Responses of arthropod communities to invasive rat suppression in diverse mesic forests of Hawaiʻi

Invasive rats (Rattus spp.) are known to strongly impact large, flightless arthropods, but impacts on wider arthropod communities are less clear, with weak and inconsistent effects on most species being reported. However, logistical challenges often impede accurate measurement of arthropod population dynamics associated with rat invasions. In two diverse mesic forests on Oʻahu, Hawaiʻi, we conducted Before-After Control-Impact monitoring to assess arthropod population changes due to continuous rat trapping for 3–4 years. While communities in trapped and control sites were significantly different even before trapping was initiated, dissimilarity of the communities generally increased after trapping. Like prior studies, we found strong evidence of rat impacts on larger, flightless native Orthoptera (both trigonidiin crickets and Banza katydids), as well as on endemic predatory Eupithecia caterpillars. Additionally, many taxonomic groups exhibited significant abundance changes at trapped sites relative to adjacent control sites in both study areas. This was true for 36–41% of groups in arboreal communities and 21–31% in ground-dwelling communities; 67–90% of these significant changes indicated relative population increases at trapped sites. However, the identities of the responding groups frequently differed between study areas and between arboreal and ground communities within each area. Species richness for some groups, particularly native species, increased modestly in response to trapping. There was little evidence for major changes in arthropod trophic structure. Our results suggest a wide range of effects of invasive rats on ground-dwelling and arboreal arthropods, and highlight the complex nature of ecological responses that can follow rat suppression in diverse terrestrial food webs.