No Difference in Herbivory Preferences Among Ecotypes of Big Bluestem (Andropogon gerardii)

Big bluestem (Andropogon gerardii), a dominant prairie grass, exhibits a wide distribution with several genetically distinct ecotypes. Each ecotype adapts to abiotic and biotic factors within its environment. These adaptations may prove more or less desirable to herbivores in the area. For instance, plants adapted to areas with greater rainfall tend to grow larger than those adapted to drier conditions. Wetter conditions might result in greater herbivore pressure, but according to the Resource Availability Hypothesis, individual plants tend to invest little in herbivore defenses when resources are abundant. Consequently, we hypothesized A. gerardii ecotypes adapted to wetter conditions might be more susceptible to herbivory and that ecotypes adapted to drier conditions might have evolved greater herbivore defense mechanisms. We tested this with feeding preference trials involving grasshoppers and leaves from five ecotypes of A. gerardii that represented plants adapted to wet and dry conditions. Scans of leaves before and after herbivory trials indicated 43 to 78 percent of leaf area remained, with no difference among ecotypes regarding which were favored by herbivores. We also hypothesized that leaves with more tannins would be less preferable to herbivores. This was tested by measuring leaf tannin concentration from five ecotypes of A. gerardii from four common gardens across a precipitation gradient. Leaf tannin concentrations ranged from 0.8 to 2.4% of leaf dry mass and were different among both sites and ecotypes. Site differences caused the biggest difference in leaf tannins, indicating a strong environmental influence on leaf tannin concentration. There was no correlation between tannin concentration and herbivory preference among ecotypes, indicating other factors are related to herbivore defense in A. gerardii.