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item Polley, Herbert

Submitted to: Ecology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2002
Publication Date: 8/4/2002
Citation: Wilsey, B.J., Polley, H.W. Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation. Ecology Letters. 2002. v. 5. p. 676-684.

Interpretive Summary: Weeds and insect pests lessen the amount of food and fiber produced on both intensively-managed and native grasslands, and reduce the economic viability of America's ranches. Weeds and insect infestations of plants sometimes are lower in grasslands that support many plant species than those in which the number of plant species present is low. Whether the number of pests also depends on the relative abundances of grassland species is not known, however. We experimentally varied abundances of grassland plants in field plots to determine whether equalizing the number of plants of each species present (increasing species evenness) would reduce insect infestation and the invasion of weeds. Increasing evenness reduced the number of unplanted seedlings of broad-leaf species that established in plots during the period that desirable plants were actively growing. Infestation of goldenrod by spittle bugs also was reduced in plots in which species were equally abundant. These results support the view that grasslands in which many plant species are abundant are more resistant to weeds and to insect pests than those dominated by a only few plant species.

Technical Abstract: Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi-dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle-bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.