The absolute amount of natural habitat diminishes daily before human development. A related, though distinct, concern for wildlife conservation is the breakup of habitat into discrete blocks, isolated pockets and "habitat islands", forcing extinction on species which have large ranges. In the longer term, habitat fragmentation also threatens entire ecosystems which can remain viable only in pieces larger than a certain size. The most sensitive are complex ecological systems, like rainforests, which have high degrees of persistence if left undisturbed, but which begin irreversible collapse, losing internal structure and diversity, with human interference.
The isolation of natural habitats through development practices that subdivide the landscape (roads, urbanization, agriculture, logging operations, hydro corridors) further contributes to loss of biodiversity, and may the primary cause of the present extinction crisis. Fragmented pockets of habitat, though useful for many species, are not sufficient for those organisms that require large home ranges, have complex life cycles or are sensitive to human disturbance. When habitat is fragmented, populations of a particular species can become isolated, leading to inbreeding and a loss of genetic diversity; this loss reduces a species' ability to adapt to other types of environmental stresses.
Experiments to estimate the smallest workable size for a Brazilian tropical rainforest suggest that reserves smaller than 100 hectares will lose their butterflies, and reserves smaller than 1000 hectares are unstable in the longer term.