Arthropod rain in a lowland tropical forest

Resource subsidies link habitats, facilitating the flow of energy across ecological boundaries. In lowland tropical forests, "arthropod rain" (i.e., arthropods falling from the canopy to the understory) represents a potentially important but understudied terrestrial nutrient flux. We investigated the composition, biomass, and environmental drivers of arthropod rain on Barro Colorado Island, Panama. Pairs of traps–pan traps and pole traps–placed 1 m above the ground, respectively collected fallen arthropods and arthropods potentially climbing to the canopy. Average (± SE) arthropod biomass density in pan traps was dominated by Hymenoptera (primarily ants; 0.501 ± 0.023 mg dry mass m-2 day-1) and Lepidoptera larvae (0.228 ± 0.001 mg m-2 day-1). Total dry biomass density in pan traps was 0.896 ± 0.033 mg m-2 day-1; thus, ca. 27 kg of arthropod biomass rains into the understory per km2 per month during the wet season in this forest. Arthropod abundance in pan traps declined with increasing canopy cover and increased with increasing frequency of high wind events. By contrast, arthropod biomass density showed no relationship with canopy cover or environmental variables. Arthropod abundance was higher in pole traps than in pan traps and was dominated by Collembola and Acari. Compositional overlap between pan and pole trap contents suggests that some fallen arboreal arthropods regularly return to the canopy. These findings illustrate an understudied pathway linking canopy and understory food webs within tropical forests, and the complex interactions between environmental conditions and arthropod rain.