Coexisting generalist herbivores occupy unique nutritional feeding niches

Abstract

A mainstay of ecological theory and practice is that coexisting species use different resources, leading to the local development of biodiversity. However, a problem arises for understanding coexistence of multiple species if they share critical resources too generally. Here, we employ an experimental framework grounded in nutritional physiology to show that closely related, cooccurring and generalist-feeding herbivores (seven grasshopper species in the genus Melanoplus; Orthoptera: Acrididae) eat protein and carbohydrate in different absolute amounts and ratios even if they eat the same plant taxa. The existence of species-specific nutritional niches provides a cryptic mechanism that helps explain how generalist herbivores with broadly overlapping diets might coexist. We also show that performance by grasshoppers allowed to mix their diets and thus regulate their protein-carbohydrate intake matched optimal performance peaks generated from no-choice treatments. These results indicate the active nature of diet selection to achieve balanced diets and provide buffering capacity in the face of variable food quality. Our empirical findings and experimental approach can be extended to generate and test predictions concerning the intensity of biotic interactions between species, the relative abundance of species, yearly fluctuations in population size, and the nature of interactions with natural enemies in tritrophic niche space.

Fig. 1.

Protein–carbohydrate intake targets (means ± SEM) for each of the seven coexisting species. Each number in the figure corresponds to a particular species, and species that are statistically different from one another have different colored error bars [only species 1 and 7 (M. angustipennis and M. sanguinipes) were statistically similar; for details, see Table 1]. The gray dashed lines represent the two most extreme foods, p7:c35 and p35:c7, and the area between them represents the available nutrient space. The different colored solid lines define the range of nutrient space occupied by a particular species (as determined by the SEM of the intake targets). Overlap in nutrient space is only obvious for species 1 and 7. For a description of how nutrient space was determined for each species, see the Materials and Methods.

Fig. 2.

Development time and growth rate for each of the seven coexisting species on treatments with single diets (filled circles) and on the mixed diets (open circles). Development time was measured as the time from the beginning of the final stadium until molting to the adult stage; growth rate was measured as mass gain (in milligrams) in the final stadium divided by development time in the final stadium. In total, there were five single diets that differed in their protein:carbohydrate ratio. The labels on the x axis are the percentage dry mass of protein and carbohydrate, respectively, in each diet. Total macronutrient content for the five single-diet treatments was 42%. For each panel, a best-fit curve is plotted to the data. In the case of M. flavidus developmental time, the curve is only fit through four data points because inclusion of the p7:c35 diet gives an exaggerated optimal diet point. In most instances, development time and growth rate were best on the mixed-diet treatment.