Spatial patterns of inter‐island plant and bird species movements in the Galápagos Islands

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Abstract
An asymmetric similarity index, defined as the proportion of species that one island has in common with a second, was introduced to infer inter‐island colonization patterns from species lists. The matrix comparison or Mantel test was used to analyze the index for spatial patterns of species composition. Directional inter‐island colonization was inferred from the skew‐symmetric component of the index. Four biogeographical hypotheses were tested for the inter‐island colonization of plants among 17 Galapagos islands and among 15 islands for birds. Plant and bird species composition and their inter‐island colonization patterns are distance‐dependent. More specifically, species density gradients are a function of the configuration of islands in the archipelago. The net direction of bird and plant density gradients for the archipelago increases from the south. Plant density gradients are significantly correlated with the prevailing northwest flow of the Humboldt Current, but bird density gradients are uncorrelated with the Humboldt Current or with the gradients of plant species. These results corroborate the most popular model of colonization for the Galapagos Archipelago: an initial colonization of islands in the southeast, and subsequent colonization of islands to the north and northwest.