One or two years of high death or low birth rates can drive a small population to extinction. Population models have calculated a rough estimate of minimum population sizes for conservation of populations. In general at least 50 individuals are needed to prevent inbreeding depression, while 500 individuals are needed to maintain proper genetic diversity. This estimate is based on all individual in a population being able to breed, which due to age, physiological factors and social factors is usually not the case. 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.