Resource availability determines abundance, predator body size, and food web structure in terrestrial arthropod communities at Palmyra Atoll

Background/Question/Methods

In this study, we tested the role of ecosystem productivity, size, and the combination of these factors on multiple metrics of arthropod community composition and structure at Palmyra Atoll, National Wildlife Refuge. Based on previous research, mainly in aquatic systems, we expected a strong positive relationship between both ecosystem size and productivity and 1) arthropod abundance, 2) arthropod species richness, 3) predatory arthropod body size, and 4) predator trophic level. We tested these hypotheses by examining variation in arthropod communities across a set of islets in Palmyra Atoll, National Wildlife Refuge, which consists of more than 20 small islets that range more than an order of magnitude in both size and estimated primary productivity. Specifically, we collected all arthropods from one kilogram of terminal plant material of a common tree (Tournefortia argentea) on 18 islets and then conducted additional focal surveys for all spiders, including the cane spider (Heteropoda venatoria), and one common herbivore, the coconut flat-moth larvae (Agonoxena argaula).

Results/Conclusions

Arthropod species richness did not show systematic changes with either island size or primary productivity. While abundance of some herbivores increased with productivity, some did not respond to either primary productivity or island size. In contrast, abundance and size of the omnivorous katydid Phisis holdhausi as well as the size of one spider (Heteropoda venatoria) were positively correlated with islet productivity, but not with islet size. No other spider species body sizes were influenced by primary productivity or islet size. Total biomass of arthropods per survey increased with primary productivity, a trend strongly driven by increases in abundance and body size of P. holdhausi. Additionally, the trophic level of H. venatoria increased with increased productivity. The observed responses in several trophic levels suggest that increased resource availability increases food quantity and quality for both herbivores and predators and that shifts in prey identity associated with increased primary productivity drive changes in predator populations and influences species interaction webs. This research underscores the significance of productivity for community and food web structure in natural terrestrial ecosystems as well as in systems impacted by human activities.