Extending the Resource Concentration Hypothesis to Plant Communities: Effects of Litter and Herbivores

We extend the resource concentration hypothesis (herbivorous insects are more likely to find and stay in more dense and less diverse patches of their host plants) to plant communities. Specifically, whenever superior plant competitors spread to form dense stands, they will be found and attacked by their specialist insect enemies. This will decrease host plant abundance, causing a reduction in standing crop biomass, which will indirectly increase subordinate competitors and plant species richness. In this study, we found that a native, specialist chrysomelid beetle (Trirhabda virgata) in an old-field community decreased total standing crop biomass, leading to an increase in plant species richness. This reduction in biomass was due solely to a reduction in the biomass of the beetle's host plant, meadow goldenrod (Solidago altissima), which was the dominant plant species in this community. Our results demonstrate that when a superior competitor increases in density, the per-stem impact of herbivores increases due to a buildup of these herbivores in high-density host patches. Specifically, we found that as goldenrod density increased, the per-stem abundance of Trirhabda virgata also increased. In turn, species richness increased as the negative effect of insects on goldenrod biomass increased. Recent research suggests that litter accumulation could negate or cancel the effect of herbivorous insects on plant communities because litter accumulation increases with standing crop biomass, causing a decline in species richness. The litter accumulation hypothesis states that, in productive communities, the increase in the abundance of the superior competitor will lead to a dense accumulation of plant litter, causing a decline in species richness. Consistent with this hypothesis, we found that as the biomass of the dominant plant species increased, litter mass also increased. In turn, species richness decreased as the negative effects of litter on stem density increased. Interestingly, the effect of litter on stem density depended on whether insects were present. Our results suggest the potential for a general rule: specialist insect herbivores will function as classic keystone species in plant communities whenever host species form large, dense aggregations if host plants are dominant species. Corresponding Editor: D. A. Spiller