Native herbivores and environmental heterogeneity as mediators of an exotic grass invasion

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1 Received: 1 Septemer 216 Revised: 7 Decemer 216 Accepted: 18 Decemer 216 DOI: 1.12/ece ORIGINAL RESEARCH Ntive herivores nd environmentl heterogeneity s meditors of n exotic grss invsion Cody L. Ender 1 Croline E. Christin 2 J. Hll Cushmn 1 1 Deprtment of Biology, Sonom Stte University, Rohnert Prk, CA, USA 2 Deprtment of Environmentl Studies & Plnning, Sonom Stte University, Rohnert Prk, CA, USA Correspondence J. Hll Cushmn, Deprtment of Biology, Sonom Stte University, Rohnert Prk, CA, USA. Emil: cushmn@sonom.edu Funding informtion Cliforni Ntive Plnt Society; Sonom Stte University. Astrct Given tht mny exotic plnt species throughout the world re hving lrge ecologicl nd economic effects, it is vitl to understnd the forces tht medite their success in novel lndscpes. Both ntive herivores nd recipient ecosystems cn hve sustntil effects on the performnce of exotic plnt species, nd my interct with ech other or vry in their effects over time. Unfortuntely, few studies hve evluted the importnce of these kinds of context- dependent effects. Here, we use 17- yer- old exclosure experiment strtified cross costl grsslnd in northern Cliforni to ddress the reltive importnce of reintroduced mmmlin herivore, tule elk (Cervus cndensis nnnodes), nd environmentl heterogeneity in mediting the growth, undnce, nd recruitment of prolemtic grss invder, Holcus lntus. We found tht elk reduced Holcus undnce, oveground iomss, percent cover, frequency, nd seedling recruitment, ut tht these effects often vried mong hitt types, with effects eing greter in open grsslnds thn shru- dominted grsslnds. The performnce of Holcus popultions lso vried significntly mong hitt types, with the invder usully hving the gretest success in Bcchris- dominted grsslnds. Our results suggest tht environmentl heterogeneity hd much greter influence on Holcus success thn elk, nd tht these effects were due lrgely to soil ph nd moisture. The negtive effects of elk on Holcus ppered fter 4 yers nd did not intensify fter n dditionl 13 yers. Furthermore, despite their negtive effects, these prominent herivores did not prevent the spred of Holcus into previously uninvded res. Our reserch highlights the importnce of ssessing the individul nd interctive effects of ntive herivores nd environmentl heterogeneity on the success of invsive, exotic plnt species. It emphsizes the relity tht the negtive effects of herivores on exotic plnt species will often vry cross heterogeneous lndscpes nd my e insufficient to prevent the expnsion of these invders. KEYWORDS drivers of invsion, environmentl heterogeneity, Holcus lntus, invsive exotic grss, long-term effects, ntive ungulte herivores, soil chrcteristics This is n open ccess rticle under the terms of the Cretive Commons Attriution License, which permits use, distriution nd reproduction in ny medium, provided the originl work is properly cited. 217 The Authors. Ecology nd Evolution pulished y John Wiley & Sons Ltd. Ecology nd Evolution 217; 7:

2 1562 ENDER et l. 1 INTRODUCTION Given tht mny exotic plnt species throughout the world re hving lrge ecologicl nd economic effects, it is vitl to understnd the forces tht medite the success of invders in their recipient lndscpes. A wide vriety of hypotheses hve een proposed to explin the success of exotic species including enemy relese (Agrwl et l., 25; Inderjit & Chill, 215; Kene & Crwley, 22), iotic resistnce (Levine, Adler, & Yelenik, 24; Prker, Burkepile, & Hy, 26; Person, Potter, & Mron, 212), invder life history trits (Corin & D Antonio, 21; Rejmánek & Richrdson, 1996), nd resource vilility (Colutti, Grigorovich, & McIsc, 26; Dvis, Grime, & Thompson, 2; Koerner et l., 215). These iotic nd iotic drivers of invsion my operte simultneously in the sme system nd my interct with ech other in importnt wys, shedding light on the forces controlling the success of invders (Ctford, Jnsson, & Nilsson, 29). Herivores re n importnt iotic chrcteristic of recipient communities tht cn influence dominnt plnt invders through their ctivities s consumers, disturnce gents, dispersers, nd fertilizers (Mron & Vil, 21; Vvr, Prks, & Wisdom, 27). Given their potentil to impct exotic plnt species, herivores could e useful in mnging invsive popultions. For exmple, domestic livestock hve een incresingly used to restore degrded grsslnds dominted y exotic nnul grsses (Sker, Grydon, & Cushmn, 213; Sthlheer & D Antonio, 213). Once extirpted, reintroduced herivores lso hve the potentil to e effective tools for mnging plnt invsions (Johnson & Cushmn, 27; Polk & Sltz, 211). However, for oth domestic nd ntive herivores, studies hve reported widely vrying results of their impcts on exotic plnt popultions. Herivores cn promote (Dávlos, Nuzzo, & Blossey, 215; Klisz, Spigler, & Horvitz, 214; Knight, Dunn, Smith, Dvis, & Klisz, 29; Relv, Nunez, & Simerloff, 21; Vvr et l., 27), deter (Cse & Crwley, 2; Cushmn, Lortie, & Christin, 211; Eckerg, Tenhumerg, & Loud, 214; Prker et l., 26; Person et l., 212), or hve no effect on the dominnce of exotic plnt species (Stohlgren, Schell, & Vnden Heuvel, 1999). Predicting the conditions under which herivores suppress versus promote invsion is criticl not only for effective use in restortion, ut lso for the study of iologicl invsions in generl. Differences in iotic nd iotic chrcteristics of recipient ecosystems my drive much of the oserved vrition in the success of exotic plnts. By themselves, iotic conditions of given environment cn hve strong influences on the success of plnt invders (Dvis et l., 2; Weiher & Keddy, 1995). In ddition, positive nd negtive interctions with memers of the recipient community cn further medite the success nd spred of exotic plnt species (Bdno, Villrroel, Bustmnte, Mrquet, & Cvieres, 27; Cushmn et l., 211; Mron & Connors, 1996). Both iotic nd iotic chrcteristics cn exhiit tremendous sptil heterogeneity, which is likely to medite the effects of herivores on invders (Bisws, Kotnen, Kmo, & Wgner, 214; Cushmn et l., 211; Mron & Vil, 21). However, few studies hve previously explored the degree to which effects of herivores on plnt invders vry cross heterogeneous lndscpes (ut see Bisws et l., 214; Stohlgren et l., 1999). Thus, to improve our ility to generlize out the importnce of herivores nd recipient environments in mediting the distriution of invsive species, it is impertive to exmine the effects of herivores long environmentl grdients nd in multiple hitt types. Plnt invsions hve strong temporl dimension nd the effects of herivores nd recipient communities on them re lso likely to vry over time (Wilson et l., 27). For exmple, there re often time lgs etween the introduction of n exotic tx nd the species ecoming invsive (Aikio, Duncn, & Hulme, 21; Lrkin, 212; Pyšek, Hulme, & Repulic, 25). Climtic vriility cn lso ply role in the spred of invsive plnts nd interct with the effects of other drivers (Cr- Rivs, Sldñ, Cstro- Díez, & Gllien, 215; Tylor & Kumr, 216). Due to this vrition, the effects of herivores nd recipient environments on exotic plnts my increse, decrese, or chnge direction over time (Britton- Simmons & Aott, 28; Klisz et l., 214; Tkgi & Miyshit, 215; Tng, Go, Wng, Zho, & Li, 212). In ddition, the impcts of herivores on plnt invders my rise soon fter introduction or my ecome pprent (or intensify) only fter some mount of time hs pssed. Thus, incorporting temporl dimension hs the potentil to forwrd our understnding of how herivores nd recipient environments influence invsion processes. In this study, we used 17- yer- old experiment strtified cross heterogeneous lndscpe to exmine the effects of tule elk (Cervus cndensis nnnodes), reintroduced ntive herivore, on n invsive exotic perennil grss, Holcus lntus, long the cost of northern Cliforni. Our reserch ddressed the following three questions: (1) Wht is the reltive importnce of ntive herivores nd recipient environments in mediting the growth, undnce, nd recruitment of dominnt exotic grss? (2) Cn the effects of heterogeneous lndscpe on invder success e explined y iotic soil differences? (3) Hs the Holcus invsion chnged over time nd hve herivores, environmentl heterogeneity, or their interction medited this trjectory? Answers to these questions will forwrd our generl understnding of the drivers of plnt invsions nd will help guide more effective efforts to control invsive perennil grsses in ltered lndscpes. Through their ctivities s herivores nd disturnce gents, we predict tht elk will hve negtive effects on the growth nd undnce of Holcus, ut positive effects on recruitment y incresing the vilility of sfe sites. We further predict tht elk will slow Holcus invsion ut tht their influences will e highly context dependent, eing greter in more ccessile open grsslnds thn in shru- dominted lndscpes where elk might e deterred y dense shru growth. 1.1 Study system Our reserch ws conducted on Tomles Point in Point Reyes Ntionl Seshore, pproximtely 65 km northwest of Sn Frncisco. Bordered y the Pcific Ocen nd Tomles By, Tomles Point is 1,3- h peninsul tht experiences Mediterrnen- type climte, with moderte riny winters nd cool, foggy summers with very little precipittion. The costl grsslnds on Tomles Point consist of oth ntive nd exotic herceous plnt species interspersed with

3 ENDER et l ntive shrus. Three distinct hitt types occur within our 3- h study re: Bcchris- dominted grsslnds, Lupinus- dominted grsslnds, nd open grsslnds. grsslnds occur on the Kehoe soil formtion (derived from Cretceous grnitic prent rock; Kshiwgi, 1985) nd re dominted y herceous species nd lrgely devoid of shrus (Johnson & Cushmn, 27). Bcchris- dominted grsslnds occur on suvrint of the Kehoe formtion (Kshiwgi, 1985) nd re chrcterized y herceous ptches mixed with dense stnds of Bcchris pilulris (Astercee), long- lived ntive shru (Johnson & Cushmn, 27). Lupinus- dominted grsslnds re locted on mix of soil formtions, either completely in Sirdrk snd (derived from Quternry dune sndstone prent rock) or mixture of Sirdrk snd nd Kehoe vrint (Kshiwgi, 1985). The ltter soils re extremely well- drined, resulting in much drier conditions in the Lupinusdominted grsslnds thn in Bcchris- dominted or open grsslnds (V. Dodge nd J. H. Cushmn, unpulished dt). Lupinus- dominted grsslnds re predominntly open res interspersed with shortlived, ntive, nitrogen- fixing shru, Lupinus roreus (Fcee). Tule elk (C. cndensis nnnodes) is ntive ungulte tht previously dominted much of costl nd centrl Cliforni. These herivores once numered 5, individuls, ut hunting nd lnd conversion during the Gold Rush rought them to the rink of extinction y the mid- 18s (McCullough, 1969). The drmtic decline prompted efforts to protect elk, olster their numers, nd reintroduce popultions to over 2 different sites in Cliforni. In 1978, 1 tule elk were reintroduced to designted wilderness re on Tomles Point. Following their reintroduction, the tule elk popultion grew rpidly for two decdes, reching pproximtely 45 individuls efore leveling off. Since 1998, the herd hs typiclly fluctuted etween 4 nd 6 individuls, lthough censuses etween 214 nd 216 indicted tht the popultion hs declined to fewer thn 3 nimls, possily due to prolonged drought (D. Press, unpulished dt). The diet of tule elk t Tomles Point consists primrily of herceous fors nd grsses, ut they lso consume shru folige during the winter months when there is less herceous mteril ville (Gogn & Brrett, 1995). The exotic perennil grss H. lntus (velvet grss; herefter referred to s Holcus) is originlly ntive to Eursi ut hs een introduced widely throughout the world. It is prticulrly prolemtic nd widespred throughout costl regions of Austrli nd the United Sttes, where it ws likely introduced s forge seed either intentionlly or s contminnt (Thompson & Turkington, 1988). The erliest record of Holcus in Cliforni is from Sn Frncisco in 1886 nd herri records show tht this grss ws found on the Point Reyes Peninsul y 1898 (dt provided y the prticipnts of the Consortium of Cliforni Herri; ucjeps.erkeley.edu/consortium/). This grss grows est in moist conditions, ut is le to withstnd moderte periods of drought nd is more susceptile to trmpling thn most psture plnts (Thompson & Turkington, 1988). Holcus hs ecome prolemtic nd widespred invder in Cliforni nd the Cliforni Invsive Plnt Council hs designted it s hving sustntil ecologicl impcts due to its ility to form dense monocultures tht reduce species richness of ntive grsses nd fors (Cl- IPC, 26; Corin & D Antonio, 21; Deck, Muir, & Struss, 213). 