Document: NIC-3-52-9

Contrasting effects of plant diversity and composition on insect abundances.

HADDAD, N.* ¹, D.TILMAN ², J.HAARSTAD ², M.RITCHIE ³ and J.KNOPS ⁴

North Carolina State University, Raleigh, NC 27695 USA ¹
University of Minnesota, St. Paul, MN 55108 USA ²
Utah State University, Logan, UT 84322 USA ³
University of Nebraska, Lincoln, NE 68588 USA ⁴

Abstract:
In an experiment that directly controlled plant species richness and functional group richness, we found that insect abundances increased as plant species richness increased, but decreased as plant functional group richness increased. To separate the effects of plant species richness and plant functional group richness, we created experimental plots that contained 1, 2, 4, 8, 16, or 32 plant species from 1 - 5 functional groups. The contrasting effects of plant species richness and plant functional group richness on insect abundances were caused by three factors. First, lower insect abundances at higher plant functional group richness were explained by a sampling effect, caused by the increased likelihood that C₄ grasses would be present at higher levels of plant diversity and reduce average insect abundances by 25%. Second, higher plant productivity increased the abundance of insect resources, which increased insect abundances. The effects of plant productivity may be related indirectly to the level of plant diversity because plant productivity was positively related to plant functional group richness. Third, insect abundances were positively related to plant species richness, possibly due to greater availability of alternate plant resources, greater seasonal availability of plant resources, or greater vegetational structure at higher levels of plant species richness. These results clarify the frequently observed negative relationship between plant diversity and insect abundances, and highlight the impact of plant composition on insect community structure in natural ecosystems.

Keywords: Abundance, community, composition, diversity, functional groups, insects, species richness, trophic structure

This abstract is being presented at: 10:15 AM in session:
Oral Session #4: Herbivore Responses to Plants.