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.
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.
The environment can directly affect the survival of a small population. Some detrimental effects include stochastic variation in the environment, (year to year variation in rainfall, temperature) which 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 become extinct due to these environmentally generated population fluctuations than the large populations.
The environment can also introduce beneficial traits to a small population that promote its persistence. In the small, fragmented populations of the acorn woodpecker, minimal immigration is sufficient for population persistence. Despite the potential genetic consequences of having a small population size, the acorn woodpecker is able to avoid extinction and the classification as an endangered species because of this environmental intervention causing neighboring populations to immigrate. Immigration promotes survival by increasing genetic diversity, which will be discussed in the next section as a harmful factor in small 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:
Examples of genetic consequences that have happened in inbred populations are high levels of hatching failure, bone abnormalities, low infant survivability, and decrease in birth rates. Some populations that have these consequences are cheetahs, who suffer with low infant survivability and a decrease in birth rate due to having gone through a population bottleneck. Northern elephant seals, who also went through a population bottleneck, have had cranial bone structure changes to the lower mandibular tooth row. The wolves on Isle Royale, a population restricted to the island in Lake Superior, have bone malformations in the vertebral column in the lumbosacral region. These wolves also have syndactyly, which is the fusion of soft tissue between the toes of the front feet. These types of malformations are caused by inbreeding depression or genetic load.
Island populations often also have small populations due to geographic isolation, limited habitat and high levels of endemism. Because their environments are so isolated gene flow is poor within island populations. Without the introduction of genetic diversity from gene flow, alleles are quickly fixed or lost. This reduces island populations' ability to adapt to any new circumstances and can result in higher levels of extinction. The majority of mammal, bird, and reptile extinctions since the 1600s have been from island populations. Moreover, 20% of bird species live on islands, but 90% of all bird extinctions have been from island populations. Human activities have been the major cause of extinctions on island in the past 50,000 years due to the introduction of exotic species, habitat loss and over-exploitation
The Galapagos penguin is an endangered endemic species of the Galapagos islands. Its population has seen extreme fluctuations in population size due to marine perturbations, which have become more extreme due to climate change. The population has ranged from as high as 10,000 specimens to as low as 700. Currently it is estimated there are about 1000 mature individuals.
Conservation efforts for small populations at risk of extinction focus on increasing population size as well as genetic diversity, which determines the fitness of a population and its long-term persistence. Some methods include captive breeding and genetic rescue. Stabilizing the variance in family size is an effective way can double the effective population size and is often used in conservation strategies.