Living things are a part of larger systems of interaction. Ecosystems tend to have cyclic fluctuations around a state of equilibrium. In the long run, however, ecosystems always change when climate changes or when one or more new species appear as a result of migration or local evolution. Human beings are part of the earth's ecosystems. Human activities can, deliberately or inadvertently, alter the equilibrium in ecosystems.
Ecosystems can be reasonably stable over hundreds or thousands of years.
If a disaster such as flood or fire occurs, the damaged ecosystem is likely to recover in stages that eventually result in a system similar to the original one.