Environmental Science Unit 2

Scientists use artificial selection to change the genetic characteristics of populations with similar genes. First, they select one or more desirable genetic traits that already exist in the population of a plant or animal such as a type of wheat, dog, or fruit. Then they use selective breeding, or crossbreeding, to control which members of a population have the opportunity to reproduce to increase the numbers of individuals in a population with the desired traits. Artificial selection involves the crossbreeding of species that are close to one another genetically. Artificial selection is limited to crossbreeding between genetic varieties of the same species or between species that are genetically similar to one another, and is not a form of speciation. However, traditional crossbreeding is a slow process. Scientists have learned how to speed this process of change by manipulating genes to get desirable genetic traits or eliminate undesirable ones in a plant or animal. They do this by transferring segments of DNA with the desired trait from one species to another through a process called genetic engineering. In this process, also known as gene splicing, scientists alter an organism's g

A new and rapidly growing form of genetic engineering is synthetic biology. It enables scientists to make new sequences of DNA and use such genetic information to design and create artificial cells, tissues, body parts, and organisms not found in nature. Proponents of this new technology want to use it to create bacteria that can use sunlight to produce clean-burning hydrogen gas, which can be used to fuel motor vehicles. They also view it as a way to create new vaccines to prevent diseases and drugs to combat parasitic diseases such as malaria. Synthetic biology might also be used to create bacteria and algae that would break down spilled oil, industrial wastes, toxic heavy metals, pesticides, and radioactive waste in contaminated soil and water. Scientists are a long way from achieving such goals but the potential is there. The problem is that like any technology, synthetic biology can be used for good or bad. For example, it could also be used to create biological weapons such as deadly bacteria that spread new diseases, to destroy existing oil deposits, or to interfere with the chemical cycles that keep us alive. This is why many scientists call for increased monitoring and regulation of this new technology to help control its use.

Large areas of tropical rain forest need to be protected because they serve as centers for terrestrial biodiversity, their loss contributes to atmospheric warming and climate change as a result of the loss of large areas of vegetation that remove carbon dioxide from the atmosphere. In addition, removing large areas of tropical forest can sometimes change regional weather patterns, which can prevent the regrowth of rain forest in cleared or degraded areas.

A tipping point in the disappearance of the tropical rain forests would be the change in regional weather patterns after clearing the forests, which prevents their return A greenhouse gas produced by raising cattle CH4 The Geosphere has the most carbon and sulfur In the carbon cycle, aerobic respiration by producers, consumers, and decomposers steadily releases carbon dioxide (CO₂) into the atmosphere. Conversely, photosynthesis by producers takes in carbon dioxide (CO₂) from the atmosphere to form energy-rich carbohydrates.

Weather is the set of physical conditions of the lower atmosphere, including temperature, precipitation, humidity, wind speed, cloud cover, and other factors, in a given area over a period of hours to days. The most important factors in the weather in any area are atmospheric temperature and precipitation. Meteorologists use equipment mounted on weather balloons, aircraft, ships, and satellites, as well as radar and stationary sensors, to obtain data on weather variables. They feed these data into computer models to draw weather maps for various parts of the world. Other computer models project upcoming weather conditions based on probabilities that air masses, winds, and other factors will change in certain ways. Much of the weather we experience results from interactions between the leading edges of moving masses of warm air and cold air. Weather changes when one air mass replaces or meets another. The most dramatic changes in weather occur along a front, the boundary between two air masses with different temperatures and densities. A warm front is the boundary between an advancing warm air mass and the cooler one it is replacing. Because warm air is less dense (weighs less per unit

Atmospheric pressure results from molecules of gases in the atmosphere (mostly nitrogen and oxygen) zipping around at very high speeds and bouncing off everything they encounter. Atmospheric pressure is greater near the earth's surface because the molecules in the atmosphere are squeezed together under the weight of the air above them. An air mass with high pressure, called a high, contains cool, dense air that descends slowly toward the earth's surface and becomes warmer. Because of this warming, water molecules in the air do not form droplets—a process called condensation. Thus clouds, which are made of droplets, usually do not form in the presence of a high. Fair weather with clear skies follows as long as this high remains over the area. A low-pressure air mass, called a low, contains low-density, warm air at its center. This air rises, expands, and cools. When its temperature drops below a certain level, called the dew point, moisture in the air condenses and forms clouds. The condensation process usually requires that the air contain suspended tiny particles of dust, smoke, sea salts, or volcanic ash, called condensation nuclei, around which water droplets can form. If the droplets in the clouds coalesce into larger drops or snowflakes heavy enough to fall from the sky, precipitation occurs. Thus, a low tends to produce cloudy and sometimes stormy weather. Movement of these air masses is influenced strongly by jet streams—powerful winds that circle the globe near the top of the troposphere. They are like fast-flowing rivers of air moving west to east, one in each hemisphere somewhere above and below the equator. They form because of the temperature difference between the equator and the poles, which causes air to move. As the air moves away from the equator, north and south, it is deflected by the earth's rotation and flows generally west to east. The greater the temperature difference, the faster the flow of these winds. Jet streams can influence weather by moving moist air masses from one area to another.

Ecosystem diversity is the variety of ecosystems across the earth's surface. The locations of continents and oceanic basins have greatly influenced the earth's climate and thus have helped to determine where plants and animals can live. Speciation occurs most commonly when a barrier or distant migration separates two or more populations of a species and prevents the flow of genes between them. Endemic species are especially vulnerable to extinction. In the taxonomic classification system, all organisms within a kingdom have one or several common features. Amphibians—frogs, toads, and salamanders—were among the first vertebrates (animals with backbones) to leave the earth's waters and live on land.

Based on their cell structure, organisms can be classified as eukaryotic or prokaryotic. All organisms except bacteria are eukaryotic. Their cells are encased in a membrane and have a distinct nucleus (a membrane-bounded structure containing genetic material in the form of DNN) and several other internal parts enclosed by membranes Bacterial cells are prokaryotic, enclosed by a membrane but containing no distinct nucleus or other internal parts enclosed by membranes.

Eukaryotic cells are the building blocks of tissues and organ systems that make up the individual organisms that populate a species. A species is a group of living organisms with characteristics that distinguish it from other groups of organisms. In sexually reproducing organisms, individuals must be able to mate with similar individuals and produce fertile offspring in order to be classified as a species. Estimates of the number of species range from 7 million to 100 million, with a best guess of 7-10 million species. Biologists have identified about 2 million species Almost half of the world's identified species are insects, and there may be 10-30 million insect species on the planet. Pollination is a vital ecosystem service that allows flowering plants to reproduce. When pollen grains are transferred from the flower of one plant to a receptive part of the flower of another plant of the same species, reproduction occurs. Many flowering species depend on bees and other insects to pollinate their flowers. In addition, insects that eat other insects—such as the praying mantis who help to control the populations of at least half the species of insects that we call pests.

Biodiversity, or biological diversity, is the variety of life on the earth. variety of different species (species diversity), genetic variability among individuals within each species (genetic diversity), variety of ecosystems (ecological diversity), and functions such as energy flow and matter cycling needed for the survival of species and biological communities (functional diversity). One component of biodiversity is species diversity, the number and abundance of the different kinds of species living in an ecosystem. Species diversity has two components, one being species richness, the number of different species in ecosystem. The other is species evenness, a measure of the comparative abundance of all species in an ecosystem.

Humans alter the water cycle in three major ways. First, sometimes we withdraw freshwater from rivers, lakes, and aquifers at rates faster than natural processes can replace it. As a result, some aquifers are being depleted and some rivers no longer flow to the ocean. Second, we clear vegetation from land for agriculture, mining, road building, and other activities, and cover much of the land with buildings, concrete, and asphalt. This increases water runoff and reduces infiltration that would normally recharge groundwater supplies. Third, we drain and fill wetlands for farming and urban development. Left undisturbed, wetlands provide the ecosystem service of flood control. They act like sponges to absorb and hold overflows of water from drenching rains or rapidly melting snow.

Carbon is the basic building block of the carbohydrates, fats, proteins, DNA, and other organic compounds required for life. Various compounds of carbon circulate through the biosphere, the atmosphere, and parts of the hydrosphere, in the carbon cycle. A key component of the carbon cycle is carbon dioxide gas. It makes up about 0.040% of the volume of the earth's troposphere. Carbon dioxide (along with water vapor in the water cycle) affects the temperature of the atmosphere through the greenhouse effect and thus plays a major role in determining the earth's climate. If the carbon cycle removes too much CO2 from the atmosphere, the atmosphere will cool, and if it generates too much CO2, the atmosphere will get warmer. Thus, even slight changes in this cycle caused by natural or human factors can affect the earth's climate, which helps determine the types of life that can exist in various places. Carbon is cycled through the biosphere by a combination of photosynthesis by producers that removes CO2 from the air and water, and aerobic respiration by producers, consumers, and decomposers that adds CO2 in the atmosphere. Typically, CO2 remains in the atmosphere for 100 years or more. Some of the CO2 in the atmosphere dissolves in the ocean. In the ocean, decomposers release carbon that is stored as insoluble carbonate minerals and rocks in bottom sediment for long periods. Marine sediments are the earth's largest store of carbon, mostly as carbonates. Over millions of years, some of the carbon in deeply buried deposits of dead plant matter and algae have been converted into carbon-containing fossil fuels such as coal, oil, and natural gas. In a few hundred years, we have extracted and burned huge quantities of fossil fuels that took millions of years to form. This has added large quantities of CO2 to the atmosphere and altered the carbon cycle. In effect, we have been adding CO2 to the atmosphere faster than the carbon cycle can remove it. As a result, levels of CO2 in the atmosphere have been rising sharply since about 1960. There is considerable scientific evidence that this disruption of the carbon cycle is helping to warm the atmosphere and change the earth's climate. Another way in which we alter the cy

Functional diversity is the variety of chemical and biological functions that cycle matter and move energy through ecosystems. Ecosystem diversity is the variety of aquatic and terrestrial ecosystems that provide habitat for the vast number of species that inhabit the earth. Species diversity is the number and abundance of different species in an ecosystem, and genetic diversity is the genetic variability within a population.

Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as its ecological niche. It is a species' way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. The niche of a species differs from its habitat—the place, or type of ecosystem, in which a species lives and obtains what it needs to survive. Ecologists use the niches of species to classify them as generalists or specialists. Generalist species, such as raccoons, have broad niches. They can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species include flies, cockroaches, rats, coyotes, white-tailed deer, and humans. In contrast, specialist species, such as the giant panda, occupy narrow niches. They may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of shorebirds feed on crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands. Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources. Because of their narrow niches, specialists are more prone to extinction when environmental conditions change. For example, China's giant panda is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mostly of bamboo. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist usually is better off.

Ecological efficiency is the percentage of energy transferred from one trophic level to the next in an ecosystem. If only 10 percent of the energy is transferred from trophic level to trophic level, where does the rest of the energy go? It ends up a low-quality heat in the environment because of the second law of thermodynamics. According to this law, when high-quality energy is transferred by chemical reactions in the cells of organisms from one trophic level to another trophic roughly 90% of it is degraded to lower quality energy that flows into the environment as heat. While most of the energy at each trophic level will be converted into waste heat that results from metabolism, some energy will be contained in detritus or metabolic waste (indigestible biomass, feces, dead organisms that were not food for other organisms) that can be used as an energy source for decomposers and detritus feeders. As these detritivores consume the detritus, they too will generate waste heat through their own metabolism and within their own food chain. Energy flow pyramids explain why the earth could support more people if they all ate at a low trophic level by consuming grains, vegetables, and fruits

Ecologists also measure biomass—the total mass of organisms in each trophic level / Organic matter produced by plants and other photosynthetic producers; total dry weight of all living organisms that can be supported at each trophic level in a food chain or web; dry weight of all organic matter in plants and animals in an ecosystem; plant materials and animal wastes used as fuel. In natural ecosystems, most consumers feed on more than one type of organism, and most organisms are eaten or decomposed by more than one type of consumer. Because of this, organisms in most ecosystems form a complex network of interconnected food chains called a food web. Food chains and food webs show how producers, consumers, and decomposers are connected to one another as energy flows through trophic levels in an ecosystem. There are two major types of food webs. In a grazing food web, energy captured by producers is transferred to herbivores then to carnivores. Decomposers get energy from the remains of organisms at all levels. In a detrital food web, energy captured by producers flows directly to decomposers and detritivores. In most ecosystems, detrital and grazing food webs interconnect.

About 60% of the world's major terrestrial ecosystems are being degraded or used unsustainably, as the human ecological footprint gets bigger and spreads across the globe. The growing season is continuous in the tropical forest. There is a constant supply of warm temperatures and accessible water, so that it is not an advantage to drop leaves until these limiting factors become available. In a temperate deciduous forest where water may be frozen and temperatures drop to freezing, it is an energy-saving advantage to drop leaves until conditions are favorable as the seasons change.

El Niño-southern oscillation (ENSO) events: they change the distribution patterns of pelagic marine species the warming of the western Pacific causes a reduction in upwelling nutrients and an increase in rainfall the prevailing winds in the tropical Pacific weaken and change directions

As energy flows through ecosystems in food chains and food webs, there is a decrease in the high-quality chemical energy available to organisms at each successive feeding level. Chemical energy, stored as nutrients in the bodies and wastes of organisms, flows through ecosystems from one trophic (feeding) level to another. A sequence of organisms with each one serving as a source of nutrients or energy for the next level of organisms is called a food chain

Every use and transfer of energy by organisms from one feeding level to another involves a loss of some high-quality energy to the environment as low-quality energy in the form of heat, in accordance with the second law of thermodynamics. A graphic display of the energy loss at each trophic level is called a pyramid of energy flow - Diagram representing the flow of energy through each trophic level in a food chain or food web. With each energy transfer, only a small part (typically 10%) of the usable energy entering one trophic level is transferred to the organisms at the next trophic level.. The large loss in chemical energy between successive trophic levels explains why food chains and webs rarely have more than four or five trophic levels.

Another factor affecting the number and types of species on the earth is extinction. Extinction occurs when an entire species ceases to exist. When environmental conditions change dramatically, a population of a species faces three possible futures: adapt to the new conditions through natural selection, migrate (if possible) to another area with more favorable conditions, or become extinct. Species found in only one area, called endemic species, are especially vulnerable to extinction. They exist on islands and in other isolated areas. For example, many species in tropical rain forests have highly specialized roles and are vulnerable to extinction. These organisms are unlikely to be able to migrate or adapt to rapidly changing environmental conditions. Many of these endangered species are amphibians. Fossils and other scientific evidence indicate that all species eventually become extinct. In fact, the evidence indicates that 99.9% of all species that have existed on the earth have gone extinct. Throughout most of the earth's long history, species have disappeared at a low rate, called the background extinction rate. Based on the fossil record and analysis of ice cores, biologists es

Evidence indicates that life on the earth has been sharply reduced by several periods of mass extinction during which there is a significant rise in extinction rates, well above the background rate. In such a catastrophic, widespread, and often global event, large groups of species (30-90% of all species) are wiped out. The causes of such extinctions are unknown, but possible events include gigantic volcanic eruptions and collisions with giant meteors and asteroids. Such events would trigger drastic environmental changes on a global scale, including massive release of debris into the atmosphere that would block sunlight for an extended period. This would kill off most plant species and the consumers that depend on them for food. Fossil and geological evidence indicates that there have been five mass extinctions (at intervals of 20-60 million years) during the past 500 million years.

Phosphorus is another nutrient that supports life. The cyclic movement of phosphorus (P) through water, the earth's crust, and living organisms is called the phosphorus cycle. The major reservoir for phosphorus in this cycle is phosphate rocks that contain phosphate ions PO43-, which are an important plant nutrient. Phosphorus does not cycle through the atmosphere because few of its compounds exist as a gas. Phosphorus cycles more slowly than water, carbon, and nitrogen. As water runs over exposed rocks, it slowly erodes away inorganic compounds that contain phosphate ions. Water carries these ions into the soil, where they are absorbed by the roots of plants and by other producers. Phosphate compounds are then transferred by food webs from producers to consumers and eventually to detritus feeders and decomposers. When phosphate and other phosphorus compounds wash into the ocean, they are deposited as marine sediment and can remain trapped for millions of years. Over time, geological processes can uplift and expose these seafloor deposits, from which phosphate can be eroded and freed up to reenter the phosphorus cycle. Most soils contain little phosphate, which often limits plant grow

For most of the past 10,000-12,000 years, humans have been living in an era called the Holocene. During this era, we have enjoyed a favorable climate and other environmental conditions. This general stability allowed the human population to grow, develop agriculture, and take over a large share of the earth's land and other resources. Most geologists contend that we are still living in the Holocene era, but some scientists disagree. According to them, when the Industrial Revolution began (around 1750) we entered an era called the Anthropocene, an era dominated by humans. In this new era, our ecological footprints have expanded significantly and are changing and stressing the earth's life-support system, especially since 1950. In 2015 an international team of 18 leading researchers in their fields published a paper estimating how close we are to exceeding nine major planetary boundaries, or ecological tipping points, because of human activities. They warn that exceeding them could change how the planet operates and could trigger abrupt and long-lasting or irreversible environmental changes. This could seriously degrade the earth's life-support system and our economies. The researchers estimated that we have exceeded four of these planetary boundaries. They are disruption of the nitrogen and phosphorus cycles, mostly from greatly increased use of fertilizers to produce food; biodiversity loss from replacing biologically diverse forests and grasslands with simplified fields of single crops; land system change from agriculture and urban development; climate change from disrupting the carbon cycle, mostly by overloading the atmosphere with carbon dioxide produced by the burning of fossil fuels.

Life on the earth depends on three interconnected factors: One-way flow of high-quality energy from the sun. The sun's energy supports plant growth, which provides energy for plants and animals, in keeping with the solar energy principle of sustainability. As solar energy interacts with carbon dioxide , water vapor, and several other gases in the troposphere, it warms the troposphere—a process known as the greenhouse effect. Without this natural process, the earth would be too cold to support most of the forms of life we find here today. Cycling of nutrients through parts of the biosphere. Nutrients are chemicals that organisms need to survive. Because the earth does not get significant inputs of matter from space, its fixed supply of nutrients must be recycled to support life. This is in keeping with the chemical cycling principle of sustainability. Gravity allows the planet to hold on to its atmosphere and enables the movement and cycling of chemicals through air, water, soil, and organisms.

Greenhouse effect: Natural effect that releases heat in the atmosphere near the earth's surface. Water vapor, carbon dioxide, methane, and other gases in the lower atmosphere (troposphere) absorb some of the infrared radiation (heat) radiated by the earth's surface. Their molecules vibrate and transform the absorbed energy into longer-wavelength infrared radiation in the troposphere. If the atmospheric concentrations of these greenhouse gases increase and other natural processes do not remove them, the average temperature of the lower atmosphere will increase. High-quality solar energy flows from the sun to the earth. It is degraded to lower-quality energy (mostly heat) as it interacts with the earth's air, water, soil, and life forms, and eventually returns to space. Certain gases in the earth's atmosphere retain enough of the sun's incoming energy as heat to warm the planet in what is known as the greenhouse effect. The greenhouse effect is a natural process in which certain gases in the atmosphere absorb some of the sun's energy flowing from the earth back into space and release it as heat into the lower atmosphere. Without this natural warming effect, the earth would be so cold that most of its current forms of life (probably including humans) would become extinct.

