Variance and skew of the distribution of plant quality influence herbivore population dynamics

Individual genotypes, phenotypes, or species of plants can vary in their effects on herbivore population dynamics. In this study I used a simulation model to explore how the distribution of plant quality in a plant population affects the population dynamics of insect herbivores feeding,on those plants. The model considers an herbivore population moving among patches (plants) with differing parameters for growth and carrying capacity. Observed population sizes across all patches were compared to the expectation that, if variance in parameters had no effect, population size should equal the sum of the carrying capacities of all plants. Results showed that variance in K alone, or in r and K together, strongly affected herbivore population size; increasing variance led to progressively stronger effects. The direction of the deviation of herbivore population size from the expectation in the absence of variance could be positive, negative, or both for different combinations of variance in r and K. Increasing skew also increased the deviation of observed population sizes from the expectation. Herbivore mobility increased the effect of variance in patch (plant) quality, whereas herbivore selectivity decreased the effects of variance by decreasing herbivore movement. These results are consistent with observations from agricultural studies that polycultures can either increase or decrease herbivore population sizes relative to monocultures.