The main unit of study in ecology is the ecosystem. Ecosystems are communities of plants and animals within a given habitat that provide the necessary means of survival, including food and water. Ecosystems are defined by such physical conditions as climate, altitude, latitude, and soil and water characteristics. Examples include forests, tundra, savannas (grasslands), and rainforests.

There are many complex and delicate interrelationships within an ecosystem. For example, green plants use the energy of sunlight to make carbohydrates, fats, and proteins; some animals eat these plants and acquire part of the energy of the carbohydrates, fats, and proteins; other animals eat these animals and acquire a smaller part of that energy. Cycles of photosynthesis, respiration, and nitrogen fixation continuously recycle the chemicals of life needed to support the ecosystem. Anything that disrupts these cycles, such as droughts, or the pollution of air or water, can disrupt the delicate workings of the entire ecosystem.

Therefore a primary concern of ecologists today is to study and attempt to find solutions for disruptions in various ecosystems. Increasingly an area of expertise is the reconstruction of ecosystems—that is, the restoration of ecosystems that are destroyed or almost completely destroyed because of pollution, overuse of land, or other action.

A key area of work for ecologists is in land and water conservation. They help to restore damaged land and water as well as to preserve wild areas for the future. Understanding the links between organisms and their physical environments can be invaluable in such efforts.

Imagine that there is a large pond at the edge of a town. A woman out jogging one day notices that hundreds of small, dead fish have washed up at the edge of the pond; a "fish kill," in environmental language. Clearly something is wrong, but what? A nearby factory discharges its wastewater into the pond. Is there something new in the wastewater that killed the fish? Or did something else kill the fish? A professional who understands the fish, the habitat (the pond), the possible reasons for the fish kill, and the potential solutions clearly would be useful here.

This is also true for environmental planning and resource management. Planning involves studying and reporting the impact of an action on the environment. For example, how might the construction of a new federal highway affect the surrounding ecosystem? A planning team may go to the site to view the physical geography and environment, the plants, and the animals. It also may recommend alternative actions that will have less damaging effects.

Resource management means determining what resources already exist and working to use them wisely. Professionals may build databases cataloging the plants, animals, and physical characteristics of a given area. They also may report on what can be done to ensure that the ecosystem can continue to sustain itself in the future. If an ecosystem has been completely destroyed, ecologists can help reconstruct it, getting the physical environment back up to par and reintroducing the species that used to live there.

Ecologists work in many areas of specialization. Limnologists study freshwater ecology; hydrogeologists focus on water on or below the surface of the earth; paleontologists study the remains of ancient life-forms in the form of fossils; geomorphologists study the origin of landforms and their changes; and geochemists study the chemistry of the earth, including the effect of pollution on the earth's chemistry. Other specialties are those of the endangered species biologists and wetlands ecologists.

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