Nitrogen is a critical nutrient for all forms of life. Nitrogen gas, N2, makes up 78% of the volume of the atmosphere, but cannot be used as a nutrient by plants. It becomes a plant nutrient only as a component of nitrogen-containing ammonia NH3, ammonium ions NH4+, and nitrate ions NO3-. These chemical forms of nitrogen are created in the nitrogen cycle by lightning, which converts N2 to NH3 and by specialized bacteria in topsoil. Other bacteria in topsoil and in the bottom sediments of aquatic systems convert NH3 to NH4+ and nitrate ions NO3- that are taken up by the roots of plants. The plants then use these forms of nitrogen to produce various proteins, nucleic acids, and vitamins. Animals that eat plants consume these nitrogen-containing compounds, as do detritus feeders and decomposers. Bacteria in waterlogged soil and bottom sediments of lakes, oceans, swamps, and bogs convert nitrogen compounds into nitrogen gas N2. The gas is released to the atmosphere to begin the nitrogen cycle again.

Humans intervene in the nitrogen cycle in several way. When we burn gasoline and other fuels, the resulting high temperatures convert some of the N2 and O2 in air to nitric oxide NO. In the atmosphere, NO can be converted to nitrogen dioxide gas NO2 and nitric acid vapor HNO3, which can return to the earth's surface as damaging acid deposition, commonly called acid rain. Acid rain damages buildings, statues, and forests. We remove large amounts of N2 from the atmosphere and combine it with H2 to make ammonia (N2 + 3H2 -> 2NH3) and ammonium ions NH4+ used in fertilizers. We add nitrous oxide N2O to the atmosphere through the action of anaerobic bacteria on nitrogen-containing fertilizer or organic animal manure applied to the soil. This greenhouse gas can warm the atmosphere and take part in reactions that deplete stratospheric ozone, which keeps most of the sun's harmful ultraviolet radiation from reaching the earth's surface. We alter the nitrogen cycle in aquatic ecosystems by adding excess nitrates NO3- to these systems. The nitrates contaminate bodies of water through agricultural runoff of fertilizers, animal manure, and discharges from municipal sewage treatment systems. This plant nutrient can cause excessive growth of algae that disrupt aquatic systems. Human nitrogen inputs into the environment have risen sharply and are projected to continue rising.

A rain shadow desert forms when warm air carrying moisture is forced upward along a mountain (windward side) where it cools and drops its moisture as precipitation. The resulting air mass is dry as it moves over the top of the mountain range, depriving the land on the other side (leeward side) of moisture, leading to desert or semiarid conditions.

Identify the four factors that determine regional climates around the world. 1. Cyclical movement of air driven by solar energy that forms convection cells. These warm air cells pick up moisture and release it as precipitation as the air moves upward and cools. 2. Uneven heating by the earth's surface by the sun results in varied intensity of insolation energy heating a region according to latitude. 3. Tilt of the earth's axis results in the earth's seasons due to widely varying amounts of incoming solar energy. 4. Rotation of the earth on its axis deflects the rising and cooling convection cells creating prevailing winds.

Different climates based on long-term average annual precipitation and temperatures, global air circulation patterns, and ocean currents, lead to the formation of tropical (hot), temperate (moderate), and polar (cold) deserts, grasslands, and forests. Average precipitation and average temperature, acting together as limiting factors over a long time, help to determine the type of desert, grassland, or forest in any particular area, and thus the types of plants, animals, and decomposers found in that area (assuming it has not been disturbed by human activities). biomes—large terrestrial regions, each characterized by a certain type of climate and dominant forms of plant life. The variety of biomes and aquatic systems is one of the four components of the earth's biodiversity—a vital part of the earth's natural capital. In reality, biomes are not uniform. They consist of a variety of areas, each with somewhat different biological communities but with similarities typical of the biome. These areas occur because of the irregular distribution of the resources needed by plants and animals and because human activities have removed or altered the natural vegetation in many areas. There are als

In a desert, annual precipitation is low and often scattered unevenly throughout the year. During the day, the baking sun warms the ground and evaporates water from plant leaves and from the soil. At night, most of the heat stored in the ground radiates quickly into the atmosphere. This explains why in a desert, you might roast during the day but shiver at night. A combination of low rainfall and varying average temperatures over many decades creates a variety of desert types—tropical, temperate, and cold. Tropical deserts such as the Sahara and the Namib of Africa are hot and dry most of the year. They have few plants and a hard, windblown surface strewn with rocks and sand. In temperate deserts, daytime temperatures are high in summer and low in winter and there is more precipitation than in tropical desert. The sparse vegetation consists mostly of widely dispersed, drought-resistant shrubs and cacti or other succulents adapted to the dry conditions and temperature variations. In cold deserts such as the Gobi Desert in Mongolia, vegetation is sparse. Winters are cold, summers are warm or hot, and precipitation is low. In all types of deserts, plants and animals have evolved adaptations that help them to stay cool and to get enough water to survive.

A mass extinction provides an opportunity for the evolution of new species that can fill unoccupied ecological niches or newly created ones. Scientific evidence indicates that each past mass extinction has been followed by an increase in species diversity. This happens over several million years as new species rise to occupy new habitats or to exploit newly available resources. As environmental conditions change, the degree of balance between speciation and extinction determines the earth's biodiversity. The existence of millions of species today means that speciation, on average, has kept ahead of extinction. However, evidence indicates that the global extinction rate is rising dramatically. Many scientists see this is as evidence that we are experiencing the beginning of a new sixth mass extinction. There is also considerable evidence that much of the current rise in the extinction rate and the resulting loss of biodiversity are primarily due to human activities. As our ecological footprint spreads over the planet, so does extinction. Research indicates that the largest cause of the rising rate of species extinctions is the loss, fragmentation, and degradation of habitats.

In a rapidly changing environment, it can be difficult for many species to survive changing conditions. Essentially, when the environment changes abruptly, organisms have three options: adapt, migrate, or go extinct. Environmental factors that impact species and lead to extinction include habitat loss, competition with invasive species, pollution, population growth, climate change, and overexploitation.

Grasslands occur primarily in the interiors of continents in areas that are too moist for deserts to form and too dry for forests to grow. Grasslands persist because of a combination of seasonal drought, grazing by large herbivores, and occasional fires—all of which keep shrubs and trees from growing in large numbers. The three main types of grassland—tropical, temperate, and cold (arctic tundra)—result from long-term combinations of low average precipitation and varying average temperatures. One major type of tropical grassland is savanna. It contains widely scattered clumps of trees and usually has warm temperatures year-round with alternating dry and wet seasons. Herds of grazing and browsing animals migrate across the savanna to find water and food in response to seasonal and year-to-year variations in rainfall and food availability. Savanna plants, like those in deserts, are adapted to survive drought and extreme heat. Many have deep roots that tap into groundwater. Elephants dig for water during drought periods, creating or enlarging waterholes that are used by other animals. Conservation scientists classify the African elephant as vulnerable to extinction. In 1979 there were an

In many coastal regions that border on deserts, we find a biome known as temperate shrubland or chaparral (Spanish for thicket). Because it is close to the sea, it has a slightly longer winter rainy season than the bordering desert has and experiences fogs during the spring and fall seasons. Chaparral is found along coastal areas of southern California, the Mediterranean Sea, central Chile, southern Australia, and southwestern South Africa. This biome consists mostly of dense growths of low-growing evergreen shrubs and occasional small trees with leathery leaves. Its animal species include mule deer, chipmunks, jackrabbits, lizards, and a variety of birds. The soil is thin and not very fertile. During the long, hot, and dry summers, chaparral vegetation dries out. In the late summer and fall, fires started by lightning or human activities spread swiftly. Research reveals that chaparral is adapted to and maintained by occasional fires. Many of the shrubs store food reserves in their fire-resistant roots and have seeds that sprout only after a hot fire. With the first rain, annual grasses and wildflowers spring up and use nutrients released by the fire. New shrubs grow quickly and crowd out the grasses. People like living in this biome because of its moderate, sunny climate. As a result, humans have moved in and modified this biome so much that little natural chaparral exists. The downside is that people living in chaparral assume the high risk of frequent fires, which are often followed by flooding during winter rainy seasons. When heavy rains come, torrents of water pour off the unprotected burned hillsides to flood lowland areas, often causing mudslides.

The earth's surface has changed dramatically over its long history. Scientists discovered that huge flows of molten rock within the earth's interior have broken its surface into a number of gigantic solid plates, called tectonic plates. For hundreds of millions of years, these plates have drifted slowly on the planet's mantle. Rock and fossil evidence indicates that 200-250 million years ago, all of the earth's present-day continents were connected in a supercontinent called Pangaea. About 175 million years ago, Pangaea began splitting apart as the earth's tectonic plates moved. Eventually tectonic movement resulted in the present-day locations of the continents. The drifting of tectonic plates has had two important effects on the evolution and distribution of life on the earth. First, the locations of continents and oceanic basins have greatly influenced the earth's climate, which plays a key role in where plants and animals can live. Second, the breakup, movement, and joining of continents has allowed species to move and adapt to new environments. This led to the formation of large number of new species through speciation. Sometimes tectonic plates that are grinding along next to on

In reproductive isolation, mutation and change by natural selection operate independently in the gene pools of geographically isolated populations. If this process continues long enough, members of isolated populations of sexually reproducing species can become different in genetic makeup. Then they cannot produce live, fertile offspring if they are rejoined with their earlier population and attempt to interbreed. When that is the case, speciation has occurred and one species has become two.

Excess use of ammonia-rich fertilizers results in an excess of ammonia and nitrogen oxides being released into the ecosystem; the added excess nitrogen to the nitrogen cycle contributes more greenhouse gases to the atmosphere. A decrease in phosphate-rich fertilizer runoff from golf courses will likely result in decreased algae population explosions and thus limit disturbances to food web dynamics in nearby waterways.

In the nitrogen cycle, molecular nitrogen exists as a gas in the atmosphere. Only lightning and nitrogen-fixing bacteria are able to convert this form of nitrogen into ammonium. In the soil, ammonium can be taken up by producers or undergo nitrification by bacteria, a process in which ammonium is converted to nitrate ions in the soil. These soil nitrate ions also have two paths: They can either be taken up by producers or undergo denitrification by bacteria, a process in which nitrates are used for energy, and as a result, gaseous nitrogen is returned to the atmosphere. In ammonification, another part of the nitrogen cycle, decomposer bacteria convert nitrogen-rich detritus into ammonia and water-soluble salts containing ammonium ions.

