Exploring the role of chance events in the introduction success of the island canary (Serinus canaria) to Midway Atoll

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Jonah Levy, Biology, Tufts University, Medford, MA and J. Michael Reed, Tufts University, Boston, MA

Background/Question/Methods
The factors promoting introduction success are multifaceted and poorly understood, despite widespread exploration of species introductions. Propagule size (number of individuals introduced) has received the most consistent support, with the expectation that extinction is more likely for smaller introduced populations. However, notable examples of small introductions succeeding call into question the universality of propagule size as a determinant of extinction probability, and suggest a greater role of chance events than has been accounted for. To explore this relationship, we identified a successful introduction event of known size of few individuals (two adult males and eleven juveniles) and retrospectively evaluated its probability of establishment success using population viability analysis (PVA). Using VORTEX stochastic modeling software, we populated a baseline model with optimistic vital rates drawn from studies of wild canaries and congenerics. We projected population trajectories for 30 years, and used two sensitivity analyses to assess the relative influence of parameter values on probability of success: first, we perturbed individual rates singly within a range of less optimistic values, and second, we used Latin Hypercube Sampling to vary multiple parameters simultaneously. Sensitivity indices for both approaches were calculated in R.
Results/Conclusions
Our baseline model, which represents a best-case scenario for vital rates, returned a probability of introduction success of 49%. Projected population sizes after 30 years were highly variable and showed a bimodal distribution peaking near zero and K. Both sensitivity analyses were dominated by mortality, with juvenile mortality emerging as the most sensitive parameter. Standard deviations in juvenile and adult mortality, driven by environmental stochasticity, were the second and fifth most sensitive rates. Given that the highest sensitivities occurred at the smallest perturbations of juvenile mortality, we postulate that a small decrease in this rate or in its variability, possibly aided by known supplemental feeding, could have improved the probability of success for this introduction event. We also found that persistence was significantly affected by initial (introduction) sex ratio and mating system, with monogamy reducing establishment success; thus the possibility for functional plasticity in mating system merits further exploration in the study of small introductions. What’s more, the importance of juvenile mortality is consistent with published PVAs of native-range passerines and raptors, and may represent an as-yet understudied driver of introduction success across taxa.