OSWALD J. SCHMITZ 1. School of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, Connecticut USA

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1 Reports Ecology, 90(9), 2009, pp Ó 2009 by the Ecological Society of America Effects of predator functional diversity on grassland ecosystem function OSWALD J. SCHMITZ 1 School of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, Connecticut USA Abstract. Predator species individually are known to have important effects on plant communities and ecosystem functions such as production, decomposition, and elemental cycling, 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 combine to influence ecosystem function. I report on an experiment conducted in a New England grassland ecosystem that quantified the net effects of a sit-and-wait and an actively hunting spider species on the plant composition and functioning of a New England grassland ecosystem. I manipulated predator functional diversity by varying the dominance ratio of the two predator species among five treatments using a replacement series design. Experimentation revealed that predator functional diversity effects propagated down the live plant-based chain to affect the levels of plant diversity, and plant litter quality, elemental cycling, and production. Moreover, many of these effects could be approximately by the weighted average of the individual predator species effects, suggesting that this kind of predator diversity effect on 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 be highly dependent on their functional identity determined by their traits (Chapin et al. 1997, Duffy 2002, Chalcraft and 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 contemporary ecology is to resolve how combinations of species with different functional traits a form of species diversity influence ecosystem properties and functions (Loreau et al. 2001, Schmid et al. 2001, Hooper et al. 2005, Wright et al. 2006). Most research directed toward understanding this interplay focuses on functional diversity at the plant soil interface (Wardle 2002, Hattenschwiler et al. 2005, Hooper et al. 2005). Yet, assessments of biodiversity ecosystem functioning relationships will be incomplete without considering diversity within higher trophic levels of ecosystems (Cardinale et al. 2006, Duffy et al. 2007). Manuscript received 15 October 2008; revised 31 March 2009; accepted 7 April Corresponding Editor: B. J. Fox. 1 oswald.schmitz@yale.edu 2339 For instance, predator species individually can have important indirect effects on the species composition of plant communities (Schmitz 2008b). Because plant species composition is an important regulating factor of ecosystem function (Chapin et al. 1997, Loreau et al. 2001), it follows that predator species should have important indirect effects on ecosystem functions, which they do (Downing and Leibold 2002, Duffy 2003, Fukami et al. 2006, Maron et al. 2006, Canuel et al. 2007, Schmitz 2008a). But, predators can propagate these indirect effects in at least two ways (Schmitz 2007). They can alter the numerical abundance of herbivore prey by capturing and consuming them. Alternatively, their mere presence in a system can trigger herbivore prey to modify foraging activity in a manner that reduces predation risk. These different kinds of effect are related to one particular functional trait, predator hunting mode, irrespective of taxonomic identity (Schmitz 2007). Sit-and-wait ambush predators cause largely evasive behavioral responses in their prey because prey species respond strongly to persistent, point-source cues of predator presence. Widely roaming, actively hunting predators may reduce prey density, but they exert highly variable predation risk cues and are thus unlikely to

2 2340 OSWALD J. SCHMITZ Ecology, Vol. 90, No. 9 cause chronic behavioral responses in their prey. Natural systems contain predators with both kinds of functional traits (Schmitz 2007), yet it remains uncertain what the net effect of such functional diversity will be on ecosystem function (Schmitz 2007, Bruno and Cardinale 2008). Here I report on a three-year experiment in a New England grassland ecosystem that quantified the collective effects two hunting spider predators with different hunting modes on plant community composition and three ecosystem functions: aboveground net primary productivity (ANPP), plant litter decomposition rate (decomposition), and nitrogen mineralization rate (mineralization). I evaluated the role of predator functional diversity by comparing effects of each predator species singly and in combination. Following recommendations (Petchey and Gatson 2006), I also manipulated predator species dominance (evenness) by changing the relative proportion of the different species. METHODS Natural history The experiment, carried out in a grassland ecosystem in northeastern Connecticut, USA, focused on the dominant interacting species in this system (Schmitz 2003): old-field plants, the generalist grasshopper Melanoplus femurrubrum and hunting spider predators Pisaurina 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 of M. femurrubrum (Schmitz 2003); (2) the herb Solidago rugosa, which provides M. femurubrum refuge from spider predation (Schmitz 2003) and, because of its competitive dominance, is an important determinant of plant species diversity and level of ecosystem function (Schmitz 2008a); and (3) a variety of other herb species including Trifolium repens, Potentilla simplex, Rudbekia hirta, Crysanthemum leucanthemum, and Daucus carota that 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 canopy of the field (Schmitz 2007). Grasshopper mortality due to predation is compensatory to natural