APPLICATIONS OF THE MENGE-SUTHERLAND MODEL TO ACID-STRESSED LAKE COMMUNITIES

The predictions of the Menge-Sutherland model with respect to the effe cts of environmental stress on species richness, food web complexity, food chain length, and the relative importance of predation, competiti on, and disturbance in structuring communities were examined using mid summer zooplankton communities of 46 lakes near Sudbury, Ontario with pHs of 3.8-7.2. The lakes were sampled in the 1970s and in 1990 yieldi ng a total of 92 food webs. Species richness increased linearly with p H rather than peaking at intermediate levels of stress as predicted by the model. Food web complexity and food chain length increased with p H, as predicted. The importance of competition in food web structure w as variable at low pH and consistently high at moderate pH, a result t hat does not support model predictions that disturbance, competition, and predation should sequentially predominate with increasing pH. High variability in the relative importance of competition in acidic lakes was explained by fish presence or absence; in lakes without fish, a l arger proportion of links in the zooplankton food web was due to preda tion by invertebrates. Persistence of predators (fish) in some acidic lakes reduces the applicability of the Menge-Sutherland model.