vol. 170, no. 6 the american naturalist december 2007 ൴ Spatial Dynamics of Communities with Intraguild Predation: The Role of Dispersal Strategies Priyanga Amarasekare* Department of Ecology and Evolutionary Biology, University of 2004, 2005). This interplay is well established for com- California, Los Angeles, California 90095 munities with one or two trophic levels (e.g., resource, consumer; Levin 1974; Holt 1985; Murdoch et al. 1992; Submitted November 1, 2006; Accepted July 11, 2007; Electronically published October 16, 2007 Amarasekare and Nisbet 2001; Jansen 2001; Abrams and Wilson 2004) but not for the more common situation of Online enhancements: appendixes. communities with multiple trophic levels (e.g., resource, consumer, natural enemy). Multitrophic communities present quite a challenge for spatial ecology. Studies of spatial coexistence typically fo- abstract: I investigate the influence of dispersal strategies on in- cus on one type of species interaction (nontrophic or pair- traguild prey and predators (competing species that prey on each wise trophic interactions) and situations where species other). I find an asymmetry between the intraguild prey and predator cannot coexist in the absence of dispersal (e.g., competitive in their responses to each other’s dispersal. The intraguild predator’s dominance, predator overexploitation, Allee effects in- dispersal strategy and dispersal behavior have strong effects on the duced by the absence of a mutualistic partner). Dispersal intraguild prey’s abundance pattern, but the intraguild prey’s dis- can allow coexistence, given spatial variation in the persal strategy and behavior have little or no effect on the intraguild predator’s abundance pattern. This asymmetry arises from the dif- strength of species interactions (Levin 1974; Holt 1985; ferent constraints faced by the two species: the intraguild prey has Amarasekare and Nisbet 2001; Codeco and Grover 2001; to acquire resources while avoiding predation, but the intraguild Amarasekare 2004; Leibold et al. 2004). Multitrophic com- predator only has to acquire resources. It leads to puzzling distri- munities do not fit this mold, and not merely because they bution patterns: when the intraguild prey and predator both move contain both trophic and nontrophic interactions. It is away from areas of high density, they become aggregated to high- because they involve situations where species within a density habitats, but when they both move toward areas of high resource productivity, they become segregated to resource-poor and trophic level can coexist in the absence of dispersal, but resource-rich habitats. Aggregation is more likely when dispersal is the operation of such coexistence mechanisms varies over random or less optimal, and segregation is more likely as dispersal space and time. The influence of dispersal on diversity is becomes more optimal. The crucial implication is that trophic con- therefore fundamentally different. There is now the po- straints dictate the fitness benefits of using dispersal strategies to tential for simultaneous operation of local and spatial co- sample environmental heterogeneity. A strategy that affords greater existence mechanisms, and for emergent properties to arise benefits to an intraguild predator can lead to a more optimal outcome for both the intraguild predator and prey than a strategy that affords from the interaction between the two classes of mech- greater benefits to an intraguild prey. anisms. Two examples of natural multitrophic interactions serve Keywords: competition, dispersal strategies, intraguild predation, life- to illustrate these differences. Intraguild predation (IGP) history trade-offs, multitrophic communities, productivity. occurs when species that compete for a common resource also prey on or parasitize one another (e.g., Polis et al. 1989; Arim and Marquet 2004); selective predation (SP) It is widely appreciated that diversity maintenance in spa- occurs when species that compete for a common resource tially structured environments results from the interplay also share a natural enemy (e.g., Sih et al. 1985; Navarrete between species interactions and dispersal (Leibold et al. and Menge 1996). In both cases, the two consumer species * E-mail: [email protected]. can coexist via a trade-off that allows for resource parti- Am. Nat. 2007. Vol. 170, pp. 819–831. ᭧ 2007 by The University of Chicago. tioning. In IGP, the trade-off is such that the inferior re- 0003-0147/2007/17006-42191$15.00. All rights reserved. source competitor gains a second resource by preying on DOI: 10.1086/522837 its competitor; in SP, the inferior competitor gains more 820 The American Naturalist of the common resource by being less susceptible to the inhabited by invertebrate