concentrated in clumps. A population's growth rate determines whether the population size increases, decreases, or stays the same. Hydrilla populations in their native habitats tend to stay more or less the same size over time. These populations have a growth rate of around zero; they neither increase nor decrease in size. The hydrilla population in Florida, by contrast, has a high growth rate—which means that it increases in size. Populations can also decrease in size, as cod populations have been doing. The cod population has a negative growth rate. To fully understand a plant or animal population, researchers need to know the population's age stnicture —the number of males and females of each age a population contains. Most plants and animals cannot reproduce until they reach a certain age. Also, among animals, only females can produce offspring.
What factors affect population growth
The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population. A population will increase or decrease in size depending on how many individuals are added to it or removed from it. The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population. A population can grow when its birthrate is higher than its death rate. If the birthrate equals the death rate, the population may stay the same size. If the death rate is greater than the birthrate, the population is likely to shrink A population may grow if individuals move into its range from elsewhere, a process called immigration. A population may decrease in size if individuals move out of the population's range, a process called emigration. What happens during exponential growth? Under ideal conditions with unlimited resources, a population will grow exponentially. If you provide a population with all the food and space it needs, protect it from predators and disease, and remove its waste products, the population will grow. The population will increase because members of the population will be able to produce offspring, and after a time, those offspring will produce their own offspring. Under ideal conditions with unlimited resources, a population will grow exponentially. In exponential growth, the larger a population gets, the faster it grows. The size of each generation of offspring will be larger than the generation before it. In a hypothetical experiment, a single bacterium divides to produce two cells every 20 minutes. After 20 minutes, under ideal conditions, the bacterium divides to produce two bacteria. After another 20 minutes, those two bacteria divide to produce four cells. After three 20-minute periods, we have 2 x 2 x 2, or 8 cells. Another way to describe the size of the bacteria population is to use an exponent: 2' cells (three 20-minute periods). In another hour (six 20-minute periods), there will be 2', or 64 bacteria. In one day, this bacterial population will grow to 4,720,000,000,000,000,000,000 individuals. If this growth continued without slowing down, this bacterial population would cover the planet within a few days! If you plot the size of this population on a graph over time, you get a 1-shaped curve that rises slowly at first, and then rises faster and faster. If nothing were to stop this kind of growth, the population would become larger and larger, faster and faster, until it approached an infinitely large size. Many organisms grow and reproduce much more slowly than bacteria. For example, a female elephant can produce a single offspring only every 2 to 4 years. Newborn elephants take about 10 years to mature. Sometimes, when an organism is moved to a new environment, its population grows exponentially for a time. When a few European gypsy moths were accidentally released from a laboratory near Boston, these plant-eating pests spread across the northeastern United States within a few years.
How has human population size changed over time
The human population like populations of other organisms tends to increase. The rate of that increase has changed dramatically over time. For most of human existence the population grew slowly because life was harsh. Food was hard to find. Predators and diseases were common and life threatening. These limiting factors kept human death rates very high. Until fairly recently only half the children in the world survived to adulthood.
Demography
The scientific study of human populations is called demography. Demography examines characteristics of human populations and attempts to explain how those populations will change over time Human societies had equally high death rates and birthrates during most of history. But over the past century, population growth in the united states Japan Europe have decreased dramatically.
What happens when competition takes place between different species
This type of competition is a major force behind evolutionary change.
How will population grow and how do scientists determine it
To predict how the population will grow demographers consider many factors include the age structure of each country and the effects of diseases on death rates- especially AIDS in Africa and parts of Asia Current studies suggest that by 2050 the worlds population will reach 9 billion The human population may level out to a logistic growth curve and become stable if countries that are currently growing rapidly complete the demographic transition Current dats suggest that global human population will grow more slowly over the next 50 years
Thomas maltas ideas
Two centuries ago English economist Thomas Malthus suggested that only war famine and disease could limit human population growth. Malthus thought that human populations would be regulated by competition (war) limiting resources (famine) parasitism (disease) and other density dependent factors. Malthus' work was vitally important to the thinking of Charles Darwin.
Competition
When populations become crowded, individuals compete for food water space sunlight and other essentials. Some individuals obtain enough to survive and reproduce. Others may obtain just enough to live bit not enough to enable them to raise offspring Still others may starve to death or die from lack of shelter Competition can lower birthrates increase death rates or both. Competition is a density dependent limiting factor. The more individuals living in an area the sooner they use up the available resources.
