American Society of Naturalists

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“Factors that can affect the spatial positioning of large and small individuals in clusters of sit-and-wait predators”

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Inon Scharf (Apr 2020)

The spatial distribution of large and small predators living in clusters is not random but depends on various factors

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What are the best positions in a cluster of sit-and-wait predators and are small or large predators better at reaching such positions?

The two photos and the drawing present a cluster of the pit-building antlion <i>Euroleon nostras</i> below a balcony in the city of Mainz, Germany. Although smaller pits seem to be more common next to the cluster periphery, reaching a firm conclusion requires a more thorough analysis.<br />(Credit: Inon Scharf)
The two photos and the drawing present a cluster of the pit-building antlion Euroleon nostras below a balcony in the city of Mainz, Germany. Although smaller pits seem to be more common next to the cluster periphery, reaching a firm conclusion requires a more thorough analysis.
(Credit: Inon Scharf)

Not all predators chase after their prey but instead simply choose an ambush site and then sit and wait for prey to enter their detection range. Such predators usually live in clusters and are restricted to the type of area that provides the best conditions for ambush. In many of these clusters, the best positions are in the periphery, because prey arrive there first. Those predators in the central positions, therefore, catch less prey, as most prey are already intercepted by the predators in peripheral positions. Consequently, whether small or large, all predators should prefer peripheral locations in the cluster. It is common, nonetheless, to observe a difference in the positions within a cluster of small and large predators. A simulation model presented the spatial arrangement of such sit-and-wait predators. Small predators only occupied the cluster periphery under certain conditions, such as high prey abundance or low predator density. The reason is that such conditions moderate their need to frequently relocate, as too frequent relocation generally pushes the small predators away from the favorable peripheral positions. Large predators, in contrast, do not relocate so frequently, even when they are located in the cluster’s center, because they are able in any case to capture more prey than small predators. Moreover, any condition that triggers more relocation by large predators will bring them closer to the periphery. The model provides a possible mechanism for the distinct positions within the cluster of small and large predators. The model should be easily testable in systems of sit-and-wait predators, such as pit-building antlion larvae, which construct pit-traps in sand and ambush ants.


Abstract

Shadow competition, the interception of prey by sit-and-wait predators closest to the source of prey arrival, is prevalent in clusters of sit-and-wait predators. Peripheral positions in the cluster receive more prey and should thus be more frequently occupied. Models predicting spatial positioning in groups, however, usually ignore variability among group members. Here, I used a simulation model to determine conditions under which small and large sit-and-wait predators, which differ in their attack range, should differ in their spatial positions in the cluster. Small predators occupied peripheral positions more frequently than large predators at the simulation beginning, while the opposite held true as time advanced. Due to the large and small attack range of large and small predators respectively, small predators mistakenly relocated away from peripheral positions, while large predators did not relocate fast enough from inferior central positions. Any factor that moderated the frequent relocations of small predators or had the opposite effect on large predators assisted small or large predators respectively to reach the more profitable peripheral positions. Furthermore, any factor elevating shadow competition led to longer occupation of the periphery by large predators. This model may explain why sit-and-wait predators are not homogenously distributed in space according to size.