Effects of omnivorous invaders on arboreal arthropod communities in naturally fragmented Hawaiian forests

Background/Question/Methods

Networks of interactions among species help maintain the biodiversity and ecosystem services on which humans depend. However, invasive species may disrupt these ecological networks with potentially cascading effects on ecosystem processes. Island ecosystems are particularly vulnerable to invasive species, in part because of their high levels of endemism, lack of evolved defenses common in continental species, and low functional redundancy. Because of the general lack of mammals on islands and often available niche space, rats are exceptionally successful invaders on islands. In a model island system, we examine how invasive rats (Rattus rattus) affect native predator-prey networks and trophic functioning. Experimental predator removal was conducted in the Upper Waiākea Forest Reserve (Big Island of Hawaii), where recent lava flows created a replicated system of forest fragments (kīpuka) which host diverse assemblages of native bird and arthropod species. Combining predator removal, experimental exclusion of insectivorous birds and stable isotope analysis of canopy arthropods, we explicitly investigated the interactive effects of forest fragment size and omnivorous rats on arboreal arthropod food webs.

Results/Conclusions

Following experimental rat removal in Metrosideros-dominated kīpuka, we documented shifts in arthropod communities both in the understory and throughout the canopy. Notably, some charismatic endemic taxa, such as the Hawaiian happy face spider, increase in local abundance following rat removal. Distance to nearest neighbor kīpuka may further influence the responses of native taxa to rat removal. In addition, stable isotope analyses of canopy arthropods indicate that the trophic position of top predatory spiders decreases with increasing kīpuka size. This suggests that rats alter food web complexity differentially as a function of kīpuka size. Impacts of rat predation are likely strongest in these small kīpuka because rats forage more heavily in the short stature canopies of smaller kīpuka than the tall canopies of larger kīpuka. Continuing work examines the hypotheses that rats alter food web complexity directly by reducing the density of spiders that exploit rare prey or indirectly via competitive or predatory impacts on insectivorous birds.