Archeological evidence indicates that our species emerged from African savannas. Early humans lived largely in trees but eventually came down to the ground and learned to walk upright. This freed them to use their hands for using tools such as clubs and spears. After the last ice age, about 10,000 years ago, the earth's climate warmed and humans began their transition from hunter-gatherers to farmers growing food on the savanna and on other grasslands. Later, they cleared patches of forest to expand farmland and created villages and eventually towns and cities. Today, vast areas of African savanna have been plowed up and converted to cropland or used for grazing livestock. Towns are also expanding there, and this trend will continue as the human population in Africa—the continent with the world's fastest population growth—increases. As a result, populations of elephants, lions, and other animals that roamed the savannas for millions of years have dwindled. Many of these animals face extinction in the next few decades because of the loss of their habitats and because people kill them for food and their valuable parts such as the ivory tusks of elephants.

Key factors that influence weather are moving masses of warm and cold air, changes in atmospheric pressure, and occasional shifts in major winds.

Adaptations for survival in the desert have two themes: beat the heat and every drop of water counts. Desert plants have evolved a number of strategies based on such adaptations. During long hot and dry spells, plants such as mesquite and creosote drop their leaves to survive in a dormant state. Succulent (fleshy) plants such as the saguaro ("sah-WAH-ro") cactus have no leaves that can lose water to the atmosphere through transpiration. They also store water and synthesize food in their expandable, fleshy tissue and they reduce water loss by opening their pores only at night to take up carbon dioxide. The spines of these and many other desert plants guard them from being eaten by herbivores seeking the precious water they hold. Some desert plants use deep roots to tap into groundwater. Others such as prickly pear and saguaro cacti use widely spread shallow roots to collect water after brief showers and store it in their spongy tissues. Some desert plants conserve water by having wax-coated leaves that reduce water loss. Others such as annual wildflowers and grasses store much of their biomass in seeds that remain inactive, sometimes for years, until they receive enough water to germin

Most desert animals are small. Some beat the heat by hiding in cool burrows or rocky crevices by day and coming out at night or in the early morning. Others become dormant during periods of extreme heat or drought. Some larger animals such as camels can drink massive quantities of water when it is available and store it in their fat for use as needed. In addition, the camel's thick fur helps it keep cool because the air spaces in the fur insulate the camel's skin against the outside heat. In addition, camels do not sweat, which reduces their water loss through evaporation. Kangaroo rats never drink water. They get the water they need by breaking down fats in seeds that they consume. Insects and reptiles such as rattle-snakes have thick outer coverings to minimize water loss through evaporation, and their wastes are dry feces and a dried concentrate of urine. Many spiders and insects get their water from dew or from the food they eat. Desert ecosystems are vulnerable to disruption because they have slow plant growth, low species diversity, slow nutrient cycling, low bacterial activity in their soils, and very little water. It can take decades to centuries for their soils to recover from disturbances such as off-road vehicle traffic, which can also destroy the habitats for a variety of animal species that live underground. The lack of vegetation, especially in tropical and polar deserts, also makes them vulnerable to heavy wind erosion from sandstorms.

The four major components of the earth's life-support system are the atmosphere (air), the hydrosphere (water), the geosphere (rock, soil, and sediment), and the biosphere (living things). Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.

Natural capital: The earth consists of a land sphere (geosphere), an air sphere (atmosphere), a water sphere (hydrosphere), and a life sphere (biosphere). The atmosphere is a spherical mass of air surrounding the earth's surface. Its innermost layer, the troposphere, extends about 17 kilometers (11 miles) above sea level at the equator and about 7 kilometers (4 miles) above the earth's North and South Poles. The troposphere contains the air we breathe. It is 78% nitrogen and 21% oxygen . The remaining 1% of air is mostly water vapor, carbon dioxide, and methane. It contains about 75% of the mass of earth's air. The next layer of the atmosphere is the stratosphere. It reaches 17 to 50 kilometers (11-31 miles) above the earth's surface. The layer of the stratosphere closest to the earth's surface contains enough ozone gas to filter out about 95% of the sun's harmful ultraviolet (UV) radiation. This global sunscreen allows life to exist on the surface of the planet. The hydrosphere includes all of the water on or near the earth's surface. It is found as water vapor in the atmosphere, as liquid water on the surface and underground, and as ice—polar ice, icebergs, glaciers, and ice in frozen soil-layers called permafrost. Salty oceans that cover about 71% of the earth's surface contain 97% of the planet's water and support almost half of the world's species. About 2.5% of the earth's water is freshwater and three-fourths of that is ice. The geosphere contains the earth's rocks, minerals, and soil. It consists of an intensely hot core, a thick mantle of very hot rock, and a thin outer crust of rock and soil. The crust's upper portion contains soil chemicals or nutrients that organisms need to live, grow, and reproduce. It also contains nonrenewable fossil fuels—coal, oil, and natural gas—and minerals that we extract and use. The biosphere consists of the parts of the atmosphere, hydrosphere, and geosphere where life is found. If you compare the earth with an apple, the biosphere would be as thick as the apple's skin.

The input of solar energy in a given area, called insolation, varies with latitude. This partly explains why tropical regions are hot, polar regions are cold, and temperate regions generally alternate between warm and cool temperatures. The amount of solar radiation reaching the earth typically varies about every 11 years because of changes in solar magnetic activity that can warm or cool the planet. The warm air that does not descend in the Hadley cells at 30 north and 30 south continues moving toward the poles and curving to the east due to the Coriolis effect. These prevailing winds that blow generally from the west in temperate regions of the globe are known as westerlies. This complex movement of air results in six huge regions between the equator and the poles in which warm air rises and cools, then falls and heats up again in great rolling patterns. The two nearest the equator are the Hadley cells. These convection cells and the resulting prevailing winds distribute heat and moisture over the earth's surface, thus helping to determine regional climates.

Other long-term factors affecting the earth's climate are: slight changes in the shape of the Earth's orbit around the sun from mostly round to more elliptical over a 100,000 year cycle, slight changes in the tilt of Earth's axis over a 41,000-year cycle, and slight changes in Earth's wobbly orbit around the sun over a 20,000-year cycle. These three long-term factors are known as the Milankovitch cycles. Water also moves vertically in the oceans as denser water sinks while less dense water rises. This creates a connected loop of deep and shallow ocean currents. This loop acts somewhat like a giant conveyer belt that moves heat from the surface to the deep sea and transfers warm and cold water between the tropics and the poles

Ecologists supplement their field research by conducting laboratory research. In laboratories, scientists create, set up, and observe models of ecosystems and populations. They create such simplified systems in containers such as culture tubes, bottles, aquariums, and greenhouses, and in indoor and outdoor chambers. In these structures, they control temperature, light, CO2, humidity, and other variables. These systems make it easier for scientists to carry out controlled experiments. Laboratory experiments are often faster and less costly than similar experiments in the field. However, scientists must consider how well their scientific observations and measurements in simplified, controlled systems in laboratory conditions reflect what takes place under the more complex and often-changing conditions found in nature. Since the late 1960s, ecologists have developed mathematical models that simulate ecosystems, and they run the models on high-speed supercomputers. The models help them understand large and complex systems, such as lakes, oceans, forests, and the earth's climate, that cannot be adequately studied and modeled in field or laboratory research. Ecologists call for greatly incr

Percent change is calculated by finding the difference between two values being compared. The difference is divided by the original number and then multiplied by 100. If the result is a negative number, we have a percentage decrease. Percent difference is used when comparing two values or measurements. It is the positive difference between the values, divided by the average of the two values, multiplied by 100.

Terrestrial life depends on soil, one of the most important components of the earth's natural capital. The minerals that make up your muscles, bones, and most other parts of your body come almost entirely from soil. Soil also supplies most of the nutrients needed for plant growth and purifies water. Through aerobic respiration, organisms living in soil remove some of the carbon dioxide in the atmosphere and store it as organic carbon compounds, thereby helping to control the earth's climate. Soil is a complex mixture of rock pieces and particles, mineral nutrients, decaying organic matter, water, air, and living organisms that support plant life, which supports animal life. Life on land depends on roughly 15 centimeters (6 inches) of topsoil—the earth's living skin. Soil is a renewable resource but it is renewed very slowly and becomes a nonrenewable resource if we deplete it faster than nature can replenish it. The formation of just 2.5 centimeters (1 inch) of topsoil can take hundreds thousands of years. Removing plant cover from soil exposes its topsoil to erosion by water and wind. This explains why protecting and renewing topsoil is a key to sustainability.

Plants (producers) convert the light energy of the sun to chemical bond energy in organic compounds produced by photosynthesis. These compounds serve as food (nutrients) for the plants. This energy is then passed on to animals that eat plants (primary consumer herbivores), animals that eat other animals (secondary and tertiary consumer omnivores and carnivores), and decomposers. Detritivores and decomposers cycle back nutrients to producers.

They are microbes, or microorganisms, catchall terms for many thousands of species of bacteria, protozoa, fungi, and floating phytoplankton. Though most of them are too small to be seen with the naked eye, they are the biological rulers of the earth and play key roles in the earth's life-support system and in our bodies. Most of us view them primarily as threats to our health in the form of infectious bacteria or fungi that cause athlete's foot and other skin diseases, and protozoa that cause diseases such as malaria. But these harmful microbes are in the minority. Researchers have identified more than 10,000 species of bacteria, fungi, and other microbes that live in or on our bodies. Many of them provide us with vital services. Bacteria in our intestinal tracts help break down the food we eat, and microbes in our noses help prevent harmful bacteria from reaching our lungs. Bacteria and fungi in the soil decompose organic wastes into nutrients that can be taken up by plants that are then eaten by humans and other plant eaters. Without these tiny creatures, we would go hungry and be up to our necks in waste matter. Some microorganisms, particularly phytoplankton in the ocean, provide

Producers, consumers, and decomposers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells, this energy is released by aerobic respiration, which uses oxygen to convert glucose (or other organic nutrient molecules) back into carbon dioxide and water. To summarize, ecosystems and the biosphere are sustained by the one-way energy flow from the sun and the nutrient cycling of key materials—in keeping with two of the scientific principles of sustainability. Natural capital: The main components of an ecosystem are energy, chemicals, and organisms. Nutrient cycling and the flow of energy—first from the sun, then through organisms, and finally into the environment as low-quality heat link these components.

