Oecologia (2012) 170:1045–1052

DOI 10.1007/s00442-012-2357-1

COMMUNITY ECOLOGY - ORIGINAL RESEARCH

EVects of an invasive plant transcend ecosystem boundaries through a dragonXy-mediated trophic pathway

Laura A. Burkle · Joseph R. Mihaljevic · Kevin G. Smith

Received: 2 September 2011 / Accepted: 1 May 2012 / Published online: 24 May 2012© The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract Trophic interactions can strongly inXuence thestructure and function of terrestrial and aquatic communi-ties through top-down and bottom-up processes. Specieswith life stages in both terrestrial and aquatic systems maybe particularly likely to link the eVects of trophic interac-tions across ecosystem boundaries. Using experimentalwetlands planted with purple loosestrife (Lythrum sali-caria), we tested the degree to which the bottom-up eVectsof Xoral density of this invasive plant could trigger a chainof interactions, changing the behavior of terrestrial Xyinginsect prey and predators and ultimately cascading throughtop-down interactions to alter lower trophic levels in theaquatic community. The results of our experiment supportthe linkage of terrestrial and aquatic food webs through thishypothesized pathway, with high loosestrife Xoral densitytreatments attracting high levels of visiting insect pollina-tors and predatory adult dragonXies. High Xoral densities

were also associated with increased adult dragonXy ovipo-sition and subsequently high larval dragonXy abundance inthe aquatic community. Finally, high-Xower treatmentswere coupled with changes in zooplankton species richnessand shifts in the composition of zooplankton communities.Through changes in animal behavior and trophic interac-tions in terrestrial and aquatic systems, this work illustratesthe broad and potentially cryptic eVects of invasive species,and provides additional compelling motivation for ecolo-gists to conduct investigations that cross traditional ecosys-tem boundaries.

Keywords Bottom-up · Consumer–resource · Purple loosestrife · Top-down · Trophic interactions

Introduction

Consumer–resource interactions can strongly inXuencecommunity structure and function via both top-down (pred-ator-controlled) and bottom-up (producer-controlled) inter-actions (e.g., Fretwell 1987; Schmitz 2010). Predator- andproducer-driven trophic patterns have been independentlydocumented in both aquatic and terrestrial ecosystems(reviewed in Power 1992; Shurin et al. 2002). However,increasing evidence suggests that ecological interactionscan cross these habitat and ecosystem boundaries, resultingin surprisingly strong direct and indirect eVects among spe-cies in distinct environments (Wallace et al. 1997; HelWeldand Naiman 2001; Baxter et al. 2004). For instance, over80 % of all animal species have complex life cycles andundergo ontogenetic shifts in habitat aYnity (Werner1988), often alternating between the use of terrestrial andaquatic systems (Wilbur 1980; Werner and Gilliam 1984).Given this prevalence of complex life cycles, linkages

Communicated by Scott Peacor.

Electronic supplementary material The online version of this article (doi:10.1007/s00442-012-2357-1) contains supplementary material, which is available to authorized users.

L. A. Burkle · J. R. Mihaljevic · K. G. SmithTyson Research Center, Washington University in St. Louis, Eureka, MO 63025, USA

Present Address:L. A. Burkle (&)Department of Ecology, Montana State University, Bozeman, MT 59717, USAe-mail: Laura.A.Burkle.Adv08@alum.dartmouth.org

Present Address:J. R. MihaljevicDepartment of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309, USA

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across habitat and ecosystem boundaries are more commonthan previously recognized (McCoy et al. 2009; Wesner2010).

Due to the propensity of many species to undergoaquatic-to-terrestrial life-stage progression, terrestrial sys-tems may be particularly sensitive to local aquatic trophicdynamics. For instance, lentic Wsh can dramaticallyincrease the Wtness of nearby terrestrial plants via indirecteVects of larval dragonXy consumption in the aquatic envi-ronment (Knight et al. 2005). By preying upon the larvallife-stage, Wsh reduce the density of emerging adult dragon-Xies, which are important predators of terrestrial plant poll-inators. Though the potential for reciprocal trophic eVectsbetween these ecosystems clearly exists, if and how terres-trial trophic interactions inXuence aquatic food webs islargely unexplored (see Carpenter et al. 2005; McCoy et al.2009; Sato et al. 2011 for exceptions). Furthermore, a morethorough understanding of trophic linkages among habitatsmay reveal additional direct and indirect eVects of habitatalteration.

