PREDICTIONS OF SPECIES INTERACTIONS FROM CONSUMER-RESOURCE THEORY - EXPERIMENTAL TESTS WITH GRASSHOPPERS AND PLANTS

We tested the ability of consumer-resource theory to predict direct an d indirect interactions among species, using an experimental system of insect herbivores and herbaceous plants. Specifically, we examined in teractions among three species of grasshoppers (Melanoplus femur-rubru m, Spharagemon collare, and Phoetaliotes nebrascensis; Orthoptera, Acr ididae) and herbaceous plants in experimental field cages placed over existing fertilized or unfertilized vegetation in a Minnesota old fiel d. For the conditions inside these cages, we addressed whether (1) gra sshopper diet predicted the presence of competition among grasshopper species, and (2) direct effects of grasshoppers on plants produced ind irect interactions among plants, grasshoppers and soil nitrogen. Overa ll, M. femur-rubrum ate a greater proportion of forbs in cages, while the other two species ate primarily grasses. As expected, a pair of gr asshopper species competed if they had similar diets. However, there w ere important exceptions that could be explained from observed indirec t effects, although alternative explanations were also possible. First , all three grasshopper species significantly shifted their diets in t he presence of other species, and these shifts occurred most often whe n competition was expected or occurred. Second, the two grass-feeding species reduced the biomass of the dominant grass (Schizachyrium scopa rium) and increased available soil nitrogen and biomass of forbs. This effect may explain why the grass-feeding P. nebrascenis had a positiv e effect on the forb-feeding M. femur-rubrum on unfertilized plots. Ov erall, we show that direct effects of consumers on resources can predi ct competition and other important indirect interactions within a comm unity.