International Association for Ecology Dragonfly Predators Influence Biomass and Density of Pond Snails Author(s): Andrew M. Turner and Michael F. Chislock Reviewed work(s): Source: Oecologia, Vol. 153, No. 2 (Aug., 2007), pp. 407-415 Published by: Springer in cooperation with International Association for Ecology Stable URL: http://www.jstor.org/stable/40210876 . Accessed: 07/03/2012 13:17 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Springer and International Association for Ecology are collaborating with JSTOR to digitize, preserve and extend access to Oecologia. http://www.jstor.org Oecologia (2007) 153:407-415 DOI 10.1007/s00442-007-0736-9 COMMUNITYECOLOGY Dragonfly predators influence biomass and density of pond snails Andrew M. Turner • Michael F. Chislock Received: 7 October 2006 /Accepted: 20 March 2007 /Published online: 25 April 2007 © Springer-Verlag 2007 Abstract Studies in lakes show that fish and crayfish not have a strong effect on species composition. A model predatorsplay an importantrole in determiningthe abun- suggested that A. junius and P. hymenaeahave the largest dance of freshwatersnails. In contrast,there are few studies effects on snail biomass in the field. Given that both pul- of snails and their predatorsin shallow ponds and marshes. monate snails and dragonfly nymphs are widespread and Ponds often lack fish and crayfishbut have abundantinsect abundantin marshesand ponds, snail assemblagesin these populations. Here we present the results of field surveys, water bodies are likely regulated in large part by odonate laboratory foraging trials, and an outdoor mesocosm predation. experiment, testing the hypothesis that insects are impor- • • • tant predatorsof pulmonate snails. In laboratoryforaging Keywords Food webs Predation Gastropoda trials, conducted with ten species of insects, most insect Odonata • Snail taxa consumed snails, and larval dragonflies were espe- cially effective predators.The field surveys showed that dragonfliesconstitute the majorityof the insect biomass in Introduction fishless ponds. More focused foraging trials evaluated the ability of the dragonfliesAnax junius and Pantala hyme- Ecologists seek to identify mechanisms that regulate the naea to prey upon different sizes and species of pulmonate distributionand abundanceof species along environmental snails (Helisoma trivolvis, Physa acuta, and Stagnicola gradients.A key gradientin freshwateris the environmental elodes). Anax junius consumed all three species up to the continuumfrom small, temporaryponds and marshes largely maximum size tested. Pantala hymenaea consumed snails lacking predatorsto deep, permanentlakes containing fish with a shell height of 3 mm and smaller, but did not kill (Wellborn et al. 1996). Freshwater gastropods are key larger snails. P. acuta were more vulnerable to predators players in many freshwaterfood webs, and given suitable than were H. trivolvis or S. elodes. In the mesocosm abiotic conditions are ubiquitousinhabitants of lentic sys- experiment, conducted with predatortreatments of A. ju- tems spanningthe habitatgradient (Pip 1986; Jokinen 1987; nius, P. hymenaea, and the hemipteranBelostoma flumi- Dillon 2000). Lodge et al. (1987) argue that abiotic condi- neum, insect predatorshad a pronouncednegative effect on tions (Ca2+availability) limit gastropodsat a regional scale, snail biomass and density. A. junius and B. flumineumre- but biotic interactions determine snail distribution and duced biomass and density to a similar degree, and both abundanceat a local scale. A numberof experiments,con- reducedbiomass more than did P. hymenaea.Predators did ducted in deep permanentlakes, show that food limitation (Osenberg 1989) and predationby fish and crayfish (Bron- market al. 1992; Martinet al. 1992; Lodge et al. 1994) can Communicated by Jay Rosenheim. regulatesnail abundanceand determine the structureof snail assemblages. In contrast, there are few studies of snail Turner • M. F. Chislock A. M. (El) regulation and community structurein shallow of Clarion Clarion, population Department Biology, University, and marshes. Such habitats lack fish and PA 16214, USA ponds typically e-mail: [email protected] crayfish but have large insect populations,many of which £l Springer 408 Oecologia(2007) 153:407^15 function as top predatorsin pond food webs (Zaret 1980; species of carnivorousaquatic insects in orderto determine McPeek 1990; Wellbornet al. 1996). if they were capable of feeding on snails. We chose to Larval dragonflies are particularlyabundant in fishless screen