Life history traits predict insect species responses to large herbivore overabundance: a multitaxonomic approach

There is increasing concern over the ecological impact of markedly increasing numbers of large herbivores (hereafter large herbivore overabundance) on forest ecosystems. To predict the ecological consequences of large herbivore overabundance, it is first necessary to understand how biological communities respond to large herbivore overabundance. Here, we examined the relationships between the life history traits of five insect taxonomic groups (moths, dung beetles, longicorn beetles, carabid beetles, and carrion beetles) and their responses to deer overabundance in Hokkaido, northern Japan. Insects were collected from three study sites: enclosure (20 deer/km²), control (10 deer/km²), and exclosure (0 deer/km²). The different taxonomic and functional insect species differed in their response to deer overabundance. The abundance (number of individuals) of longicorn beetles, dung beetles, and arbor-feeding moths was higher in the enclosure site than in the control site, whereas that of carabid beetles, carrion beetles, and herb- or shrub-feeding moths was higher in the control site than in the enclosure site. These results suggest that the type of food and the level of dependence on the understory are key traits determining insect sensitivity to deer overabundance. In addition, large or flightless species responded negatively to deer overabundance. Overall, this study demonstrated a significant change in insect communities following experimental deer overabundance, suggesting that large herbivore overabundance leads to the homogenization of biological communities. Unfortunately, because insects have diverse functions in forest ecosystems, such marked changes in both abundance and composition of insect communities will decrease ecosystem functions and resilience.