Baseline data about freshwater crab populations and their roles in ecosystem processes (e.g., detrital processing) are needed to predict how freshwater crabs, and the tropical streams in which they live, may respond to multiple stressors from urbanization and climate change. Crabs are often the largest macroconsumers in neotropical headwater streams and may comprise a large portion of invertebrate biomass. In this dissertation, I focus on overlooked and understudied freshwater crabs in the family Pseudothelphusidae. First, I conducted a capture-mark-recapture survey across headwater stream sites spanning land uses from forest reserves to an urban center in Monteverde, Costa Rica. Site-specific crab abundances, estimated using hierarchical Bayesian models that included alternative environmental covariates, revealed substantially fewer crabs in sites near concentrated human development. In contrast, crab densities reached ~5 crabs/m in cobble-dominated, forested streams with low conductivity. In-stream enclosures and laboratory trials showed that crabs increase rates of leaf breakdown through manipulation, shredding, and consumption of leaves, suggesting that where abundant, crabs can substantially contribute to the breakdown of terrestrially derived plant matter. Finally, I synthesized literature and proposed a conceptual shift to broaden the scope for understanding the effects of climate warming on detritivory in tropical streams. This is important in light of a prevailing ecological prediction that shredding insects will experience declines in abundance and richness under warming conditions, particularly in tropical systems. Current evidence, however, suggests that if the thermal preferences of freshwater crabs and other non-insect macroconsumers (e.g., shrimps, crayfishes, fishes, snails, tadpoles) are included in these predictions, it is not clear that shredders as a whole – insects and macroconsumers – will contribute less to organic matter processing in tropical streams under warming conditions. Accounting for the thermal preferences and ecological roles of freshwater crabs and other macroconsumers, in addition to those of shredding insects, is important for making more nuanced predictions about the effects of climate warming on detrital processing and carbon cycling in streams.