Steven D. Thompson

Strategic planning for captive populations: projecting changes in genetic diversity

Maintenance of genetic diversity is critical for the preservation of small populations. Most captive populations are small and managers focus their efforts on reducing extinction risks by minimising loss of genetic diversity. To project changes in genetic diversity, managers use a genetic drift model. We used data from 40 managed captive populations to test whether this drift model is a good predictor of change in genetic diversity by comparing projected 10-year trajectories of annual loss in expected heterozygosity with the changes estimated from pedigree analysis. We found that the drift model frequently overestimated the rate at which genetic diversity would be lost and could not predict increases, which we observed in 22 species. We suggest that the drift model does not adequately reflect the potential for management strategies, such as prioritised breeding recommendations and recruitment of potential founders, to slow the loss of genetic diversity. In many cases, the drift model may be overly pessimistic, leading managers to seek solutions such as importation, increased population size or the abandonment of a captive population, that may be unnecessary or premature. Managers should be cautious when interpreting results from genetic drift models, taking care to assess how management actions might lead to more optimistic prognoses.

Birth sex ratio in captive mammals: Patterns, biases, and the implications for management and conservation

Sex allocation theory predicts that a female should produce the offspring of the sex that most increases her own fitness. For polygynous species, this means that females in superior condition should bias offspring production toward the sex with greater variation in lifetime reproductive success, which is typically males. Captive mammal populations are generally kept in good nutritional condition with low levels of stress, and thus populations of polygynous species might be expected to have birth sex ratios biased toward males. Sex allocation theory also predicts that when competition reduces reproductive success of the mother, she should bias offspring toward whichever sex disperses. These predicted biases would have a large impact on captive breeding programs because unbalanced sex ratios may compromise use of limited space in zoos. We examined 66 species of mammals from three taxonomic orders (primates, ungulates, and carnivores) maintained in North American zoos for evidence of birth sex ratio bias. Contrary to our expectations, we found no evidence of bias toward male births in polygynous populations. We did find evidence that birth sex ratios of primates are male biased and that, within primates, offspring sex was biased toward the naturally dispersing sex. We also found that most species experienced long contiguous periods of at least 7 years with either male- or female-biased sex ratios, owing in part to patterns of dispersal (for primates) and/or to stochastic causes. Population managers must be ready to compensate for significant biases in birth sex ratio based on dispersal and stochasticity. Zoo Biol 19:11–25, 2000.

Zoo and aquarium research: priority setting for the coming decades

Scientific study within contemporary zoos and aquariums has developed ad hoc as an extremely broad, academically oriented mixture of basic and applied research spanning a wide array of concepts and disciplines. Several papers have considered prioritization of present or future research efforts within disciplines, but only a few have touched on prioritization across institutions, disciplines, and species. This lack of prioritization across institutions and disciplines is surprising given the growing interdependence of zoos and aquariums to maintain populations through exchange of animals, standardization of animal care procedures, and maintenance of self-sustaining populations. The purpose of this paper is to explore prioritization of scientific research, and support of that prioritized research, within and among the professionally managed zoo and aquarium members of the Association of Zoos and Aquariums (AZA). Zoo Biol 27:488-497, 2008. (c) 2008 Wiley-Liss, Inc.

Extinction risk assessment for the species survival plan (SSP®) population of the Bali mynah (Leucopsar rothschildi)

The Bali mynah Species Survival Plan (SSP), an Association of Zoos and Aquariums program, strives to maintain the genetic and demographic health of its population, avoid unplanned changes in size, and minimize the risk of population extinction. The SSP population meets current demographic and genetic objectives with a population size of 209 birds at 61 institutions and 96% genetic diversity (GD) retained from the source population. However, participating institutions have expressed concerns regarding space allocation, target population size (TPS), breeding restrictions, inbreeding depression, and harvest in relation to future population availability and viability. Based on these factors, we assess five questions with a quantitative risk assessment, specifically a population viability analysis (PVA) using ZooRisk software. Using an individual-based stochastic model, we project potential population changes under different conditions (e.g. changes in TPS and genetic management) to identify the most effective management actions. Our projections indicate that under current management conditions, population decline and extinction are unlikely and that although GD will decline over 100 years the projected loss does not exceed levels acceptable to population managers (less than 90% GD retained). Model simulations indicate that the combination of two genetic management strategies (i.e. priority breeding based on mean kinship and inbreeding avoidance) benefits the retention of GD and reduces the accumulation of inbreeding. The current TPS (250) is greater than necessary to minimize the risk of extinction for the SSP population but any reduction in TPS must be accompanied by continued application of genetic management. If carefully planned, birds can be harvested for transfer to Bali for a reintroduction program without jeopardizing the SSP population.