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ReportsEcology, 90(9), 2009, pp. 2339–2345! 2009 by the Ecological Society of America

Effects of predator functional diversityon grassland ecosystem function

OSWALD J. SCHMITZ1

School of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, Connecticut 06515 USA

Abstract. Predator species individually are known to have important effects on plantcommunities and ecosystem functions such as production, decomposition, and elementalcycling, the nature of which is determined by a key functional trait, predator hunting mode.However, it remains entirely uncertain how predators with different hunting modes combineto influence ecosystem function. I report on an experiment conducted in a New Englandgrassland ecosystem that quantified the net effects of a sit-and-wait and an actively huntingspider species on the plant composition and functioning of a New England grasslandecosystem. I manipulated predator functional diversity by varying the dominance ratio of thetwo predator species among five treatments using a replacement series design. Experimen-tation revealed that predator functional diversity effects propagated down the live plant-basedchain to affect the levels of plant diversity, and plant litter quality, elemental cycling, andproduction. Moreover, many of these effects could be approximately by the weighted averageof the individual predator species effects, suggesting that this kind of predator diversity effecton ecosystems is not highly nonlinear.

Key words: active vs. sit-and-wait predators; biodiversity and ecosystem function; hunting mode;nitrogen cycling; old-field spiders; Phidippus rimator; Pisaurina mira; plant dominance; primary production;top-down control.

INTRODUCTION

The nature of species impacts on ecosystems can behighly dependent on their functional identity determinedby their traits (Chapin et al. 1997, Duffy 2002, Chalcraftand Resetarits 2003, Hooper et al. 2005, McGill et al.2006, Petchey and Gaston 2006, Wright et al. 2006,Violle et al. 2007). Accordingly, a major thrust of con-temporary ecology is to resolve how combinations ofspecies with different functional traits—a form of speciesdiversity—influence ecosystem properties and functions(Loreau et al. 2001, Schmid et al. 2001, Hooper et al.2005, Wright et al. 2006). Most research directed towardunderstanding this interplay focuses on functionaldiversity at the plant–soil interface (Wardle 2002,Hattenschwiler et al. 2005, Hooper et al. 2005). Yet,assessments of biodiversity–ecosystem functioning rela-tionships will be incomplete without considering diver-sity within higher trophic levels of ecosystems (Cardinaleet al. 2006, Duffy et al. 2007).

For instance, predator species individually can haveimportant indirect effects on the species composition ofplant communities (Schmitz 2008b). Because plantspecies composition is an important regulating factorof ecosystem function (Chapin et al. 1997, Loreau et al.2001), it follows that predator species should haveimportant indirect effects on ecosystem functions, whichthey do (Downing and Leibold 2002, Duffy 2003,Fukami et al. 2006, Maron et al. 2006, Canuel et al.2007, Schmitz 2008a). But, predators can propagatethese indirect effects in at least two ways (Schmitz 2007).They can alter the numerical abundance of herbivoreprey by capturing and consuming them. Alternatively,their mere presence in a system can trigger herbivore preyto modify foraging activity in a manner that reducespredation risk. These different kinds of effect are relatedto one particular functional trait, predator huntingmode, irrespective of taxonomic identity (Schmitz2007). Sit-and-wait ambush predators cause largelyevasive behavioral responses in their prey because preyspecies respond strongly to persistent, point-source cuesof predator presence. Widely roaming, actively huntingpredators may reduce prey density, but they exert highlyvariable predation risk cues and are thus unlikely to

Manuscript received 15 October 2008; revised 31 March2009; accepted 7 April 2009. Corresponding Editor: B. J. Fox.