Within terrestrial systems, invasive plants can have sig-niWcant consequences for native plant community composi-tion through competition and species replacement at a localscale (e.g., Vitousek et al. 1997; Levine et al. 2003). Nativeplant–pollinator interaction networks are also inXuenced byplant invasion (e.g., Aizen et al. 2008; Padron et al. 2009;Vila et al. 2009). However, little is known about how inva-sive plants might alter terrestrial and aquatic food webssimultaneously, despite the potential for strong consumer–resource interactions that could inXuence system dynamicsvia complex life cycle organisms. For example, the Xowersof highly fertile invasive plants may subsidize insect popu-lations by attracting large numbers of pollinators (Brownet al. 2002). Locally abundant insect pollinator prey maythen attract a greater abundance of predatory adult dragon-Xies. This terrestrial, bottom-up eVect resulting fromincreased prey resources could alter the food webs ofnearby wetlands through increased dragonXy ovipositionand recruitment of their aquatic larvae. DragonXy larvae arevoracious consumers of aquatic macroinvertebrates, andlarge changes in their abundances and foraging pressurecould cascade through the aquatic community to inXuencelower trophic level taxa (Benke 1976, 1978; Batzer andWissinger 1996).

Here, we investigate if and how purple loosestrife(Lythrum salicaria), a common, noxious invasive plant ofwetland–terrestrial interfaces, aVects terrestrial and aquatictrophic interactions in a manipulative pond study. Purpleloosestrife can occur in high densities and dominate largeexpanses of wetland habitat, is highly fertile, and attractslarge numbers of terrestrial, pollinating insects (Brownet al. 2002). Uninvaded wetlands typically have low Xoraldensities relative to those invaded by purple loosestrife,

through the displacement of native plants with no or lowXowering (e.g., cattails, Mal et al. 1997; or native congenerof L. salicaria, Brown et al. 2002). Thus, loosestrife isfunctionally diVerent from the native plants it replaces, andthis may have important community-level consequences.We chose purple loosestrife for this study to (1) investigate,in a basic ecological sense, the degree to which experimen-tal diVerences in Xoral densities can cross the terrestrial–aquatic ecotone, and (2) provide a realistic ecological sce-nario in which such eVects may occur. We hypothesizedthat the introduction of Xowering purple loosestrife plantsto artiWcial wetlands would stimulate a series of terrestrialtrophic interactions in which bottom-up eVects of loose-strife plants on secondary consumers would in turn gener-ate top-down eVects in the aquatic community, cascadingdown to inXuence the abundance and diversity of zooplank-ton communities (Fig. 1). Although Polis et al. (1997)argue that these types of community alterations could resultfrom direct inputs of resources across an ecotone, in thissystem we expect loosestrife Xoral resource density to indi-rectly inXuence the aquatic community via changes in thefrequency of dragonXy oviposition events and reproduc-tion. By experimentally manipulating Xoral density, weaddressed this gap in knowledge of trophic interactions atthe terrestrial–aquatic boundary (e.g., Polis and Strong1996; Baxter et al. 2005) by focusing on dragonXies,hypothesized key players in this system.

Methods

Eight artiWcial wetlands were created at Washington Uni-versity in the Tyson Research Center, St. Louis, MO, USA,in June of 2009. Each wetland was located in an old Weldhabitat within a matrix of oak–hickory forest, and com-prised a central vinyl stock tank (»1,300 L capacity) andfour smaller surrounding pools (»100 L capacity) Wlledwith well water on June 12. These artiWcial wetlands wereeach positioned within 80 m of existing water bodies acrossthe landscape (mean distance to existing water: 45 m, range27–77 m). The experimental wetlands were an average of306 m from each other (range 163–516 m). Neither dis-tance to nearest water nor distance to nearest experimentalwetland inXuenced any of the response metrics (P > 0.15 inall cases). The central tanks were stocked on June 22 withapproximately equivalent amounts of six species of aquaticmacrophytes and three species of snails. We collected zoo-plankton and phytoplankton from local ponds using an80 �m plankton net, and used aliquots of this mixture toinoculate each central tank. The remainder of the aquaticcommunity, including amphibians, odonates, dipterans,coleopterans, and hemipterans, was permitted to assemblenaturally via dispersal from the local species pool. We

