Different responses of herbivore abundance to plant genotypic diversity depending on herbivore host specificity

Key message

The response of herbivore abundance to birch genotypic diversity varies depending on the host specificity of the herbivores. Specialist, but not generalist, herbivores were affected by birch genotypic diversity.

Abstract

Biological control is an important ecosystem service mediated through plant diversity. Responses of herbivore abundance to plant genotypic diversity may depend on host specificity, especially the host-feeding type. We conducted a common-garden experiment by manipulating plot-level genotypic diversity (number of source populations per plot) of Erman’s birch (Betula ermanii) to understand how birch genotypic diversity affected the abundance of three herbivore feeding types (chewers, galls, and suckers). Specifically, we investigated whether the effects of plant genotypic diversity on herbivore abundance were additive or non-additive. Furthermore, we examined this mechanism as a possible change in plant phenotype (plant productivity and/or leaf traits) and/or herbivore foraging behavior (associational resistance with neighboring plant individuals). We found that genotypic diversity had a significant positive effect on condensed tannin concentration; however, it did not affect plant growth. There was no effect of increasing birch genotypic diversity on the density of chewers (generalists) and chewing herbivory, whereas the density of galls and suckers (specialists) decreased significantly. A negative non-additive effect was observed with regard to gall density and a negative additive effect on sucker density was observed. Gall density was not related to the increase in condensed tannins affected by birch genotypic diversity. This implies that associational resistance is more likely to explain the negative non-additive effects on gall density. Our study shows that the loss of birch genotypic diversity due to shrinking population sizes will strongly affect specialist herbivores in the boreal forests of Hokkaido.