Facing Truths About Population Management

Even without considering genetic diversity, managing zoo populations sustainably without applying controlled mortality faces a predictable dilemma: Holding space is limited, and zoos worth their salt will inevitably become better at reducing accidental (uncontrolled) mortality. For most species, longevity attainable under good husbandry conditions is considerably longer than generation time, and even though most animals can reproduce until death, there is reproductive senescence, where the probability of successful reproduction decreases with age. Thus, over time, available holding space will inevitably be filled by older animals of lower fecundity. Breeding recommendations that aim at exact numerical replacement to avoid surplus at all cost rather than accounting for average success rates (so that the number of offspring will overshoot the target by up to 50% of the time) will stochastically lead to declines in population size. For small populations, extinction in zoos thus predictably becomes a matter of time. Only large populations will be able to exist at high proportions of old animals, with waves of replacement cycles. Therefore, gaining public acceptance for respectful and humane controlled mortality is essential for the long-term sustainability of small zoo populations. Concepts of continuously increasing holding capacities, using frozen gametes to generate offspring when needed, or nonspecific hopes that compassionate conservation will align individual longevity and conservation goals, detract from the necessity to act. Details of welfare and education can support the call for controlled mortality, but it is the inevitability of the outlined process that obliges us to choose honestly between being empathetic observers of extinction or taking responsibility to conserve animal species.