Relative importance of plant communities and nutrient concentrations in regulating prairie herbivore communities

Background/Question/Methods

Efforts to manage grasslands in general, and coastal tallgrass prairie in particular, often focus on managing for native plant diversity, assuming that a diverse plant community will support higher trophic levels that are often conservation targets. This assumption is based upon a prevailing hypothesis in ecology that suggests herbivores should be more diverse and abundant where plants are more diverse and abundant; however, this prevailing hypothesis has led ecologists to overlook other factors that are potentially important to herbivore communities, such as micronutrient concentrations. We used a natural experiment to examine the relative importance of factors affecting herbivore community structure by measuring plant and herbivore biomass and diversity and plant, litter and soil macro- and micro-nutrients across a range of human influence in a coastal tallgrass prairie south of Houston.

Results/Conclusions

These data show that plant community attributes alone do not adequately predict density or richness of herbivores, but that plant micro-nutrients (specifically calcium) are important in mediating herbivore community structure.  Most strikingly, areas with low plant richness and biomass due to high amounts of calcium in soil have grasshopper diversity equal to areas with high plant species richness and biomass. Surprisingly, these areas with low plant richness and biomass had higher grasshopper abundance than areas with high plant richness and biomass. These results suggest that herbivore communities are not regulated solely by plant diversity and abundance, and therefore, managing for grassland plant diversity may not be sufficient to achieve high diversity and density of higher prairie trophic levels.