Density-Dependent Effects of Multiple Predators Sharing a Common Prey in an Endophytic Habitat
Total Page：16
File Type：pdf, Size：1020Kb
Oecologia (2004) 139: 418–426 DOI 10.1007/s00442-004-1511-9 COMMUNITY ECOLOGY Brian H. Aukema . Murray K. Clayton . Kenneth F. Raffa Density-dependent effects of multiple predators sharing a common prey in an endophytic habitat Received: 13 June 2003 / Accepted: 15 January 2004 / Published online: 13 February 2004 # Springer-Verlag 2004 Abstract Multiple predator species feeding on a common female basis. The combined effects of these predators did prey can lead to higher or lower predation than would be not enhance or reduce prey establishment or reproduction expected by simply combining their individual effects. in unexpected manners, and these predators were entirely Such emergent multiple predator effects may be especially substitutable. The herbivore’s net replacement rate was prevalent if predators share feeding habitat. Despite the never reduced significantly below one at prey and predator prevalence of endophagous insects, no studies have densities emulating field conditions. Similar numbers of examined how multiple predators sharing an endophytic each predator species emerged from the logs, but predator habitat affect prey or predator reproduction. We investi- reproduction suffered from high intraspecific interference. gated density-dependent predation of Thanasimus dubius The net replacement rate of P. cylindrica was not affected (Coleoptera: Cleridae) and Platysoma cylindrica (Coleop- by conspecifics or T. dubius. In contrast, the net tera: Histeridae) on a bark beetle prey, Ips pini (Coleop- replacement rate of T. dubius decreased with the presence tera: Scolytidae), in a laboratory assay. I. pini utilize of conspecifics or P. cylindrica. Combinations of both aggregation pheromones to group-colonize and reproduce predators led to an emergent effect, a slightly increased net within the stems of conifers. T. dubius and P. cylindrica replacement rate of T. dubius. This may have been due to exploit these aggregation pheromones to arrive simulta- predation by larval T. dubius on pupal P. cylindrica,asP. neously with the herbivore. Adult T. dubius prey cylindrica develops more rapidly than T. dubius within this exophytically, while P. cylindrica adults enter and prey shared habitat. within the bark beetle galleries. Larvae of both predators prey endophytically. We used a multiple regression Keywords Multiple predator effects . Endophagous . analysis, which avoids confounding predator composition Density dependence . Linear effects . Substitutable effects with density, to examine the effects of varying predator densities alone and in combination on herbivore establish- ment, herbivore reproduction, and predator reproduction. Introduction Predators reduced colonization success by both sexes, and decreased I. pini reproduction on a per male and per Most studies of predator-prey interactions have explored single paired relationships (Kareiva 1994). However, there B. H. Aukema (*) . K. F. Raffa is increasing emphasis on understanding multiple predator Department of Entomology, University of Wisconsin, effects that more closely emulate natural systems (Polis 345 Russell Laboratories, 1630 Linden Drive, and Strong 1996; Relyea and Yurewicz 2002). Studies of Madison, WI 53706, USA multiple predators on shared prey have identified “emer- e-mail: [email protected] ” Tel.: +1-608-2624755 gent properties that challenge strict notions of top-down Fax: +1-608-2623322 trophic cascades erected by collapsing different predators into single functional units (Polis and Strong 1996). B. H. Aukema Emergent multiple predator effects arise when the realized Biometry Program, University of Wisconsin (Madison), 345 Russell Laboratories, 1630 Linden Drive, effects of two predators deviate from expected combina- Madison, WI 53706, USA tions of their individual impacts (Sih et al. 1998). e-mail: [email protected] Predicting such outcomes is especially important for biological control, where an introduced predator’sor M. K. Clayton ’ Department of Statistics, University of Wisconsin (Madison), parasitoid s interactions with other natural enemies could 1210 West Dayton Street, inadvertently increase herbivore populations (Rosenheim Madison, WI 53706, USA 1998; Fagan et al. 2002; Van Lenteren et al. 2003). 