1 E-mail: [email protected]

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cause chronic behavioral responses in their prey. Naturalsystems contain predators with both kinds of functionaltraits (Schmitz 2007), yet it remains uncertain what thenet effect of such functional diversity will be onecosystem function (Schmitz 2007, Bruno and Cardinale2008).Here I report on a three-year experiment in a New

England grassland ecosystem that quantified the collec-tive effects two hunting spider predators with differenthunting modes on plant community composition andthree ecosystem functions: aboveground net primaryproductivity (ANPP), plant litter decomposition rate(decomposition), and nitrogen mineralization rate (min-eralization). I evaluated the role of predator functionaldiversity by comparing effects of each predator speciessingly and in combination. Following recommendations(Petchey and Gatson 2006), I also manipulated predatorspecies dominance (evenness) by changing the relativeproportion of the different species.

METHODS

Natural history

The experiment, carried out in a grassland ecosystemin northeastern Connecticut, USA, focused on thedominant interacting species in this system (Schmitz2003): old-field plants, the generalist grasshopperMelanoplus femurrubrum and hunting spider predatorsPisaurina mira (see Plate 1) and Phidippus rimator.The plant species may be assigned to three groups: (1)

the grass Poa pratensis, which is a preferred resource ofM. femurrubrum (Schmitz 2003); (2) the herb Solidagorugosa, which provides M. femurubrum refuge fromspider predation (Schmitz 2003) and, because of itscompetitive dominance, is an important determinant ofplant species diversity and level of ecosystem function(Schmitz 2008a); and (3) a variety of other herb speciesincluding Trifolium repens, Potentilla simplex, Rudbekiahirta, Crysanthemum leucanthemum, and Daucus carotathat are dominated by S. rugosa (Schmitz 2003).The spider species are functionally distinct. P. mira is

a sit-and-wait predator that resides in the upper canopyof the field (Schmitz 2007). Grasshopper mortality dueto predation is compensatory to natural mortality in thepresence of P. mira (Sokol-Hessner and Schmitz 2002).The spider causes grasshoppers to reduce their foragingon grasses and to seek refuge in and forage on the leafierS. rugosa (Schmitz 2003) which in turn leads to apositive indirect effect on grasses, a negative indirecteffect on S. rugosa and a positive indirect effect on otherherbs owing to competitive release from S. rugosa(Schmitz 2008a). The widely roaming active hunting P.rimator does not cause chronic foraging shifts bygrasshoppers (Sokol-Hessner and Schmitz 2002). In-stead, this predator has an additive effect on grasshop-per mortality (Sokol-Hessner and Schmitz 2002) thattranslates into a positive indirect effect on grass and S.rugosa and a negative indirect effect on other herbs(Schmitz 2008a).

The experiment was motivated by observations(Appendix A) that the abundance of the competitivedominant plant S. rugosa and plant species diversityvary linearly with the relative proportion of P. mira andP. rimator spiders among several fields in the vicinity ofthe experimental field site, suggesting that predatordiversity effects may explain the field pattern.

Study design

I examined the indirect effects of predator functionaldiversity on plant community composition and decom-position, N-mineralization, and ANPP. Thus, thepatterns of plant diversity and ensuing ecosystemfunctions were deliberately allowed to emerge as aconsequence of the manipulations; they were notmanipulated directly as part of the experiment.The experiment involved 35 cylindrical mesocosms,

1.5 m high 3 2 m2, placed over naturally growingvegetation in the field. The mesocosms were arranged inseven replicate blocks with five treatments (differentcombinations of the predator species) randomly as-signed to mesocosms within each block. The first year ofthe study (2005) was devoted to assigning plots formesocosm placement and measuring initial conditionswithin each plot. The subsequent two years involvedmanipulation of predator diversity and measurement ofecosystem responses within the mesocosms.Initial conditions.—I measured seven community and

ecosystem properties and three ecosystem functionswithin each plot: soil moisture, soil temperature, totalplant biomass, S. rugosa biomass grass biomass, otherherb biomass, plant diversity, decomposition, N miner-alization, and ANPP.I measured soil surface temperature using a Digi-

