Ecosystems are all things in one location living and non-living. An ecosystem is a very fine balance of life and death which holds all the places in the world together.
A way to disturb an ecosystem is by introducing an invasive species or by taking away some food source. An ecosystem can be illustrated by a fox eating a rabbit that was fed by clover growing on the bank of a river in an open patch of forest. This ecosystem would fail if humans polluted the river, or if they introduced wolves into this ecosystem.
Each animal in an ecosystem have what is called a "niche", which is something that an animal does that helps the ecosystem. An example of a niche is a fox lowering the population of rabbits, and thus saving the clover on the banks of the river from extinction.