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assume that these methods provide each mesocosm with thesame species pool, but allow for some natural stochasticityin colonization and extinction dynamics. We thereforeexpect that initial community composition was variableamong replicates, but do not expect that initial compositionwould be biased with respect to experimental Xoral treat-ment.

We placed 25 loosestrife plants in pots in each of thefour small pools around each central pond on June 12.Plants were grown from cuttings derived from Wve parentplants. Plants from the Wve parent lineages were dividedequally among the wetland replicates to account for poten-tial genetic eVects. The artiWcial wetlands were randomlyassigned one of Wve loosestrife Xowering treatments: 0 %Xowers (n = 2), 25 % Xowers (n = 1), 50 % Xowers (n = 2),75 % Xowers (n = 1), or 100 % Xowers (n = 2). Once theloosestrife plants began Xowering (July 6), we maintainedthese treatments until September 1 by removing the appro-priate number of Xowers at each wetland by hand threetimes per week, relative to the number of open Xowers atthe 100 % Xowers treatment wetlands. By September 1, theloosestrife plants were Wnished Xowering. We did not clipXowering stalks outright because we wanted to maintainequivalent plant structure at all wetlands, which is knownto aVect odonate oviposition behavior (Remsburg andTurner 2009). Occasionally, we removed some Xowers atone of the 100 % Xowers wetlands in order for the two rep-licates to have an equal number of open Xowers. The spatialseparation of the experimental loosestrife pools from thecentral sampling pool prevented the input of plant litter andpollen to the central pool. Each wetland was surrounded byfencing to prevent loosestrife decimation by deer herbivory.Thus, the spatial scale of each artiWcial wetland representedinitial stages of loosestrife invasion, with small patches eas-ily accessible to deer. The fencing may have allowed spe-cies interactions to occur that would typically manifest at

moderate stages of loosestrife invasion that are less accessi-ble to deer.

We observed all small pools equally for visiting insectsduring peak activity (0900–1500) for eight weeks (July 6 toSeptember 1, 2009). All insects were identiWed in the Weldto species or morphospecies [see Electronic supplementarymaterial (ESM) 1], and their behavior was recorded in theform of time spent at the pool and number of Xowers vis-ited. Insects were also classiWed into one of Wve size cate-gories, ranging from very small (e.g., some sweat bees andsyrphid Xies) to very large (e.g., carpenter bees and somebutterXies). Each pool was observed for 10 min once perweek during the eight weeks of the experiment, for a totalof 320 observation minutes at each wetland. Pools wereobserved individually in order to accurately assess smallXoral insect visitors, and data were pooled for each wet-land. Each wetland was also observed for dragonXy activity(10 min per week for eight weeks) in sunny, hot weather inrandom order. DragonXy individuals were counted andidentiWed to species (ESM 2) through binoculars, and theirbehaviors were recorded as the amount of time spent Xying,perching, or ovipositing. DragonXy abundance and behav-iors (e.g., number of oviposition events, quantiWed as whenan individual approached the tank and repeatedly dippedher abdomen in the water) were summed over the totalobservation time for each wetland.

