How likely was the successful introduction of the island canary to Midway Atoll?

Abstract

The positive relationship between number of introduced individuals and introduction success has been widely proposed in invasion biology on theoretical and empirical grounds. Still, successful establishment has been reported for smaller-than-expected founding populations, thus we suspected that the role of chance in species introductions had been understated. To evaluate this proposition, we modeled the likelihood of success of a known small-introduction event, that of island canaries (Serinus canaria) to Midway Atoll. We built an individual-based, stochastic simulation model, populated with vital rates from wild canaries and related species, and projected population growth for 30 years under a variety of conditions. We used perturbation analysis and logistic regression to evaluate model sensitivities. Our optimistic baseline model returned a probability of establishment of roughly 50%, although the true probability would have been lower if any realized vital rates were below their optimistic values. Both sensitivity analyses were dominated by juvenile mortality, suggesting that a small change in this value has an outsized impact on the likelihood of introduction success. As such, we propose three factors that may have increased the likelihood of success for this introduction: behavioral plasticity in mate selection allowing opportunistic polygamy, resistance to inbreeding depression that is common in species with a history of captive breeding, and a reduction in juvenile mortality rates via human assistance. Our models suggest that chance can play a role in establishment likelihood for small founding populations even under optimistic conditions, but that case-specific factors are likely important in most successful small introductions.