Long-term demographic balance in the Broadstone stream insect community

1. Population models based on Lotka-Volterra-type differential equations wi th logistic prey were made for a simple stream community including two ston efly prey Leuctra nigra Olivier and Nemurella pictetii Klapalek, and two pr edators, the caddisfly Plectrocnemia conspersa (Curtis) and the alderfly Si alis fuliginosa Pictet. In order to assess the importance of predation in t his system, we constructed both an explicit four-species model and a simpli fied model with two functional groups which was more amenable to analytical treatment. 2. The models were parameterized using new data on adult emergence and recr uitment combined with previously published data on larval densities and pre y uptake. The models were falsified if parameterizations led either to nega tive prey carrying capacities or to unstable dynamics. 3. Both the functional group and four-species models predict asymptotically stable interactions, with feasible carrying capacities. The models are con sistent in predicting that the observed prey are in excess of 70% of their carrying capacities. The four-species model indicates that predation impact is not evenly shared between the two prey, with L. nigra being depressed f urther from its carrying capacity than N. pictetii. 4. Sensitivity analysis shows that the results of the full four-species mod el remain very robust to realistic levels of stochastic variation in the in put data. 5. The four-species model is used to predict the outcome of an ongoing larg e-scale field experiment involving the transfer of all S. fuliginosa eggs f rom one stretch of the stream to another. Although the equilibrial prey pop ulations are barely affected by the manipulation, the model predicts marked transient prey-release and prey-depression of L. nigra in the predator add ition and removal areas, respectively.