Ecosystems are the biological community of organisms as well as the physical components of a specific area. A biological community is all of the populations of different species that live in a certain environment. An ecosystem can be as large as an entire forest or as small as a clump of moss that provides a habitat for plants, microscopic invertebrates, and bacteria.

Ecosystems can be thought of and studied in various contexts. Ecologists often study the flow of energy through ecosystems. Another focus of study of ecosystems involves understanding the interactions between the organisms that live within a community. Finally, ecological researchers study the interactions between organisms and the physical environment.

There are several important types of ecosystems on Earth. These ecosystems are often defined by the amount of precipitation they receive and the typical temperatures that they experience. Some of the more important ecosystems are the tropical rain forest, desert, temperate forest, tundra, and savanna or grasslands. Common aquatic ecosystems include lakes, rivers, estuaries, coral reefs, kelp forests, the open ocean, and the deep ocean.

An ecosystem is the collection of organisms, or the biological community, that lives in a certain area as well as the physical components of the environment. The biological community includes all of the populations of animals, plants, fungi, and bacteria. A population is all the members of a certain species that live in a location. In terrestrial ecosystems, the physical environment includes the type and conditions of soil or rocky terrain, the general climatic conditions, the amount of precipitation, and the amount of sunlight available. In aquatic ecosystems, the physical environment includes the salinity, temperature, and acidity of the water, the amount of sunlight available, the amount of sedimentation in the water, the amount of nutrients dissolved in the water, and the type of substrate below the water.

The flow of energy through an ecosystem plays a key role in the stability and functioning of an ecosystem. With few exceptions, the energy for life originates with light energy from the sun. Plants convert this energy to chemical energy stored in carbohydrates through photosynthesis. Herbivores then consume the carbohydrates in plants and use the energy from the chemical bonds for growth and reproduction. Carnivores consume the herbivores and assimilate the energy stored in the chemical bonds of the herbivores. Finally, the carnivores die, and their organic remains are decomposed by bacteria and fungi. These decomposers use the stored chemical energy in organis’s chemical bonds and convert the organic material into inorganic material that plants require to perform photosynthesis. With each successive step in the food chain, or food web in complex ecosystems, energy is lost as heat to the environment. Approximately 10% of the energy harvested from the sun by plants gets passed on to the herbivores. Similarly, only 10% of the energy stored in the chemical bonds of the herbivores is passed to the carnivores. As a result, most ecosystems have significantly more plants, or primary producers, than predators.

Every organism in an ecosystem depends on many other organisms in order to grow and reproduce. In order to grow, organisms require food. Both the struggle to obtain food and the struggle to avoid becoming food are major goals of the biological interactions in ecosystems. Predation is the consumption of one species by another species. The predator is the species that consumes the prey. Predators have developed adaptationsPage 243  |  Top of Article that allow them to hunt their prey more effectively. For example, tuna have special muscles that allow them to swim quickly after smaller fish. Lions have large jaws and sharp teeth to capture prey. Angler fish have special outgrowths from their foreheads that look like worms and lure smaller fish into striking distance. Halibut can disguise themselves so that they are nearly invisible to prey swimming near the seafloor. However, prey has developed adaptations to avoid predation. Roses and cacti have thorns that discourage grazing. Tobacco produces nicotine, which is toxic to many insects. Skunks spray acrid chemicals on potential predators.

Another type of ecological interaction is competition for resources. Resources are anything that an organism needs to grow and reproduce such as shelter, territory, food, sunlight, and water. Members of the same species may also compete for mates. Just as organisms have developed numerous adaptations in response to predator and prey relationships, so too have they developed numerous adaptations to improve their ability to compete in an ecosystem. Tubeworms have developed long tentacles to sieve more plankton out of the water than their competitors. Several species of desert plants release chemicals into the soil that prevent the growth of other plants nearby. This decreases the competition for soil nutrients and water.

The sum of the resource requirements for an organism is called its ecological niche. When organisms have ecological niches that overlap, competition is more intense. Because individuals of the same species have identical resource requirements, competition within a species is often quite intense. Any resource that is in such short supply in an ecosystem that it prevents the growth of a population is called a limiting factor. Limiting factors often control the available ecological niches within an ecosystem. The number of available ecological niches within an ecosystem is often related to the biodiversity, or species richness of an ecosystem. When an ecosystem has more ecological niches, the number of species in the ecosystem is greater.

