Interspecific Interactions Among Phytophagous Insects of Tallgrass Prairie: An Experimental Test

Densities of the grasshopper Phoetaliotes nerbrascensis (Acrididae) were manipulated in native tall grass prairie at Konza Prairie, Kansas, to measure competitive and facilitative effects on densities of co—occuring phytophagous insects. In late June 1987, P. nebrascensis nymphs were transferred among enclosed plots (each 550—900 m 2 ) to produce three pairs of plots with grasshoppers either removed or added. Pretransfer densities in the six plots were estimated at °5 nymphs/m 2 , 2—2.5 nymphs/m 2 were then removed or added. Densities of the manipulated species and other abundant phytophagous insects were subsequently monitored (by sweep net) through mid—August. The manipulations in late June resulted in mean densities of P. nebrascensis nymphs 1.5–4 times as large on addition plots as on removal plots; these differences persisted through mid—August. No significant differences between treatments were found, however, in the numbers of individuals of other Acrididae or Tettigoniidae on four post—transfer sampling dates (July to mid—August). Significant differences were also not found in the numbers of individuals of Phasmatidae, Homoptera, or phytophagous Hemiptera and Coleoptera, or in the biomasses of live grass, dead grass, or forbs present in mid—August. 1987 was year of moderate primary production on Konza Prairie. The manipulations produced experimental treatments that mimicked intermediate vs. relatively high natural densities of P. nebrascensis. The results indicate that under these conditions, changes in P. nebrascensis density have little net short—term impact, competitive or otherwise, on the population dynamics of co—occurring grasshoppers or other phytophagous insects.