2 METHODS 2.1 Herivore- exclosure experiment This study centers round lrge- scle elk exclosure experiment locted on Tomles Point in Point Reyes Ntionl Seshore. Estlished y the Ntionl Prk Service nd US Geologicl Survey in 1998, the experiment occurs within 3- h re nd consists of m plots distriuted eqully mong three hitt types Bcchrisdominted, Lupinus- dominted, nd open grsslnds. Ech plot in the experiment is locted m from the Pcific Ocen. Within ech of the three hitt types, there re four pirs of plots, with one plot within ech pir rndomly ssigned fencing to exclude elk nd nother plot spced 3 m wy left unfenced to serve s control. The fencing tht surrounds ech exclosure plot is 2.5- m tll nd effectively excludes elk, ut not other smll- or mid- sized herivores such s deer or hres (J. H. Cushmn, personl oservtion). Other studies using this exclosure experiment hve shown tht elk exert mjor influences on the plnt community (Johnson & Cushmn, 27; Lee, Spsojevic & Cushmn, unpulished dt; Richter, Spsojevic & Cushmn, in review), smll mmmls (Ellis & Cushmn, in review), ground- dwelling rthropods (Cecil & Cushmn, unpulished dt), plnt functionl trits (Lee, Spsojevic & Cushmn, unpulished dt), nd soil chrcteristics (Dodge, Eviner & Cushmn, unpulished dt). 2.2 Holcus responses To ssess the effects of elk on n exotic perennil grss, we quntified growth, undnce, nd recruitment responses of Holcus in the exclosure experiment in lte My of 215, just efore this grss strted to senesce. We strtified cm qudrts within ech of the 24 plots in the exclosure experiment, uniformly spcing them in 4 3 grid. We restricted smpling to the center 3 3 m of ech plot to reduce edge effects nd smpled only in res without shru cover, relocting qudrts lnding eneth shru cnopies to the nerest open spce in order to remin consistent with methods used y Johnson nd Cushmn (27). In ech of the 12 qudrts per plot, we quntified the percent cover, frequency, nd undnce of Holcus. We ssessed percent cover using stndrd point- intercept smpling t 16 points within ech qudrt nd frequency within 25 cells of ech qudrt. Due to the clonl nture of Holcus, it ws often difficult to distinguish individul plnts. Following Johnson nd Cushmn (27), when estimting plnt undnce, we defined n individul s clump of culms nd tillers (young vegettive shoots) unttched to other clumps y stolons. We lso ssessed frequency nd undnce of Holcus seedlings nd juveniles smll, non- flowering plnts with no rnched culms in order to estimte successful recruitment from seed. We quntified oveground iomss using five cm qudrts strtified within ech plot in the center of ech plot nd in the center of ech plot qurter. We clipped ll non- woody plnts t ground level, seprted Holcus iomss from other plnt mtter, nd then dried the iomss for 48 hr t 6 C prior to weighing.

4 1564 ENDER et l. To ssess individul plnt responses, we quntified height, inflorescences per dult, nd specific lef re (SLA; lef re/dry mss). We mesured mximum height of Holcus nd the numer of inflorescences in ech of the 12 previously descried 5 5 cm qudrts per plot. In erly June of 215, we hrvested 1 fully formed, undmged Holcus leves from ech plot in order to quntify SLA, which is positively relted to numer of mesures of plnt performnce, including photosynthetic rte, lef nitrogen, nd reltive growth rte, nd negtively relted to lef longevity nd secondry compounds such s lignin (Pérez- Hrguindeguy et l., 213). We kept leves cool nd moist during trnsporttion from the field to the l nd llowed leves to rehydrte in wet pper towels efore processing. In the l, we scnned rehydrted leves nd mesured re of the lef lmin, excluding the lef sheth from scnned the plnt mteril using Imge- J softwre. After weighing dried leves (dried t 6 C for 48 hr), we clculted SLA s lef re per unit dry mss (cm 2 /g). 2.3 Dung deposition To generte n estimte of elk ctivity, we determined the mount of dung deposited in ech of the 12 control plots of the exclosure experiment in nine surveys conducted etween Jnury nd My 215. Ech survey consisted of whole- plot count nd quntified the length nd width of ech dung pile. The re of n ellipse ws used to estimte the re of ech dung pile (in our system, dung counts nd dung re were highly correlted). As pointed out y Riginos nd Grce (28), Young, Plmer, nd Gdd (25), nd others, dung counts re relile method for estimting the level of ctivity of mmmlin herivores within their hitts. 2.4 Soil ph nd moisture We quntified verge soil ph nd grvimetric moisture in ech of the 24 plots in the exclosure experiment to test whether these vriles predicted the success of Holcus. Between Ferury 215 nd April 216, we quntified soil ph t nine loctions on 3 3 grid in ech plot using Kelwy ph meter (Kelwy Products, Wyckoff, NJ, USA). In Mrch 215, we collected soil smples from these sme nine loctions strtified within ech plot to ssess soil grvimetric moisture. We used soil corer to collect smples from the top 1 cm of soil nd weighed the soil efore nd fter drying for 72 hr t 6 C. We quntified soil moisture s mss of oven- dried soil divided y mss of field- wet soil (g/g). 