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Keywords:

  • Age and area;
  • community assembly;
  • host–pathogen coevolution;
  • John C. Willis;
  • phylogenetic effects;
  • phylogeography;
  • range size;
  • taxon cycle;
  • West Indies

Abstract

During my graduate studies, I characterized patterns of geographical distribution and taxonomic differentiation in birds of the West Indies, which suggested that species undergo phases of expansion and contraction similar to the taxon cycles that E. O. Wilson had described for Melanesian ants. Fieldwork in the early 1970s with George Cox confirmed that these phases were associated with variation in habitat distribution and abundance on individual islands, tying together local ecology and biogeography. Because taxon-cycle stage was independent of taxonomic or ecological relationships among birds of the West Indies, George and I postulated that whether a species was in a phase of expansion or contraction reflected the outcome of coevolved relationships with antagonists, including pathogens. The taxon cycle concept had a cool reception initially, but subsequent phylogeographical analyses, beginning in the early 1990s with Eldredge Bermingham, provided a time scale that confirmed the relationship between taxon cycle stage and the relative age of the most recent population expansion. The discrete nature of islands allows one to visualize taxon cycles in island systems, but the principle should apply in a continental biota as well. The absence of strong phylogenetic effects in distribution and abundance is consistent with evolutionary lability caused by coevolutionary outcomes with specialized antagonists. Related species appear to compete for resources on a more-or-less equal footing across a broad range of environments, and their distribution at any particular time is likely to be determined primarily by their relationships with pathogens, among other antagonists. This model of distribution and abundance within a regional community is consistent with much of what we know about the interactions between pathogens and their host populations, but testing the model will require the development of a new research programme focused on endemic pathogen effects in natural communities.