Multiple predator species feeding on a common prey can lead to higher or lower predation than would be expected by simply combining their individual effects. Such emergent multiple predator effects may be especially prevalent if predators share feeding habitat. Despite the prevalence of endophagous insects, no studies have examined how multiple predators sharing an endophytic habitat affect prey or predator reproduction. We investigated density-dependent predation of Thanasimus dubius (Coleoptera: Cleridae) and Platysoma cylindrica (Coleoptera: Histeridae) on a bark beetle prey, Ips pini (Coleoptera: Scolytidae), in a laboratory assay. I. pini utilize aggregation pheromones to group-colonize and reproduce within the stems of conifers. T. dubius and P. cylindrica exploit these aggregation pheromones to arrive simultaneously with the herbivore. Adult T. dubius prey exophytically, while P. cylindrica adults enter and prey within the bark beetle galleries. Larvae of both predators prey endophytically. We used a multiple regression analysis, which avoids confounding predator composition with density, to examine the effects of varying predator densities alone and in combination on herbivore establishment, herbivore reproduction, and predator reproduction. Predators reduced colonization success by both sexes, and decreased I. pini reproduction on a per male and per female basis. The combined effects of these predators did not enhance or reduce prey establishment or reproduction in unexpected manners, and these predators were entirely substitutable. The herbivore's net replacement rate was never reduced significantly below one at prey and predator densities emulating field conditions. Similar numbers of each predator species emerged from the logs, but predator reproduction suffered from high intraspecific interference. The net replacement rate of P. cylindrica was not affected by conspecifics or T. dubius. In contrast, the net replacement rate of T. dubius decreased with the presence of conspecifics or P. cylindrica. Combinations of both predators led to an emergent effect, a slightly increased net replacement rate of T. dubius. This may have been due to predation by larval T. dubius on pupal P. cylindrica, as P. cylindrica develops more rapidly than T. dubius within this shared habitat.
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Acknowledgements We thank the Wisconsin Department of Natural Resources for providing the sites for tree and insect collection. Field and laboratory assistance by B. Burwitz and J. Ludden is greatly appreciated. This study was supported by the US Department of Agriculture USDA NRI AMD 96 04317, USDA CSREES 2001–35302–10952, the Wisconsin Department of Natural Resources, the University of Wisconsin-Madison College of Agricultural and Life Sciences, S.C. Johnson and Son Inc., and an Elsa and Louis Thomsen Wisconsin Distinguished Fellowship awarded to B.H.A. We thank J. Reeve, Department of Zoology, Southern Illinois University for helpful discussions on T. dubius biology. This manuscript was greatly improved by the comments of C. Gratton, A. Ives, R. Lindroth, J. Handelsman, J. Cronin, and two anonymous reviewers.
- Density dependence
- Linear effects
- Multiple predator effects
- Substitutable effects