Although the flow of energy moves through ecosystems in one direction, from plants to herbivores and then to carnivores, many of the physical components of ecosystems are recycled. For example, chemicals that are critical to growth and reproduction in animals, water, carbon, nitrogen, and phosphorus all flow through both the biotic, or living, and abiotic, or nonliving, components of ecosystems. The patterns of flow are called biogeochemical cycles. Each of these cycles varies depending on the chemical properties of the matter. For example, the generalized biogeochemical cycle for carbon begins with carbon dioxide from the atmosphere becoming assimilated into carbohydrates during plant photosynthesis. It is passed to animals as the plants are consumed. The animals release carbon dioxide to the atmosphere through respiration. If plants are not con-

sumed, they die and become buried. In some cases, over long periods of time, these buried plants become fossil fuels. When fossil fuels are burned, carbon dioxide is returned to the atmosphere. Alternatively, the hydrologic cycle involves evaporation of water from lakes and oceans. Precipitation returns water to Earth where plants and animals use it to grow. A process known as transpiration returns water to the atmosphere from plants. Although water is in the atmosphere, the movement of air transports water from region to region, controlling the climate.

Ecosystems vary in size and structure depending on the physical properties of an ecosystem. In addition, ecosystems can be thought of as nested within ecosystems. For example, the ocean as a whole is an ecosystem. So too, a rocky shoreline is an ecosystem. Further, a tide pool within a rocky shoreline consists of animals, plants, and decomposers that make it an ecosystem unto itself. Within a tide pool, the back of a decorator crab might be an ecosystem, complete with moss, barnacles, limpets, shrimp, and even a parasite or two.

Throughout the world, several broad types of ecosystems are recognized. Because these forms of ecosystems are found in many different locations, they are also referred to as biomes. On land, the major biomes include the tropical rain forest, desert, temperate forest, tundra, and savanna or grasslands. Terrestrial ecosystems depend on the amount of precipitation they receive as well as the typical temperatures they experience.

The wettest and warmest type of ecosystem is the tropical rain forest. These ecosystems are the most diverse in the world, which means that they contain the most species as well as the most interactions among species. Rain forests are particularly diverse because they have

Efforts to restore the fragile ecosystem of coastal wetlands were dealt a severe setback by Hurricane Katrina (2005). AP Images

abundant ecological resources and therefore possess many different environmental niches. Rain forests receive more than 80 in (200 cm) of precipitation per year. In addition, temperatures are consistently warm, so organisms do not need adaptations to survive extremes. Rain forests are characterized by trees that form multiple layers of canopy. Within the trees live numerous species of insects, birds, reptiles, amphibians, sloths, and monkeys.

The driest ecosystems on Earth are desert ecosystems. Deserts are found in places where the climate is hot or temperate. Because there is very little water in

desert ecosystems, plants have evolved adaptations to decrease water loss to evaporation. Instead of leaves, which have a large surface area over which water can evaporate, desert plants have tiny leaves or spines. Other plants only grow leaves during rainy periods and then shed them during dry periods. Many desert plants have a waxy coating on the outsides of their stems and leaves to prevent evaporation. Animals that live in deserts tend to be small. Many are nocturnal, hunting at night and hiding in shadows where it is cooler during the day. Typical animals include desert reptiles like tortoises, iguanas, and other lizards. Mammals include rodents like kangaroo rats and gerbils. Many species of birds and cats also hunt in deserts.

The coldest ecosystems are known as taiga or tundra. These places have long, cold winters. The snow only melts for a short time each year. There is not much precipitation, less than 10 in (25 cm) a year. Because the soil is frozen, trees have difficulty taking root and most of the plants are small and scrublike. Mosses, lichens, grasses, and sedges are the most common plants. Animals include those that are adapted to the harsh climate, such as lemmings, arctic foxes, snowshoe hares, and musk oxen. Many species of birds migrate to the tundra during the warm season. During this time, the tundra swarms with insects that provide food for the migrating birds.

Temperate forests are found in places where winter is cold and summer is warm, generally at mid-latitudes throughout North America, Europe, and Asia, and to a smaller extent in the southern portion of South America. Precipitation tends to be significant, between 30 and 50 in (75 to 125 cm) a year. Typically the trees in temperPage 245  |  Top of Articleate forests lose their leaves each winter. The nutrients from these leaves are recycled into the soil by decomposers like bacteria and fungi. Many large mammals such as puma, bear, wolves, and bison are native to temperate forests. Numerous species of insects, reptiles, and amphibians are also found in temperate forests.

Grasslands are found in places where temperatures are moderate, but where the precipitation is somewhat less than that of temperate forests, approximately 10 to 30 in (25 to 75 cm) per year. They are common throughout central Asia, Australia, Africa, central North America, and South America. The major vegetation is grass, and few trees are found near water reservoirs. Grasslands are often home to large herds of herbivores like bison and elk. Fewer numbers of predators, like coyotes and wolves, hunt these grazers.