At the end of the experiment in October 2009, each ofthe eight central tanks was thoroughly sampled for zoo-plankton and macroinvertebrates using methods similar tothose employed by Chase et al. (2009). By sampling overten weeks after the Wrst oviposition events were observed(July 21), we provided ample time for developing dragonXylarvae to inXuence the aquatic community. We exhaustivelysampled invertebrates within two 0.2 m2 chimney samplersand collected and preserved all individuals in ethanol forlater identiWcation in the laboratory. We sampled tank walls

Fig. 1 Conceptual diagram of the hypothesized trophic pathway inthis study. We predicted that introductions of the proliWcally bloominginvasive plant purple loosestrife (L. salicaria) would attract insect poll-inators and, subsequently, adult dragonXies, which prey upon the

smaller insects. Through increased dragonXy oviposition and greaterabundance of predaceous larval dragonXies in the wetland, these bot-tom-up terrestrial trophic interactions could then translate to top-downeVects on the aquatic zooplankton community

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for surface-dwelling invertebrates with sweeps of a 25 cmwide rectangular net from the bottom to the top of each tankwall at four locations in each tank. We sampled zooplank-ton from each central tank with Wve collections using anintegrated tube sampler to sample the entire water column.These Wve samples were combined (»15 L total) and wereWltered through an 80 �m mesh zooplankton net into a50 mL sample for later laboratory identiWcation. All taxawere identiWed in the laboratory using standard keys andguides. When identiWcation to species level was not possi-ble, taxa were identiWed to morphospecies (ESM 3 and 4).

To determine the relationships between the loosestrifeXower treatments and insect visitors, dragonXy abun-dance, dragonXy oviposition, and trophic levels of theaquatic community, we performed individual regressionsusing JMP (version 4.0.4). We used a partially replicatedregression design to maximize our statistical power (Cot-tingham et al. 2005). There was a large amount of varia-tion in the abundance of zooplankton among treatments(range: 87–1245 sampled zooplankton individuals pertank). To ensure that diVerences among treatments in zoo-plankton richness were not caused by diVerences in thenumber of individuals, we conducted an individual-basedrarefaction analysis on the zooplankton data by samplingdown to the lowest common abundance value. Becausethe number of loosestrife Xowers naturally Xuctuated overthe course of the season, and we manipulated the numberof Xowers to maintain our intended Xoral treatments, weused the percent of loosestrife Xowers (i.e., the treatment)in our analyses, because it is a clear and constant indepen-dent variable. A single wetland (25 % Xowers treatment)was excluded from all aquatic community analyses due toaccidental contamination by Wsh larvae during zooplank-ton inoculation.

We described and quantiWed diVerences in zooplanktoncommunities among treatments using descriptive and infer-ential multivariate methods. We Wrst ordinated the zoo-plankton communities using nonmetric multidimensionalscaling (NMDS) based on Bray–Curtis similarity values.Because our experimental design had very low (or no) rep-lication within treatment groups, we could not formally testfor diVerences in zooplankton community structure amongtreatment groups. Instead, we tested for a correlationbetween diVerence in treatment group (percent of loose-strife Xowers) and Bray–Curtis community similarity via aMantel’s test. Finally, to identify key diVerences in zoo-plankton communities between treatment groups, wepooled all zooplankton into four major functional/taxo-nomic groups (rotifers, cladocerans, copepods, or ostrac-ods) and then conducted a SIMPER analysis to identify thegroups that contributed the greatest change in Bray–Curtissimilarity values among treatments. The SIMPER analysiswas restricted to the 100, 50, and 0 % Xower treatment cat-

egories, each of which were replicated twice. Multivariatecommunity analyses were conducted in PAST (Hammeret al. 2001) and R (R Development Core Team, version2.13.1).

Results

Insect visitation rates were higher in wetlands with greaternumbers of loosestrife Xowers available (Fig. 2a,F1,6 = 14.13, P = 0.0094, r2 = 0.65, standardized regressioncoeYcient = 0.84). There were no species-speciWc or size-speciWc trends in pollinator abundance or behavior acrossthe Xoral density treatment (P > 0.20 in all cases). In addi-tion, we found evidence of a positive relationship betweenloosestrife Xower treatment and number of adult dragon-Xies that visited the experimental wetlands (Fig. 2b,F1,6 = 5.37, P = 0.059, r2 = 0.38, standardized regressioncoeYcient = 0.69) and number of dragonXy ovipositionevents (Fig. 2c, F1,6 = 11.37, P = 0.015, r2 = 0.60, standard-ized regression coeYcient = 0.81).