Some organisms produce the nutrients they need, others get the nutrients they need by consuming other organisms, and some recycle nutrients back to producers by decomposing the wastes and remains of other organisms. Soil is a renewable resource that provides nutrients that support terrestrial plants and helps purify water and control the earth's climate.

Scientists classify matter into levels of organization ranging from atoms to galaxies. Ecologists study five levels of matter—the biosphere, ecosystems, communities, populations, and organisms. Biosphere: Parts of earth's air, water, and soil where life is found. Ecosystem: A community of different species interacting with one another and with their nonliving environment of matter and energy. Community: Populations of different species living in a particular place, and potentially interacting with each other. Population: A group of individuals of the same species living in a particular place. Organism: An individual living being. The biosphere and its ecosystems are made up of living (biotic) and nonliving (abiotic) components. Nonliving components include water, air, nutrients, rocks, heat, and solar energy. Living components include plants, animals, and microbes.

Evolutionary biologists attribute our ability to dominate the planet to three major adaptations: Strong opposable thumbs that allowed humans to grip and use tools better than the few other animals that have thumbs The ability to walk upright, which gave humans agility and freed up their hands for many uses A complex brain, which allowed humans to develop many skills, including the ability to talk, read, and write in order to transmit complex ideas Evolutionary biologists study patterns of evolution by examining the similarities and differences among species based on their physical and genetic characteristics. They use this information to develop branching evolutionary tree, or phylogenetic tree, diagrams that depict the hypothetical evolution of various species from common ancestors. They use fossil, DNA, and other evidence to hypothesize the evolutionary pathways and connections among species

Scientists in this field say not likely because of two limitations on adaptation through natural selection. First, a change in environmental conditions leads to adaptation only for genetic traits already present in a population's gene pool, or if they arise from mutations—which occur randomly. Second, even if a beneficial heritable trait is present in a population, the population's ability to adapt may be limited by its reproductive capacity. Populations of genetically diverse species that reproduce quickly can often adapt to a change in environmental conditions in a short time (days to years). Examples are dandelions, mosquitoes, rats, bacteria, and cockroaches. By contrast, species that cannot produce large numbers of offspring rapidly—such as elephants, tigers, sharks, and humans—take thousands or even millions of years to adapt through natural selection.

Bacteria fix nitrogen into a form that can be utilized by plants, convert ammonia to nitrate, the preferred form that plants can assimilate, and convert nitrate back into gaseous

Scientists use field research, laboratory research, and mathematical and other models to learn about ecosystems and how much stress they can take. Field research, sometimes called "muddy-boots biology," involves going into forests, oceans, and other natural settings to study the structure of ecosystems and to learn what happens in them. Most of what we know about ecosystems has come from such research. Scientists use a variety of methods to study tropical forests. Some build tall construction cranes to reach the canopies. This, along with climbing trees and installing rope walkways between treetops, helps them identify and observe the diversity of species living or feeding in these treetop habitats. Ecologists carry out controlled experiments by isolating and changing a variable in part of an area and comparing the results with nearby unchanged areas. Scientists also use aircraft and satellites equipped with sophisticated cameras and other remote sensing devices to scan and collect data on the earth's surface. They use geographic information system (GIS) software to capture, store, analyze, and display this information. For example, GIS software can convert digital satellite images into global, regional, and local maps. These maps show variations in vegetation, gross primary productivity, soil erosion, deforestation, air pollution emissions, water usage, drought, flooding, pest outbreaks, and other variables. Some researchers attach tiny radio transmitters to animals and use global positioning systems (GPSs) to track where and how far animals go. This technology is important for studying endangered species. Scientists also study nature by using cell phone cameras and mounting time lapse cameras or video cameras on stationary objects or small drones.

Key factors that influence an area's climate are incoming solar energy, the earth's rotation, global patterns of air and water movement, gases in the atmosphere, and the earth's surface features. Weather is the set of short-term atmospheric conditions over hours to days to years, whereas climate is the general pattern of atmospheric conditions in a given area over periods ranging from at least three decades to thousands of years. Weather often fluctuates daily, from one season to another, and from one year to the next. However, climate tends to change slowly because it is the average of long-term atmospheric conditions over at last 30 years. Climate varies among the earth's different regions primarily because of global air circulation and ocean currents, or mass movements of ocean water. Global winds and ocean currents distribute heat and precipitation unevenly between the tropics and other parts of the world. Scientists have described the various regions of the earth according to their climates.

Several major factors help determine regional climates. The first is the cyclical movement of air driven by solar energy. It is a form of convection, the movement of matter (such as gas or water) caused when the warmer and less dense part of a body of such matter rises while the cooler, denser part of the fluid sinks due to gravity. In the atmosphere, convection occurs when the sun warms the air and causes some of it to rise, while cooler air sinks in a cyclical pattern called a convection cell. For example, the air over an ocean is heated when the sun evaporates water. This transfers moisture and heat from the ocean to the atmosphere, especially near the hot equator. This warm, moist air rises, then cools and releases heat and moisture as precipitation. Then the cooler, denser, and drier air sinks, warms up, and absorbs moisture as it flows across the earth's surface to begin the cycle again. Convection cells play a key role in transferring energy (heat) and moisture through the atmosphere from place to place on the planet. A second major climatic factor is the uneven heating of the earth's surface by the sun. Air is heated much more at the equator, where the sun's rays strike directly, than at the poles, where sunlight strikes at an angle and spreads out over a much greater area. Thus, solar heating varies with latitude—the location between the equator and one of the poles. Latitudes are designated by degrees north or south. The equator is at 0, the poles are at 90 north and 90 south, and areas between range from to 0-90. Global air circulation: Air rises and falls in giant convection cells. Air flowing away from the equator is deflected to the east and air flowing toward the equator is deflected west, due to the Coriolis effect. This creates global patterns of prevailing winds that help to distribute heat and moisture in the atmosphere, which leads to the earth's variety of forests, grasslands, and deserts. A third major factor is the tilt of the earth's axis and resulting seasonal changes. The earth's axis—an imaginary line connecting the north and south poles—is tilted with respect to the sun's rays. As a result, regions north and south of the equator are tipped toward or away from the sun at differ

Every few years, normal wind patterns in the Pacific Ocean are disrupted and this affects weather around much of the globe. This change in wind patterns is called the El Niño-Southern Oscillation, or ENSO El Niño: Normal trade winds blowing east to west cause shore upwellings of cold, nutrient-rich bottom water in the tropical Pacific Ocean near the coast of Peru. A zone of gradual temperature change called the thermocline separates the warm and cold water. Every few years, a shift in trade winds known as the El Niño-Southern Oscillation (ENSO) disrupts this pattern for 1 to 2 years. In an ENSO, often called simply El Niño, winds that usually blow more-or-less constantly from east to west weaken or reverse direction. This allows the warmer waters of the western Pacific to move toward the coast of South America. A horizontal zone of gradual temperature change called the thermocline, separating warm and cold waters, sinks in the eastern Pacific. These changes result in drier weather in some areas and wetter weather in other areas. A strong ENSO can alter weather conditions over at least two-thirds of the globe—especially on the coasts of the Pacific and Indian Oceans. An ENSO is a 1- to

Sometimes we experience weather extremes. Two examples are violent storms called tornadoes (which form over land) and tropical cyclones (which form over warm ocean water and sometimes pass over coastal land areas). Tornadoes, or twisters, are swirling, funnel-shaped clouds that form over land. They can destroy houses and cause other serious damage in areas where they touch down. The United States is the world's most tornado-prone country, followed by Australia. Tornadoes in the plains of the Midwestern United States often occur when a large, dry, cold front moving southward from Canada runs into a large mass of warm humid air moving northward from the Gulf of Mexico. As the large warm front moves rapidly over the denser cold-air mass, it rises swiftly and forms strong vertical convection currents that suck air upward. Scientists hypothesize that the interaction of the cooler air nearer the ground and the rapidly rising warmer air above causes a spinning, vertically rising air mass, or vortex. Most tornadoes in the American Midwest occur in the spring and summer when cold fronts from the north penetrate deeply into the Great Plains and the Midwest. Tropical cyclones are spawned by the formation of low-pressure cells of air over warm tropical seas. Hurricanes are tropical cyclones that form in the Atlantic Ocean. Those forming in the Pacific Ocean usually are called typhoons. Hurricanes and typhoons kill and injure people, damage property, and hinder food production. Unlike tornadoes, however, tropical cyclones take a long time to form and gain strength. This allows meteorologists to track their paths and wind speeds, and to warn people in areas likely to be hit by these violent storms. For a tropical cyclone to form, the temperature of ocean water has to be at least ‍80f to a depth of 46 meters (150 feet). Areas of low pressure over these warm ocean waters draw in air from surrounding higher-pressure areas. The earth's rotation makes these winds spiral counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Moist air, warmed by the heat of the ocean, rises in a vortex through the center of the storm until it becomes a tropical cyclone The intensities of tropical cyclones are