Sense 8523 thermistor thermometer (Cole-Parmer In-strument Company, Chicago, Illinois, USA), coupled toa soil probe accurate to 0.18C that was immersed 5 cminto the soil. I measured soil moisture (percentage ofwater content) using a Dynamax ML2x Theta Probe(Dynamax, Inc., Houston, Texas, USA). I measuredeach variable at five random locations within each plotand then estimated the plot average to obtain anindependent temperature and moisture value for a plot.I sampled plant biomass within a plot using anondestructive method. I counted the number of plantspecies within each plot and estimated the percent of theplot area covered by each species. At the time ofsampling, I also estimated the percentage of a 0.1-m2

quadrat area covered by monocultures of each plantspecies outside of the 2-m2 plots and clipped those plantsat ground level, dried them at 608C for 48 hours andweighed them to estimate plant species biomass persquare meter. I obtained five random samples andestimate their average. This value was then multiplied bythe plot estimate of percentage cover to estimate plantspecies biomass and total plant biomass in each 2-m2

plot. I estimated plant species evenness (an index ofdiversity that accounts for plant dominance effects) for

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each enclosure using the standard Shannon index J 0 !("R pi log pi )/log S where pi is the proportion of totalenclosure plant biomass represented by plant species iand S is the total number of plant species within anenclosure.I measured decomposition using a standard litterbag

method. Samples of loose, dead plant matter werecollected from the soil surface within each mesocosm inearly spring before the onset of growing conditions.Random subsamples of plant matter were weighed andsealed into 535 cm litter bags made of fiberglass windowscreening. The bags were returned to their respectivemesocosms. Each month from April until September oneset of litter bags was collected from each plot, dried andweighed to measure decay rates (Appendix A). Beginningin June, I measured N mineralization by obtaining fromeach enclosure two 10 cm23 15 cm long soil cores takenbelow the organic layer. One core was taken to the laband within 24 hours was extracted with 2 mol/L KCl tomeasure ammonium and nitrate content using anautomated flow analyzer. A companion core was sealedin a polyethylene bag, returned it to its original hole toincubate in the field for 60 days after which it wasextracted for analysis of ammonium and nitrate content.N mineralization rate was estimated by subtracting theinitial quantity of inorganic N from the post-incubationquantity and dividing by the length of the incubationperiod (Hart et al. 1994). I measured ANPP by randomly

selecting one 0.05-m2 circular area within each plot inMay, clipping all aboveground green biomass and thenplacing a 0.05 m2 3 1.5 m circular cage covered withaluminum screening over the clipped area to excludeherbivores. This caging method is necessary (McNaugh-ton et al. 1996) to remove biases in net primaryproduction estimates caused by direct herbivory. InAugust, I removed each cage, clipped all formerlyenclosed live biomass to ground level, dried thevegetation at 608C for 48 h and then weighed it. ANPPwas estimated as the final biomass within each cagedivided by the growth period.

Experimental stocking.—Experimental manipulationbegan in late May 2006 by enclosing each plot with a1.5 m high 3 2 m2 wire frame cylindrical mesocosmcovered with 65 mm mesh aluminum window screensunk 6 cm into the ground. I stocked the mesocosmswith predator species using a replacement series design,which holds total predator density constant but variesrelative abundance of the different predators, for threereasons. First, natural history sampling within the field(O. J. Schmitz, unpublished data) revealed that totalpredator density among 2-m2 sampling plots varied little(CV ! 0.16, n ! 10) whereas the relative abundance ofthe predator species (i.e., predator species dominance)varied much more (CV ! 0.54, n ! 10). Second, it hasbeen suggested that changing species dominance is animportant way to understand diversity–function rela-

PLATE 1. The sit-and-wait hunting spider Pisaurina mira is a key predator of herbivore grasshoppers in a New England old-fieldecosystem. By scaring grasshoppers into refuge habitats, it causes important indirect effects on plant community structure andecosystem function. This spider, together with an actively hunting spider, is the focus of research on predator functional diversityeffects on an old-field ecosystem function. Photo credit: Brandon Barton.

