Over half of the world's original forests have been cleared, and much of the forests that remain are degraded and constitute poor habitat for many native species. Whole ecosystems are effectively extinct, and some habitat types, such as Mediterranean-climate shrublands, temperate grasslands, and temperate rainforests are endangered globally. Intact assemblages of large vertebrates are rare and dwindling worldwide, and larger predators are all severely threatened by habitat loss and poaching. Freshwater and coastal marine ecosystems are estimated to be even more threatened than terrestrial systems. Over the next 40 years, conservative estimates predict an average of 100 species per day will become extinct largely from habitat loss.
Loss of habitat can adversely affect biodiversity by reducing or eliminating available habitat for individual species, altering population dynamics and ecological interactions, or changing biophysical processes that ultimately sustain critical ecological and evolutionary processes. Habitat fragmentation contributes both to habitat loss and habitat degradation. Ecological and population integrity can be compromised through habitat degradation. Specific forms and effects of habitat degradation will vary in different major habitat types, but impacts on species ranges, population dynamics, ecological interactions, and biophysical drivers can be generally derived for every ecoregion.
Habitat is defined by the structure and processes associated with one or more natural communities. An ecoregion is usually classified under one major habitat type, but may encompass multiple habitat types.