2601 Ecology, 85(9), 2004, pp. 2601–2615  2004 by the Ecological Society of America DOES INTRAGUILD PREDATION ENHANCE PREDATOR PERFORMANCE? A STOICHIOMETRIC PERSPECTIVE MASAYA MATSUMURA, 1 GENEVIEVE M. TRAFELET-SMITH, 2 CLAUDIO GRATTON, 3 DEBORAH L. FINKE, 4 WILLIAM F. FAGAN, 5 AND ROBERT F. DENNO 4,6 1 National Agricultural Research Center for Kyushu Okinawa Region, Nishigoshi, Kumamoto, Japan 2 Old Mill High School, Millersville, Maryland 21108 USA 3 Department of Entomology, University of Wisconsin, Madison, Wisconsin 53706 USA 4 Department of Entomology, University of Maryland, College Park, Maryland 20742 USA 5 Department of Biology, University of Maryland, College Park, Maryland 20742 USA Abstract. Ecological stoichiometry provides a novel context for elucidating the oc- currence of intraguild predation. Recent data show that predators on average have a higher nitrogen content and lower C:N ratio than potential herbivorous prey. Thus, many predators may be nitrogen limited, and intraguild predation may allow them to increase their nitrogen intake and growth by supplementing a diet of herbivores with more nitrogen-rich intraguild prey. We tested this hypothesis using an assemblage of salt-marsh-inhabiting arthropods. First, we determined the nitrogen content and C:N ratio of taxa in four trophic groups (plants, herbivores, omnivores, and predators). Second, we fed an intraguild predator, the wolf spider Pardosa, one of three diets (herbivores, intraguild prey, or an alternating mix of the two) and measured spider survival, growth, capture rate, and biomass and nitrogen intake. In general, body nitrogen content increased and C:N ratio decreased from lower to higher trophic levels for marsh-inhabiting species, with predators having a higher nitrogen content and lower C:N ratio than herbivores. Performance experiments showed that in one case Pardosa ingested more biomass and nitrogen and grew faster on a diet of intraguild prey (the planthopper egg predator Tytthus) than on a diet of herbivores (the planthopper Prokelisia dolus). This occurred because Pardosa captured more Tytthus than Prokelisia and not because Tytthus (a stoichiometric exception) was higher in nitrogen content. In another case, Pardosa grew slower on a diet of intraguild prey (the web-building spider Grammonota) than on a planthopper diet even though Grammonota was more nitrogen rich, a result we attribute to prey behavior and risk of predation. Mass gain and nitrogen intake in Pardosa were highly correlated with the biomass of prey consumed. However, after accounting for the biomass of prey consumed across all diet treatments, we found little evidence that either the nitrogen content or C:N stoichiometry of prey contributed to Par- dosa’s growth. Thus, there was little support for the hypothesis that the nitrogen stoichi- ometry of prey directly confers a performance advantage to Pardosa and in itself promotes intraguild predation. In this system, characteristics other than the nitrogen stoichiometry of prey play a significant role in prey capture and predator performance. Nonetheless, by supplementing their diet with readily captured intraguild prey, predators such as Pardosa can increase their nitrogen intake and performance. Key words: C:N ratio; diet mixing; ecological stoichiometry; intraguild predation; nitrogen content; omnivory; predator performance; prey behavior; prey nutrition; risk of predation; salt marsh; trophic level. INTRODUCTION Omnivory is widespread in terrestrial arthropods, oc- curring in a diverse group of taxa that occupy both nat- ural and agricultural habitats (Alomar and Wiedenmann 1996, Coll and Guershon 2002). Broadly defined, om- nivores feed across two or more trophic levels (Polis and Strong 1996) and include ‘‘herbivores’’ that obtain nutrients from resources other than their host plants (e.g., Manuscript received 22 September 2003; revised 9 January 2004; accepted 26 January 2004. Corresponding Editor: J. T. Cronin. 6 E-mail: rd12@umail.umd.edu cannibalism), ‘‘predators’’ that feed on certain plant tis- sues (e.g., pollen, seeds, and meristems) in addition to prey, and predators that feed on herbivores as well as other predators (e.g., intraguild predators) (Coll 1998, Rosenheim 1998, Coll and Guershon 2002, Finke and Denno 2002). Because omnivory is thought to have sig- nificant consequences for predator–prey interactions (Rosenheim 1998, Eubanks and Denno 2000a, Finke and Denno 2003), food-web dynamics (Polis and Strong 1996, Fagan 1997, McCann et al. 1998), ecosystem func- tion (Ostrom et al. 1997), and biological control (Ro- senheim et al. 1995, Hodge 1999), it becomes paramount to understand factors that promote this feeding strategy.