Ecosystem

An ecosystem is all the organisms living in an area, from microbes to trees and animals, together with the nonliving, or abiotic, parts of their surroundings, such as water, soil, rock, light, temperature and humidity. The living and nonliving parts constantly affect each other, so ecologists study them as one working system. Ecosystems come in every size, from a tide pool or a single layer of rainforest to a lake or a whole forest, and they are often grouped as freshwater, marine and terrestrial.

Two processes hold an ecosystem together. Energy flows through it in one direction: producers such as plants and algae capture sunlight, consumers eat them or each other, and at every step much of the energy is lost as heat. Matter, by contrast, is recycled: decomposers break down dead material and return carbon, nitrogen and other nutrients to the soil or water for producers to use again.

Because the parts are connected, removing or adding one species can change the whole system. When wolves were eliminated from Yellowstone National Park, elk overgrazed the land; after wolves were reintroduced in 1995, beaver populations later grew.