insects that were relatively large and commonly ponds and can play an importantrole in structuringpond encounteredin the ponds of northwestPennsylvania, USA. communities (McPeek 1990, 1998; Wissinger and McGr- Insect species screened were the dragonfliesAnax junius, ady 1993; Skelly 1996). Although a wide variety of ani- Pantala hymenaea, Pachydiplax longipennis, Tramealac- mals are known to prey on freshwatergastropods (reviewed erata, and Erythemis simplicicollis, the damselfly Lestes in Lodge et al. 1987; Brown 1991; Dillon 2000), there are vigilax, the water bugs Belostomaflumineum, Ranatra ni- no studies examining whether dragonflies prey on snails. gra and Pelecoris fermoratus, and the larvae of the beetle Here we present the results of laboratory experiments, Dytiscus verticalis. Prey in the screening study were Physa mesocosm experiments, and field surveys, testing the acuta between 3.0 and 6.0 mm shell height. Due to their hypothesis that dragonflies are important predators of small size and thin shell, this size class and species is pulmonate snails. Our larger goal is to evaluate the po- among the most vulnerableof the snails commonly found tential importance of predation in structuring the snail in ponds. Trials were conducted indoors in polypropylene assemblages of fishless ponds. shoeboxes (20 x 35 x 14 cm deep) filled to a depth of Our study focuses on three species of pulmonatesnails; 4 cm (2.8 1) with aged well water. We chose to use a rel- Physa acuta, Stagnicola elodes, and Helisoma trivolvis. P. atively small foraging arena so as to maximize predator- acuta and H. trivolvis are both widespreadand abundantin prey encounter rates and minimize the role of any behav- the ponds of northwest Pennsylvania (Mower and Turner ioral avoidance by snails. Fifteen snails and one insect 2004). S. elodes has a narrow habitat distribution,being predatorwere stocked into each containerand we recorded restrictedto shallow ponds with forest cover, but within snail survivorshipafter 48 h, along with the method of kill such ponds they are also quite abundant (Mower and (shell crushedor body extractedfrom shell). Four replicate Turner2004). All three snail species commonly co-occur trails were conducted for each predatortaxa. In addition, with the insect predators screened in this study. Of the for each set of four predatortrials we ran a no-predator three species, S. elodes is the largest and has a relatively control trial in orderto assurethat there was no background thick shell, whereas P. acuta is the smallest and has a mortalityin the absence of predators.The environmentwas relatively thin shell (Mower and Turner2004; A. Turner, controlled with a temperatureof 22°C and 14:10 h day:- unpublisheddata). Because these three snail species differ night lighting. in body size, degree of shell development, and anti-pred- ator behavior (Mower and Turner2004), we expected that Field survey their vulnerabilityto dragonflieswould also differ. We first screened ten species of carnivorousinsects in The impactof a particularspecies of insect predatoron snail order to determine which might be the most important assemblages will depend both on the feeding rates of indi- predatorsof pulmonate snails. Based on these results, and vidual predatorsand on the abundanceof the predator.We informedby the results of a field survey, we then chose for surveyed the biomass of predatoryinsects in 12 ponds in furtherstudy three species that likely are the most impor- orderto providean assessmentof the relative abundancesof tant snail predators:Pantala hymenaea, a libellulid drag- different sorts of insect predators.Sampled ponds were lo- onfly, Anax junius, an aeshnid dragonfly, and the cated in CrawfordCounty, Pennsylvania, USA. All 12 ponds hemipteranBelostoma flumineum. A. junius and B. flumi- were more or less permanentbut lacked well-developedfish neum are abundantin permanent,fishless ponds (Dunkle communities, though a couple contained scatteredfathead 1989; Kesler and Munns 1989; McPeek 1990; Chase 1999). minnows(Pimephales promelas). Insects were sampledwith P. hymenaea are largely restricted to and abundant in a stove-pipe-typesampler, 25 cm diameter,which has been ephemeralponds, but we view them as broadly represen- shown to perform well in sampling vegetated habitats tative of the many species of libellulid dragonfliesthat are (Turner and Trexler 1997). Eight replicate samples were especially abundantin fishless ponds. taken from randomly selected locations in each pond, and samples were pooled for furtheranalyses.