Pit relocation by antlion larvae: A simple model and laboratory test

We generated a computer model to analyse the effects of 'shadow' competition for sit-and-wait predators, particularly antlion larvae. The model used a simple foraging assessment rule to determine the quality of an antlion's location, and antlions relocated randomly in their habitat when a location proved to be of low quality. Shadow competition, or competition for food caused when one sit-and-wait predator intercepts moving prey before a second sit-and-wait predator is encountered, was incorporated into the model by restricting antlions to a bounded arena, and having prey for the antlions enter from the arena periphery. Antlions responded to shadow competition by relocating their pits to peripheral areas of their habitat. This peripheral accumulation of pits was most pronounced when antlion densities were high, and when prey availabilities were intermediate. An experimental test with the antlion Myrmeleon immaculatus supported the importance of shadow competition as a cause of observed pit distributions. Only the treatment which incorporated shadowing had pit distributions near the periphery, while the pit distributions in the control treatments did not differ from randomly generated distributions. We conclude that shadowing can influence sit-and-wait predator distributions when the prey distributions and movement patterns generate the conditions necessary for shadowing. But when prey availability is unpredictable, making assessment of patches difficult, or when prey do not originate in the periphery of the habitat, other factors, such as temperature or moisture, could be more important.