Small populations behave differently from larger populations. Often this has various harmful consequences for the survival of that population.
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The influence of stochastic 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. They are subject to a higher chance of extinction because they are more vulnerable to genetic drift, resulting in stochastic variation in their gene pool, their demography and their environment.
One demographic consequence of a small population size, the probability that all offspring in a generation are of the same sex, and where males and females are equally likely to be produced (see sex ratio), is easy to calculate: it is given by 1 / 2^{n − 1} (The chance of all animals being females is 1 / 2^{n}; the same holds for all males, thus this result). This can be a problem in very small populations. In 1977, the last 18 Kakapo on a Fiordland island in New Zealand were all male, though the probability of this would only be 0.0000076 if determined by chance (however, females are generally more predated than males and kakapo may be subject to sex allocation). With a population of just three individuals the probability of them all being the same sex is 0.25. Put another way, for every four species reduced to three individuals (or more precisely three individuals in the effective population), one will go extinct within one generation just because they are all the same sex. If the population remains at this size for several generations, such an event becomes almost inevitable.
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.
Conservationists are often worried about a loss of genetic variation in small populations. There are two types of genetic variation that are important when dealing with small populations.
There are two mechanisms operating in small populations that influence these two types of genetic variation.
There are two types of potential consequence of loss of genetic variation:
The effective population size is commonly lower than the actual population size.