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tionships (Petchey and Gaston 2006). Third, a replace-ment series design serves as a benchmark for predatordiversity studies (Sih et al. 1998, Schmitz 2007) becausethe combined species effects should be the weightedmean of the corresponding individual species effects, ifpredator effects are linear. Deviations from this averageindicate nonlinear effects (Sih et al. 1998, Schmitz 2007).Predator species dominance was changed by varying

the ratio of actively hunting and sit-and-wait spidersamong five treatments (4:0; 3:1; 2:2; 1:3; 0:4). In earlyJune, focal spider and grasshopper species were stockedinto the cages according to their preassigned treatments.I stocked four predators and 10 grasshoppers to eachmesocosms, densities that approximate average Junefield densities of 2 spiders/m2 and 5 grasshoppers/m2

(Schmitz and Sokal Hessner 2002). The treatments wereallowed to run their course for the season.

The spider and grasshopper species typically undergoannual life cycles in which they emerge as juveniles andin spring, grow to adults over the course of the growingseason, reproduce, and die. The mesocosm size waschosen to offer a balance between obtaining a detailedunderstanding of species abundances and function andenabling ecosystem dynamics to run their course. Onelimitation of the mesocosm size is that spiders andgrasshoppers may not have reproduced sufficiently tostart conditions anew at the beginning of 2007. Itherefore monitored the number of emerging grasshop-pers and spiders in spring 2007 and stocked additionalindividuals as needed to reproduce average Junedensities.Sampling response variables.—Each year for two years

I used the methods described above to make monthlymeasurements of soil moisture and soil temperature,total plant biomass, plant species biomass and plantspecies diversity in each mesocosm between May andOctober. In 2007, I also measured decomposition, Nmineralization and ANPP using methods describedabove. Additional samples of plant litter were analyzedfor quality (C:N ratio) using a CHN autoanalyzer.Statistical analyses.—I tested for differences in initial

conditions among treatment plots using ANOVA inSYSTAT 9 for Windows (Systat, Chicago, Illinois,USA). I evaluated whether or not there were directionaltrends in the magnitude of community and ecosystemproperties and ecosystem functions along the predatorspecies dominance gradient using linear regression inSYSTAT 9 for Windows. For all significant trends, Iestimated the expected treatment effect as the average ofthe individual predator species effects weighted by theirproportion in each treatment. I then compared theexpected and observed mean treatment values usinglinear regression: a nonsignificant relationship wouldindicate nonlinear predator functional diversity effects.

RESULTS

ANOVA revealed that initially there were no signifi-cant differences among treatment locations in any of theseven biotic and abiotic variables or ecosystem functions(Appendix A: all P . 0.20). Regression analysis revealedthat total plant biomass (Fig. 1a) did not differ amongexperimental treatments after two years of predatormanipulation (P! 0.25). But, S. rugosa abundance (Fig.1b) decreased (P, 0.0008; F!8.355; df!1, 33) and plantspecies evenness (Fig. 1c) increased (P , 0.004; F! 9.26;df!1, 33) with decreasing proportion of actively huntingspiders in the system. Regression revealed that litter C:Nratio increased (i.e., litter quality declined) significantly(P!0.004; F!9.15; df!1, 33) with declining proportionof active hunting predators (Fig. 2). There was nosignificant treatment effect on decomposition (P! 0.53).But, there was a significant decline in N mineralizationrate (P! 0.05; F! 4.5; df! 1, 33) and ANPP (P! 0.01;F! 7.58; df! 1, 33) with declining proportion of activehunting predators in the system (Fig. 2).

FIG. 1. Effect of experimentally manipulating the relativeabundance of two spider predator species with differentfunctional identities (active hunting vs. sit-and-wait) on (a)total plant biomass, (b) the percentage of the total plant biomassrepresented by the competitive dominant plant Solidago rugosa,and (c) plant species evenness in the Connecticut grasslandecosystem. The dotted lines in panels (b) and (c) representexpected effects based on the weighted mean of the individualpredator species’ effects. Values are means and SE.