Niches can be classified further in terms of specific roles that certain species play within ecosystems. Ecologists describe these roles as native, nonnative, indicator, and keystone. Any given species may play one or more of these roles in a particular ecosystem. Native species normally live and thrive in a particular ecosystem. Other species that migrate into or that are deliberately or accidentally introduced into an ecosystem are called nonnative species. They are also referred to as invasive, alien, and exotic species. People often think of nonnative species as threatening. In fact, most nonnative species, including certain food crops, trees, flowers, chickens, cattle, fish, and dogs, have certainly benefitted people. However, some nonnative species compete with and reduce an ecosystem's native species, causing unintended and unexpected consequences. For example, in 1957 Brazil imported wild African honeybees to help increase honey production. The opposite occurred. The more aggressive African bees displaced some of Brazil's native honeybee populations, which led to a reduced honey supply. African honeybees have since spread across South and Central America and into the souther

Species that provide early warnings of changes in environmental conditions in an ecosystem are called indicator species. They are like biological smoke alarms. Scientists who study reptiles and amphibians have identified natural and human-related factors that can cause the decline and disappearance of these indicator species. One natural threat is parasites such as flatworms that feed on certain amphibian eggs. Research indicates that this has caused birth defects such as missing limbs or extra limbs in some amphibians. Another natural threat comes from viral and fungal diseases. For example, the chytrid fungus infects a frog's skin and causes it to thicken. This reduces the frog's ability to take in water through its skin and leads to death from dehydration. Such diseases can spread easily, because adults of many amphibian species congregate in large numbers to breed. Habitat loss and fragmentation is another major threat to amphibians. It is mostly a human-caused problem resulting from the clearing of forests and the draining and filling of freshwater wetlands for farming and urban development. Another human-related problem is higher levels of UV radiation from the sun. Ozone that forms in the stratosphere protects the earth's life from harmful UV radiation emitted by the sun. During the past few decades, ozone-depleting chemicals released into the troposphere by human activities have drifted into the stratosphere and have destroyed some the stratosphere's protective ozone. The resulting increase in UV radiation can kill embryos of amphibians in shallow ponds as well as adult amphibians basking in the sun for warmth. International action has been taken to reduce the threat of stratospheric ozone depletion, but it will take about 50 years for ozone levels to recover to those in 1960. Pollution from human activities also threatens amphibians. Frogs and other species are exposed to pesticides in ponds and in the bodies of insects that they eat. This can make them more vulnerable to bacterial, viral, and fungal diseases and to some parasites. Climate change is also a concern. Amphibians are sensitive to even slight changes in temperature and moisture. Warmer temperatures may lead amphibians to breed too

A keystone is the wedge-shaped stone placed at the top of a stone archway. Remove this stone and the arch collapses. In some communities and ecosystems, ecologists hypothesize that certain species play a similar role. A keystone species has a large effect on the types and abundance of other species in an ecosystem. Without the keystone species, the ecosystem would be dramatically different or might cease to exist. Keystone species play several critical roles in helping to sustain ecosystems. One is the pollination of flowering plant species by butterflies, honeybees, hummingbirds, bats, and other species. In addition, top predator keystone species feed on and help to regulate the populations of other species. Examples are wolves, leopards, lions, some shark species, and the American alligator. The loss of a keystone species in an ecosystem can lead to population crashes and extinctions of other species that depend on them for certain ecosystem services. This is why it so important for scientists to identify keystone species and work to protect them.

The American alligator is a keystone species in subtropical wetland ecosystems in the southeastern United States. These alligators play several important ecological roles. They dig deep depressions known as gator holes. During dry periods, these depressions hold freshwater and serve as refuges for aquatic life. The depressions supply freshwater and food for fishes, insects, snakes, turtles, birds, and other animals. The large nesting mounds alligators build provide nesting and feeding sites for some herons and egrets. Red-bellied turtles lay their eggs in old gator nests. When alligators excavate holes and build nesting mounds, they help keep vegetation from invading shorelines and open water. Without this ecosystem service, freshwater ponds and coastal wetlands fill in with shrubs and trees, and dozens of species can disappear from these ecosystems, due to such changes. The alligators also help ensure the presence of game fish such as bass and bream by eating large numbers of gar, a predatory fish that hunts these species. In the 1930s, hunters began killing American alligators for their exotic meat and their soft belly skin. People used the skin to make expensive shoes, belts, and purses. Other people hunted alligators for sport or out of dislike for the large reptile. By the 1960s, hunters and poachers had wiped out 90% of the alligators in the state of Louisiana, and the alligator population in the Florida Everglades was near extinction. In 1967 the U.S. government placed the American alligator on the endangered species list. By 1977, because it was protected, its populations had made a strong comeback and the alligator was removed from the endangered species list. Today, there are more than a million alligators in Florida, and the state allows property owners to kill alligators that stray onto their land. To conservation biologists, the comeback of the American alligator is an important success story in wildlife conservation. Recently, however, large and rapidly reproducing Burmese and African pythons released deliberately or accidently by humans have invaded the Florida Everglades. These nonnative invaders feed on young alligators, and could threaten the long-term survival of this keystone species i

The process of convection involves warmer, less dense fluids rising, while cooler, denser fluids sink. Water moves vertically in the oceans. The tundra biome exists in two places: the arctic and high alpine areas. Much of the weather we experience results from interactions between the leading edges of moving masses of warm and cold air. Atmospheric pressure is created by gas molecules in the atmosphere moving at high speeds and bouncing off everything they encounter. The El Nino-Southern Oscillation is a change in wind patterns that affects temperatures of Pacific Ocean water.

The earth has a great diversity of species and habitats, or places where these species can live. Some species live in terrestrial, or land, habitats. Why do grasslands grow on some areas of the earth's land while forests and deserts form in other areas? The answer lies largely in differences in climate, the average short-term weather conditions in a given region over at least three decades to thousands of years. Differences in climate result mostly from long-term differences in weather, based primarily on average annual precipitation and temperature. These differences lead to three major types of climate—tropical (areas near the equator, receiving the most intense sunlight), polar (areas near the earth's poles, receiving the least intense sunlight), and temperate (areas between the tropical and polar regions). Throughout these regions, we find different types of ecosystems, vegetation, and animals adapted to the various climate conditions. For example, in tropical areas, we find a type of grassland called a savanna. This biome typically contains scattered trees and usually has warm temperatures year-round with alternating dry and wet seasons. Savannas in East Africa are home to grazing (primarily grass-eating) and browsing (twig- and leaf-nibbling) hoofed animals and their predator's.

Large areas of forest and other biomes tend to have a core habitat and edge habitats with different environmental conditions and species, called edge effect (the changes in population or community structures that occur at the boundary of two habitats.). For example, a forest edge is usually more open, bright, and windy and has greater variations in temperature and humidity than a forest interior. Humans have fragmented many forests, grasslands, and other biomes into isolated patches with less core habitat and more edge habitat that supports fewer species. Natural ecosystems within biomes rarely have distinct boundaries. Instead, one ecosystem tends to merge with the next in a transitional zone called an ecotone. It is a region containing a mixture of species from adjacent ecosystems along with some migrant species not found in either of the bordering ecosystems.

The fourth component of biodiversity is functional diversity—the variety of processes such as energy flow and matter cycling that occur within ecosystems as species interact with one another in food chains and food webs. This component of biodiversity includes the variety of ecological roles organisms play in their biological communities and the impacts these roles have on their overall ecosystems. A more biologically diverse ecosystem with a greater variety of producers can produce more plant biomass, which in turn can support a greater variety of consumer species. Biologically diverse ecosystems also tend to be more stable because they are more likely to include species with traits that enable them to adapt to changes in the environment, such as disease or drought. We use biodiversity as a source of food, medicine, building materials, and fuel. Biodiversity also provides natural ecosystem services such as air and water purification, renewal of topsoil, decomposition of wastes, and pollination. In addition, the earth's variety of genetic information, species, and ecosystems provide raw materials for the evolution of new species and ecosystem services, as they respond to changing environmental conditions. Biodiversity is the earth's ecological insurance policy.

Matter, in the form of nutrients, cycles within and among ecosystems and the biosphere, and human activities are altering these chemical cycles. The elements and compounds that make up nutrients move continually through air, water, soil, rock, and living organisms within ecosystems, in cycles called nutrient cycles, or biogeochemical cycles (life-earth-chemical cycles). They represent the chemical cycling principle of sustainability in action. These cycles are driven directly or indirectly by incoming solar energy and by the earth's gravity and include the hydrologic (water), carbon, nitrogen, and phosphorus cycles. Human activities are altering these important components of the earth's natural capital. Nutrient cycles connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of an oak leaf, a dinosaur's skin, or a layer of limestone rock. Your grandmother, George Washington, or a hunter-gatherer who lived 25,000 years ago may have inhaled some of the nitrogen molecules you just inhaled.

The hydrologic cycle, also called the water cycle, collects, purifies, and distributes the earth's fixed supply of water. The sun powers the water cycle. Incoming solar energy causes evaporation—the conversion of some of the liquid water in the earth's oceans, lakes, rivers, soil, and plants to vapor. This water vapor rises into the atmosphere, where it condenses into droplets. Gravity then draws the water back to the earth's surface as precipitation (rain, snow, sleet, and dew). Most precipitation falling on terrestrial ecosystems becomes surface runoff. This water flows into streams, rivers, lakes, wetlands, and oceans, from which it can evaporate to repeat the cycle. Some precipitation seeps into the upper layers of soils and is used by plants, and some evaporates from the soils back into the atmosphere. Some precipitation also sinks through soil into underground layers of rock, sand, and gravel called aquifers. This water stored underground is called groundwater. Some precipitation is converted to ice that is stored in glaciers. Because water is good at dissolving many different compounds, it can easily be polluted. However, natural processes in the water cycle can purify water—an important and free ecosystem service. Only about 0.024% of the earth's huge water supply is available to humans and other species as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest of the planet's water is too salty, is too deep underground to extract at affordable prices, or is stored as ice.