At the end of the experiment, the abundance of dragonXylarvae in our aquatic samples was positively related toloosestrife Xower treatment (Fig. 2d, F1,5 = 8.87, P = 0.031,r2 = 0.57, standardized regression coeYcient = 0.80). Atthe lower aquatic trophic level, raw (Fig. 3, F1,5 = 21.25,P = 0.0058, r2 = 0.77, standardized regression coeYcient =0.90) and rareWed (F1,5 = 20.87, P = 0.0060, r2 = 0.81, stan-dardized regression coeYcient = 0.90) zooplankton speciesrichness was highly positively associated with loosestrifeXower treatment. Neither larval dragonXy species richness(F1,5 = 1.39, P = 0.29) nor zooplankton abundance(F1,5 = 2.392, P = 0.18) were signiWcantly associated withloosestrife Xower treatment. Analysis of all other macroin-vertebrates present in the aquatic community showed nosigniWcant relationships between Xower treatment and eithermacroinvertebrate abundance (F1,5 = 0.32, P = 0.60) or rich-ness (F1,5 = 0.077, P = 0.79). Likewise, when the predatorysubset of macroinvertebrates was considered, we found nosigniWcant relationships between their abundance or richnessand Xower treatment (F1,5 = 0.077, P = 0.42 and F1,5 = 0.008,P = 0.93, respectively).

Zooplankton communities in similar Xower treatmentcategories were more similar to one another than those inwidely diVering treatments (ESM Fig. 1, observed correla-tion value = ¡0.28, mean simulated value § SD =0.0045 § 0.2060, P = 0.0731). DiVerences in zooplanktoncommunities among treatments were primarily driven bydiVerences in the abundance of rotifers, with greater rotiferabundance in 0 % Xower treatments compared to 50 and100 % Xower treatments (Table 1). Copepods and cladocer-ans were also more abundant in the 0 % Xower treatmentthan in the 100 % Xower treatment.

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Discussion

Organisms, nutrients, and energy are increasingly observedto cross traditional ecosystem boundaries (Power andRainey 2000; Baxter et al. 2005). Here, we showed thecapacity of the invasive wetland plant purple loosestrife(L. salicaria) to spark a series of terrestrial-to-aquatic tro-phic interactions, enhancing pollinator and predatory adultdragonXy local abundance, increasing dragonXy oviposi-tion events in our experimental wetlands, increasing preda-tory larval dragonXy abundance, and altering zooplanktonspecies richness as well as community structure in theaquatic community.

We documented a signiWcant positive relationshipbetween the density of purple loosestrife Xowers and thevisitation of Xying insects, including many potential pollin-ators. Previous research has demonstrated that purple loose-

strife is highly attractive to pollinating insects, and usurpspollinators from native congeners (Brown et al. 2002). Ourresults further suggest that, by attracting relatively highlevels of pollinating insects where there might otherwise belittle insect activity (Fig. 2a), purple loosestrife Xowerspotentially created a new resource base for novel trophicinteractions. For instance, we documented a concomitantincrease in adult dragonXy abundance with increasingloosestrife Xower levels. Adult dragonXies are predators ofmany small Xying insects (Corbet 1999; Knight et al.2005). These results suggest that adult dragonXies respondwith increased abundance and activity to the presence ofXying insect prey, to the visual cue of the Xowers them-selves, or to both. The nature of our experimental designdoes not allow us to separate and assess the relative impor-tance of these potential eVects. We are conWdent, however,that by removing only Xowers and not plant stems we were

Fig. 2 The abundances of a pollinating insects, b adult dragonXies,c dragonXy oviposition events, and d dragonXy larvae in wetlandsincreased with purple loosestrife (L. salicaria) Xower abundance.

Abundances were summed over equal sampling eVort for each tank.The data points in each panel were Wtted with a linear regression (solidline)

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0.0

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1.2

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1.6

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0

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Loosestrife flower abundance (% flowers available)0 20 40 60 80 100A

du

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0 20 40 60 80 100Lar

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(n

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0

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r2 = 0.65 r2 = 0.38

r2 = 0.60 r2 = 0.57

Table 1 Results of a SIMPER analysis on diVerences in zoo-plankton functional/taxonomic groups among treatments. Anal-ysis is based on Bray–Curtis dis-tances among zooplankton communities

Taxonomic/functional group

Contribution Cumulative % Mean abundance within treatment

100 % 50 % 0 %

Rotifers 38.61 71.67 79 24.5 601

Copepods 9.45 89.22 38.5 75 73

Cladocerans 5.22 98.91 5.5 11 18.5

Ostracods 0.59 100 2.5 1.5 0

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able to rule out the known eVects of plant structure on drag-onXy activity and abundance (Remsburg and Turner 2009).