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The expected trends in community properties orecosystem functions are plotted in Figs. 1 and 2 forthose variables that varied significantly with predatorfunctional dominance. In all cases, except ANPP, theexpected and observed relationships were significant (P, 0.05, df ! 1, 3). The expected values explained muchvariation in community properties (S. rugosa abundanceR2 ! 0.91; plant species evenness R2 ! 0.89) but lessvariation in ecosystem properties and functions (litterquality R2 ! 0.87; mineralization R2 ! 0.68).

DISCUSSION

Predators influence the functioning of this grasslandecosystem via the plant-based chain running frompredators, to grasshoppers to S. rugosa to plantcommunity composition (Schmitz 2003, 2008a). Plantcommunity composition in turn determines the qualityand quantity of plant matter entering the soil organicmatter pool to be decomposed and mineralized asnitrogen which in turn affects primary production(Schmitz 2008a). This study revealed that changingpredator functional identity and dominance (diversity)caused quantitative changes in community and ecosys-tem properties and levels of ecosystem functions alongthe effect chain. Moreover, for many of the variables, theweighted average of the individual predator effectsoffered a good approximation of the observed average

effect of predator functional diversity. But, the degree ofreliability in the approximation (i.e., variation explainedby the expected values) diminished the further down thecausal chain of effect one measured the response. Inretrospect, such an outcome is expected. The spiderpredators directly influence the way M. femurrubrumgrasshoppers impact the plant community and therebyhave a strong indirect effect on the quantity and qualityof plant material entering the soil organic matter pool(Schmitz 2008a). But, soil organisms and biophysical soilproperties will increasingly come into play to determinelitter breakdown, mineralization, and resource availabil-ity to plants for production, thereby weakening top-down indirect effects propagated along the plant-basedchain (Wardle 2002, Hattenschwiler et al. 2005).

The pronounced changes in ecosystem properties andfunctions across the predator dominance gradient arosefrom seemingly small changes in plant species evenness(0.75–0.84: Fig. 1c). Nevertheless, this range of values inthe experimental enclosures matches that observedacross various fields in the vicinity of the study site(Appendix A) and other systems reporting appreciableeffects of top predator manipulations on plant commu-nity diversity (Appendix B). Moreover, the levels ofANPP observed across the range of plant evennessvalues in this study match those in a similar grasslandsystem but for which plant species evenness was

FIG. 2. Effect of experimentally manipulating the relative abundance of two spider predator species with different functionalidentities (active hunting vs. sit-and-wait) on (a) plant litter quality (C:N ratio), (b) plant litter decomposition, (c) nitrogenmineralization, and (d) aboveground net primary production (ANPP). The dotted lines represent expectations based on theweighted mean effects of the individual predator species. Expected trend lines are presented only for ecosystem functions orproperties that showed a significant treatment effect. A dagger represents a marginally significant deviation from the expected trend(t tests: 0.10 . P . 0.05). Values are mean and SE.


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explicitly manipulated in the absence of consumers(Wilsey and Potvin 2000). This reinforces the pointmade by Duffy (2003) that cascading effects of predatorsmay cause magnitudes of changes in plant compositionand ecosystem function that rival those observed instudies simply manipulating plant species diversity in theabsence of consumers.Theoretically, in a replacement series experiment the

