A community is all the populations interacting in an area. When one considers the living or biotic components as well as the non-living or abiotic components, this is known as the ecosystem. An ecosystem is the relationship between the living organisms and the non-living factors such as available food, shelter, soil, light, water, oxygen, temperature, weather, etc. that they need to survive. Biodiversity refers to species richness--a greater diversity of living things leads to a healthier ecosystem. Different organisms carry out different jobs or niches. For instance, if a marsh is filled in and a housing project is placed there, important ecosystem services are lost. The marsh plants help to purify the water on its way to the sea. When forests are cut down and the land is developed into farmland, habitat is lost to creatures that are adapted to a forest biome. This can lead to extinction. When native short grass prairies were removed and replaced with crops, during times of drought, the crops die and the soil erodes. This can lead to desertification. The native plants were uniquely adapted to periods of drought, unlike the crops that replaced them. Organisms in an ecosystem rely on one another to provide habitat, food, services and when there are less organisms, food webs can collapse and endangerment and extinction may occur.