National Recovery Plan for the Christmas Island Goshawk Accipiter fasciatus natalis
Small population sizeSmall population size
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Small population size
The risk of extinction varies inversely with population size (Lacy 1987) due primarily to demographic and environmental variability the effects of which can become marked in small populations. Demographic variability is the individual variation in reproductive success, which is masked when population size is large. Very small populations (< 30 individuals) can easily become extinct as a result of random demographic variation between individuals (Caughley & Sinclair 1994).
The reproductive success of a population is also likely to vary with fluctuations in the environment. The degree of this fluctuation is determined by how much seasonal and annual variation in habitat quality there is. The influence of this on the risk of extinction increases markedly with decreasing population size. For example, a cyclone, an extreme example of environmental variation, causing the death of half the population of 10000 animals has an insignificant affect on the probability of extinction of that population. However, if half a population of 200 animals dies, the risk of extinction of that population increases significantly (Caughley & Sinclair 1994).