Taxon cycles in the Lesser Antillean avifauna

Patterns of taxonomic differentiation and geographical distribution of West Indian birds suggest that taxa may pass through phases of increasing and decreasing geographical range. Such sequences are referred to as taxon cycles. Although decline may often lead to extinction, new phases of expansion, accompanied by increased or shifted habitat distribution are also possible. The taxon cycle is a historical hypothesis, but its existence was formerly inferred from contemporary, indirect evidence. Molecular studies of land birds in the West Indies now provide relative ages for taxa based on genetic differentiation among island populations. These age estimates confirm that older populations tend to have restricted geographical and ecological distributions. That ecology and geography are strongly correlated with age across taxa also suggests that the time course for evolutionary change through the taxon cycle is relatively consistent among independently evolving populations. Because young taxa have continuous distributions within the West Indies, gaps in the ranges of older taxa indicate extinctions of island populations. Starting from this premise, one may estimate exponential extinction rates of about 50% per million years for island populations in the Lesser Antilles, which indicates an average population life span of two million years. The relative timing of expansion phases suggests that cyclic dynamics of populations are not driven by extrinsic factors such as climate and associated habitat change during glacial cycles. Alternatively, such cycles may be intrinsic, perhaps driven by lags in the evolutionary responses of host and parasite populations. This is suggested by observed variation in the prevalence of blood parasites among island populations of the same species. The taxon cycle concept provides a useful paradigm for understanding variation among species in geographical range, ecological distribution, and vulnerability to extinction.