Effects of added nutrients, plant biomass, plant species richness, and plant quality on Orthoptera abundance and distribution across a small grassland area

Background/Question/Methods

Grassland plant community composition and structure can vary widely along environmental gradients over a relatively small area. Typically these plant community differences can have a direct effect on insect herbivore abundance and species diversity. Previous studies have shown that vegetation structure can have a large influence on habitat selection by grasshoppers (Orthoptera); however it is not clear which vegetation characteristics are most important in determining grasshopper abundance.  Thus we explored how grasshoppers responded to variations in plant biomass, plant tissue quality, and plant species richness on a mixed-grass prairie site at the University of Nebraska’s Cedar Point Biological Station over the summer of 2008. A three factorial experiment with nutrient additions of nitrogen, phosphorus, and potassium with 6 replicates of each treatment in 5m x 5m plots was set up. We hypothesized that increased soil nutrient levels, which leads to higher plant productivity and increased plant quality, may lead to increases in grasshopper abundance. In addition, plant species diversity, may also lead to increased grasshopper abundance. Grasshopper counts were conducted weekly in each plot during June and July.  We measured aboveground plant biomass, conducted cover estimates, and analyzed soil and biomass samples for percent nitrogen and percent carbon.

Results/Conclusions

Nitrogen addition within the first year did not have a significant impact on plant biomass or plant species richness, but nitrogen did increase graminoid tissue quality. Existing variations in plant biomass and the number of plant species present were shown to be of particular importance to grasshopper abundance and distribution. Graminoid biomass was found to be highly significant in explaining grasshopper abundance among certain plots (R²=0.333, P<0.001). Overall, plant species diversity was also found to be significant in explaining grasshopper abundance (R²=0.395, P<0.020).  However, graminoid tissue quality was not significantly related to grasshopper abundance (P>0.18).  These findings provide evidence that grasshoppers specifically seek out areas of higher graminoid biomass and likely mix their diets with a few forbs and other plants. The tissue quality of the plants in this case is not important to grasshopper habitat selection. We also found that throughout the season, grasshoppers respond to different aspects of the vegetation. Early in the season (early June) it was found that grasshoppers are randomly distributed, mid-season they select areas of higher plant diversity, and late-season (late July) they select areas with higher graminoid biomass.  Thus, grasshoppers tend to prefer different vegetation aspects during different life stages.