COS 26-6 - Nutrient niche hypothesis: Investigating correlations between plant quality and generalist grasshopper communities

Tuesday, August 9, 2011: 9:50 AM
12B, Austin Convention Center
Paul A. Lenhart, Department of Entomology, Texas A&M University, Spencer T. Behmer, Departement of Entomology, Texas A&M University, College Station, TX and Micky D. Eubanks, Entomology, Texas A&M University, College Station, TX

While coexistence of specialized herbivores can be explained by differing host plant use, the diversity of generalist herbivore communities requires other mechanisms of resource partitioning. Resource partitioning may occur at the nutrient rather than host plant level, whereby different herbivores require different ratios of important macronutrients (i.e. proteins, carbohydrates) and feed on differing ratios of various plant parts and species. In grasslands, grasshoppers are the dominant invertebrate herbivores with communities that are usually diverse and contain many generalist species existing in close proximity with broadly overlapping diets. The existence of differing required macronutrient ratios has been verified with coexisting species of Melanoplus and may follow similar patterns in entire assemblage of grasshoppers. To determine to what extent grasshopper communities correlate to plant nutrient content I surveyed the plant and grasshopper community at 16 sites in the Balcones Canyonlands NWR from June to September in 2009 and 2010.


By assaying the levels of digestible protein and carbohydrate in abundant forbs and grasses at each site I was able to produce a graphical ‘nutrient landscape’ available to a foraging herbivore. This nutrient landscape appears to be variable both in time and space. As the drought of 2009 progressed, the range of protein and carbohydrate contents decreased. The mean and range of protein and carbohydrate content were significantly correlated to grasshopper density, but not grasshopper species richness. Further use of this macronutrient approach to plant quality may provide important insights into herbivore population dynamics and community assembly rules.

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