Our results also indicate that the adult dragonXy abun-dances associated with high-Xower treatments resulted in ahigher frequency of dragonXy oviposition events and a sub-sequently higher abundance of dragonXy larvae in theexperimental ponds as compared to low-Xower treatments.This is a key link between the terrestrial and aquatic foodwebs, with a bottom-up eVect of increased adult dragonXiesresulting in higher densities of aquatic predators (dragonXylarvae) and in the potential for a top-down aquatic trophiccascade. Further experiments are required to mechanisti-cally conWrm each step of this cascade beyond what weobserved from Xoral manipulation alone.

Finally, we document a strong positive relationshipbetween Xower treatment level and zooplankton speciesrichness. Although we hypothesized that Xower treatmentlevel would inXuence lower trophic levels in the aquaticenvironment, we did not predict a priori that this eVectwould manifest itself as elevated zooplankton richness inhigh-Xower wetlands, with no overall eVect on zooplanktonabundance. Although a bottom-up eVect of increased inputsof pollen or detritus from purple loosestrife is one possibleexplanation for this result, we consider it to be unlikely forthree reasons. First, purple loosestrife is primarily insect,not wind, pollinated. Second, the loosestrife plants werehoused in separate aquatic mesocosms and could thereforenot deposit detritus in the central mesocosms that were thefocus of our aquatic sampling. Third, rareWed zooplanktonrichness also increased with loosestrife Xower treatment,which suggests that diVerences in richness among the treat-ments were not driven by diVerences in abundance; thiscontrasts with a bottom-up (more individuals) eVect onzooplankton.

A second, and more likely, explanation is that higherzooplankton richness in high-Xower treatment ponds wasmediated by higher densities of dragonXy larvae, possiblyvia a keystone eVect of predation by dragonXy larvae. Wehave evidence of large shifts in abundance patterns of zoo-plankton taxonomic categories (Table 1), which in combi-nation with the positive relationship between Xowertreatment and zooplankton richness, is consistent with akeystone eVect. We also found evidence of a shift in zoo-plankton community composition associated with Xowertreatment level, such that similar Xower treatments (andsimilar abundance of larval dragonXies) resulted in moresimilar zooplankton communities (Mantel’s test and ESMFig. 1). This general pattern suggests that in this experi-ment, predation by dragonXy larvae was selective andaltered the structure of the zooplankton communities,potentially toward more similar, species-rich communities.This result is further supported by evidence that this changein aquatic community structure was primarily driven by areduction in the abundance of rotifers. Although signiWcanteVects of dragonXy predation on zooplankton assemblageshave been documented relatively rarely and are variableamong studies (e.g., Hampton and Gilbert 2001; Burkset al. 2001; Magnusson and Williams 2009), dragonXy lar-vae are known to prey on rotifers to varying degrees(Hampton and Gilbert 2001; Walsh et al. 2006). This eVectmay have been particularly strong in our study, in which alarge proportion of dragonXy larvae were of small size.

Other mechanisms may also contribute to the positiverelationship between Xoral treatment and zooplankton rich-ness, including increased zooplankton colonization oppor-tunities associated with increased dragonXy ovipositionevents (Havel and Shurin 2004). Intraguild predation, seenas the suppression of other macroinvertebrate predators byhigh densities of dragonXy larvae, is also a possible mecha-nism. However, we did not detect any associations betweenXower treatments and the abundance or richness of non-odonate macroinvertebrate predators in this study. Finally,we cannot rule out other indirect eVects of larval odonateabundance on zooplankton richness. The majority of thedragonXy larvae present in our experimental ponds weresmall in size, supporting eVects on zooplankton and notother taxa.

