Viable captive populations – the numbers game

Abstract

‘Setting conservation thresholds at a few hundred individuals only is a subjective and non-scientific decision, not an evidence-based biological one…. Many existing conservation programs might therefore be managing inadvertently or implicitly for extinction’ (Lochran Traill) Introduction Measuring the potential for zoos to house and breed threatened species has been a cause of concern for some time, but is as yet unresolved. However, progress has been made since the 1980s around the ‘small population paradigm’, as Caughley (1994) termed it, the study of the dynamics of small populations that have declined owing to some (deterministic) perturbation and which are more susceptible to extinction via chance (stochastic) events (Chapter 1). Past theoretical and empirical work predicts that population viability should increase with increasing initial population size both among and within species (Reed et al. , 2003a). Individuals in small or fragmented populations may have fewer opportunities to locate mates because of a skewed local sex ratio and/or physical isolation from conspecifics, henceforth the Allee effect (Courchamp et al. , 1999, 2008). But also low demographic rates, i.e. low annual adult survival and fecundity (Beissinger, 2000) or high annual variability in these demographic rates in response to environmental variation (Stacey & Taper, 1992), can lead to extinction of small populations.