Effects of an invasive alien tree on the diversity and temporal dynamics of an insect assemblage on an oceanic island

Native vegetation is frequently replaced by alien plants on isolated oceanic islands. The effects of such replacements by invasive plants on the diversity and temporal dynamics of island-endemic insects remain unclear. We examined flying insect communities using Malaise traps on the small island of Nishi-jima in the oceanic Ogasawara Archipelago in the northwestern Pacific. On the island, an alien tree, Casuarina equisetifolia, has become dominant, occupying 57.3 % of the vegetation area. The species richness, composition, and abundance of pollinators (bees), predators (wasps), and wood-boring beetles (cerambycids, mordellids, and elaterids) were compared in each summer season of 4 years among three vegetation types: C. equisetifolia forest, natural forest, and grassland. In the traps, 82.3 % of species captured were endemic to the archipelago. The grassland harbored the highest species richness of native bees and wasps, whereas the natural forest had the highest species richness of native wood-boring beetles. The C. equisetifolia forest had the poorest species richness for most insect groups. Principal response curves indicated that differences in species composition among the three vegetation types were consistent through time for all insect groups. Most insect species were more abundant in natural forest or grassland than in C. equisetifolia forest. Standard deviations in both the numbers of individuals and species estimated under a Bayesian framework suggested that annual fluctuations of abundance and species density were similar among vegetation types (except for elaterid abundance). Therefore, replacement by C. equisetifolia has likely altered insect species composition but has not necessarily dramatically affected the temporal dynamics of insect assemblages on the island.