Studies have shown that these minimum population estimates are probably an order of magnitude too small for real situations. Isolation is also a major problem for fragmented habitats. Being isolated can prevent the immigration of individuals from other areas, which may be needed to maintain a small population. If a local population is wiped out by disease, fire or predation they cannot be replaced by immigration from other areas. Immigration is also brings new genes into the population allowing for more variation and preventing inbreeding depression.
Isolation also prevents dispersal of organisms away from their birth sites.
This is often needed to prevent inbreeding which causes loss of genetic diversity. Many animals cannot or will not cross through different habitats to find new, suitable habitats. Southern flying squirrels for example will not cross agricultural fields. Some songbirds such as the Blue-gray Gnatcatcher, have territorial boundaries that coincide with habitat boundaries. Isolation is also a problem for pollination of plants.