page_title: Small population size
section_title: Demographic effects
hierarchical_section_title: Small population size / Demographic effects
caption_attribution_description: Kakapo Strigops habroptila "Sirocco" amongst renga renga lillies. Maud Island, New Zealand. Photo: Chris Birmingham, 2012.
context_page_description: Small populations can behave differently from larger populations. They are often the result of population bottlenecks from larger populations, leading to loss of heterozygosity and reduced genetic diversity and loss or fixation of alleles and shifts in allele frequencies. A small population is then more susceptible to demographic and genetic stochastic events, which can impact the long-term survival of the population. Therefore, small populations are often considered at risk of endangerment or extinction, and are often of conservation concern.
context_section_description: 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. This variation could be a result of unequal sex ratios, high variance in family size, inbreeding or fluctuating population size. The smaller the population the greater the probability that fluctuations will lead to extinction.
  One demographic consequence of a small population size is 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  (the chance of all animals being females is ; 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 preyed upon more often 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 become 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.
caption: Kakapo
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