Geological age, ecosystem development, and resource constraints on arthropod community structure in the Hawaiian Islands

Abstract

An ongoing debate in evolutionary ecology concerns the relative role of contemporary versus historical processes in determining local species richness and community structure. At sites along a 4-Myr geological chronosequence on Hawai‘i, Moloka‘i and Kaua‘i, numerous extrinsic factors can be held constant, but ecosystem fertility and nutrient availability are low at both very young and very old sites, peaking at intermediate geological age across islands. Thus, contemporary resource traits are shared among sites with different biogeographic legacies, and these opposing gradients allowed a test of their relative importance for arboreal arthropod community structure. Pyrethrum knockdown was used to sample arboreal arthropods from Metrosideros polymorpha (Myrtaceae), the dominant tree throughout the Hawaiian Islands. Arthropod abundances and sample-based species richness peaked at more productive, intermediate-aged sites, but did not correlate with geological age. The proportions of individuals and biomass in trophic groups and in different taxonomic orders differed widely across sites, but proportions of species in trophic groups were more regular than chance expectation. Species richness in local communities did not accumulate or pack more tightly with increasing geological age to the oldest island. Intermediate-aged islands may be contemporary peaks of richness, mediated by ecosystem development and senescence. Although historical and evolutionary processes generate diversity at broad scales, local communities converged in trophic structure and composition, and ecosystem resource availability constrained arthropod numbers and richness at local scales.