communities (Chase and Leibold predator. A key feature of these trade-offs is that their 2002; Chase 2003; Chase and Ryberg 2004). I consider expression depends on species occupying other trophic spatial heterogeneity in habitat quality to be permanent, levels within the community. In IGP, it is the common as would be the case with differences in soil, nutrient resource; in SP, it is the common resource and/or natural availability, or moisture content that make some ponds or enemy. In the absence of dispersal or other ameliorating host plant patches more productive than others. The spa- factors, spatial variation in resource productivity or pred- tial scale on which such heterogeneity occurs is within the ator mortality can eliminate the trade-off and cause ex- dispersal ranges of the organisms that occupy these hab- clusion of the species that has the overall disadvantage. itats. For instance, a set of ponds (host plant patches) in For instance, when resource productivity is low (predator a given area may vary in quality but occur in sufficiently mortality is high), exploitative competition dominates, and close proximity to allow dispersal between ponds (host the inferior resource competitor is excluded; when re- plant patches). source productivity is high (predator mortality is low), Each habitat patch supports a local community with predation dominates, and the species more susceptible to unidirectional IGP: two species compete for the same lim- predation is excluded (Holt and Polis 1997; Diehl and iting resource, but one species (“IGPredator”) can prey on Feissel 2000; Noonburg and Abrams 2005). Thus, the or parasitize its competitor (“IGPrey”). Unidirectional IGP trade-off between competition and predation operates commonly occurs in aquatic invertebrates (Wissinger et only at intermediate productivity/mortality levels. This il- al. 1996; MacNeil et al. 2004) and insect parasitoids (Zwol- lustrates another feature that distinguishes multitrophic fer 1971; Polis et al. 1989; Amarasekare 2000, 2003; Arim interactions. In nontrophic or pairwise trophic interac- and Marquet 2004). Coexistence is possible within a given tions, environmental variability in species’ traits is gen- habitat patch if the IGPrey and IGPredator exhibit a trade- erally conducive to coexistence (Leibold et al. 2004). In off that allows resource partitioning, as would occur if multitrophic interactions, environmental variability that the IGPrey is the superior resource competitor but the affects the resource or predator trophic level can constrain IGPredator’s ability to prey on the IGPrey gives it an ad- coexistence at the intermediate consumer trophic level. ditional resource (Holt and Polis 1997). Coexistence, how- Diversity maintenance therefore depends crucially on ever, is not guaranteed. The trade-off is expressed only at whether dispersal by intermediate consumers can coun- intermediate resource productivity; if productivity changes teract the diversity-reducing effects of spatial variation that such that it becomes too low or too high, one species gains act through a shared resource or natural enemy. an overall advantage and excludes the other (Holt and Here I investigate this issue using IGP, a multitrophic Polis 1997). Coexistence in variable environments thus interaction that occurs in a wide variety of taxa from mi- requires additional mechanisms besides the competition- crobes to mammals (Polis et al. 1989; Arim and Marquet IGP trade-off. 2004). I consider the worst-case scenario for coexistence: These features define a landscape that is patchy and there is spatial variation in resource productivity but no spatially heterogeneous, and in which local coexistence spatial variation in the consumers’ life-history traits. How- within a habitat patch is determined by the ambient level ever, the consumer species can sample spatial variation in of resource productivity. Here I consider the simplest productivity by adopting different dispersal strategies. This mathematical representation of such a system, a three- study is novel in two respects. First, it presents a theoretical patch model with each patch exhibiting a level of resource framework for spatial dynamics of multitrophic com- productivity that leads to a qualitatively different outcome: munities, an area of spatial community ecology that has (1) resource productivity is too low for the IGPredator to hitherto received little attention. Second, it investigates the invade when rare, (2) resource productivity is too high for impact of dispersal strategies on species that interact via the IGPrey to invade when rare, and (3) resource pro- competition and predation, an aspect of spatial dynamics ductivity is within the range that allows both species to that has not been studied. invade