Do macroconsumers play a role in leaf litter breakdown in tropical urban streams?

Allochthonous detritus represent a major resource for aquatic food webs.  In forested streams, leaf litter breakdown is an ecosystem process that involves the participation of different biotic (e.g. fishes, invertebrates, microbes) and abiotic (e.g., flow, temperature) components.  Urbanization decreases aquatic biodiversity and in urban streams abiotic factors may play a large role controlling leaf breakdown.  However, urban streams in Puerto Rico that lack large dams still maintain their natural macroconsumer (i.e., fish and shrimp) fauna.  Given this characteristic we designed an experiment to determine whether macroconsumers play a role in controlling leaf breakdown in urban streams in Puerto Rico.  Leak packs were secured inside 1 cm mesh cages, open cages work as controls and closed cages as macroconsumer exclusion. Leaf packs were collected at 0, 4, 7, 14, 21, 28, 42, 56, 70 and 84 days. Overall, we found no significant differences between treatments in breakdown rates and percent mass remaining. Although native macroconsumers are still abundant in this tropical, they do no play a major role in processing leaf litter.  Results are in general agreement with previous studies of non-urban streams, suggesting that abiotic factor might be the driving factors in lowland Puerto Rican streams.