When fossil bivalves and gastropods of late Cretaceous from the Atlantic and Gulf coastal areas of the United States were divided into small, medium and large geographic ranges, those with small ranges were found to suffer a high rate of extinction than those with larger ranges (Jablonski, 1986).
2. Body Size and Longevity:
Rate, at which population returns to its equilibrium, is referred to as population resilience. Long-lived animals generally have larger bodies and lower resilience. Small bodied animals have comparatively higher population resilience. Thus, an interaction between life history characteristics and population size is recorded.
In case of small populations, all else being equally large, long-lived animals may be exposed to less risk of extinction than small, short-lived species. Small, short- lived species with large population get advantage owing to their greater resilience (Pimm, 1991).
3. Decreased Competitive Ability:
After establishing on an island, immigrant population losses its competitive ability, becomes restricted in distribution along with a decrease in local population densities (Ricklefs and Cox, 1978). Such change in distribution and differentiation of species over time is known as taxon cycle (Wilson, 1961) that ultimately leads to extinction. Taxon cycle is not accepted as a reasonable hypothesis by some biologists (Pregill and Olson, 1981). It is also evident that island provides the place for extinction.
4. Minimum Viable Population:
Population size required to escape the risk of extinction is called Minimum Viable Population (MVP). MVP is calculated by population viability analysis (PVA), otherwise known as population vulnerability analysis (Shaffer, 1990).
Three constantly changing and interacting aspects, namely, phenotype of population, environment, population structure and fitness are accounted in PVA (Gilpin and Soule, 1986). Most study on MVP and PVA has been carried out on animals and it is extremely difficult for many reasons to extrapolate methodology and results desired there from to plants (vide given, 1996).
PVA is a powerful tool for predicting probability of persistence of a species over a specified period of time with a specified probability. As a theoretical instance, it can be said that a population is found to have 95% probability of surviving for 100 years, on the basis of PVA.
Populations in a fragmented system show a number of ecological traits that hasten extinction. Ecological specialists are more vulnerable to extinction than generalists. Species able to tolerate the condition at interface between fragments will survive for a longer period. Stable populations are less vulnerable to extinction than fluctuating population (Krishnamurthy, 2004).