Leaf litter decomposition rates in streams are influenced by ambient water quality as well as colonizing microbial and macroinvertebrate communities with often predictable and well understood outcomes. The portion of decomposition due to active consumption by aquatic insects relative to microbial processing and leaf availability is less understood, particularly on a species-specific basis for aquatic insects. To investigate leaf shredding patterns of potential ecological significance we compared consumption rates of two common leaf shredding insect genera (Pteronarcys and Tipula) from different orders (Plecoptera and Diptera) for two common riparian tree species (Acer rubrum and Quercus ruba) across a gradient of available food quantities for each tree species. Insects were placed in isolated recirculating 3 L tanks for 21 days with a known leaf quantity from one species. If all labile leaf portions were consumed prior to 21 days the insect was immediately removed and weighed to avoid any food limitation effects of consumption rate. Leaf consumption and insect growth rates were compared to initial leaf mass, insect and leaf species, and initial insect mass via multiple linear regression.
Results/Conclusions
Insect genus and initial leaf mass were significant predictors of consumption rate with a strong interactive effect, with Pteronarcys and higher initial leaf quantities resulting in faster consumption rates. Leaf species was marginally significant in predicting consumption rate with Acer leaves consumed faster. In addition, insect growth rate significantly increased with increasing initial leaf mass available, and this relationship was species-specific based on initial insect mass. While we expect consumption rate to differ between leaf and insect species, the influence of initial leaf mass on consumption rate was unexpected. This result indicates consumption (and therefore decomposition) rates in streams might be dependent on leaf litter density where consumption rates decrease with decreasing food availability prior to clear food limitation. In addition, this indicates these taxa (particularly Pteronarcys) might be able to assess ambient food availability and decrease their consumption rates before food limitation occurs. To our knowledge, this ability has not been observed before in aquatic insects and has implications for our understanding of both aquatic insect behavior and decomposition rates in streams.