A phylogenetic basis for species–area relationships among three Pacific Island floras

Abstract

The angiosperm floras of the Hawaiian, Society, and Marquesas archipelagoes are remarkably comparable ecologically and evolutionarily, a result of similar geologic history, climate, and isolation. We characterized variation in species richness among islands and whole archipelagoes by analyzing species-area relationships (SARs). By partitioning each flora into putative phylogenetic lineages each derived from a given colonization event, we explored several ways in which speciation contributes to SARs. Specifically, these groups exhibit expected island SARs and a whole archipelago SAR characterized by a steep slope. The number of species added by net cladogenesis increases with area much more quickly than the number contributed by net colonization from outside. In each of the three archipelagoes, most colonists do not speciate, while many species occur in a few diverse colonist lineages. Colonization events that are unique to a given archipelago are in more prone to speciation than lineages with close relatives in the other archipelagoes. Most lineages with relatives in all three archipelagoes have one species in each, suggesting a similar tendency not to diversify. On the other hand, a correlation between lineage size in one archipelago and that of related lineages in other archipelagoes suggests a consistent tendency among diverse groups to speciate extensively. Lineages with multiple species in each archipelago also tend to have far more species in the largest archipelago, the Hawaiian Islands. The most diverse lineages exhibit a strong response to archipelago area. These diverse, area-sensitive lineages contribute substantially to the slope of the inter-archipelago SAR. Regional species pools elsewhere may exhibit similar steep-sloped SARs; thus, these findings may inform how the behavior of lineages with different responses to increasing shapes these patterns.