2.5 Sttisticl nlyses We nlyzed ech of the Holcus response vriles using liner mixed models in JMP 12 (SAS Institute, Cry, NC, USA), with elk (present or excluded), hitt type (Bcchris- dominted, Lupinus- dominted, nd open grsslnds) nd their interction s fixed effects nd plot pir (1 12) nested within hitt type s rndom effect. For ll response vriles except SLA, we nested qudrt within plot pir nd treted it s rndom effect. We used the Kenwrd Roger method (Kenwrd & Roger, 1997) to estimte error degrees of freedom, which is widely recognized s the most rigorous pproch when using liner mixed models (Bolker et l., 29; Kenwrd & Roger, 1997; Schlje, McBride, & Fellinghm, 22). For SLA, we used plot- level verges in our sttisticl nlyses. To ensure tht ssumptions for liner mixed models were met, we visully ssessed ll model residuls for pproximte normlity nd checked for homoscedsticity of residul plots. Totl undnce nd seedling undnce were squre- root- trnsformed nd seedling frequency ws squre- root- log- trnsformed to meet normlity ssumptions. If hitt type or ny interction terms were significnt in our models, we followed up with Tukey multiple comprison tests to evlute differences mong the mens. As outlined in Edwrds, Muller, Wolfinger, Qqish, nd Schenerger (28), we clculted the reltive effect size of ll fixed fctors in mixed models s mrginl R 2 vlues (=[no. of fixed effects F/df Den]/[1 + no. of fixed effects F/df Den], where F is the F rtio sttistic, nd df Den is denomintor degrees of freedom). These vlues descrie the proportion of vrince explined y fixed fctor on its own. In ddition to ssessing the influence of elk on Holcus vriles using ctegoricl predictor vrile (herivores present vs. excluded), we hve evluted whether vrying levels of elk ctivity, s estimted y dung cover in the control plots of the exclosure experiment, predicted the mgnitude of chnge in Holcus undnce, iomss, or cover due to elk. For ech of the 12 plot pirs (control nd exclosure plots), we quntified the mgnitude of the elk effect on Holcus using logresponse rtios (LRR: ln[holcus vlue in control plot/holcus vlue in exclosure plot]). We then nlyzed LRR vlues for Holcus undnce, iomss, cover, nd frequency using ANCOVAs, with dung re, hitt type, nd their interction s fixed effects. We used multiple regression nlysis to determine the degree to which soil ph nd moisture predicted Holcus undnce, independent of elk. The regression consisted of soil ph nd grvimetric moisture s independent vriles nd Holcus undnce s the dependent vrile. We log- trnsformed Holcus undnce to meet normlity ssumptions. We tested whether the presence of Holcus in ech of the 24 plots vried y yer using contingency tle nlysis. We originlly used hitt type, yer (22 or 215), nd Holcus presence (yes or no) s the focl vriles, ut due to solute presence or sence of Holcus in some hitt types in some yers, this nlysis resulted in unstle F nd p vlues. Insted, we performed seprte nlyses for Lupinusdominted nd open grsslnds to look for vrition mong hitt types with yer nd Holcus presence s the focl vriles. Becuse the Bcchris- dominted grsslnd ws perfect predictor of Holcus (1% presence in 22 nd 215), we did not include this hitt type in the contingency nlysis. We lso evluted the effects of elk on the invder over time using dt collected y Johnson nd Cushmn (27), whose methods were comprle to the ones descried in this study. We clculted the elk effect on Holcus undnce nd oveground iomss s the LRR (=ln [elk present/elk excluded]). We nlyzed the Holcus response to elk in mixed model with yer (22/23 or 215) nd hitt type s fixed effects nd pir nested within hitt type s rndom effect.

5 ENDER et l () Aundnce (/m 2 ) Elk present Elk excluded () Biomss (g/m 2 ) FIGURE 1 Men (±1SE) undnce (), oveground iomss (), frequency (c), nd percent cover (d) of Holcus lntus s function of elk (present or excluded) nd hitt type (Bcchris- dominted, Lupinusdominted, or open grsslnd). Letters ove rs correspond to the results from Tukey multiple comprison tests. Although Holcus ws present in some of the Lupinusdominted grsslnd plots, our iomss qudrts did not cpture those individuls, resulting in zero Holcus iomss (c).4 Frequency Bcchris Lupinus Hitt type (d) 12 % Cover Bcchris Lupinus Hitt type 3 RESULTS Our results document tht elk significntly reduced Holcus undnce, oveground dry iomss, frequency, nd percent cover (Figure 1, Tle 1 d). In ddition, undnce, frequency, nd percent cover ut not oveground iomss vried significntly mong hitt types, with vlues eing gretest in Bcchris- dominted grsslnds, lest in Lupinus- dominted ones, nd intermedite in open grsslnds (Figure 1, Tle 1 d). The effect of elk on Holcus undnce vried significntly mong hitt types, with similr trends for iomss nd frequency (Figure 1 c, Tle 1 c). In ech cse, the effect of elk ws gretest in open grsslnds nd wek or sent in Bcchris- nd Lupinusdominted grsslnds. In contrst to these ptterns, the effect of elk on percent cover did not vry mong hitt types (Figure 1d, Tle 1d). We filed to detect n effect of elk, hitt type, or their interction on Holcus SLA or men inflorescences per dult (Figure 2,c, Tle 1e nd g). We detected trend for elk to decrese plnt height, ut neither hitt type nor the elk hitt interction hd n influence (Figure 2, Tle 1f). In ll models, the mrginl R 2 vlue for significnt effects ws lrgest for hitt type (R 2 =.72.9), intermedite for elk (R 2 =.14.2), nd smllest for the elk hitt interction (R 2 =.6; Tle 1). We detected trend for elk to reduce the undnce of Holcus seedlings nd this effect vried significntly mong hitt types, with differences rising only in open grsslnds (Figure 3, Tle 1h). Seedling undnce vried significntly mong hitt types, with levels highest in Bcchris- dominted grsslnds, lowest in Lupinus- dominted ones, nd intermedite in open grsslnds (Figure 3, Tle 1h). Elk lso significntly decresed seedling frequency nd vlues vried mong hitt type, with the highest frequencies found in Bcchris- dominted grsslnds, followed y open grsslnds, nd then Lupinus- dominted grsslnds (Figure 3, Tle 1i). We lso detected trend for the effect of elk to vry mong hitts, following the sme pttern s seen for seedling undnce (Figure 3, Tle 1i). Agin, the mrginl R 2 vlues for oth seedling undnce nd frequency were greter for hitt type thn elk or the elk hitt interction (Tle 1h nd i). Despite the significnt effect of elk treted s ctegoricl vrile (presence, sence) on mny Holcus response vriles, we filed to detect n effect of dung cover (n estimte of elk ctivity level) on the mgnitude of chnge in