Density-dependent recruitment structures a heterogeneous distribution of herbivores among host plants
A growing body of evidence indicates that plants can influence the survival and reproduction of the insect herbivores they host via both herbivore density-dependent and density-independent processes. A remaining challenge is identifying how density-dependent and density-independent processes in herbivores contribute to the distribution of herbivores in natural populations. I tested which herbivore recruitment parameters—the intrinsic rate of increase, carrying capacity, or shape of density dependence—contributed to variance in the distribution of a gall-making fly among individuals of its host plant by experimentally manipulating herbivore density on plants in the field. I used model selection to determine the relationships between herbivore demographic parameters and the natural, pre-experimental pattern of herbivore abundances. The naturally occurring pattern of herbivore abundances before the experiment covaried positively with the herbivore carrying capacity, a parameter inversely related to the strength of density dependence, but not with the shape of density dependence or the intrinsic reproductive rate. This means that plants with high natural herbivore abundances had lower herbivore density dependence but not higher rates of herbivore reproduction at low abundances. More generally, these results suggest that density dependence mediated through the host plant was responsible for the significant spatial variance in abundance of this herbivore among host plants. This also means that the processes influencing the spatial variance in the abundance of this herbivore occur at high, but not at low, herbivore density. This suggests that when measuring parameters of herbivore preference for, or performance on, plants with different genotypes or phenotypes, ecologists should use a range of herbivore densities to ensure that they capture density-dependent processes. Density-dependent recruitment at the scale of host plants could be a widespread determinant of abundance patterns for the many insect herbivores that have high heterogeneity in abundances among host plants and low variance in that pattern through time.