Predation as a factor mediating resource competition among rotifer sibling species

The relevance of predation as a factor mediating the competitive interaction among ecologically very similar species is investigated by experimentally analyzing the effect of the copepod predator Diacyclops bicuspidatus odessanus on three sibling rotifer species belonging to the Brachionus plicatilis species complex. These rotifer species are similar in shape but show notable differences in body size. Predator and prey species co-occur in brackish waterbodies close to the Mediterranean coast of Spain. First, we characterized differential vulnerability of rotifers to predation. A consistent tendency of higher predation rates on smaller prey (i.e., smaller species and younger individuals) was observed. Analysis of predation showed that predator contact rate did not differ significantly among prey species, but that attack, capture probabilities, and handling time did. Second, we performed population dynamics experiments with two prey species competing for a single resource at different levels of predation. Predation extended coexistence of the competing rotifers, whereas the inferior competitor was excluded in the absence of the predator. In some pairwise experiments, we found that the greater the predation level, the larger the relative increase in density of the inferior competitor. Our results suggest that predation can affect the dynamics of very similar competing species in natural aquatic communities, promoting coexistence.