Nutrient Niches: an Investigation of Nutritional Ecology in a Generalist Herbivore Community



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Understanding how diversity is maintained is a classic question in ecology. A diverse group of organisms can often be found utilizing the same resource. For example, in grasslands there are communities of grasshoppers containing many generalist species with overlapping diets that are likely competing for resources. To explore how species that overlap in host plant use can coexist, I investigated a recent hypothesis in nutritional ecology that species-specific macronutrient requirements in generalist insect herbivores could represent different nutrient niches. As a model system I used a community of grasshoppers in Central Texas.

First, I surveyed variation in plant macronutrient content and compared this data to the grasshopper community. By assaying levels of digestible protein and carbohydrate in abundant forbs and grasses at different sites, I produced a ?nutrient landscape? available to foraging herbivores and found significant correlations between plant nutrients and grasshopper abundance.

To further explore the role of plant macronutrient shifts in controlling grasshopper populations, I manipulated water availability in plots of grassland during a severe drought. Total grasshopper density and diversity were lower in water-stressed plots despite previous observations of drought-induced outbreaks. The effect of water stressed plants on grasshoppers depended on their diet, and how different plant groups responded to water stress.

I then compared host plant use to macronutrient requirements among 11 dominant grasshopper species. I found differences associated with functional diet groupings. I also found intake differences among mixed-feeders with highly overlapping diets, which could potentially represent nutrient niches.

Finally, I tested the nutrient niche hypothesis in a greenhouse competition experiment using three species of generalist grasshoppers with overlapping diets. I found mixed support for the nutrient niche hypothesis. Body size was more important for predicting competitive outcomes.

Understanding community-wide patterns of nutrient regulation in insect herbivores is in its infancy. While the plant nutrient landscape plays a large role in consumer populations, we are far from understanding how species-specific nutrient regulation differences might impact communities. Perhaps the potential effects of nutrient intake differences are inconsequential next to other ecological factors. Future comparative studies should determine what evolutionary factors shape nutrient requirements.