A Lotka-Volterra theory of predation-competition Interactions in variable environments
Recent theory on the interaction between predation and competition has shown many possibilities for the effects of predation on species coexistence. Predation can promote coexistence if it strengthens intraspecific apparent competition relative to interspecific apparent competition, but can undermine coexistence if reduces the overall magnitude of competition for species that partition resources. For temporal partitioning via the storage effect, previous work has shown that the timescales for resource consumption compared with environmental change are critical to coexistence, and that coexistence due to temporal predator niches cannot occur unless predators respond very rapidly to changes in prey abundance. Potential effects of a predator are enormous accounting for diverse predator behaviors, and life-histories, ranging from “predators” as diseases to predators as large long-lived species. In an attempt to bring order to the range of possibilities and so produce a predictive theory, we developed a general modified Lotka-Volterra framework with three trophic levels in a variable environment and asked about coexistence of species in the middle trophic level. This framework allowed a variety of life-histories to be investigated, with a variety of timescales for resource consumption by the middle trophic level, and predation on that trophic level.
We found several different ways in which predation undermines coexistence of species temporally partitioning resources. These ways include, as has been previously shown, reduction in prey densities and hence the overall magnitude of competition, and also by reducing the effective temporal variation of resource consumption. This latter possibility occurs when predators focus on newly produced prey biomass. However, these outcomes need not occur if predator life-histories allow rapid increases in population size, creating strong apparent competition. In that case, apparent competition is temporally partitioned and promotes coexistence. In addition, a predator can promote coexistence if it has seasonal activity patterns not directly dependent on new production by the prey. As expected, timescales for environmental change compared with timescales for the dynamics of the various species are critically important. In general coexistence is promoted by rapid population dynamics relative to environmental change. Using unique methods of quantifying species coexistence, we show that the diversity of outcomes can be understood within a general predictive framework.