Environment, all of the external factors affecting an organism. These factors may be other living organisms (biotic factors) or nonliving variables (abiotic factors), such as water, soil, climate, light, and oxygen. All interacting biotic and abiotic factors together make up an ecosystem. Organisms and their environment constantly interact, and both are changed by this interaction. Additionally, environmental factors, singly or in combination, ultimately limit the size that any population may attain. This limit, a population's carrying capacity, is usually reached because needed resources are in short supply. Occasionally, carrying capacity may be dictated by the direct actions of other species, as when predators limit the number of their prey in a specific area. Like all other living beings, humans have clearly changed their environment, but they have done so generally on a grander scale than have other species. Some of these changes-such as the destruction of the world's tropical rain forests to create grazing land for cattle or the drying up of almost three-quarters of the Aral Sea, once the world's fourth-largest freshwater lake, for irrigation purposes-have led to altered climate patterns, which in turn have changed the distribution of species of animals and plants. Scientists are working to understand the long-term consequences that human actions have on ecosystems, while environmentalists-professionals in various fields, as well as concerned citizens in the United States and other countries-are struggling to lessen the impact of human activity on the natural world.
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