grass → rabbit → fox
If the foxes in this food chain were killed, the population of rabbits would increase because they are no longer prey to the foxes. As a result, the amount of grass would decrease because the increased population of rabbits would be eating it.
Very small changes to ecosystems have large consequences, which can be difficult to predict. This means that all the organisms in an ecosystem are dependent upon each other. We call this interdependence.
A stable community is one in which the size of the populations of all species remain relatively constant over time. In the example above, the amount of grass, and the numbers of rabbits and foxes would all remain relatively constant. The different populations are living in a healthy balance within their environment.
Often, the organisms within an ecosystem compete for resources.
Plants compete for:
Animals might compete for:
Competition creates a 'struggle for existence'. Individuals within a species that cannot compete effectively are unlikely to reproduce, and may die. This can be referred to as 'survival of the fittest' and is a driver for evolution. Species that are unable to effectively compete may become extinct.