Holcus undnce, iomss, cover, or frequency, s ssessed y LRRs (F 1,6 =.42, p =.5416; F 1,6 = 3.56, p =.18; F 1,6 =.69, p =.439; F 1,6 =.5, p =.828, respectively). We lso filed to detect n interction etween dung ctivity hitt type on the mgnitude of chnge in Holcus vriles (undnce, F 2,6 =.6, p =.9385; iomss, F 2,6 = 1.13, p =.382; cover, F 2,6 =.34, p =.726; frequency, F 2,6 =.23, p =.8). Holcus undnce decresed significntly with incresing soil lklinity (Figure 4; F 1,21 = 12.4, p =.23) nd incresed significntly with incresing soil moisture (Figure 4; F 1,21 = 5.68, p =.266). Overll, these two effects ccounted for 58% of the vrition in Holcus undnce (F 2,21 = 14.71, p =.1). Holcus ws present in ll of the Bcchris- dominted grsslnd plots in 22 s well s 215, incresed from 75% to 1% in the open grsslnd plots nd incresed from % to 5% of the plots in Lupinusdominted grsslnds (Figure 5). We detected trend for the presence of Holcus to vry etween yers in open grsslnds (LR χ 2 = 3.6, df = 1, p =.83) nd this reltionship ws significnt in Lupinusdominted grsslnds (LR χ 2 = 6.9, df = 1, p =.86). Interestingly, the elk effect on Holcus undnce nd oveground iomss did not vry etween the two smple yers ut did vry mong hitt types, with there eing negtive effects in Bcchris- dominted nd open

6 1566 ENDER et l. Response Fixed effect df F p Mrginl R 2 () Aundnce (sqrt) Elk 1, Hitt type 2, E HT 1, () Biomss Elk 1, Hitt type 2, E HT 2, (c) Frequency Elk 1, Hitt type 2, E HT 2, (d) Percent cover Elk 1, Hitt type 2, E HT 2, (e) SLA Elk 1, Hitt type 2, E HT 2, (f) Height Elk 1, (g) Inflorescences per dult (h) Seedling undnce (sqrt) (i) Seedling frequency (sqrt ln) (j) Elk effect on undnce (k) Elk effect on iomss Hitt type 2, E HT 2, Elk 1, Hitt type 2, E HT 2, Elk 1, Hitt type 2, E HT 1, Elk 1, Hitt type 2, E HT 1, Yer 1, Hitt type 2, Y HT 2, Yer 1, Hitt type 2, Y HT 2, TABLE 1 Results from liner mixed models evluting the effects of tule elk nd hitt type on Holcus () undnce (squre- root- trnsformed), () oveground iomss, (c) frequency, (d) percent cover, (e) seedling undnce (squre- roottrnsformed), (f) seedling frequency (squre- root- log- trnsformed), (g) specific lef re (SLA), (h) height, (i) verge inflorescences per dult, (j) elk effect on undnce, nd (k) elk effect on oveground iomss grsslnds ut no effect in Lupinus- dominted grsslnds (Figure 6, Tle 1j nd k). However, we did detect trend for the effect of elk on Holcus iomss to decrese over time in Bcchris- dominted grsslnds, while it remined constnt in Lupinus- dominted nd open grsslnds (Figure 6, Tle 1k). The elk effect on Holcus undnce did not vry neither mong hitt types nor with the interction of hitt nd yer (Figure 6, Tle 1j). 4 DISCUSSION Using 17- yer- old exclosure experiment, we hve shown tht reintroduced ntive herivore nd environmentl heterogeneity oth ply importnt roles in ffecting the dominnce of n extremely invsive exotic grss. Our findings demonstrte tht elk negtively influenced Holcus popultions, decresing their locl distriution, undnce, percent cover, oveground iomss, nd seedling recruitment. We lso found tht the popultion prmeters we mesured commonly vried sustntilly mong different hitt types nd tht the effects of elk on Holcus vried mong them s well. Soil moisture nd ph explined much of the vrition in Holcus undnce nd these vriles were importnt fctors driving the heterogeneity mong the different hitt types. The effect of elk on Holcus in 215 ws no different thn tht detected 13 yers erlier. The reltive effect size of hitt type ws much greter thn tht for elk, nd this my explin why the negtive impcts of elk were not sufficient to prevent the expnsion of this dominnt invder into the more fvorle hitt types found in our system during the pst 13 yers.

7 ENDER et l SLA (cm 2 /g) Height (cm) Inflorescences per dult () () (c) Bcchris Elk present Lupinus Hitt type Elk excluded FIGURE 2 Men (±1SE) specific lef re (SLA) (), plnt height (), nd verge numer of inflorescences per dult individul, (c) of Holcus lntus s function of elk (present or excluded) nd hitt type (Bcchris- dominted, Lupinus- dominted, or open grsslnd). Three of four of the control plots in Lupinus- dominted grsslnds contined no Holcus, so rs for SLA nd inflorescences per dult represent the plot- wide verges of the one remining plot nd lck error rs The enemy relese hypothesis (ERH, sensu Drwin, 1859; Elton, 1958) predicts tht exotic tx should thrive in their new rnges ecuse they re freed from control y ntive predtors, pthogens, nd herivores. Although not direct test of ERH, our findings tht ntive herivores cn reduce the success of exotic plnt popultions joins growing numer of studies tht fil to support this hypothesis. In greement with our findings, mny other studies hve reported tht ntive herivores reduce the success of exotic plnt popultions (Cse & Crwley, 2; Colutti, Riccirdi, Grigorovich, & McIsc, 24; Cushmn et l., 211; Kene & Crwley, 22; Prker et l., 26). Although elk exerted n overll negtive effect on Holcus popultions in our study, this influence often vried mong hitt types Seedling undnce (/m 2 ) Seedling frequency (Figure 1 nd 3, Tle 1). Elk typiclly hd strong negtive effects on Holcus in open grsslnds nd sent or wek effects in oth Bcchris- nd Lupinus- dominted grsslnds. We do not think tht these hitt- specific results re explined y sptil vrition in the level of elk ctivity in our exclosure experiment. This is ecuse we found tht the mount of dung deposited y elk in plots n estimte of their ctivity level did not predict the mgnitude of the elk effect on Holcus. Rther, we hypothesize tht the effects of elk were miniml in Bcchris- dominted grsslnds ecuse the dense shru cover of this hitt type protected the invder from herivores (Johnson & Cushmn, 27). Alterntively, the invsion in Bcchris- dominted grsslnds my hve reched its full sptil extent t the time the experiment ws estlished. Elk my e more effective t slowing the spred of the invsion rther thn reducing Holcus in plots lredy hevily invded () () Bcchris Elk present Elk excluded FIGURE 3 Men (±1SE) seedling undnce () nd seedling frequency () of Holcus lntus s function of elk (present or excluded) nd hitt type (Bcchris- dominted, Lupinus- dominted, or open grsslnd). Letters ove rs correspond to the results from Tukey multiple comprison tests We hypothesize tht the negtive effects of elk on Holcus popultions were cused y the comined effects of herivory nd disturnce, resulting in greter mortlity of seedlings nd juvenile plnts nd thus decresed recruitment. In support of this hypothesis, previous studies t our field site hve reported tht, lthough not preferred food plnt, elk will consume Holcus, with the species constituting up to 12% of its diet during the summer months (Gogn & Brrett, 1995; Roerts, 2). In New Zelnd nd Englnd, oth Crwford nd Liddle (1977) nd Edmond (1964) lso report tht Holcus ws reduced y livestock nd humn trmpling, nd ws more sensitive to these c Lupinus Hitt type

