Introduction of captive breeders to the wild: Harmful or beneficial?

This work focuses on the consequences on the genetic load and the risk of extinction when an endangered population is exposed to recurrent introductions from a captive population where selection is somewhat relaxed. Our findings suggest that, although selection pressures might be reduced in captivity, which leads to higher frequency of deleterious alleles in natural populations (Lynch and O'Hely 2001), such a population structure could have positive effects on population fitness when three conditions are met: (i) the time length of the supplementation program does not exceed a reasonable time frame, e. g., 20 generations (ii) introduction of captive individuals is kept at a low level, i.e., one or two individuals per generation (iii) the size of the captive population is reasonably large, e.g., more than 20 individuals. The positive effect is due to the fact that the supplementation program delays the increase of homozygosity of the natural population. When migration from the wild towards captivity is also allowed, the benefits with regard to genetic load increase significantly even for larger numbers of captive immigrants and a higher number of generations. We also worked out a model with explicit demographic considerations (fluctuating population sizes, captive migrants increase the size of the wild population), which shows that the probability of extinction decreases significantly with the number of introduced individuals when short-term supplementation programs are applied (up to twenty generations). Furthermore, an appropriate genetic management of the captive population, such as the equalization of family sizes, could enhance the positive effects of such supplementation programs.