Leaf breakdown in a tropical headwater stream (Puerto rico): The role of freshwater shrimps and detritivorous insects

Current ecological theory suggests that some organisms have overlapping roles, such that species loss may not disrupt ecosystem processes. In a tropical headwater stream, we hypothesized that the loss of major shredders (two species of shrimps) would decrease leaf breakdown rates. We tested our hypothesis by conducting a leaf breakdown experiment using Cecropia schreberiana leaves under the following treatments: (1) shrimp exclusion, (2) invertebrate exclusion (i.e., shrimp and insect), and (3) control, where all groups had access to leaves. Treatments were replicated in three pools in Quebrada Prieta, Luquillo Experimental Forest, Puerto Rico. Leaf packs were collected from each treatment on days 1, 3, 6, 13, 25, and 35. We found that breakdown rates between the control and shrimp exclusion treatments were not significantly different, whereas the total invertebrate exclusion treatment broke down the slowest. However, ash-free dry mass remaining at day 35 was significantly different among all treatments; control treatments (with shrimps) had the lowest mass remaining and the total invertebrate exclusion had the largest mass remaining. Our study demonstrated that even in a tropical island stream dominated by large populations of decapods, there was a significant degree of leaf breakdown by small, less abundant groups of detritivorous insects. In the long-term, however, shrimps may have a crucial role in the ecosystem process of leaf breakdown by maintaining faster rates of organic matter recycling.