Print PageOntogenetic changes in individual size or morphology can significantly change the type and strength of ecological interactions. Accounting for such ontogenetic niche shifts could help explain the outcome of species interactions, particularly in predator-prey systems. In many communities, prey species can exert strong competitive effects on early life stages of their predators, which temporarily overlap in resource use. This “juvenile bottleneck” can significantly reduce predator recruitment (and future predation pressure), potentially leading to alternative stable community states. We conducted two experiments to explore the joint effects of predation and juvenile competition using a predatory copepod, Mesocyclops edax, and the freshwater crustacean Daphnia pulex. These taxa compete for algal resources during the predator’s larval (nauplius) stage, after which predators metamorphose into carnivorous (and cannibalistic) adults. We assessed the independent and interactive effects of cannibalism, predation, and competition by housing predator larvae (nauplii) and prey either 1) alone, 2) together, 3) each separately with adult predators, and 4) all together simultaneously. In a second experiment, we repeated these treatments while varying the initial density of D. pulex to determine 1) how initial prey density affects predator recruitment, and 2) whether prey density influences predator establishment, potentially leading to alternative community states.
Results/Conclusions
As expected, the presence of competitors and adult predators independently reduced the abundance of predator larvae, indicating a significant bottleneck effect in experiment 1. However, predator larvae performed better than additive predictions when both competitors and adult predators were present, although survival was still lower than in treatments with larvae alone. In contrast, D. pulex performed significantly better when housed with both life stages of the predator. These results indicate strong non-linear interactions between cannibalism, predation, and competition in a simple aquatic community. We investigated these mechanisms further in our second experiment by following nauplii from birth to adult recruitment, while simultaneously varying the initial abundance of D. pulex. In experiment 2, initial results suggest that high cannibalism rates reduced recruitment to the adult stage, though the strength of this effect varied with initial prey density. Higher prey abundance altered the recruitment curves of M. edax, indicating a level of density dependence in the observed juvenile bottleneck. Though recruitment varied, predators were not initially excluded from communities. Overall, our results indicate that stage-structured interactions can lead to complex dynamics between predation, cannibalism, and juvenile competition, altering the relationship between predators and prey in natural communities.
