Testing a simple rule for dominance in resource competition.

Competition for limiting resources long has been considered an important factor generating community structure. A minimal model of resource competition predicts that the species that reduces the limiting resource R to the lowest level ([Formula: see text]) will exclude its competitors. Whether this "[Formula: see text] rule" is robust to violations of model assumptions remains largely unknown. I conducted a competition experiment with four species of bacterivorous protists in laboratory microcosms and predicted the outcome from each species' [Formula: see text] value. I also examined how the outcome of competition, species abundances, and the effect of protists on bacterial density varied with productivity. Microcosms were unstirred batch cultures containing a variety of bacteria, challenging the robustness of the simplest competition models. Protists with low [Formula: see text] values were less affected by competition, although competing protists often coexisted. The values of [Formula: see text] can predict competitive dominance, even in the absence of competitive exclusion. Other model predictions were less robust. Contrary to expectation, densities of grazed bacteria increased with productivity, and the effect of some protists on bacterial density did not vary with productivity. Bacterial heterogeneity may account for deviations from model predictions. Further experiments should examine the conditions under which simple rules can be expected to identify dominant species.