mortality in the presence of P. mira (Sokol-Hessner and Schmitz 2002). The spider causes grasshoppers to reduce their foraging on grasses and to seek refuge in and forage on the leafier S. rugosa (Schmitz 2003) which in turn leads to a positive indirect effect on grasses, a negative indirect effect on S. rugosa and a positive indirect effect on other herbs owing to competitive release from S. rugosa (Schmitz 2008a). The widely roaming active hunting P. rimator does not cause chronic foraging shifts by grasshoppers (Sokol-Hessner and Schmitz 2002). Instead, this predator has an additive effect on grasshopper mortality (Sokol-Hessner and Schmitz 2002) that translates 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 competitive dominant plant S. rugosa and plant species diversity vary linearly with the relative proportion of P. mira and P. rimator spiders among several fields in the vicinity of the experimental field site, suggesting that predator diversity effects may explain the field pattern. Study design I examined the indirect effects of predator functional diversity on plant community composition and decomposition, N-mineralization, and ANPP. Thus, the patterns of plant diversity and ensuing ecosystem functions were deliberately allowed to emerge as a consequence of the manipulations; they were not manipulated directly as part of the experiment. The experiment involved 35 cylindrical mesocosms, 1.5 m high 3 2 m 2, placed over naturally growing vegetation in the field. The mesocosms were arranged in seven replicate blocks with five treatments (different combinations of the predator species) randomly assigned to mesocosms within each block. The first year of the study (2005) was devoted to assigning plots for mesocosm placement and measuring initial conditions within each plot. The subsequent two years involved manipulation of predator diversity and measurement of ecosystem responses within the mesocosms. Initial conditions. I measured seven community and ecosystem properties and three ecosystem functions within each plot: soil moisture, soil temperature, total plant biomass, S. rugosa biomass grass biomass, other herb biomass, plant diversity, decomposition, N mineralization, and ANPP. I measured soil surface temperature using a Digi- Sense 8523 thermistor thermometer (Cole-Parmer Instrument Company, Chicago, Illinois, USA), coupled to a soil probe accurate to 0.18C that was immersed 5 cm into the soil. I measured soil moisture (percentage of water content) using a Dynamax ML2x Theta Probe (Dynamax, Inc., Houston, Texas, USA). I measured each variable at five random locations within each plot and then estimated the plot average to obtain an independent temperature and moisture value for a plot. I sampled plant biomass within a plot using a nondestructive method. I counted the number of plant species within each plot and estimated the percent of the plot area covered by each species. At the time of sampling, I also estimated the percentage of a 0.1-m 2 quadrat area covered by monocultures of each plant species outside of the 2-m 2 plots and clipped those plants at ground level, dried them at 608C for 48 hours and weighed them to estimate plant species biomass per square meter. I obtained five random samples and estimate their average. This value was then multiplied by the plot estimate of percentage cover to estimate plant species biomass and total plant biomass in each 2-m 2 plot. I estimated plant species evenness (an index of diversity that accounts for plant dominance effects) for

3 September 2009 PREDATOR FUNCTIONAL DIVERSITY 2341 each enclosure using the standard Shannon index J 0 ¼ ("R pi log pi )/log S where pi is the proportion of total enclosure plant biomass represented by plant species i and S is the total number of plant species within an enclosure. I measured decomposition using a standard litterbag method. Samples of loose, dead plant matter were collected from the soil surface within each mesocosm in early spring before the onset of growing conditions. Random subsamples of plant matter were weighed and sealed into cm litter bags made of fiberglass window screening. The bags were returned to their respective mesocosms. Each month from April until September one set of litter bags was collected from each plot, dried and weighed to measure decay rates (Appendix A). Beginning in June, I measured N mineralization by obtaining from each enclosure two 10 cm cm long soil cores taken below the organic layer. One core was taken to the lab and within 24 hours was extracted with 2 mol/l KCl to measure ammonium and nitrate content using an automated flow analyzer. A companion core was sealed in a polyethylene bag, returned it to its original hole to incubate in the field for 60 days after which it was extracted for analysis of ammonium and nitrate content. N mineralization rate was estimated by subtracting the initial quantity of inorganic N from the post-incubation quantity and dividing by the length of the incubation period (Hart et al. 1994). I measured ANPP by randomly selecting one 0.05-m2 circular area within each plot in May, clipping all aboveground green biomass and then placing a 0.05 m m circular cage covered with aluminum screening over the clipped area to exclude herbivores. This caging method is necessary (McNaughton et al. 1996) to remove biases in net primary production estimates caused by direct herbivory. In August, I removed each cage, clipped all formerly enclosed live biomass to ground level, dried the vegetation at 608C for 48 h and then weighed it. ANPP was estimated as the final biomass within each cage divided by the growth period. Experimental stocking. Experimental manipulation began in late May 2006 