The relative importance of competition and predation in environment characterized by resource pulses - an experimental test with a microbial community

Background: Resource availability and predation are believed to affect community dynamics and composition. Although the effects of resource availability and predation on prey communities are usually studied in isolation, these factors can also have interactive effects, especially since the outcome of competition under shared predation is expected to depend on resource availability. However, there are few experimental studies that test the interactive roles of resources and predation on dynamics of more complex multispecies communities. Here, we examine the importance of competition and predation on microbial community dynamics in a resource pulse environment. Results: We manipulated resource availability and predation simultaneously in a microbial microcosm experiment, where a bacterial community was exposed to the protozoan predator Tetrahymena thermophila in three different resource concentrations (low, intermediate and high). The prey community consisted of three heterotrophic bacterial species: Bacillus cereus, Serratia marcescens and Novosphingobium capsulatum, all feeding on a shared plant detritus medium. In fresh culture media, all species grew in all resource concentrations used. However, during experiments without any addition of extra resources, the existing resources were soon depleted to very low levels, slowing growth of the three bacterial species. Prior to the microcosm experiment, we measured the competitive ability and grazing resistance, i.e. reduced vulnerability to predation, of each prey species. The three species differed in allocation patterns: in general, N. capsulatum had the best competitive abilities and B. cereus had good grazing resistance abilities. In the long-term microcosm experiment, N. capsulatum dominated the community without predation and, with predation, B. cereus was the dominant species in the intermediate and high resource environments. Conclusions: Short-term, single-species assays revealed significant differences in the allocation of competitive and defensive traits among the prey species. Based on these differences, we were, to some extent, able to predict how the long-term community structure, e. g. species dominance, is modified by the resource availability and predation interaction in pulsed resource environments. Our results are consistent with theoretical predictions and also highlight the importance of interactive effects of resource competition and predation, suggesting that these factors should not be studied in isolation.