419 Emergent effects may be “risk enhancing” to the prey if Materials and methods predators kill more prey than expected together than by themselves (Losey and Denno 1998; Swisher et al. 1998), Study animals or “risk reducing” if predators kill fewer prey than expected (Spiller 1986; Rosenheim et al. 1993). Emergent Ips pini (Say) reproduces within the subcortical tissues of stressed and recently dead pine trees across North America (Thomas 1955). multiple predator effects may be trait- or density-mediated. Male beetles chew through the bark, and construct “nuptial” Trait-mediated effects may occur when prey exhibit a chambers in which they are joined by females. While boring, males foraging strategy or defensive behavior in the presence of emit a pheromone blend that attracts both genders (Wood 1982) and one predator that predisposes predation by another pred- varies transcontinentally (Seybold et al. 1995; Miller et al. 1997). Male I. pini typically mate with two or three females (Schenk and ator species (Swisher et al. 1998; Okuyama 2002; Relyea Benjamin 1969; Kirkendall 1983). Final settlement densities average and Yurewicz 2002). Density-mediated effects may arise 1 male and 2 females/dm2 of bark surface (Schenk and Benjamin through intraguild predation among the predators, as their 1969; Poland and Borden 1994; Robins and Reid 1997). Each satiation may reduce threats to prey (Polis et al. 1989; female constructs one ovipositional gallery radiating from the nuptial chamber, and oviposits in niches that she chews along these Rosenheim et al. 1999; Snyder and Ives 2001). galleries and subsequently packs with frass. Larvae feed and tunnel Many factors influence trait- and density-mediated throughout the phloem tissue before pupating in oval chambers. interactions and the potential for emergent multiple Adults emerge to seek new hosts. Development requires approxi- predator effects, such as body size, growth rates, repro- mately 30 days, and adults can only live for a few days outside host ’ ductive phenology, feeding breadth, and the degree of trees. Thus almost all of I. pini s life cycle (except during overwintering in soil) occurs within an endophytic habitat. overlap of predator habitats (Polis et al. 1989; Sih et al. Two beetles, Thanasimus dubius (F.) (Cleridae), and Platysoma 1998). To date, however, predators that share endophytic cylindrica (Paykull) (Histeridae) are the numerically dominant habitats have never been studied. All multiple-predator predators attacking I. pini. Together, they represent approximately studies on terrestrial arthropods have involved exophytic 79% of all predators sampled in funnel traps arriving to trees being colonized by this herbivore (Aukema et al. 2000b). Both are highly herbivores, despite the ubiquity of endophytic insects effective at exploiting the aggregation pheromones of I. pini to across insect and plant taxa, geographic regions, and locate prey (Raffa and Klepzig 1989; Herms et al. 1991; Aukema et biomes (Cornell et al. 1998). An endophytic lifestyle can al. 2000a, 2000b; Ayres et al. 2001). For example, T. dubius has provide substantial protection from natural enemies been shown to be four times more attracted to I. pini colonizing red pine, Pinus resinosa Aiton, than is this herbivore to itself (Aukema relative to external feeding (Hawkins et al. 1997; Cornell et al. 2000b). Adult T. dubius prey on adult I. pini on the bark et al. 1998), although many specialist predators and surface, mate, and oviposit in bark crevices. Larvae enter the parasitoids exploit host plant compounds and herbivore galleries and prey on the developing I. pini brood (Thatcher and pheromones to effectively locate prey, including endo- Pickard 1966; Aukema and Raffa 2002). Adult P. cylindrica average phages (Vet and Dicke 1992). Once inside the confines of 1.2 mm in width and 4.5 mm in length, approximately the size of I. pini, and are heavily sclerotized. They enter bark beetle chambers the plant however, both predators and prey may suffer and use the gallery architecture to confine adult beetles, which they high density-dependent mortality (Aukema and Raffa crush and consume (Aukema and Raffa 2004). The impact of P. 2002). The effects of additional predator species on cylindrica on I. pini has never been quantified. However, behavioral these relationships are unknown. observations in the laboratory, the high number arriving to pheromones of I. pini, and its numerical association with I. pini We used a multivariate regression approach to examine indicate that it is a potentially effective predator (Erbilgin et al. the density-dependent effects of two predators by 2002; Aukema and Raffa 2004). Both T. dubius and P. cylindrica are themselves and in combination on an endophytic bark habitat specialists in that they feed almost exclusively within trees beetle (Coleoptera: Scolytidae) prey. This approach has colonized by bark beetles, but are feeding generalists in that they feed on most primary and secondary species within this community certain advantages over conventional 2×2 factorial designs (Savely 1939; Thomas 1955). that use the presence/absence of two predators (i.e., 1 We obtained I. pini from a laboratory culture raised on white pine, control, 1 A, 1 B, 1 A + 1 B). Multivariate regression