Dangerous neighbors: interactive effects of factors influencing cannibalism in pit-building antlion larvae

Cannibalism is a common trophic interaction in size-structured populations, having important ramifications for population and community dynamics. Although the main factors potentially influencing cannibalism are apparently well investigated, relatively little is known on how these factors interact to influence the frequency of cannibalism. We performed 2 complementary experiments to test if cannibalism in larvae of the pit-building antlion Myrmeleon hyalinus is influenced by size differences within and between developmental stages, as well as by sand depth (potential refuge), conspecific density, and hunger level. Our working hypothesis was that cannibalism should act as a threshold event governed by the interactive (rather than additive) effects of intrinsic and extrinsic factors. As expected, the frequency of cannibalism was higher between than within developmental stages, with more advanced instar larvae feeding on smaller instars. Notably, decreased density significantly lowered the frequency of cannibalism; however, this pattern was evident only in deep sand, which provides a potential refuge from predators. Deep sand also lessened the positive effect of body size difference on the frequency of cannibalism. Hunger level increased the frequency of cannibalism; however, the identity of the cannibal was almost exclusively determined by its initial size advantage rather than by its hunger level. Our results demonstrate that, despite their sedentary nature, the frequency of cannibalism in pit-building antlion larvae can be relatively high. Furthermore, this trophic interaction is governed by the interactive rather than additive effects of intrinsic and extrinsic factors influencing cannibalism.