Biological evolution (or simply evolution)—the process by which the earth's life forms change genetically over time. These changes occur in the genes of populations of organisms from generation to generation. According to this scientific theory, species have evolved from earlier, ancestral species through natural selection. Through this process, individuals with certain genetic traits are more likely to survive and reproduce under a specific set of environmental conditions. These individuals then pass these traits on to their offspring. A large body of scientific evidence supports this idea. As a result, biological evolution through natural selection has become a widely accepted scientific theory. It explains how the earth's life has changed over the past 3.8 billion years and why we have today's diversity of species. Most of what we know about the history of life on the earth comes from fossils—the remains or traces of past organisms. Fossils include mineralized or petrified replicas of skeletons, bones, teeth, shells, leaves, and seeds, or impressions of such items found in rocks. Scientists have discovered fossil evidence in successive layers of sedimentary rock such as limestone a

The idea that organisms change over time and are descended from a single common ancestor has been discussed since the time of the early Greek philosophers. No one had developed an explanation of how this happened until 1858 when naturalists Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently proposed the concept of natural selection as a mechanism for biological evolution. Darwin gathered evidence for this idea and published it in his 1859 book, On the Origin of Species by Means of Natural Selection. Biological evolution by natural selection involves changes in a population's genetic makeup through successive generations. Populations—not individuals—evolve by becoming genetically different. The first step in this process is the development of genetic variability: a variety in the genetic makeup of individuals in a population. This occurs primarily through mutations, which are changes in the coded genetic instructions in the DNA in a gene. During an organism's lifetime, the DNA in its cells is copied each time one of its cells divides and whenever it reproduces. In a lifetime, this happens millions of times and results in various mutations. Most mutations result from random changes in the DNA's coded genetic instructions that occur in only a tiny fraction of these millions of divisions. Some mutations also occur from exposure to external agents such as radioactivity, ultraviolet radiation from the sun, and certain natural and human-made chemicals called mutagens. Mutations can occur in any cell, but only those that take place in the genes of reproductive cells are passed on to offspring. Sometimes a mutation can result in a new genetic trait, called a heritable trait, which can be passed from one generation to the next. In this way, populations develop genetic differences among their individuals. Some mutations are harmful to offspring and some are beneficial. The next step in biological evolution is natural selection, which explains how populations evolve in response to changes in environmental conditions by changing their genetic makeup. Through natural selection, environmental conditions favor increased survival and reproduction of certain individuals in a population. These

Some of the world's most spectacular environments are high on mountains, steep or high-elevation lands that cover about one-fourth of the earth's land surface. Mountains are places where dramatic changes take place over very short distances. In fact, climate and vegetation vary according to elevation, or height above sea level, just as they do with latitude. About 1.2 billion people (16% of the world's population) live in mountain ranges or in their foothills, and 4 billion people (54% of the world's population) depend on mountain systems for all or some of their water. Because of the steep slopes, mountain soils are easily eroded when the vegetation holding them in place is removed by natural disturbances such as landslides and avalanches, or by human activities such as timber cutting and agriculture. Many mountains are islands of biodiversity surrounded by a sea of lower-elevation landscapes transformed by human activities. Mountains play an important ecological role. They contain a large portion of the world's forests, which are habitats for much of the planet's terrestrial biodiversity. They often are habitats for endemic species—those that are found nowhere else on earth. They al

The monetary value of natural capital, the natural resources and ecological services that keep us alive and support our economies, has been estimated by ecological economist Robert Costanza and his colleagues. For example, they estimate that the value of all the ecosystem services provided by the earth's forests is worth at least $15.6 trillion a year. This is many times greater than the estimated value of their economic services, showing why intact forests are far more valuable than the price of the timber they would yield. Underpricing the value of natural resources is the basic problem in unsustainable practices of resource harvesting. The estimated economic value of ecosystem services provided by forests, oceans, and other ecosystems are not included in the market prices of timber, fish, and other goods we get from them. Until this underpricing is corrected, unsustainable use of forests, oceans, the atmosphere, and many of nature's other irreplaceable forms of natural capital will continue. Ecosystem services are important benefits or functions that nature provides for us and other species. Some of the most important services include the following: Pollination Clean water Seed dispersal Soil fertility Decomposition of organic waste Pest control Flood control Climate regulation Cycling of nutrients

Biodiversity is a biological indicator of ecosystem health. A highly diverse ecosystem provides a large variety of ecosystem services. Characteristics of biodiversity are species richness, the total number of different species in an ecosystem, and species evenness, the number of individuals of each species. Simpson's Diversity Index measures both species richness and species evenness. As both of these factors increase, so does overall diversity. Ecologists use this biological measure as a way to objectively quantify ecosystem diversity. The species diversity of ecosystems varies with their geographical location. For most terrestrial plants and animals, species diversity (primarily species richness) is highest in the tropics and declines as we move from the equator toward the poles. The most species-rich environments are tropical rain forests, large tropical lakes, coral reefs, and the ocean-bottom zone.

The second component of biodiversity is genetic diversity, which is the variety of genes found in a population or in a species. Genes contain genetic information that give rise to specific traits, or characteristics, that are passed on to offspring through reproduction. Species have a better chance of surviving and adapting to environmental changes if their populations have greater genetic diversity. The third component of biodiversity, ecosystem diversity, refers to the earth's diversity of biological communities such as deserts, grasslands, forests, mountains, oceans, lakes, rivers, and wetlands. Biologists classify terrestrial (land) ecosystems into biomes—large regions such as forests, deserts, and grasslands characterized by distinct climates and certain prominent species (especially vegetation). Biomes differ in their community structure based on the types, relative sizes, and stratification of their plant species.

The second major type of forest is the temperate forest, the most common of which is the temperate deciduous forest. Such forests typically have warm summers, cold winters, and abundant precipitation—rain in summer and snow in winter months. They are dominated by a few species of broadleaf deciduous trees such as oak, hickory, maple, aspen, and birch. Animal species living in these forests include predators such as wolves, foxes, and wildcats. They feed on herbivores such as white-tailed deer, squirrels, rabbits, and mice. Warblers, robins, and other bird species live in these forests during the spring and summer, mating and raising their young. In these forests, most of the trees' leaves, after developing their vibrant colors in the fall, drop off the trees. This allows the trees to survive the cold winters by becoming dormant. Each spring, the trees sprout new leaves and spend their summers growing and producing until the cold weather returns. Because they have cooler temperatures and fewer decomposers than tropical forests have, temperate forests also have a slower rate of decomposition. As a result, they accumulate a thick layer of slowly decaying leaf litter, which becomes a store

The third major forest type is the cold, or northern coniferous forests, also called boreal forests or taigas. They are found south of arctic tundra in northern regions across North America, Asia, and Europe and above certain altitudes in the Sierra Nevada and Rocky Mountain ranges of the United States. In the subarctic, cold, and moist climate of the northernmost boreal forests, winters are long and extremely cold, with winter sunlight available only 6 to 8 hours per day. Summers are short, with cool to warm temperatures, and the sun shines as long as 19 hours a day during midsummer. Most boreal forests are dominated by a few species of coniferous evergreen trees or conifers such as spruce, fir, cedar, hemlock, and pine. Plant diversity is low because few species can survive the winters when soil moisture is frozen. Beneath the stands of trees in these forests is a deep layer of partially decomposed conifer needles. Decomposition is slow because of low temperatures, the waxy coating on the needles, and high soil acidity. The decomposing conifer needles make the thin, nutrient-poor topsoil acidic, which prevents most other plants (except certain shrubs) from growing on the forest floor. Year-round wildlife in this biome includes bears, wolves, moose, lynx, and many burrowing rodent species. Caribou spend winter in the taiga and summer in the arctic tundra. During the brief summer, warblers and other insect-eating birds feed on flies, mosquitoes, and caterpillars.

In the phosphorus cycle, phosphorus is bound as phosphate in rocks on land. Weathering, or erosion, of these rocks releases phosphate into the soils. Runoff carries and dispenses phosphates into streams, rivers, and ultimately the ocean, where they can enter marine food webs or come out of solution and settle as deposits along marine sediments. If phosphates in the ocean undergo deposition, then these deposits will uplift, via plate tectonics, onto land as the earth's crust moves through geologic time.

The use of cameras mounted on aircraft and satellites to scan and collect data on the earth's surface is called remote sensing. In nitrogen fixation, bacteria combine gaseous N2 with hydrogen to make ammonia (NH3).

Ecologists assign each organism in an ecosystem to a feeding level, or trophic level, based on its source of nutrients. Organisms are classified as producers and consumers. Producers (also called autotrophs) are organisms, such as green plants, that make the nutrients they need from compounds and energy obtained from their environment. In the process known as photosynthesis, plants capture solar energy that falls on their leaves. They use it to combine carbon dioxide and water and form carbohydrates such as glucose , to store chemical energy that plants need and emit oxygen gas into the atmosphere. This oxygen keeps us and most other animal species alive. (Organism that uses solar energy (green plants) or chemical energy (some bacteria) to manufacture the organic compounds it needs as nutrients from simple inorganic compounds obtained from its environment.) About 2.8 billion years ago, producer organisms called cyanobacteria, most of them floating on the surface of the ocean, started carrying out photosynthesis, which added oxygen to the atmosphere. After several hundred million years, oxygen levels reached about 21%—high enough to keep oxygen-breathing animals alive. Today, most p

There are several types of consumers. Primary consumers, or herbivores (plant eaters), are animals that eat mostly green plants. Carnivores (meat eaters) are animals that feed on the flesh of other animals. Some carnivores, including spiders, lions, and most small fishes, are secondary consumers that feed on the flesh of herbivores. Other carnivores such as tigers, hawks, and killer whales (orcas) are tertiary (or higher-level) consumers that feed on the flesh of herbivores and other carnivores. Omnivores such as pigs, rats, and humans eat both plants and animals. Decomposers are consumers that get nourishment by releasing nutrients from the wastes or remains of plants and animals. These nutrients return to the soil, water, and air for reuse by producers. Most decomposers are bacteria and fungi. Other consumers, called detritus feeders, or detritivores, feed on the wastes or dead bodies (detritus) of other organisms. Examples are earthworms, soil insects, hyenas, and vultures. Detritivores and decomposers can transform a fallen tree trunk into simple inorganic molecules that plants can absorb as nutrients. In natural ecosystems, the wastes and dead bodies of organisms are resources for other organisms in keeping with the chemical cycling principle of sustainability. Without decomposers and detritivores, many of which are microscopic organisms, the planet's land surfaces would be buried in plant and animal wastes, dead animal bodies, and garbage.

