Contrasting effects of plant richness and composition on insect communities: A field experiment

We experimentally separated the effects of two components of plant diversit y-plant species richness and plant functional group richness-on insect comm unities. Plant species richness and plant functional group richness had con trasting effects on insect abundances, a result we attributed to three fact ors. First, lower insect abundances at higher plant functional group richne ss were explained by a sampling effect, which was caused by the increasing likelihood that one low-quality group, C-4 grasses, would be present and re duce average insect abundances by 25%. Second, plant biomass, which was pos itively related to plant functional group richness, had a strong, positive effect on insect abundances. Third, a positive effect of plant species rich ness on insect abundances may have been caused by greater availability of a lternate plant resources or greater vegetational structure. In addition, a greater diversity of insect species, whose individual abundances were often unaffected by changes in plant species richness, may have generated higher total community abundances. After controlling for the strong, positive inf luence of insect abundance on insect diversity through rarefaction, insect species richness increased as plant species richness and plant functional g roup richness increased. Although these variables did not explain a high pr oportion of variation individually, plant species richness and plant functi onal group richness had similar effects on insect diversity and opposing ef fects on insect abundances, and both factors may explain how the loss of pl ant diversity influences higher trophic levels.