Common aquatic ecosystems include lakes, rivers, estuaries, coral reefs, kelp forests, the open ocean, and the deep ocean. The plants include phytoplankton, which are photosynthetic microorganisms that live in the surface layers of the water where sunlight is available. Other plants include kelp and sea grasses, many of which have adaptations that hold them firmly to the sea floor, especially in places where wave action is powerful. Many aquatic animals have adaptations that allow them to filter food out of the water. Clams extend a tube into the water, corals and anemones have tentacles, barnacles use modified leg appendages that look like feathers, and ctenophores spin mucous webs. Small fish often feed on these filter-feeders. Larger fish and marine mammals feed on the smaller fish. When organisms die in aquatic systems, they sink to the ocean or lake floor where they are decomposed by bacteria, returning inorganic nutrients to the water for phytoplankton to use in photosynthesis.

Maintaining the health of ecosystems is important for numerous reasons. Natural ecosystems break down pollutants, recycle wastes, provide flood and erosion control, and create freshwater in aquifers. They also provide habitat for organisms, including pests, diverting these organisms from urban centers. The diversity of plants in the many ecosystems is the source for many medicines that humans use to treat disease. All of the oxygen on the planet results from photosynthesis from plants in forests and phytoplankton in the oceans.

Ecosystems throughout the world are threatened by human activity. Development, agriculture, mining, and grazing by ranchers have the most significant impact on land because they destroy native habitats. Pollution and climate change threaten the health of organisms in ecosystems, just as it threatens the health of humans. Because organisms within ecosystems depend on one another, impacts to one type of organism have repercussions throughout the entire ecosystem. Although every

ecosystem is affected by these human activities, each type of biome has its own challenges due to its specific structure. Some of these more important impacts to specific ecosystems are discussed below.

Desert soils are usually thin and easily damaged. In addition, desert soils are slow to recover from damage. For example, tire tracks left by military vehicles training during World War II (1939–1945) are still visible in California deserts. Many grasslands are being converted to deserts through a process known as desertification. Desertification occurs when grasslands in semiarid regions are overgrazed. This overgrazing exposes the soil to wind, which removes the fertile top layers. Plants are unable to take root and the land becomes unable to support biological communities. Agriculture and grazing are no longer viable on desertified lands and people living nearby often migrate away. It has been estimated that up to 90% of all semiarid land in North America has been moderately to severely desertified. Conservation and management have attempted to slow the effects of desertification. These practices include controlling the distriPage 246  |  Top of Articlebution of grazing animals on rangeland and seeding in places where plant cover is sparse. Although these management practices have contributed to some range recovery, they are both slow and costly.

Forests, both tropical and temperate, are threatened by deforestation, which occurs when trees are removed for economic use, either for the wood itself or to clear the land for grazing or farming. Not only does deforestation remove trees that function as a basis for the ecosystems, tree removal exposes soils leading to erosion and it stops the influx of organic materials into soils making them less fertile. In wet areas, erosion leads to sedimentation of waterways, harming aquatic ecosystems. In dry areas, erosion can lead to desertification. Many species have become extinct as a result of deforestation, and deforestation affects the migratory patterns of birds and butterflies. Between 2000 and 2005, the United Nations Food and Agriculture Organization (FAO) estimates that 10.4 million hectares of tropical rain forest were destroyed, which is about double the rate of destruction during the prior decade.

More than half of the world’s population lives along coastlines. Aquatic ecosystems are threatened by urbanization, engineering (projects such as dams and levees), pollution, mining, and overfishing. Wetlands, in particular, are disappearing at alarming rates. In the contiguous 48 states, only about half the original wetlands remain undeveloped. In 1972 a section of the Clean Water Act was aimed at preventing additional loss of coastal wetlands. In 1986 the Emergency Wetlands Resource Act further authorized the U.S. Fish and Wildlife Service to acquire important wetlands. Though the loss of wetlands has slowed, questions of authority over wetlands complicate application of the legislation. In the open ocean, pollution is a major issue threatening ecosystems. In particular plastics break down into small spheres that both float and resemble fish eggs. Many organisms ingest these plastic spheres, which become incorporated into their tissues. In the center of the North Pacific Ocean, where winds and currents are slack, plastics have accumulated into a large flotilla. Net samples through this region collect six times more plastic than biological material. In addition, overfishing and bycatch of large marine mammals disrupt aquatic food webs.

The National Biological Service undertook a broad study of endangered ecosystems as a means of assessing human impact on the environment. The study found that 85% of the forests had been destroyed by the 1980s. More than 90% of the old growth forests were destroyed. About 98% of all streams were degraded so as to be unworthy of designation as wild or scenic waterways. Overall, the greatest losses to ecosystems in the United States were in the Northeast, the South, the Midwest, and in California. In response studies on threatened ecosystems, conservation agencies and governments have begun buying land and setting aside areas with particularly sensitive or endangered ecosystems. In some cases, government incentives encourage land owners to restore ecosystems to natural conditions.

Juli Berwald