The plants and animals that are found in a particular location are referred to as an ecosystem. These plants and animals depend on each other to survive. In a delicate balance, these lifeforms help to sustain one another in regular patterns. Disruptions to an ecosystem can be disastrous to all organisms within the ecosystem.
As an example, consider what happens when a new plant or animal is introduced into an ecosystem where it did not before exist. The new organism competes with the natural organisms from that location for available resources. These unnatural strangers can push other organisms out, causing them to become extinct. This can then affect still other organisms that depended on the extinct organism as a source of food.
Throughout the history of the Earth, the delicate balance found within ecosystems has been disrupted by natural disasters such as fires, floods, storms, volcano eruptions, and so forth. In recent years, however, mankind has increasingly been affecting the many ecosystems around the world.