8 1568 ENDER et l. 5 () 2 () Aundnce Log Holcus undnce (/m 2 ) Log Holcus undnce (/m 2 ) Soil ph 5 () Soil grvimetric moisture (g/g) Elk effect Elk effect Bcchris 22/3 215 () Aoveground iomss Lupinus Hitt type FIGURE 6 Men (±1SE) elk effect (log response rtio = ln [elk present/elk excluded) ]on Holcus lntus undnce () nd oveground iomss () s function of yer (22/23 or 215) nd hitt type (Bcchris- dominted, Lupinus- dominted, or open grsslnd) FIGURE 4 The log- trnsformed undnce of Holcus lntus per plot s function of soil ph () nd soil moisture () % Plots with Holcus FIGURE 5 Percent of plots contining Holcus lntus in 22 nd 215 cross three hitt types (Bcchris- dominted, Lupinusdominted, nd open grsslnd) disturnces thn other grss species. At our site, elk cuse sustntil disturnce to the soil nd vegettion (J. H. Cushmn, unpulished dt; Johnson & Cushmn, 27) nd we hypothesize tht this trmpling is mjor fctor explining the negtive effects of elk on Holcus popultions Yer Bcchris Lupinus A lck of effect on individul responses ut decrese in Holcus undnce nd seedling recruitment leds us to hypothesize tht elk reduced the survivl of young plnts ut hd little negtive effects on estlished individuls. Since elk did not ffect the numer of inflorescences per plnt, nd presumly seed production, the reduced numer of seedlings cn e ttriuted to reduction in sfe sites, due to either reduced germintion rtes or incresed seedling mortlity. Although we did not quntify germintion rtes, we hypothesized tht ground disturnce nd reduced competition due to elk ctivity would increse fvorle germintion sites in control plots. Our seedling undnce dt did not support this hypothesis. In contrst, incresed mortlity of delicte young plnts due to trmpling nd/or consumption could explin the negtive effects of elk on seedling recruitment. While few studies hve quntified the effects of lrge ntive herivores on exotic plnt recruitment, our results gree with other studies showing tht vrious smller ntive herivores nd grnivores suppressed exotic seedling recruitment. For exmple, Cse nd Crwley (2) found tht rits reduced seedling recruitment nd survivl of n invsive exotic for in Gret Britin. Additionlly, in review of 18 studies, Mron nd Vil (21) found tht ntive herivores decresed seedling performnce (seed set, seed viility, nd seedling recruitment) y third. The mgnitude of the elk effect on Holcus iomss nd undnce did not chnge with incresing durtion of the exclosure experiment,

9 ENDER et l except for trend to decrese in Bcchris- dominted grsslnds when mesured y oveground iomss. Although we do not know if or how the elk effect vried in the intervening yers, the negtive effects of elk on Holcus were evident 4 yers into the experiment if not erlier nd were still evident nd not significntly different 13 yers lter. As Levine et l. (24) nd Mron nd Vil (21) point out, generlist herivores my hve negtive effects on exotic plnt species, ut it is uncler whether they cn prevent invsive estlishment or erdicte lredy estlished exotic popultions. Our results provide support for this skepticism, since the negtive influence of elk did not increse over time nd ws not enough to prevent the dvnce of Holcus into previously uncolonized plots. We elieve this occurred ecuse, lthough elk negtively ffect recruitment of new individuls, they likely do not completely eliminte them. Furthermore, we hypothesize tht elk hve miniml effects on older, estlished plnts. Thus, we suspect tht ny Holcus seedlings tht re le to survive will persist, if not thrive, in elk grzed lndscpes. In contrst to our study, Person et l. (212) found tht grnivorous rodents reduced percent cover nd reproduction of n exotic ster (Trgopogon duius) nd provided iotic resistnce to the community y severely limiting locl undnce of this invder. Additionlly, the effect of smll mmmls incresed over time s popultions of T. duius grew within exclosures. The study y Person et l. (212) diverges from ours in tht it excluded grnivores, who my preferentilly prey upon the seeds of certin species, thus hving stronger nd more specific effects thn the generlist herivores in our experiment. In ddition to the effects of elk, we found tht environmentl heterogeneity hd mjor influence on Holcus undnce, cover, frequency, nd seedling recruitment (Figures 1 nd 2). We consistently oserved tht Holcus popultions were lest successful in Lupinusdominted grsslnds, most successful in Bcchris- dominted grsslnds, nd intermedite in open grsslnds. Our dt suggest tht much of this vrition in Holcus performnce ws due in prt to soil differences mong the three hitt types. The Lupinus- dominted grsslnds hve significntly higher proportion of corse snd, lower soil moisture, nd higher soil ph thn the other two hitt types (Dodge, Eviner & Cushmn, unpulished dt). As shown in Figure 4, we found tht Holcus undnce incresed with soil moisture nd cidity, which ws lso descried y Thompson nd Turkington (1988). These two fctors ccounted for over hlf of the vrition in Holcus undnce nd these unfvorle iotic conditions in the Lupinusdominted grsslnds hve proly een the primry fctor slowing the spred of Holcus. In our system, it ppers tht the environmentl heterogeneity