COS 85-5 - Interactions with insect herbivores are important for the biodiversity-ecosystem functioning relationship

Abstract: Global changes have cascading impacts. Loss of plant diversity can alter interactions with higher trophic levels, and be amplified by changes in resource availability. Long-term experiments have established positive plant biodiversity-ecosystem functioning (BEF) relationships. However, to predict the impacts of lost diversity on ecosystems, we need a mechanistic understanding of how and why plant diversity influences ecosystem-level processes. One mechanism driving the positive BEF relationship could be decreased herbivory, or induced defenses, in diverse plots. To address this, we manipulated plant species richness and phylogenetic diversity in 2m x 2m plots in a central Texan grassland experiment, using netting to exclude insect herbivory from 1m x 1m subplots. We quantified chemical defenses (secondary metabolites) and herbivory damage, as well as herbivory-linked traits (e.g., specific leaf area) on Centaurea americana inside and outside of insect-exclusion treatments from 4- and 6-species plots, and percent cover for all 144 plots (including monoculture, 2-, 4-, 6-, and 12-species plots).

We found interactions with the insect community influence physical and chemical defense allocations of C. americana individuals. Specifically, C. americana individuals had thicker leaves (lower specific leaf area) out of the net (19.22 m2/kg ± 0.58) than in the net (23.12 m2/kg ± 1.22). The diversity of secondary metabolites were similar for C. americana in and out of the net, however, we identified 111 metabolites (out of 2284, most classified as related to defense) that were expressed differentially between C. americana individuals from in and out of the nets (adjusted p < 0.05). Secondary metabolites of C. americana did not protect them fully, with significantly more leaf area lost to herbivory out of the net (1.67% ± 0.39) than in the net (0.57% ± 0.22). In polyculture plots (≧2sp), metabolomic diversity of C. americana (accounting for structural relationships among metabolites) was positively correlated with herbivory damage (in and out of the nets, p = 0.0564). In all communities, phylogenetic diversity significantly explained total percent cover out of the net (p = 0.03127) and not in the net (p = 0.1275), with significantly different percent cover in and out of the net in paired comparisons (p = 8.614e-07). We conclude that the interactions between C. Americana with insect herbivores exemplify a mechanism explaining the positive BEF relationship. Our work quantifies how individual allocations link to community and ecosystem processes and identifies interactions with insect herbivores as a main driver in herbaceous plant communities.