With all of the concern about human effects on the environment, a considerable amount of study has been done on the ecosystems of the world. These ecosystems can be devastated by many factors, but the one that affects them the most is population.
Population is defined as a group of organisms of one species that have the ability to interbreed and who live in the same area at the same time. Some examples are the wolf, deer or human population in an area.
A population's size is the most potent factor affecting an ecosystem. The bigger the population, the more strain it puts on the ecosystem. If a population grows too large, the ecosystem could collapse.
Populations go through cycles of growth and contraction. If a population grows too big too fast then the lack of resources usually results in a population crash. This gives the species and the ecosystem time to recover so there's no permanent damage.
A certain number of limiting factors keep populations from growing too large. One is physical space, which is especially important for plant populations. Another is food stock.
If the environmental resistance (lack of food, water and other resources) is greater than the species' ability to reproduce, than the population will shrink.