LB 27-238 - Bird loss impacts ground-dwelling arthropod communities

Birds exert top-down pressure on arthropods, therefore a change in bird communities is likely to impact arthropod communities. However, this impact is likely to be complicated due to interactions between arthropods and difficult to study on large spatial scales. Here, we use the functional extirpation of birds following the introduction of the brown tree snake to Guam to ask how bird loss impacts arthropod communities focusing on which arthropod taxa are more impacted by defaunation. We collected arthropods along transects in three islands with intact bird communities (Saipan, Tinian, Rota) and Guam. We sampled arthropods on vegetation by branch beating and ground-dwelling arthropods from the leaf litter using a leaf sifter and Winkler funnel. We then prepared DNA libraries of collected arthropods targeting the COI region for Illumina sequencing and curated an OTU table based on NCBI database. Using the OTU table, we analyzed differences in arthropod taxa across the islands. Overall species diversity and composition of soil-dwelling arthropods differed between Guam and the other islands, suggesting that landscape-level bird extirpation alters arthropod dynamics. Specifically, OTU richness and Shannon diversity was lower on Guam than on islands with birds. When mapped on an ordination space, arthropod community composition on Guam clustered distinctly. These differences are likely driven by increased abundances of lepidoptera, psocoptera, and hemiptera in litter on Guam. While overall species diversity and composition of arthropods in beating samples did not differ across islands, some taxa in beating samples were more impacted than others. Specifically, web-building spiders and ants were more abundant on Guam than on islands with birds. This suggests that arthropod mesopredators may be important for regulating arthropod populations in the absence of birds.