Interspecific Competition between Two Stream Insect Grazers Mediated by Non-Feeding Predatory Fish

Abstract

Two periphyton-grazing stream insects, a baetid mayfly and a glossosomatid caddisfly, were found to be strong exploitative competitors. The presence of predatory fish was found to alter the foraging activities of the baetid. but not of the glossosomatid. We investigated the local distribution of Baetis thermicus and Glossosoma sp. to examine the potential for competition between the two grazers in a northern Japanese stream. Further, we carried our a laboratory experiment to test the hypothesis that the freshwater sculpin Cottus nozawae, when not allowed to attack prey, mediates the resource-limited competitive influences of Baetly on the growth of Glossosoma, but not the reverse. We found a negative correlation between the densities of Baetis and Glossosoma in the stream, suggesting the strong potential for resource-limiting competitive interactions to occur between the grazers. Using a laboratory channel experiment employing a target-neighbor design, neither the presence of sculpin nor the density of the two grazers altered any life-history traits measured for the grazers. However, the proportion of Baetis individuals positioned on the upper surface of a ceramic plate substrate (with greater periphyton biomass) largely decreased in the presence of sculpin, with only a minor behavioral shift being detected in Glossasoma. This difference in behavioral shift resulted in greater growth suppression in Baetis than in Glossosoma. In the absence of sculpin, growth of both target grazers was lowered by experimentally increasing the density of neighboring competitors. In partial contradiction to the above hypothesis, the growth of not only Glossosoma but also Baetis was never influenced by the density of neighbor competitors when sculpin were present. The biomass of periphyton in the presence of sculpin was greater than when sculpin were absent, and was lower with higher densities of either grazer. Predator-avoidance behavioral modifications in both grazers may be the mechanism responsible for mediation of their exploitative competition.