found mong the different hitt types ws greter driver of Holcus invsion thn tule elk. In estimting the reltive effect size of our fixed effects, ll significnt hitt effects ccounted for t lest 72% of the vrition in Holcus response, wheres elk only ccounted for up to 2% (Tle 1). We suspect tht the lrger influence of hitt type explins why elk lone were not sufficient to slow the spred of Holcus in this system. In long- term, lrge- scle exclosure study in Rocky Mountin grsslnd, Stohlgren et l. (1999) lso found tht differences in climte nd soil chrcteristics hd greter effects on exotic plnt species richness nd cover thn grzing y ntive nd domestic herivores. However, vriles such s precipittion nd mny soil chrcteristics re eyond the control of lnd mngers, wheres grzing cn e mnipulted nd used s tool to mnge invsive plnt species. Understnding the interction etween grzing nd environmentl fctors will help in ssessing the potentil for herivores to control exotic plnt species. For exmple, we found tht elk were effective t reducing Holcus cover in the open grsslnds, ut less effective in the other two hitt types. Thus, lthough environmentl fctors my ply the lrgest role in determining the overll dynmics of the Holcus invsion, the smller ut very rel effects of ntive herivores on exotic invsive plnt species cn still e useful in mking mngement decisions, given tht these re the fctors tht mngers cn mnipulte. In conclusion, our study demonstrtes tht oth ntive herivores nd environmentl heterogeneity cn e importnt drivers of exotic plnt invsions nd cn interct with ech other to medite the success of dominnt exotic plnt species. In our system, hitt type ws the stronger driver of invsive plnt success nd medited the effects of elk, highlighting the need to ssess hitt suitility s well s iotic interctions when ttempting to understnd nd mnge the dynmics of invsive plnt popultions. Furthermore, our results show tht the negtive effects of ntive herivores on exotic plnt popultions my not trnsfer into long- term control or prevention of invsion. Thus, it is criticl to study interctions etween ntive herivores nd exotic plnts cross heterogeneous lndscpe nd over longer time periods, which will llow for greter insight out the importnce nd dynmics of context- dependent outcomes in invded systems. ACKNOWLEDGMENTS We re indeted to the following individuls for ssistnce in the field: Eric Cecil, Vness Dodge, Dn Foley, Cprice Lee, Elis Lopez, nd Shiloh Vlentyne. Thnks to Tim Bernot t Point Reyes Ntionl Seshore for his help in mintining the exclosures. Dvid Press nd Brent Johnson provided invlule logisticl support throughout the entire project. Specil thnks go to Dn Crocker for his guidnce with our sttisticl nlyses. This project hs een generously supported y grnts from the Cliforni Ntive Plnt Society (Milo Bker, Mrin County nd sttewide Chpters) nd Sonom Stte University. CONFLICT OF INTEREST None declred. REFERENCES Agrwl, A. A., Kotnen, P. M., Mitchell, C. E., Power, A. G., Godsoe, W., & Klironomos, J. (25). Enemy relese? An experiment with congeneric plnt pirs nd diverse ove- nd elowground enemies. Ecology, 86, Aikio, S., Duncn, R. P., & Hulme, P. E. (21). Lg- phses in lien plnt invsions: Seprting the fcts from the rtefcts. Oikos, 119, Bdno, E. I., Villrroel, E., Bustmnte, R. O., Mrquet, P. A., & Cvieres, L. A. (27). Ecosystem engineering fcilittes invsions y exotic plnts in high- Anden ecosystems. Journl of Ecology, 95,

10 157 ENDER et l. Bisws, S. R., Kotnen, P. M., Kmo, D., & Wgner, H. H. (214). Context- dependent ptterns, determinnts nd demogrphic consequences of herivory in n invsive species. Biologicl Invsions, 17, Bolker, B. M., Brooks, M. E., Clrk, C. J., Genge, S. W., Poulsen, J. R., Stevens, M. H. H., & White, J.-S. S. (29). Generlized liner mixed models: A prcticl guide for ecology nd evolution. Trends in Ecology & Evolution, 24, Britton-Simmons, K. H., & Aott, K. C. (28). Short- nd long- term effects of disturnce nd propgule pressure on iologicl invsion. Journl of Ecology, 96, Cr-Rivs, I., Sldñ, A., Cstro-Díez, P., & Gllien, L. (216). A multiscle pproch to identify invsion drivers nd invders future dynmics. Biologicl Invsions, 18, Cl-IPC (26) Cliforni invsive plnt inventory. Cliforni Invsive Plnt Council, Berkeley, CA. Inventory26.pdf Cse, C. M., & Crwley, M. J. (2). Effect of interspecific competition nd herivory on the recruitment of n invsive lien plnt: Conyz sumtrensis. Biologicl Invsions, 2, Ctford, J. A., Jnsson, R., & Nilsson, C. (29). Reducing redundncy in invsion ecology y integrting hypotheses into single theoreticl frmework. Diversity nd Distriutions, 15, Colutti, R. I., Grigorovich, I. A., & McIsc, H. J. (26). Propgule pressure: A null model for iologicl invsions. Biologicl Invsions, 8, Colutti, R. I., Riccirdi, A., Grigorovich, I. A., & McIsc, H. J. (24). Is invsion success explined y the enemy relese hypothesis? Ecology Letters, 7, Corin, J. D., & D Antonio, C. M. (21). Not novel, just etter: Competition etween ntive nd non- ntive plnts in Cliforni grsslnds tht shre species trits. Plnt Ecology, 29, Crwford, A. K., & Liddle, M. J. (1977). The effect of trmpling on neutrl grsslnd. Biologicl Conservtion, 12, Cushmn, J., Lortie, C. J., & Christin, C. E. (211). Ntive herivores nd plnt fcilittion medite the performnce nd distriution of n invsive exotic grss. Journl of Ecology, 99, Drwin, C. (1859). On the origin ofspecies y mens of nturl selection, or the preservtion of fvoured rces in the struggle for life. New York, NY: D. Appleton nd Compny. Dávlos, A., Nuzzo, V., & Blossey, B. (215). Single nd interctive effects of deer nd erthworms on non- ntive plnts. Forest Ecology nd Mngement, 351, Dvis, M. A., Grime, J. P., & Thompson, K. (2). Fluctuting resources in plnt communities: A generl theory of invsiility. Journl of Ecology, 88, Deck, A., Muir, A., & Struss, S. (213). Trnsgenertionl soil- medited differences etween plnts experienced or nïve to grss invsion. Ecology nd Evolution, 3, Eckerg, J. O., Tenhumerg, B., & Loud, S. M. (214). Ntive insect herivory limits popultion growth