Intraspecific competition influences population and community dynamics and occurs via two mechanisms. Exploitative competition is an indirect effect that occurs through use of a shared resource and depends on resource availability. Interference competition occurs by obstructing access to a resource and may not depend on resource availability. Our study tested whether the strength of interference competition changes with protozoa population density. We grew experimental microcosms of protozoa and bacteria under different combinations of protozoan density and basal resource availability. We then solved a dynamic predator–prey model for parameters of the functional response using population growth rates measured in our experiment. As population density increased, competition shifted from exploitation to interference, and competition was less dependent on resource levels. Surprisingly, the effect of resources was weakest when competition was the most intense. We found that at low population densities, competition was largely exploitative and resource availability had a large effect on population growth rates, but the effect of resources was much weaker at high densities. This shift in competitive mechanism could have implications for interspecific competition, trophic interactions, community diversity, and natural selection. We also tested whether this shift in the mechanism of competition with protozoa density affected the structure of the bacterial prey community. We found that both resources and protozoa density affected the structure of the bacterial prey community, suggesting that competitive mechanism may also affect trophic interactions.

Abstract. Predators have potentially strong effects on prey abundance, growth, life histories, and behavior, but the net direction and strength of these effects depend on traits in the predator population. Trait evolution that occurs on ecological timescales is common and may affect species interactions. Using laboratory microcosms of a bacterivorous ciliate (Colpidium sp.) and a diverse community of bacteria, we tested the hypothesis that predator trait evolution in response to temperature variability and nutrient concentration alters the composition of the prey community using a two-phase experiment. The first phase consisted of a selection experiment, in which microcosm communities were randomly assigned to one of eight fully factorial treatments of high or low nutrients, variable or stable temperatures, and with or without Colpidium. The second phase was a reciprocal transplant experiment, in which a subsample of each microcosm community from the selection experiment was transferred to one of four common garden environments, consisting of all combinations of high or low nutrients and variable or stable temperature. Nutrient enrichment directly decreased bacterial species richness and altered the community composition, but also indirectly increased bacterial species richness through decreases in ciliate peak density and, subsequently, ciliate abundance. Moreover, the evolutionary effects of nutrient enrichment on predator traits were more pronounced in some contemporary common garden environments, and undetectable in others. Our findings suggest that historical and contemporary environments are equally important to consider in understanding how trait evolution affects community structure.

Many species are thought to use cryptic coloration to evade predators or catch prey.However, colour patterns are historically difficult to document and compare when coloration is continuous and spatially complex. The authors studied colour variation in the green lynx spider, Peucetia viridans, a sit-and-wait predator often found in the tops of inflorescences feeding on pollinators. Spiders were collected from three distinct plant species that provided different original colour backgrounds and photographed under standard lighting and background. A novel colour analysis method (Colormesh) was used to quantify spatially explicit contributions of red, green, and blue along the spider abdomen from digital photographs. Spiders collected from green backgrounds were found to have significantly greater contributions of green abdomen colours; red and blue body colours showed no significant correlation with spider background. This is the first study of colour-matching in a natural population of this wide-spread generalist predator and also demonstrates a novel method for quantifying complex, continuous colour patterns.