net collective effect of predator species could simply bethe weighted average of the individual species effects (alinear effect); or predator species could interact syner-gistically or antagonistically leading to nonlinear effects(Sih et al. 1998, Ives et al. 2005, Casula et al. 2006,Schmitz 2007). Intuitively, the distinct functionaldifferences of each spider species would suggest anonlinear predator diversity effect. Counter to suchintuition, plant community and ecosystem variablesvaried linearly (weighted average of the individualpredator effects) with predator functional diversity.One explanation for this outcome involves considerationof the habitat domain, defined as spatial extent ofhabitat use, by the predators and prey (Schmitz 2007).The sit-and-wait P. mira occupies the upper canopy andthe actively hunting P. rimator the entire middle of thecanopy. This spatially complementary juxtapositionmeans that there is little if any opportunity for the twopredator species to engage in interspecific interactionsthat would cause nonlinear reductions in top-downeffects (Schmitz 2007). The grasshopper roams through-out the canopy and thereby effectively experiences theaverage of the predation risk posed by the two specieswithin its habitat. Thus, the mortality rates experiencedby the grasshopper vary in proportion to the weightedaverage abundance of the two predator species (Sokol-Hessner and Schmitz 2002). Moreover, the grasshopperbehavioral shifts vary in proportion to the abundance ofdifferent predator species leading to predator diversityeffects on plants, especially the dominant plant S.rugosa, that are the weighed average of the individualpredator species effects (Schmitz and Sokol-Hessner2002). Plant dominance effects in general are importantdeterminants of ecosystem structure and function (Smithet al. 2004, Wilsey et al. 2005, Hillebrand et al. 2008)which may explain why mediation of plant dominancevia trophic interactions in this system links the averagingeffects of predator diversity on the plant community toaveraging effects on ecosystem function.An alternative hypothesis for the linear trend is that the

effect of the active hunting spiderP. rimatorwas swampedout by the behavioral response of the grasshopper in theface of the sit-and-wait spider P. mira. In this case, theexpectation would be that increasing P. mira densitystrengthens the behavioral response of the grasshopperleading to a linear increase in grasshopper damage to S.rugosa with attendant effects on plant community andecosystem properties. However, evidence from previousresearch in this study system suggests that this hypothesisis probably not tenable (Appendix C).

One could argue that the averaging effects of predatorspecies observed in this experiment are a consequence ofworking in a simple system such that predator effects onthe ecosystem were effectively channeled through asingle intermediate herbivore species and a singlecompetitive dominant plant. But, averaging effects seemto play out in a system with greater intermediate speciesdiversity in which predator hunting mode and habitatdomain are also known. In streams of northern Europe,two predatory fish, stone loach (Barbatula barbatula)and brown trout (Salmo trutta) have identical huntingmodes (active) but they have complementary habitatdomains where the loach resides near the benthic zoneand the trout resides in the water column (Nilsson et al.2008). Experimentation in artificial stream channels thatemulate natural streams examined the effects of thesepredators, individually and in combination, on theirmajor invertebrate prey and on algal production. Thestudy revealed that predators enhanced algal production(Nilsson et al. 2008). Moreover, the combined predatoreffect was the average of the individual predator effectsand was brought about largely by nonconsumptiverather than consumptive predator effects.These findings add to our capacity to undertake trait-

based forecasting of biodiversity’s effect on ecosystemfunction (Naeem 2008). Even though the empiricalexamples preclude making any broad generalizations,the examples nonetheless provide some proof-of-conceptfor a conceptual framework (Schmitz 2007, 2008b)about how predator functional diversity is linked tovariation in ecosystem function. To the extent thatbehavioral traits of predators and prey offer a generalframework for understanding biodiversity–ecosystemfunction relationships, then this trait-based approach(Schmitz 2007, 2008a) makes biologically plausiblepredictions that are amenable to further testing acrossecosystem types (Naeem 2008).

ACKNOWLEDGMENTS

I thank B. Barton, N. David, D. Hawlena, and K. Kidd forhelp with the field work. D. Hawlena, H. Jones, and twoanonymous reviewers provided helpful comments. The studywas supported by NSF Grant DEB 0515014.

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APPENDIX A

Initial ecosystem properties and functions and methods used to calculate litter decomposition rate (Ecological Archives E090-163-A1).

APPENDIX B

Effects of predator manipulations on plant species evenness across ecosystems (Ecological Archives E090-163-A2).

APPENDIX C

Consideration of alternative hypotheses for linear effects of predator functional diversity on ecosystem properties and functions(Ecological Archives E090-163-A3).


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