rte of non- ntive thistle. Oecologi, 175, Edmond, D. B. (1964). Some effects of sheep treding on the growth of 1 psture species. New Zelnd Journl of Agriculturl Reserch, 7, Edwrds, L. J., Muller, K. E., Wolfinger, R. D., Qqish, B. F., & Schenerger, O. (28). An R2 sttistic for fixed effects in the liner mixed model. Sttistics in Medicine, 27, Elton, C. S. (1958). The ecology of invsions y nimls nd plnts. Chicgo, IL: University of Chicgo Press. Gogn, P., & Brrett, R. (1995). Elk nd deer diets in costl pririe- scru mosic, Cliforni. Journl of Rnge Mngement, 48, Inderjit, & Chill, J. F. (215). Linkges of plnt- soil feedcks nd underlying invsion mechnisms. AoB Plnts, 7, 1 8. Johnson, B. E., & Cushmn, J. (27). Influence of lrge herivore reintroduction on plnt invsions nd community composition in Cliforni grsslnd. Conservtion Biology, 21, Klisz, S., Spigler, R. B., & Horvitz, C. C. (214). In long- term experimentl demogrphy study, excluding ungultes reversed invder s explosive popultion growth rte nd restored ntives. Proceedings of the Ntionl Acdemy of Sciences of the United Sttes of Americ, 111, Kshiwgi, J. H. (1985). Soil survey of Mrin County, Cliforni. United Sttes Deprtment of Agriculture: Soil Conservtion Service. Kene, R. M., & Crwley, M. J. (22). Exotic plnt invsions nd the enemy relese hypothesis. Trends in Ecology nd Evolution, 17, Kenwrd, M. G., & Roger, J. H. (1997). Smll smple inference for fixed effects from restricted mximum likelihood. Biometrics, 53, Knight, T. M., Dunn, J. L., Smith, L. A., Dvis, J., & Klisz, S. (29). Deer fcilitte invsive plnt success in Pennsylvni forest understory. Nturl Ares Journl, 29, Koerner, S. E., Avolio, M. L., Chng, C. C., Gry, J., Hoover, D. L., & Smith, M. D. (215). Invsiility of mesic grsslnd depends on the time- scle of fluctuting resources. Journl of Ecology, 13, Lrkin, D. J. (212). Lengths nd correltes of lg phses in upper- Midwest plnt invsions. Biologicl Invsions, 14, Levine, J. M., Adler, P. B., & Yelenik, S. G. (24). A met- nlysis of iotic resistnce to exotic plnt invsions. Ecology Letters, 7, Mron, J. L., & Connors, P. G. (1996). A ntive nitrogen- fixing shru fcilittes weed invsion. Oecologi, 15, Mron, J. L., & Vil, M. (21). When do herivores ffect plnt invsion? Evidence for the nturl enemies nd iotic resistnce hypotheses. Oikos, 95, McCullough, D. R. (1969). The tule elk: Its history, ehvior, nd ecology. University of Cliforni pulictions in zoology (Vol. 88). 2: 29. Prker, J. D., Burkepile, D. E., & Hy, M. E. (26). Opposing effects of ntive nd exotic herivores on plnt invsions. Science, 311, Person, D. E., Potter, T., & Mron, J. L. (212). Biotic resistnce: Exclusion of ntive rodent consumers releses popultions of wek invder. Journl of Ecology, 1, Pérez-Hrguindeguy, N., Díz, S., Grnier, E., Lvorel, S., Poorter, H., Jureguierry, P., Gurvich, D. E. (213). New hndook for stndrdised mesurement of plnt functionl trits worldwide. Austrlin Journl of Botny, 61, Polk, T., & Sltz, D. (211). Reintroduction s n ecosystem restortion technique. Conservtion Biology, 25, 424. Pyšek, P., Hulme, P. E., & Repulic, C. (25). Sptio- temporl dynmics of plnt invsions: Linking pttern to process. Ecoscience, 12, Rejmánek, M., & Richrdson, D. M. (1996). Wht ttriutes mke some plnt species more invsive? Ecology, 77, Relv, M. A., Nunez, M. A., & Simerloff, D. (21). Introduced deer reduce ntive plnt cover nd fcilitte invsion of non- ntive tree species: Evidence for invsionl meltdown. Biologicl Invsions, 12, Riginos, C., & Grce, J. B. (28). Svnn tree density, herivores, nd the herceous community: Bottom- up vs. top- down effects. Ecology, 89, Roerts, S. L. (2). Forging ecology of tule elk t Tomles Point, Point Reyes Ntionl Seshore. University of Idho: Moscow, Idho. Schlje, G. B., McBride, J. B., & Fellinghm, G. W. (22). Adequcy of pproximtions to distriutions of test sttistics in complex mixed liner models. Journl of Agriculturl, Biologicl, nd Environmentl Sttistics, 7, Sker, M. J., Grydon, D. J., & Cushmn, J. (213). Community- level consequences of cttle grzing for n invded grsslnd: Vrile responses of ntive nd exotic vegettion. Journl of Vegettion Science, 24, Sthlheer, K. A., & D Antonio, C. M. (213). Using livestock to mnge plnt composition: A met- nlysis of grzing in Cliforni Mediterrnen grsslnds. Biologicl Conservtion, 157, 3 38.

11 ENDER et l Stohlgren, T. J., Schell, L. D., & Vnden Heuvel, B. (1999). How grzing nd soil qulity ffect ntive nd exotic plnt diversity in Rocky Mountin grsslnds. Ecologicl Applictions, 9, Tkgi, S., & Miyshit, T. (215). Time- scle dependency of host plnt iomss- nd trit- medited indirect effects of deer herivory on swllowtil utterfly. Journl of Animl Ecology, 84, Tng, L., Go, Y., Wng, C., Zho, B., & Li, B. (212). A plnt invder declines through its modifiction to hitts: A cse study of 16- yer chronosequence of Sprtin lterniflor invsion in slt mrsh. Ecologicl Engineering, 49, Tylor, S., & Kumr, L. (216). Will climte chnge impct the potentil distriution of ntive vine (Merremi peltt) which is ehving invsively in the Pcific region? Ecology nd Evolution, 6, Thompson, J. D., & Turkington, R. (1988). The iology of Cndin Weeds. 82. Holcus lntus L. Cndin Journl of Plnt Science, 68, Vvr, M., Prks, C. G., & Wisdom, M. J. (27). Biodiversity, exotic plnt species, nd herivory: The good, the d, nd the ungulte. Forest Ecology nd Mngement, 246, Weiher, E., & Keddy, P. A. (1995). Nordic society oikos ssemly rules, null models, nd trit dispersion: New questions from old ptterns. Oikos, 74, Wilson, J. R. U., Richrdson, D. M., Rouget, M., Proches, S., Amis, M. A., Henderson, L., & Thuiller, W. (27). Residence time nd potentil rnge:crucil considertions in modelling plnt invsion. Diversity nd Distriutions, 13, Young, T. P., Plmer, T. M., & Gdd, M. E. (25). Competition nd compenstion mong cttle, zers, nd elephnts in semi- rid svnn in Likipi, Keny. Biologicl Conservtion, 122, How to cite this rticle: Ender CL, Christin CE, nd Cushmn JH. Ntive herivores nd environmentl heterogeneity s meditors of n exotic grss invsion. Ecol Evol. 217;7: