Tropical forests undergo more carbon (C) cycling than other terrestrial systems; therefore, there is a critical need to understand how these forests respond to and drive changes in climate. In Hawaii, forest floor macroinvertebrates (isopods, millipedes, etc.) have a disproportionately large (30%) impact on decomposition, or mineralization of C. Although we know these impacts are climate dependent in other systems, the relationships between temperature, macroinvertebrate abundance and community composition, and above- and belowground C cycling in Hawaiian and other tropical forests have not been examined. In this study, we compared macroinvertebrate abundance and community composition across an elevational gradient where vegetation is similar but mean annual temperature and soil moisture varies. We found that variation in soil moisture, but not temperature, drives differences in forest floor macroinvertebrate community composition. Interestingly, millipede abundance, which was two orders of magnitude greater in the driest sites, was inversely related to macroinvertebrate diversity. If precipitation, and subsequently soil moisture, is further reduced under climate change, increases in millipede abundance may become widespread, thereby reducing macroinvertebrate species diversity across large areas. These changes in forest floor communities likely will yield alterations to C cycling in these tropical forests.