Forests are lands that are dominated by trees. The three main types of forest—tropical, temperate, and cold (northern coniferous, or boreal)—result from combinations of varying precipitation levels and temperatures averaged over three decades or longer.

Tropical rain forests are dominated by broadleaf evergreen plants, which keep most of their leaves year-round. The tops of the trees form a dense canopy that blocks most light from reaching the forest floor. Many of the relatively few plants that live at the ground level have enormous leaves to capture what little sunlight filters down to them. Some trees are draped with vines (called lianas) that grow to the treetops to gain access to sunlight. In the canopy, the vines grow from one tree to another, providing walkways for many species living there. When a large tree is cut down, its network of lianas can pull down other trees. Tropical rain forests have a high net primary productivity. They are teeming with life and possess incredible biological diversity. Although tropical rain forests cover only about 2% of the earth's land surface, ecologists estimate that they contain at least 50% of the known terrestrial plant and animal species. A single tree in these forests may support several thousand different insect species. Plants from tropical rain forests are a source of a variety of chemicals, many of which have been used as blueprints for making most of the world's prescription drugs. Rain forest species occupy a variety of specialized niches in distinct layers, which contribute to their high species diversity. Vegetation layers are structured, for the most part, according to the plants' needs for sunlight. Much of the animal life, particularly insects, bats, and birds, lives in the sunny canopy layer, with its abundant shelter and supplies of leaves, flowers, and fruits. Specialized plant and animal niches are stratified, or arranged roughly in layers, in a tropical rain forest. Filling such specialized niches enables many species to avoid or minimize competition for resources and results in the coexistence of a great variety of species. Dropped leaves, fallen trees, and dead animals decompose quickly in tropical rain forests because of the warm, moist conditions and the hordes of decomposers. About 90% of the nutrients released by this rapid decomposition are quickly taken up and stored by trees, vines, and other plants. Nutrients that are not taken up are soon leached from the thin topsoil by the freq

The geosphere consists of the earth's intensely hot core, a thick mantle composed mostly of rock, and a thin outer crust. The greenhouse effect is a natural process. Decomposers are consumers that, in the process of obtaining their own nutrients, release nutrients from the wastes or remains of plants and animals and then return those nutrients to the soil, water, and air for reuse by producers. In a food chain or web, chemical energy stored in biomass is transferred from one trophic level to another. Gross primary productivity (GPP) is the rate at which an ecosystem's producers (usually plants) convert solar energy into chemical energy in the form of biomass found in their tissues. The water cycle is a global nutrient cycle. Lightning fixes nitrogen into compounds useful for organisms. Tools like remote sensing and GIS software allow scientists to collect and analyze data on the earth's surface. Scientists can study ecosystems in the laboratory.

Tropical rain forests are found near the earth's equator and contain an amazing variety of life. These lush forests are warm year round and have high humidity because it rains almost daily. Rain forests cover only 2% of the earth's land but contain up to half of the world's known terrestrial plant and animal species. These properties make rain forests natural laboratories in which to study ecosystems—communities of organisms that interact with one another and with the physical environment of matter and energy in which they live. To date, at least half of the earth's rain forests have been destroyed or degraded by humans cutting down trees, growing crops, grazing cattle, and building settlements. The destruction and degradation of these centers of biodiversity is increasing. Ecologists warn that without protection, most of these forests will be gone or severely degraded by the end of this century. Why should we care that tropical rain forests are disappearing? Scientists give three reasons. First, clearing these forests reduces the earth's vital biodiversity by destroying the habitats for many of the earth's species. Second, destroying these forests contributes to atmospheric warming and speeds up climate change. Eliminating large areas of trees faster than they can grow back decreases the ability of the forests to remove some of the human-generated emissions of carbon dioxide , a gas that contributes to atmospheric warming and climate change. Third, large-scale loss of tropical rain forests can change regional weather and climate patterns. Sometimes such changes can prevent the regrowth of rain forests in cleared or degraded areas. When this ecological tipping point is reached, tropical rain forests in such areas become less diverse tropical grasslands.

Gross primary productivity (GPP) is the rate at which an ecosystem's producers (such as plants and phytoplankton) convert solar energy into chemical energy stored in compounds found in their tissues. To stay alive, grow, and reproduce, producers must use some of their stored chemical energy for their own respiration. Net primary productivity (NPP) is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration. NPP measures how fast producers can make the chemical energy that is stored in their tissues and that is potentially available to other organisms (consumers) in an ecosystem. Gross primary productivity is similar to the rate at which you make money, or the number of dollars you earn per year. Net primary productivity is similar to the amount of money earned per year that you can spend after subtracting your expenses such as the costs of transportation, clothes, food, and supplies. Despite its low NPP, the open ocean produces more of the earth's biomass per year than any other ecosystem or life zone. This happens because oceans cover 71% of the earth's surface

Tropical rain forests have a high net primary productivity because they have a large number and variety of producer trees and other plants. When these forests are cleared or burned to make way for crops or for grazing cattle, they suffer a sharp drop in net primary productivity. They also lose much of their diverse array of plant and animal species. Only the plant matter represented by NPP is available as nutrients for consumers. Thus, the planet's NPP ultimately limits the number of consumers (including humans) that can survive on the earth. This is an important lesson from nature.

There are a number of misconceptions about biological evolution through natural selection. Here are five common myths: Survival of the fittest means survival of the strongest. To biologists, fitness is a measure of reproductive success, not strength. Thus, the fittest individuals are those that leave the most descendants, not those that are physically the strongest. Evolution explains the origin of life. It does not. However, it does explain how species have evolved after life came into being around 3.8 billion years ago. Humans evolved from apes or monkeys. Fossil and other evidence shows that humans, apes, and monkeys evolved along different paths from a common ancestor that lived 5 million to 8 million years ago. Evolution by natural selection is part of a grand plan in nature in which species are to become more perfectly adapted. There is no evidence of such a plan. Instead, evidence indicates that the forces of natural selection and random mutations can push evolution along any number of paths. Evolution by natural selection is not important because it is just a theory. This reveals a misunderstanding of what a scientific theory is. A scientific hypothesis becomes a scientific theo

Under certain circumstances, natural selection can lead to an entirely new species. In this process, called speciation, one species evolves into two or more different species. For sexually reproducing organisms, a new species forms when a separated population of a species evolves to the point where its members can no longer interbreed and produce fertile offspring with members of another population of its species that did not change or that evolved differently. Speciation, especially among sexually reproducing species, happens in two phases: first geographic isolation, and then reproductive isolation. Geographic isolation occurs when different groups of the same population of a species become physically isolated from one another for a long time. Part of a population may migrate in search of food and then begin living as a separate population in an area with different environmental conditions. Winds and flowing water may carry a few individuals far away where they establish a new population. A flooding stream, a new road, a hurricane, an earthquake, or a volcanic eruption, as well as long-term geological processes, can also separate populations. Human activities, such as the creation of large reservoirs behind dams and the clearing of forests, can also create physical barriers for certain species. The separated populations can develop quite different genetic characteristics because they are no longer exchanging genes.

As energy flows from the sun to the earth, some of it is reflected by the earth's surface back into the atmosphere. Molecules of certain gases in the atmosphere, including water vapor, carbon dioxide, methane, and nitrous oxide, absorb some of this solar energy and release a portion of it as infrared radiation (heat) that warms the lower atmosphere and the earth's surface. These gases, called greenhouse gases, play a role in determining the lower atmosphere's average temperatures and thus the earth's climates. The earth's surface absorbs much of the solar energy that strikes it and transforms it into longer-wavelength infrared radiation, which then rises into the lower atmosphere. Some of this heat escapes into space, but some is absorbed by molecules of greenhouse gases and emitted into the lower atmosphere as even longer-wavelength infrared radiation. Some of this released energy radiates into space, and some adds to the warming of the lower atmosphere and the earth's surface. Together, these processes result in a natural warming of the troposphere, called the greenhouse effect. Without this natural warming effect, the earth would be a very cold and mostly lifeless planet. Human acti

Various topographic features of the earth's surface can create local climatic conditions that differ from the general climate in some regions. For example, mountains interrupt the flow of prevailing surface winds and the movement of storms. When moist air from an ocean blows inland and reaches a mountain range, it is forced upward. As the air rises, it cools, expands, and loses most of its moisture as rain and snow that fall on the windward slope of the mountain. When the drier air mass passes over the mountaintops, it flows down the leeward slopes (facing away from the wind) and warms up. This warmer air can hold more moisture, but it typically does not release much of it. This tends to dry out plants and soil below. This process is called the rain shadow effect. Over many decades, it results in semiarid or arid conditions on the leeward side of a high mountain range. Sometimes this effect leads to the formation of deserts such as Death Valley, a part of the Mojave Desert, which lies within the U.S. states of California, Nevada, Utah, and Arizona. The rain shadow effect is a reduction of rainfall and loss of moisture from the landscape on the leeward side of a mountain. Warm, moist air in onshore winds loses most of its moisture as rain and snow that fall on the windward slopes of a mountain range. This leads to semiarid and arid conditions on the leeward side of the mountain range and on the land beyond. Cities also create distinct microclimates based on their weather averaged over three decades or more. Bricks, concrete, asphalt, and other building materials absorb and hold heat, and buildings block wind. Motor vehicles and the heating and cooling systems of buildings release large quantities of heat and pollutants. As a result, cities on average tend to have more haze and smog, higher temperatures, and lower wind speeds than the surrounding countryside. These factors make cities heat islands.