Captive breeding and reintroduction
Captive breeding and reintroduction
In 1973, the Endangered Species Act was passed in the United States to protect species that are rapidly declining due to human influences. Captive breeding and release is one of the tools available to halt or reverse the decline of some species in the wild. Such programs may be carried out by zoos, aquaria, botanical gardens, or conservation organizations. In some cases the efforts have met with a substantial degree of success, while in others the results have been limited. It takes a relatively deep understanding of the biology and ecology of an endangered species to implement a successful program of captive breeding and release.
The primary goal of captive breeding, also known as ex situ conservation, is to develop a self-sustaining or increasing population of an endangered species in captivity, without the need to capture additional individuals from the wild. Any surplus captive-bred individuals are available to support a program of release into the wild.
Another goal of captive breeding programs is to maintain an appropriate level of genetic diversity, which can allow the population be adaptable to conditions in the environment after release. Genetic diversity refers to the numerous alleles of genes in a population. (An allele is one of several forms of a gene, the latter being the unit inherited by offspring from their parents.) If all captive-bred individuals are offspring of the same parents, then the population is likely to have low genetic diversity because of the effects of inbreeding (or breeding between closely related individuals). This can also lead to a phenomenon known as inbreeding depression, a detrimental effect on offspring that can result from mating between close relatives. Inbreeding depression is due to an accumulation of deleterious recessive alleles, which can become expressed in a high frequency in inbred populations. Inbreeding depression can be manifested as lowered fecundity, smaller numbers of offspring produced, and decreased survival after birth.
If a highly inbred population was reintroduced into the wild, its chances of survival and reproduction are likely to be relatively low. In essence, genetic diversity helps to ensure that a released population will be able to survive and grow, despite natural selection against some of its individuals.
The size of a released population is another important issue. A small population has a greater probability of extinction because of the potentially devastating effects of deaths caused by unpredictable environmental events or flaws in the reintroduction process. In addition, small populations may exhibit a phenomenon known as genetic drift, caused by the disappearance of certain alleles and fixation in the population of others. Genetic drift occurs readily in small populations, and results in a loss of genetic diversity.
It is also important that the alleles of the founder individuals, that is, the animals brought from the wild into the breeding program, are maintained so that the natural, “wild” alleles are not lost during years of captive breeding. Since the ultimate aim is to re-introduce animals back into a native habitat, maintenance of the original genetic diversity is crucial to the eventual survival of those individuals in the wild. Also, captive breeding over several generations may select for characteristics such as docility, which are not advantageous in the wild.
Many research programs are addressing these problems of captive breeding. At the Minnesota Zoo, for example, a program known as the International Species Inventory is keeping track of the pedigree of individual animals in zoos around the world. This information is used to help prevent mating among closely related individuals, and to thereby maintain the genetic diversity of captive populations.
Various methods are available for increasing the numbers of offspring that can be bred from a limited number of parents. One such method is artificial insemination, in which sperm is transferred to females by artificial means. This allows animals from different zoos to be mated without actually moving them from pace to place. Another enhancement technique involves the removal of eggs from nests of bird species that will subsequently lay replacement eggs. This allows more eggs to be produced by a female than would occur under natural conditions. Reproduction can also be enhanced by foster parenting the young of an endangered species by “parents” of a closely related one, thereby ensuring the rearing of young in a relatively natural, non-human environment. This method has been used to rear endangered whooping cranes, by foster-rearing captive-incubated nestlings by sandhill cranes.
Captive breeding programs must address the issue of adequately preparing animals behaviorally for life in a wild environment. This is an especially formidable task with animals that have a complex social system, and whose behaviors for mating, communication, foraging, predator avoidance, offspring rearing, and migration are learned by observation of the parents or other experienced individuals. A captive environment does not adequately simulate natural conditions or ensure that exposure to appropriate learning opportunities occurs. To circumvent this important problem, training programs have been developed to teach survival skills to captive-bred animals before they are introduced to the wild. For example, red wolves have been taught to hunt and kill living prey, and golden lion tamarins to find and manipulate the kinds of fruit they depend on in the wild.
Another extremely important learned behavior is the fear of potential predators, including humans. Captive-reared individuals may be taught this essential behavior using realistic dummies in situations that frighten the animals, so they learn to associate fear with the model. Imprinting on humans is another potential problem, involving impressionable young animals learning to think that they are the same as humans, while not recognizing other individuals as their own species. Imprinting on people can be avoided by using a puppet of an adult of the proper species to “interact” with the young, including during feeding. For example, peregrine falcon chicks born in captivity are fed by people wearing puppets of adult falcons on their arms, while blocking the rest of their body from view with a partition. This prevents the falcon chicks from seeing the human care-giver, and helps it to imprint on an appropriate subject.
