Flying snails: immigrant selection and the taxon cycle in Pacific Island land snails

Abstract

We tested the hypothesis that, for land snails, long-distance dispersal across oceans is primarily via aerial dispersal (i.e. wind- or bird-mediated), which likely favors so-called micromolluscs through immigrant selection for small (aerially buoyant) body size. Immigrant selection is a filtering process favoring phenotypes conferring greater capacities for long-distance dispersal. We also tested predictions of E. O. Wilson’s taxon cycle, which hypothesizes that descendant species of island colonists are subject to a series of ecological and evolutionary dynamics, resulting over time in progressively more ecologically specialized island endemics with more limited dispersal capacity. We tested predictions of immigrant selection on aerial dispersal and the taxon cycle in native Pacific Island land snails of the Samoan Islands, Mariana Islands, and Lord Howe Island and neighboring small islands using geographic range, shell size, microhabitat, and elevation data compiled from primary and secondary literature. Single-archipelago endemic species found on multiple islands within an archipelago had significantly larger shell sizes than widespread species found in multiple archipelagos and single-island endemic species. Single-archipelago endemic and single-island endemic species were associated with vegetation and ground/rock microhabitats, respectively, whereas widespread species were more likely to be microhabitat generalists. Single-island endemic species were more likely to occur at high-elevation habitats, while widespread species were more likely to be confined to low-elevation habitats. Consistent with predictions of the taxon cycle and immigrant selection on aerial dispersal, Pacific Island land snails endemic to single islands or archipelagos (i.e. those assumed to be later in the taxon cycle) are more likely to have larger body size (archipelago endemics) and to occupy higher elevations (i.e. island interiors; island endemics) in more specialized microhabitats (all endemics).