Simple models for cladoceran species interactions: how competition, predation, and evolutionary trade-offs potentially influence coexistence of Bosminid species

The balance between competition and predation prompted early research into species coexistence. Spe-cies in a sibling species complex (subgenus Bosmina) often occur together in lakes (47 %). Initial studies in Lake Washington revealed that two sibling species of Bosmina (western B. longirostris, B. liederi) overlapped across Union Bay. One species with shorter spines was the competitive dominant and was more abundant in shallow, nearshore bay waters, whereas the other species had longer defensive spines that allowed it to survive with an off-shore invertebrate predator (Epischura). Initially a simple population model developed by Slobodkin, and slightly modified by Emlen, was applied to the circumstances. The model predicted reversed outcomes over a gradient of predation and stable coexistence of species at intermediate predation levels. Here we conduct a full stability analysis of the Slobodkin/Emlen model, and then incorporate a simple evolutionary tradeoff into the equations. By "tradeoffs" we mean that defensives are purchased at a cost to competitive ability. The L-V trade-off model predicts coexistence of two species because there are two limiting factors (competition, predation), and increases the range of stability conditions. Field experiments in Lake Washington demonstrated reversal of outcomes at predation ex-tremes, whereas enclosure experiments in Lake Norford, Vermont, confirmed co-existence of sibling species over a range of intermediate predation levels. Comparable experiments in Third Sister Lake, Michigan, showed that when predation removal rates were pushed to extremes (more efficient predators), sibling species mixtures collapsed to single species and then to local extinction, a final aspect of the simple model.