Complex interactions between components of individual prey specialization affect mechanisms of niche variation in a grasshopper-hunting wasp.

1. Individual foraging behaviour defines the use of resources by a given population and its variation in different ways such as, for example, unpredictable interactions between taxon-biased and size-biased selection. Here we investigated how the environmental availability of prey and individual specialization, for both prey taxa and prey size, shape niche variation across generations in the grasshopper-hunting digger wasp Stizus continuus. 2. The population of S. continuus expressed selective predation, females mainly hunting species encountered on large bushes; diet changed across generations, due more to size increase in potential prey than to changes in the orthopteran community. 3. Individual females of both generations weakly overlapped the size and taxa of prey, and the niche width of the second generation increased for both prey size and taxa. 4. The greater variance in prey size in the environment accounted for the enlarged prey size niche of the second generation, but the load-lifting constraints of the wasps maintained individual prey size specialization constant. In contrast, the enlarged prey taxon niche paralleled a smaller overlap of diets between wasps in the second generation. 5. Increased niche width in the S. continuus population was thus achieved in two ways. Regarding prey size, all individuals shifted towards the use of the full set of available resources (parallel release). For prey taxa, according to the classical niche variation hypothesis, individuals diverged to minimize resource use overlap and perhaps intraspecific competition. These two mechanisms were observed for the first time simultaneously in a single predator population.