Overall, our results demonstrate that trophic eVects gen-erated by an emergent wetland invasive Xowering plant arepropagated into the aquatic system by a common group ofinsects with a complex lifecycle: dragonXies. Our resultswere surprising in that the eVects of purple loosestrife wereconsistently strong and operated across four distinct levelsof trophic interactions. Our study was short-term, highlymanipulative, and focused on the assembly phase of theaquatic food web, when dragonXies (for example) may berecruitment limited. Whether eVects such as those observed

Fig. 3 Zooplankton species richness increased with loosestrife Xowerabundance. Zooplankton richness was based on counts from 15 L ofwater from each tank. The data points were Wtted with a linear regres-sion (solid line)

Loosestrife flower abundance (% flowers available)

0 20 40 60 80 100Zo

op

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kto

n s

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ies

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nes

s (n

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r2 = 0.77

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here also occur in natural habitats in mature wetlands isunknown. However, the importance of assembly history indetermining the long-term trajectory of ecological commu-nities suggests that eVects similar to those we documenthere could potentially have long-lasting consequences forthe structure of aquatic communities. Documenting thestrength of longer-term reciprocal eVects (e.g., Baxter et al.2005; Massol et al. 2011) of loosestrife across the aquatic–terrestrial ecotone will be an important goal for future workin this and similar systems.

Invasive plants have well-documented eVects on terres-trial communities. Purple loosestrife can directly inXuencenative plant communities through competition for resourcesby reducing the colonization success of native species andoutcompeting rare species (Hovick et al. 2011). Purpleloosestrife can also indirectly reduce native plant Wtness byusurping pollinators (Brown et al. 2002) or altering abioticconditions that inXuence pollinator visitation, such as thelight environment (McKinney and Goodell 2010). Invasivewetland plants like purple loosestrife also have the potentialto link and disrupt native terrestrial and aquatic ecosystems(Naiman and Decamps 1997) via allochthonous resourceinputs and alterations of aquatic communities (e.g., Schulzeand Walker 1997; Bailey et al. 2001; Going and Dudley2007), leading to an overall alteration of community struc-ture and ecosystem functioning (Hladyz et al. 2011). How-ever, less studied are the multi-trophic interactions that maybe propagated across ecosystem boundaries through behav-ioral responses to resource availability and ontogenetichabitat shifts. Our results suggest that purple loosestrife, asa plant that produces much more Xoral resources than thenative plants that it replaces, has the capacity to inXuencethe attraction of predatory adult dragonXies that link ter-restrial and aquatic trophic interactions. Further experi-mentation incorporating a native Xoral community as anadditional control will help to quantify the relative magni-tude of eVects on aquatic communities. Given that purpleloosestrife can dominate large areas of wetlands, its inva-sion might have important implications for species interac-tions and trophic structure over large scales. Larger-scalestudies will help elucidate the landscape implications ofloosestrife invasion, particularly in the context of popula-tion-level pollinator and dragonXy foraging behavior. TheeVects of invasive species may propagate further andthrough more cryptic pathways than previously appreci-ated. By broadening our views of ecosystems and collabo-rating across traditional disciplines, terrestrial and aquaticecologists together may better understand the intricacies oftrophic interactions.

Acknowledgments We thank J.P. Gorham, M. Bujnak, J. Davie, M.Dust, C. Galluppi, and S. Mulhern for their help in the Weld, and M.Dyer for greenhouse assistance. Discussions with J. Chase and T.

Knight and their respective lab groups contributed to the developmentof this project. The Cross lab, A. Sepulveda, K. Gates, and A. Ray, pro-vided insightful comments on an earlier draft of this manuscript. Wealso thank E. Biro for help with specimen identiWcation, and TimBanek and the Missouri Department of Conservation for granting per-mission to work on purple loosestrife. Tyson Research Center providedfunding and logistical support. This work was also facilitated by fund-ing from NSF 06-520 DRL-0739874 to LAB and NSF DEB-0816113to KGS.

Open Access This article is distributed under the terms of the Crea-tive Commons Attribution License which permits any use, distribution,and reproduction in any medium, provided the original author(s) andthe source are credited.

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