There are virtually no ecosystems in the world that are natural in the sense of having escaped human influence. Many natural ecosystems are in this sense "modified". Natural ecosystems have to a large extent disappeared from upper-income countries and densely populated lower-income countries. They are under continuing pressure wherever they remain, being converted to agricultural, urban and industrial land uses. Once natural habitats are converted to such uses, restoration to a fully diverse natural system can take centuries, while urbanization may take even longer to reverse. Achieving a balance between the increasing needs of a rapidly growing global population, and the need for global sustainability are currently difficult at best, though clearly an urgent priority.
Conservation status is an assessment of the status of ecological processes and of the viability of species populations in an ecoregion. Status categories often used are (1) [relatively intact] indicating natural communities largely intact with species and ecosystem processes occurring within their natural ranges of variation; (2) [relatively stable] meaning that extensive areas of intact habitat remain but in which local species have declined and disruptions of ecological processes have occurred; (3) [vulnerable] indicating a good probability of persistence of remaining intact habitat (assuming adequate protection) but also the loss of some sensitive or exploited species; (4) [endangered] meaning there is a medium to low probability of persistence of remaining intact habitat; (5) [critical] meaning there is a low probability of persistence of remaining intact habitat; and (6) [extinct] meaning that the ecological system is lost.