Genetic concerns about small populations

The discussion of thresholds bemoaned the fact that small populations were at risk of extinction by demographic stochastic processes. Efforts to maximize intra-specific genetic diversity are a high priority for conservationists. In general, genetic diversity is correlated with population size. Thus larger populations should manifest a greater variety of phe-notypes—and should therefore be better able to respond to variations in environment. However, with habitat loss and fragmentation, populations of many mammalian species are declining and are being fractured into small, disconnected units. And as populations shrink, genetic variation may be lost.

Loss of genetic variability from a population is primarily a function of three interrelated phenomena: "bottlenecking," random genetic drift, and inbreeding depression.

Bottlenecks are events that greatly reduce a population's size. Such reductions can have many causes, including habitat alteration or loss, introduction of competitors or predators, and the spread of epidemic disease. The individuals that survive a bottleneck are the founders for all future generations, and only the genetic variability that is represented in these founder-individuals (plus subsequent mutations) can be preserved within the species. The cheetah (Acinonyx jubatus) is the classic example of a bottleneck species. A population crash some 10,000 years ago radically reduced genetic variability. Even today all cheetahs remain so similar, genetically, that skin transplants from one animal to another are not rejected. More significant, cheetahs may lack the genetic variability to respond, evolutionarily, to new diseases confronting the species. But a large population size can usually overcome problems of low genetic variation, as is the case with the current populations of elephant seals, for example.

In evaluating a bottleneck, conservation biologists are especially concerned about the effective population size, or Ne, which is (roughly) the number of breeding animals in a population. Ne is generally smaller than the number of individuals in the population—and if breeding animals are closely related, it can be much smaller indeed. For example, two sets of identical twins do not count as four complete animals, for genetic purposes. The rate of a population's heterozygosity-loss, per generation, is largely a function of effective population size. That is to say, the amount of genetic variability preserved from one generation to the next is approximately proportional to (1 — 0.5 Ne). Obviously, when Ne is large, the majority of genetic variability will be maintained, and when Ne is small, heterozygosity can be lost very rapidly.

Clearly, even the tightest bottlenecks need not be fatal to a population's survival. Every mammalian species began from a minimal founder-size, and if a bottlenecked population is allowed to increase greatly in numbers, any decay in genetic diversity can be balanced by new variability added through mutation. The most serious problems arise when the bottleneck-

squeeze is maintained over multiple generations, because in this case (1) loss of variability by genetic drift vastly exceeds replacement by mutation, and (2) this process is accelerated by inbreeding. An example is the lowered allozyme and DNA variability observed in the brown hares (Lepus europaeus) of New Zealand and Britain and attributed to bottlenecks.

Random genetic drift, sometimes called the Sewall Wright effect, designates changes in a population's allele frequencies due to chance fluctuations. Random genetic drift becomes important only when populations are small. Cross-generational transfer of alleles is then subject to sampling error, and a given allele can be lost (decline to 0% frequency) or fixed (increase to 100% frequency). In other words, when small populations of a species are isolated, out of pure chance the few individuals who carry certain relatively rare genes may fail to transmit them. The genes can therefore disappear and their loss may lead to the emergence of new species, although natural selection has played no part in the process. And the smaller a bottleneck, the more rapidly genetic drift can operate. The longer a bottleneck persists, the greater the potential cumulative effects of genetic drift.

Inbreeding depression can result from matings between close relatives and is more likely to occur in a small population confined to a small habitat-patch. The deleterious effects of inbreeding have been repeatedly documented in zoo populations, before the implementation of genetic management programs. For example, inbred calves of Dorcas gazelle (Gazella dorcas) suffered from high juvenile mortality and delayed sexual maturity of females. Among wild populations, inbreeding depression in Florida panthers (Felis concolor coryi) may have lowered reproductive rates and reduced the species' capacity to respond to disease.

Przewalski's horse (Equus caballus przewalskii) is the only surviving variety of wild horse. This animal is considered extinct in nature (wild individuals were last observed in 1969) and survives only because of captive breeding. The founder-stock for the captive herd was limited in number. Therefore, genetic drift and inbreeding depression have led to a loss of genetic diversity in E. c. przewalskii and are reflected by high juvenile mortality and a reduced lifespan. International management programs aim to retain 95% of the current average individual heterozygosity for at least 200 years. And a program is underway to reintroduce these animals into Mongolia.

Wildlife

Conservation medicine focuses on the changing health-relationships between people, other animals, and shared ecosystems. Of course every variety of mammal has been affected by diseases and parasites throughout its species-history. And across evolutionary time, mammalian species have established accommodations with pathogenic organisms: immune and behavioral defenses evolve in the host; responses (often including transmission "improvement" and reduced virulence) co-evolve in the pathogen. However, the present age—with its fragmented populations of wild mammals and anthropogenic mixing of previously separate species—has destabilized these

The Florida panther (Felis concolor coryi) is an endangered species. International efforts are being made to encourage the survival of all identified endangered species. (Photo by Animals Animals ©John Pon-tier. Reproduced by permission.)

dynamic equilibria. And the results are of concern to conservation biologists.

In recent years several emerging infectious diseases (EIDs) have been identified in wildlife populations. Among immune-naive populations, these EIDs may inflict direct mortality beyond a species' evolved capacity for demographic response. In addition, EIDs may affect reproduction, susceptibility to predation, and the competitive fitness of infected hosts. When these detrimental effects extend to the population level, they may in turn affect community structure by altering the relative abundance of species.

Habitat fragmentation often places wildlife species in closer proximity to domesticated animals because habitat patches may be adjacent to farms, villages, and even urban areas. Such situations can facilitate cross-species contagion of disease. People have transmitted tuberculosis, measles, and influenza to gorillas (Gorilla gorilla), orangutans (Pongo pyg-maeus), ferrets (Mustela putorius furo), and other mammals. The slaughter of primates for bushmeat in Africa was prob-

On French Island, Australia, koalas (Phascolarctos cinereus) are captured for conservation measures while they rest in eucalyptus trees. (Photo by Richard T. Nowitz/Photo Researchers, Inc. Reproduced by permission.)

ably the first exposure of people to the precursor of human immunodeficiency virus (HIV). Lions in the Serengeti have been affected by diseases of cats and dogs from Tanzanian villages. In 1984 a disease caused by calicivirus was detected among rabbits in China. The origin of the disease is not definitively known, but the pathogen soon spread westward into Europe, where it affected perhaps 90% of the rabbit populations. Rabbit calicivirus is now used as the primary means for controlling feral rabbits in Australia. (Earlier control relied mainly in the introduction of myxomatosis virus among Australian rabbits. In the 1950s, myxomatosis had a kill-efficiency of about 99%. Over time, however, Australian rabbit populations evolved immunity.) Efforts to re-establish wild populations of black-footed ferrets (Mustela nigripes) in western North America have been hampered by the spread of cat and dog diseases among the ferrets, and because enormous populations of ferret prey (prairie dogs) had been destroyed by sylvatic plague.

Because human health may be increasingly affected by diseases transferred from wildlife, conservation medicine is likely to become an important area of research for conservation biologists.

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