Linking shrimp assemblages with rates of detrital processing along an elevational gradient in a tropical stream.

Abstract: We experimentally excluded freshwater shrimp assemblages (Atyidae, Xiphocarididae, and Palaemonidae) to examine their effects on detrital processing and benthic insect biomass at three sites along an elevational gradient in a tropical stream in Puerto Rico. We also determined which shrimp taxon was responsible for leaf decay in a subsequent laboratory experiment. At the high-elevation site, the shrimp assemblage was dominated by Atya spp. and Xiphocaris elongata, and leaf decay rates were significantly faster in the presence of shrimps than in their absence. Laboratory experiments showed that this was primarily due to direct consumption of leaves by Xiphocaris. Shrimps had no effect on leaf decay rates at mid- and low-elevation sites where there were higher proportions of Macrobrachium spp. shrimps (which prey on Xiphocaris). Laboratory experiments showed that Xiphocaris consumed significantly less leaf material and experienced significantly higher mortality in the presence of Macrobrachium. Shrimp exclusion resulted in significantly less and significantly more insect biomass at the high- and low-elevation sites, respectively; no difference was found at the mid-elevation site. Insects played a minor role in leaf decay. Results show a strong linkage between shrimp assemblages and rates of detrital decay and illustrate the importance of conducting experiments at multiple sites.