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Small population size

Species with a small population size are subject to a higher chance of extinction because their small population size makes them more vulnerable to random (stochastic) variation in their demography, their gene pool and their environment.

Table of contents
1 Demographic effects
2 Environmental Effects
3 Genetic Consequences
4 See also

Demographic effects

The influence of stochastic (random) variation in demographic (reproductive and mortality) rates is much higher for small populations than large ones. Stochastic variation in demographic rates causes small populations to fluctuate randomly in size. The smaller the population the greater the probability that fluctuations will lead to extinction.

One demographic consequence of a small population size, the probability that all offspring in a generation are of the same sex, is easy to calculate: it is given by 21-n This can be a problem in very small populations, the last 18 Kukapo in Fiordland were all male, though the probability of this was only 0.0000076. With a population of just 3 individuals the probability of them all being the same sex is 0.25. Put another way, for every 4 species reduced to 3 individuals (or more precisely 3 individuals in the effective population), one will go extinct just because they are all the same sex.

Environmental Effects

Stochastic variation in the environment (year to year variation in rainfall, temperature) can produce temporally correlated birth and death rates (i.e. 'good' years when birth rates are high and death rates are low and 'bad years when birth rates are low and death rates are high) that lead to fluctuations in the population size. Again, smaller populations are more likely to go extinct due to these environmentally generated population fluctuations than are large populations.

Genetic Consequences

Conservationists are often worried about a loss of genetic variation in small populations. There are 2 types of genetic variation that are important when dealing with small populations.

There are 2 mechanisms operating in small populations that influence these 2 types of genetic variation.

There are two types of consequence of loss of genetic variation in small populations:

See also