Arthropod rain in a lowland tropical forest

Abstract In lowland tropical forests, “arthropod rain” (i.e., arthropods falling from the canopy to the understory), represents a potentially important terrestrial nutrient flux. We investigated the composition, abundance, 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 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 in pan traps was 0.891 ± 0.033 mg m −2 day −1 ; thus, ca. 27 kg of arthropod biomass rains into the understory per km 2 per month during the wet season in this forest. This equates to ca. 3 million mid‐sized ants falling from the canopy per day on BCI as a whole. Arthropod abundance in pan traps, especially ants and spiders, increased marginally with the increasing number of high‐wind events. By contrast, arthropod biomass showed no relationship with wind or rain. 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.