by enclosing each plot with a 1.5 m high 3 2 m2 wire frame cylindrical mesocosm covered with 65 mm mesh aluminum window screen sunk 6 cm into the ground. I stocked the mesocosms with predator species using a replacement series design, which holds total predator density constant but varies relative abundance of the different predators, for three reasons. First, natural history sampling within the field (O. J. Schmitz, unpublished data) revealed that total predator density among 2-m2 sampling plots varied little (CV ¼ 0.16, n ¼ 10) whereas the relative abundance of the predator species (i.e., predator species dominance) varied much more (CV ¼ 0.54, n ¼ 10). Second, it has been suggested that changing species dominance is an important way to understand diversity function rela- Reports PLATE 1. The sit-and-wait hunting spider Pisaurina mira is a key predator of herbivore grasshoppers in a New England old-field ecosystem. By scaring grasshoppers into refuge habitats, it causes important indirect effects on plant community structure and ecosystem function. This spider, together with an actively hunting spider, is the focus of research on predator functional diversity effects on an old-field ecosystem function. Photo credit: Brandon Barton.

4 2342 OSWALD J. SCHMITZ Ecology, Vol. 90, No. 9 FIG. 1. Effect of experimentally manipulating the relative abundance of two spider predator species with different functional identities (active hunting vs. sit-and-wait) on (a) total plant biomass, (b) the percentage of the total plant biomass represented by the competitive dominant plant Solidago rugosa, and (c) plant species evenness in the Connecticut grassland ecosystem. The dotted lines in panels (b) and (c) represent expected effects based on the weighted mean of the individual predator species effects. Values are means and SE. tionships (Petchey and Gaston 2006). Third, a replacement series design serves as a benchmark for predator diversity studies (Sih et al. 1998, Schmitz 2007) because the combined species effects should be the weighted mean of the corresponding individual species effects, if predator effects are linear. Deviations from this average indicate nonlinear effects (Sih et al. 1998, Schmitz 2007). Predator species dominance was changed by varying the ratio of actively hunting and sit-and-wait spiders among five treatments (4:0; 3:1; 2:2; 1:3; 0:4). In early June, focal spider and grasshopper species were stocked into the cages according to their preassigned treatments. I stocked four predators and 10 grasshoppers to each mesocosms, densities that approximate average June field densities of 2 spiders/m 2 and 5 grasshoppers/m 2 (Schmitz and Sokal Hessner 2002). The treatments were allowed to run their course for the season. The spider and grasshopper species typically undergo annual life cycles in which they emerge as juveniles and in spring, grow to adults over the course of the growing season, reproduce, and die. The mesocosm size was chosen to offer a balance between obtaining a detailed understanding of species abundances and function and enabling ecosystem dynamics to run their course. One limitation of the mesocosm size is that spiders and grasshoppers may not have reproduced sufficiently to start conditions anew at the beginning of I therefore monitored the number of emerging grasshoppers and spiders in spring 2007 and stocked additional individuals as needed to reproduce average June densities. Sampling response variables. Each year for two years I used the methods described above to make monthly measurements of soil moisture and soil temperature, total plant biomass, plant species biomass and plant species diversity in each mesocosm between May and October. In 2007, I also measured decomposition, N mineralization and ANPP using methods described above. Additional samples of plant litter were analyzed for quality (C:N ratio) using a CHN autoanalyzer. Statistical analyses. I tested for differences in initial conditions among treatment plots using ANOVA in SYSTAT 9 for Windows (Systat, Chicago, Illinois, USA). I evaluated whether or not there were directional trends in the magnitude of community and ecosystem properties and ecosystem functions along the predator species dominance gradient using linear regression in SYSTAT 9 for Windows. For all significant trends, I estimated the expected treatment effect as the average of the individual predator species effects weighted by their proportion in each treatment. I then compared the expected and observed mean treatment values using linear regression: a nonsignificant relationship would indicate nonlinear predator functional diversity effects. RESULTS ANOVA revealed that initially there were no significant differences among treatment locations in any of the seven biotic and abiotic variables or ecosystem functions (Appendix A: all P. 0.20). Regression analysis revealed that total plant biomass (Fig. 1a) did not differ among experimental treatments after two years of predator manipulation (P ¼ 0.25). But, S. rugosa abundance (Fig. 1b) decreased (P, ; F¼8.355; df¼1, 33) and plant species evenness (Fig. 1c) increased (P, 0.004; F ¼ 9.26; df ¼ 1, 33) with decreasing proportion of actively hunting spiders in the system. Regression revealed that litter C:N ratio increased (i.e., litter quality declined) significantly (P ¼ 0.004; F ¼ 9.15; df ¼ 1, 33) with declining proportion of active hunting predators (Fig. 2). There was no significant treatment effect on decomposition (P ¼ 0.53). But, there was a significant decline in N mineralization rate (P ¼ 0.05; F ¼ 4.5; df ¼ 1, 33) and ANPP (P ¼ 0.01; F ¼ 7.58; df ¼ 1, 33) with declining proportion of active hunting predators in the system (Fig. 2).

