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RESEARCH PRODUCT

Resource competition between sympatric sibling rotifer species

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Mechanisms underlying competitive interactions are important in understanding the structure of planktonic communities, particularly the coexistence of similar species. Here we present experimental results of exploitative competition among three sympatric sibling species of the Brachionus plicatilis complex for two differently-sized species of food microalgae. These three rotifer species are to be found in seasonal succession in brackish ponds on the Spanish Mediterranean coast; they can, however, co-occur for long periods. The functional and numerical responses of the three Brachionus species to both food microalgae, although similar, differed. Neither rotifer population growth nor grazing rates showed a clear positive correlation between rotifer and alga sizes. Tilman’s models were applied to these results in order to predict the outcome of competition in two rotifer, two resources systems. For testing predictions and addressing the possibility of rotifer coexistence, we performed pairwise competition experiments in semicontinuous cultures, which introduces periodic disturbance as an additional factor. Results confirmed the pattern expected from Tilman’s models regarding the competitive superiority of each rotifer species, when food composition was biased toward one of the two resources. This shows the relevance of the food threshold concept to predict the superior competitor. However, coexistence of rotifers was found in conditions in which Tilman’s models had predicted an unstable equilibrium, with the winning species depending on the initial conditions. We hypothesize that variance in food availability mediated coexistence. Our results suggest that both food partitioning and disturbance are important in explaining the coexistence of these sibling species in nature. Natural communities are structured by a network of interactions among species and of the species with their environment. Among the most important are those of a trophic nature, known as consumer-resource interactions. Food limitation and resource competition can clearly exert a strong influence on a herbivorous zooplankton community structure in natural environments (e.g., DeMott and Kerfoot 1982; Sommer et al. 1986). Resource competition is an indirect interaction that affects competitors via exploitation of common resources, which can lead finally to coexistence or exclusion. In exploitative competition, displacement occurs when one species reduces the limiting resource below that level required for sustained growth by the competing species (Tilman 1982). Exclusion of the inferior competitive species limits the community diversity by restricting the number of co-occurring species with similar use of one or more resources from their fundamental niches (Hutchinson 1957). Because closely related species may have considerable overlap in their food requirements, strong competition is to be expected. It has been proposed that zooplankton can avoid competition by spatial-temporal niche separation (e.g., Makarewicz and Likens 1975). However, spatial-temporal 1