SEXUAL SELECTION AND THE RISK OF EXTINCTION OF INTRODUCED BIRDS ON OCEANIC ISLANDS

We test the hypothesis that response to sexual selection increases the risk of extinction by examining the fate of plumage monomorphic versus plumage-dimorphic bird species introduced to the tropical islands of Oahu and Tahiti. We assume that plumage dimorphism is a response to sexual selection and we assume that the males of plumage-dimorphic species experience stronger sexual selection pressures than males of monomorphic species. On Oahu, the extinction rate for dimorphic species, 59%, is significantly greater than for monomorphic species, 23%. On Tahiti, only 7% of the introduced dimorphic species have persisted compared to 22% for the introduced monomorphic species. For the combined Oahu and Tahiti data sets, addition of plumage-by-fate interaction significantly improves the fit of the log-linear model, fate+island+plumage+(fate-by island)+(island-by-plumage). To control for phylogenetic constraint, a logistic regression model is analyzed using a data subset consisting of only the two best represented families, Fringillidae and Passeridae. Here, plumage and the plumage-by-family interaction are significant. Plumage is significantly associated with increased risk of extinction for passerids but insignificantly associated for fringillids. Thus, the hypothesis that response to sexual selection increases the risk of extinction is supported for passerids and for the data set as a whole. The probability of extinction was correlated with the number of species already introduced. Thus, species that have responded to sexual selection may be poorer interspecific competitors when their communities contain many other species.