Density-dependent effects of multiple predators sharing a common prey in an endophytic habitat

Abstract

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.

Phenotypically plastic responses of green frog embryos to conflicting predation risk

“…Responses by prey exposed simultaneously to multiple predators are often predicted poorly by the sum of pair-wise predator-prey interactions (e.g., Sih et al 1998;Relyea 2003;Aukema et al 2004;Teplitsky et al 2004), implying that the results of experiments utilizing single-predator environments may not always be applicable to natural situations. Here, however, we demonstrate that the independent and opposite eVects of two predators on prey phenotype can dampen one another: the directional outcomes of pair-wise interactions between R. clamitans embryos and two stage-speciWc predators were not observed when embryos were exposed to cues from both predators simultaneously.…”

Does aggregation benefit bark beetles by diluting predation? Links between a group‐colonisation strategy and the absence of emergent multiple predator effects

“…Like T. dubius , P. cylindrica larvae feed on I. pini and other species present. A previous laboratory study found some evidence of intraguild predation of T. dubius larvae on P. cylindrica pupae at high predator densities, but this experiment was conducted at a single, moderate I. pini density (Aukema et al ., 2004a).…”

“…These densities reflect natural colonisation patterns of 0–2.5 males per dm 2 (Schenk & Benjamin, 1969; Poland & Borden, 1994; Robins & Reid, 1997). Two females per male were added overall, with the sex ratio being 3:2 on day 1 and 1:6 on day 2, to mimic natural arrival patterns (Aukema et al ., 2004a). There were 10 logs per series, with two logs each of four and eight males, and one log each of 12, 16, 20, 24, 28, and 32 males.…”

Can chemical communication be cryptic? Adaptations by herbivores to natural enemies exploiting prey semiochemistry

“…1). The positioning of the preferences of I. pini midway between the preferences of the two major predators may arise from equal and opposing pressures they exert, i.e., they occur in approximately equal numbers (Table 1), show equivalent numerical relationships with I. pini populations in nature , and have equivalent and noninteracting per-predator eVects on I. pini reproduction within hosts (Aukema et al 2004). Incorporating a synergist into their pheromone plume gives I. pini further opportunity to communicate with only moderate or with no additional eavesdropping by predators.…”

“…Analyses of Weld populations indicate strong density-dependent relationships , and laboratory assays indicate strong eVects on beetle survival and reproduction . Further, each predator species exerts equivalent eVects on beetle reproduction, and there are no nonadditive interference or augmentation eVects (Aukema et al 2004). These predators can be considered habitat generalists, in that they develop exclusively within trees killed by bark beetles, but feed on several species of primary colonizers and secondary opportunists in such trees .…”