5 September 2009 PREDATOR FUNCTIONAL DIVERSITY 2343 FIG. 2. Effect of experimentally manipulating the relative abundance of two spider predator species with different functional identities (active hunting vs. sit-and-wait) on (a) plant litter quality (C:N ratio), (b) plant litter decomposition, (c) nitrogen mineralization, and (d) aboveground net primary production (ANPP). The dotted lines represent expectations based on the weighted mean effects of the individual predator species. Expected trend lines are presented only for ecosystem functions or properties that showed a significant treatment effect. A dagger represents a marginally significant deviation from the expected trend (t tests: P. 0.05). Values are mean and SE. The expected trends in community properties or ecosystem functions are plotted in Figs. 1 and 2 for those variables that varied significantly with predator functional dominance. In all cases, except ANPP, the expected and observed relationships were significant (P, 0.05, df ¼ 1, 3). The expected values explained much variation in community properties (S. rugosa abundance R 2 ¼ 0.91; plant species evenness R 2 ¼ 0.89) but less variation in ecosystem properties and functions (litter quality R 2 ¼ 0.87; mineralization R 2 ¼ 0.68). DISCUSSION Predators influence the functioning of this grassland ecosystem via the plant-based chain running from predators, to grasshoppers to S. rugosa to plant community composition (Schmitz 2003, 2008a). Plant community composition in turn determines the quality and quantity of plant matter entering the soil organic matter pool to be decomposed and mineralized as nitrogen which in turn affects primary production (Schmitz 2008a). This study revealed that changing predator functional identity and dominance (diversity) caused quantitative changes in community and ecosystem properties and levels of ecosystem functions along the effect chain. Moreover, for many of the variables, the weighted average of the individual predator effects offered a good approximation of the observed average effect of predator functional diversity. But, the degree of reliability in the approximation (i.e., variation explained by the expected values) diminished the further down the causal chain of effect one measured the response. In retrospect, such an outcome is expected. The spider predators directly influence the way M. femurrubrum grasshoppers impact the plant community and thereby have a strong indirect effect on the quantity and quality of plant material entering the soil organic matter pool (Schmitz 2008a). But, soil organisms and biophysical soil properties will increasingly come into play to determine litter breakdown, mineralization, and resource availability to plants for production, thereby weakening topdown indirect effects propagated along the plant-based chain (Wardle 2002, Hättenschwiler et al. 2005). The pronounced changes in ecosystem properties and functions across the predator dominance gradient arose from seemingly small changes in plant species evenness ( : Fig. 1c). Nevertheless, this range of values in the experimental enclosures matches that observed across various fields in the vicinity of the study site (Appendix A) and other systems reporting appreciable effects of top predator manipulations on plant community diversity (Appendix B). Moreover, the levels of ANPP observed across the range of plant evenness values in this study match those in a similar grassland system but for which plant species evenness was

6 2344 OSWALD J. SCHMITZ Ecology, Vol. 90, No. 9 explicitly manipulated in the absence of consumers (Wilsey and Potvin 2000). This reinforces the point made by Duffy (2003) that cascading effects of predators may cause magnitudes of changes in plant composition and ecosystem function that rival those observed in studies simply manipulating plant species diversity in the absence of consumers. Theoretically, in a replacement series experiment the net collective effect of predator species could simply be the weighted average of the individual species effects (a linear effect); or predator species could interact synergistically or antagonistically leading to nonlinear effects (Sih et al. 1998, Ives et al. 2005, Casula et al. 2006, Schmitz 2007). Intuitively, the distinct functional differences of each spider species would suggest a nonlinear predator diversity effect. Counter to such intuition, plant community and ecosystem variables varied linearly (weighted average of the individual predator effects) with predator functional diversity. One explanation for this outcome involves consideration of the habitat domain, defined as spatial extent of habitat use, by the predators and prey (Schmitz 2007). The sit-and-wait P. mira occupies