Perhaps the most difficult problem involves teaching captive-bred animals about the social hierarchy and other behavioral intricacies of their species. The most practical approach to this problem has been to keep wild-caught individuals together with captive-reared ones for some time, and to then release them together. This method has been somewhat successful in the reintroduction of the golden lion tamarin to tropical forest in Brazil.
If a successful reintroduction of an endangered species is to occur, the factors causing its decline must be understood and managed. The most common cause of endangerment is habitat destruction or degradation. Obviously, it is crucial that the habitat of endangered species is conserved before captive-bred individuals are released into the wild. This is not necessarily an easily attained goal, because the causes of habitat destruction usually involve complex social, cultural, and economic factors. Controversy has, for example, accompanied reintroduction of the critically endangered California condor to the wild. The condor is a large scavenging bird that requires an extremely large range to survive, exceeding millions of acres per bird. Initially, the U.S. Fish and Wildlife Service failed to conserve enough habitat to support the highly endangered condor, resulting in controversy over the ultimate goal and likely success of the captive-breeding program. In 1986, however, an extremely large tract of suitable land was purchased for use as the base of the reintroduction of captive-reared birds, which has since begun.
After release, captive-reared animals must be monitored to determine whether they have been able to survive the stresses of living in a wild habitat. To ease the transition from captivity to the wild, the release may be somewhat gradual. For example, a “soft release” may involve the provision of food at the release point until animals learn to forage on their own. Moreover, if environmental conditions become particularly stressful, such as a drought making water and food scarce, it may be necessary to intervene temporarily until conditions improve. Monitoring the released population is necessary for the assessment of survival and the causes of mortality, so that future releases can attempt to avoid such pitfalls.
Although releases of captive-bred animals has received most of the public attention, there have also been attempts to reintroduce endangered plants to the wild. Many of the same issues are involved, but plants also present unique problems due to their lack of mobility and specific microhabitat requirements for establishment and growth. For example, the immediate environment in the soil surrounding a seed must have appropriate conditions of light, water, nutrient availability, and temperature, and must be free of seed predators and fungal disease spores. Moreover, the microhabitat requirements for germination often involve a specific disturbance regime, such as fire or canopy gaps created by tree-falls. Consequently, even in native habitats, only a very small percentage of seeds produced by a given plant can germinate and establish. In a successful reintroduction program, the habitat should be managed to allow these periodic
Genetic diversity— Variation in the alleles, or forms of genes, present in a population of organisms natural selection acts upon this variation to select forms better able to survive and reproduce.
Genetic drift— Random change in gene frequencies in a population; this can be a problem in captive populations.
Habitat destruction— Removal or alteration of an organism’s home environment; this is the most common cause of extinction today.
Social group— Individuals of an animal species living together; such groups form the cultural basis from which individuals learn complex behavior and survival skills from each other.
disturbances to occur. Higher success rates in germination can be achieved in a greenhouse, after which the seedlings can be transplanted into the wild. This does not, however, dismiss the necessity of managing the land for future reproduction and survival of the plant in the wild; otherwise the reintroduction effort could fail.
A study was undertaken to evaluate 79 different reintroductions of birds and mammals in the U.S. It was found that certain reintroduction conditions had a higher probability of success than others. The highest probability of failure occurred when the species was a large carnivore requiring an extensive range, when the animals were released into marginal habitat, and when the released individuals were reared in captivity instead of being wild-caught and released within their lifetime. Any of these circumstances require particularly close attention if the reintroduction attempt is to be successful.
Programs of captive breeding and release can be extremely expensive, and their success may be limited because of difficulties in biology, ecology, and in addressing the ultimate cause of the species decline (such as habitat loss or excessive hunting). Moreover, reintroduction efforts should always be accompanied by a program of public education. The informed public has an influence on political decisions to attempt to reverse human-induced losses of biological diversity, and to avoid such ecological damage by preventing habitat loss, overhunting, and other destructive actions.
See also Condors.
Kleiman, D.G. “Reintroduction of Captive Mammals for Conservation.” Bioscience. 39 (1989): 152-161.
Spellerberg, I.F., and S.R. Hardes. Biological Conservation. Cambridge: Cambridge University Press, 1992.
Griffith, B., et al. “Translocation as a Species Conservation Tool: Status and Strategy.” Science 245 (1989): 477-480.