An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment. The levels of organisation run from a single organism, to a population of one species, to a community of all the populations living in one place, and finally to the whole ecosystem.
Organisms need a supply of materials from their surroundings and from the other living things there. Because the supply is limited, organisms compete. Plants in a community compete for light, space, water and mineral ions from the soil. Animals compete for food, mates and territory.
Within a community each species depends on other species for food, shelter, pollination and seed dispersal. For example, flowers are pollinated by insects, so they need them to reproduce. This is called interdependence. If one species is removed, the whole community can be affected. A stable community is one where all the species and environmental factors are in balance, so population sizes stay fairly constant.
