Applying Optimal Foraging Theory to Assess Nutrient Availability Ratios for Ants

Although the availability of food resources affects a wide range of ecological and evolutionary processes, its role in generating patterns in natural systems remains largely unresolved. Elucidating the importance of resource availability should be made easier by the development of techniques that can measure the amount of resources that are accessible to a consumer, rather than simply the abundance of resources in the environment. In this study, I develop a behavioral assay for inferring the ratio of nutrients available to organisms in the field. The assay involves measuring the responses of foragers to nutrient solutions. These responses are used to determine the mean effective concentration, or ED50, of a nutrient, defined as the concentration that is rejected in 50% of trials. Because ED50 provides a measure of the quality of a depletable patch at which a consumer would quit foraging, it is analogous to the giving-up density (GUD), a common tool in ecological research. Here, I use ED50s to estimate the ratio of carbohydrates : protein available to ant colonies. Using supplementation experiments, I show that ED50s of sucrose and casein (a source of amino acids) reveal the net rates at which a colony expects to acquire each resource elsewhere in the environment. These results demonstrate that differences in ED50s will reveal differences in the availabilities of nutrients when other factors affecting the costs and benefits of foraging are controlled. To control these factors, I measured a colony's ED50s of sucrose and casein at the same time and location. Using this technique, I found that ratios of carbohydrate : protein availability differed significantly among 10 species of ants in southeastern Arizona, USA. Ratios were highest for species that are known to collect liquids from extrafloral nectaries and phloem-feeding insects. Carbohydrate : protein ratios may be especially low for three granivorous species, Pogonomyrmex barbatus, Pogonomyrmex occidentalis, and Aphaenogaster cockerelli, which accepted sucrose solutions but rejected even highly concentrated casein solutions. Because ratios of carbohydrate : protein availability differed substantially among species in the same habitat, the results of this study suggest that measures of resource abundance may poorly predict habitat quality for consumers.