The practice of supporting weak wild populations by capturing a fraction of the wild individuals, bringing them into captivity for reproduction, and releasing their offspring into the natural habitat to mix with wild ones is called supportive breeding and has been widely applied in the fields of conservation biology and fish and wildlife management. This procedure is intended to increase population size without introducing exogenous genes into the managed population. Previous work examining the genetic effects of a single generation of supportive breeding has shown that although a successful program increases the census population size, it may reduce the genetically effective population size and thereby induce excessive inbreeding and loss of genetic variation. We expand and generalize previous analyses of supportive breeding and consider the effects of multiple generations of supportive breeding on rates of inbreeding and genetic drift. We derived recurrence equations for the inbreeding coefficient and coancestry, and thereby equations for inbreeding and variance effective sizes, under three models for selecting captive breeders: at random, preferentially among those born in captivity, and preferentially among those born in the wild. Numerical examples indicate that supportive breeding, when carried out successfully over multiple generations, may increase not only the census but also the effective size of the supported population as a whole. If supportive breeding does not result in a substantial and continuous increase of the census size of the breeding population, however, it might be genetically harmful because of elevated rates of inbreeding and genetic drift.

Genetic Effects of Multiple Generations of Supportive Breeding

Within the next decades species extinction may eliminate between 20 and 50% of the Earth's species. Captive breeding has often been claimed to be a useful tool in preservation of biodiversity. The role of zoos in conservation work and the value of captive breeding are discussed; the latter exemplified by the Peregrine Falcon (Falco peregrinus) Programme and the Arabian oryx (Oryx leucoryx) Programme. Captive breeding programmes are very resource demanding and can only be afforded for a very small number of species, which limits their value significantly. Zoos deal mainly with vertebrates, but these comprise less than 3% of the described species, and although the 878 zoos considered hold more than 20 000 specimens of 140 threatened mammal species, they probably only contribute to the conservation of 20 full species. The situation for birds, reptiles and amphibians is even worse. Zoos face serious problems with minimum viable population sizes and hybridization. However, zoos can make a major contribution to preservation of biodiversity through educating and informing the public. Today, where the crisis of extinction of species has reached such daunting dimensions, captive breeding and otherex situ conservation tools should be the last resort for preserving biodiversity, and captive breeding must not become an excuse to avoid dealing with preservation of habitats.

Captive breeding—a useful tool in the preservation of biodiversity?

In an effort to understand patterns and causes of nestling loss in Ospreys (Pandion haliaetus), I studied brood reduction in three eastern U.S. Osprey colonies during 1978 and 1979. The colonies, located in Florida Bay (1) and on coastal Long Island, N.Y. (2), differed in the average daily amount of food delivered to nestlings; Florida nests received 43% and 11% less fish per day than nests in the two N.Y. colonies, largely because latitude and season restricted day length and thus foraging time for the winter-breeding Florida Ospreys. Increased distance from stable food sources accounted for the lower rate of feeding at one of the N.Y. colonies. Variation in clutch size in the three colonies reflected differences in latitude more than in food availability; average clutch sizes in Long Island were larger than Florida clutches by 0.5 of an egg, but were similar to each other and to those in other northeastern U.S. Osprey populations.Increased nestling loss coincided with reduced food delivery rates and, in food stressed colonies, this loss was 2-3 times greater than any recorded for Ospreys. Starvation was the primary cause of nestling death, with mortality concentrated on third chicks, which hatched on average 3.9 d later and from eggs 5.6% smaller than chicks hatching first. Sibling aggression accounted for the preferential feeding of older nestmates,but only in colonies or nests where food was limited. Aggressive chicks nearly always stopped fighting after being fed. This behavior provided a reversible mechanism for controling brood reduction that was based on nutrition. Growth rates of young measured during the first half of the growth period were more variable between colonies than within nests. This is interpreted as reflecting both the differences in colony food delivery rates as well as the evolutionary pressures of sibling competition to equalize the growth of nestmates.

Brood reduction in temperate and sub-tropical ospreys.

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Tim Inskipp

Papers

Endangered Birds : Management techniques for preserving endangered species, edited by Stanley A. Temple. Croom Helm, £8.95.