Very few natural areas remain in the world and even these are threatened by increasing pressure for land to be put to other uses by fast-growing human populations. Wildlife habitats are lost by physical destruction, pollution and over-exploitation. Perhaps the most profound way that habitats are modified and redistributed, changed and 'lost' is through episodes of relatively abrupt and severe global temperature change

For an ecosystem to function naturally, there are certain habitat size and diversity conditions. When habitat within an ecoregion is lost or degraded and the total area of that habitat declines, there is a rapid loss of species and disruption of ecoogical processes. Reduced or degraded habitats threaten biodiversity at gene, species and ecosystems level, hampering the provision of key ecological products and services. Freshwater and marine habitats, especially coral reefs, are also very vulnerable

It is impossible to define a habitat more precisely than "a habitat is an environment of a particular kind". In practice, a habitat is in part defined by the dominant species of plants (which provide the main structural properties of the habitat), in part by biogeographic, climatic and soil and other environmental factors, and in part by the particular collection of species which are assembled and interact together as a community. Sometimes the term habitat is more loosely used to mean the "home" of a particular species, in the sense of the place that species is most likely to be living. Habitats rarely have hard physical boundaries; they tend to blend one with another along interface zones, called "edge zones", which may shift position over time. Certain species favour edge zones, which may themselves be considered a kind of habitat

In the last 200 years the USA has lost 50% of its wetland habitat, 90% of its north-western old-growth forests and 99% of its tallgrass prairie. Up to 490 species of native plants and animals were lost with another 9,000 now at risk