Effects of freshwater shrimp assemblages on benthic communities along an altitudinal gradient of a tropical island stream

1. In tropical island stream ecosystems freshwater shrimps are often the dominant macroconsumers and can play an important role in determining benthic community composition. However, most studies of the ecological role of shrimps are limited to high-altitude shrimp-dominated sites where other biota (fishes and snails) are absent or significantly less abundant than at lower altitudes. 2. We examined how effects of different shrimp assemblages on benthic communities changed along an altitudinal gradient in a tropical island stream in Puerto Rico. We used electroshocking and observations to quantify abundance and taxonomic composition of shrimp assemblages at three sites (300, 90 and 10 m a. s. 1) along the Rio Espiritu Santo. We also experimentally manipulated access of shrimps to the benthic environment simultaneously at each site using electric fences over a 35-day period. 3. At the high-altitude site, exclusion of shrimps (predominantly Atya spp. and Xiphocaris elongata) resulted in significantly greater accrual of organic and inorganic material, chlorophyll a and algal biovolume. In the absence of shrimps, the algal community was dominated by filamentous green algae (Chlorophyta: Oedogonium and Rhizoclonium). Excluding shrimps did not affect total insect biomass but significantly increased sessile chironomids (Diptera: Chironomidae). We observed similar treatment effects at the mid-altitude site where shrimps (primarily Macrobrachium spp. and X. elongata) occurred at lower densities. In contrast, at the low-altitude site there were no treatment differences in organic and inorganic material, chlorophyll a, algal biovolume, algal assemblage composition and insects. 4. The lack of treatment differences at the low-altitude site was probably because of very high densities of grazing snails (Thiara granifera and Neritina spp.) which reduced organic and inorganic resources and obscured potential shrimp effects. 5. This study demonstrates that freshwater shrimps can play an important role in determining benthic community composition; however, their effects vary and appear to depend on the presence of other biota. This study suggests that loss of shrimps as a result of anthropogenic disturbances will have different effects on the stream community depending upon location along the altitude gradient.