the upper canopy and the actively hunting P. rimator the entire middle of the canopy. This spatially complementary juxtaposition means that there is little if any opportunity for the two predator species to engage in interspecific interactions that would cause nonlinear reductions in top-down effects (Schmitz 2007). The grasshopper roams throughout the canopy and thereby effectively experiences the average of the predation risk posed by the two species within its habitat. Thus, the mortality rates experienced by the grasshopper vary in proportion to the weighted average abundance of the two predator species (Sokol- Hessner and Schmitz 2002). Moreover, the grasshopper behavioral shifts vary in proportion to the abundance of different predator species leading to predator diversity effects on plants, especially the dominant plant S. rugosa, that are the weighed average of the individual predator species effects (Schmitz and Sokol-Hessner 2002). Plant dominance effects in general are important determinants of ecosystem structure and function (Smith et al. 2004, Wilsey et al. 2005, Hillebrand et al. 2008) which may explain why mediation of plant dominance via trophic interactions in this system links the averaging effects of predator diversity on the plant community to averaging effects on ecosystem function. An alternative hypothesis for the linear trend is that the effect of the active hunting spider P. rimator was swamped out by the behavioral response of the grasshopper in the face of the sit-and-wait spider P. mira. In this case, the expectation would be that increasing P. mira density strengthens the behavioral response of the grasshopper leading to a linear increase in grasshopper damage to S. rugosa with attendant effects on plant community and ecosystem properties. However, evidence from previous research in this study system suggests that this hypothesis is probably not tenable (Appendix C). One could argue that the averaging effects of predator species observed in this experiment are a consequence of working in a simple system such that predator effects on the ecosystem were effectively channeled through a single intermediate herbivore species and a single competitive dominant plant. But, averaging effects seem to play out in a system with greater intermediate species diversity in which predator hunting mode and habitat domain are also known. In streams of northern Europe, two predatory fish, stone loach (Barbatula barbatula) and brown trout (Salmo trutta) have identical hunting modes (active) but they have complementary habitat domains where the loach resides near the benthic zone and the trout resides in the water column (Nilsson et al. 2008). Experimentation in artificial stream channels that emulate natural streams examined the effects of these predators, individually and in combination, on their major invertebrate prey and on algal production. The study revealed that predators enhanced algal production (Nilsson et al. 2008). Moreover, the combined predator effect was the average of the individual predator effects and was brought about largely by nonconsumptive rather than consumptive predator effects. These findings add to our capacity to undertake traitbased forecasting of biodiversity s effect on ecosystem function (Naeem 2008). Even though the empirical examples preclude making any broad generalizations, the examples nonetheless provide some proof-of-concept for a conceptual framework (Schmitz 2007, 2008b) about how predator functional diversity is linked to variation in ecosystem function. To the extent that behavioral traits of predators and prey offer a general framework for understanding biodiversity ecosystem function relationships, then this trait-based approach (Schmitz 2007, 2008a) makes biologically plausible predictions that are amenable to further testing across ecosystem types (Naeem 2008). ACKNOWLEDGMENTS I thank B. Barton, N. David, D. Hawlena, and K. Kidd for help with the field work. D. Hawlena, H. Jones, and two anonymous reviewers provided helpful comments. The study was supported by NSF Grant DEB LITERATURE CITED Bruno, J. F., and B. J. 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Schmid, and D. Tilman Conventional functional classification schemes underestimate the relationship with ecosystem functioning. Ecology Letters 9: APPENDIX A Initial ecosystem properties and functions and methods used to calculate litter decomposition rate (Ecological Archives E A1). APPENDIX B Effects of predator manipulations on plant species evenness across ecosystems (Ecological Archives E A2). APPENDIX C Consideration of alternative hypotheses for linear effects of predator functional diversity on ecosystem properties and functions (Ecological Archives E A3).