Ecology, 85(1), 2004, pp. 127±133 ᭧ 2004 by the Ecological Society of America

AN EMERGENT MULTIPLE PREDATOR EFFECT MAY ENHANCE BIOTIC RESISTANCE IN A STREAM FISH ASSEMBLAGE

BRET C. HARVEY,1 JASON L. WHITE, AND RODNEY J. NAKAMOTO U.S. Forest Service, Paci®c Southwest Research Station, Redwood Sciences Laboratory, 1700 Bayview Drive, Arcata, California 95521 USA

Abstract. While two cyprinid ®shes introduced from nearby drainages have become widespread and abundant in the Eel River of northwestern California, a third nonindigenous cyprinid has remained largely con®ned to Յ25 km of one major tributary (the Van Duzen River) for at least 15 years. The downstream limit of this species, speckled dace, does not appear to correspond with any thresholds or steep gradients in abiotic conditions, but it lies near the upstream limits of three other ®shes: coastrange sculpin, prickly sculpin, and nonindigenous Sacramento pikeminnow. We conducted a laboratory stream experiment to explore the potential for emergent multiple predator effects to in¯uence biotic resistance in this situation. Sculpins in combination with Sacramento pikeminnow caused greater mortality of speckled dace than predicted based on their separate effects. In contrast to speckled dace, 99% of sculpin survived trials with Sacramento pikeminnow, in part because sculpin usually occupied benthic cover units while Sacramento pikeminnow occupied the water column. A 10-fold difference in benthic cover availability did not detectably in¯uence biotic interactions in the experiment. The distribution of speckled dace in the Eel River drainage may be limited by two predator taxa with very different patterns of habitat use and a shortage of alternative habitats. Key words: biotic resistance; Cottidae; cover; Cyprinidae; emergent multiple predator effects; introduced species; laboratory stream; predation risk; stream ®sh. Reports

INTRODUCTION although such resistance may be limited to speci®c hab- itats or otherwise affect patterns in the abundance but The ability of communities to resist invasion is a not the large-scale distribution of invaders (e.g., Rob- signi®cant issue in community ecology and resource inson and Wellborn 1988, Reusch 1998, Byers 2002). management (Mack et al. 2000). While empirical rules for predicting invasion resistance are available for Emergent multiple predator effects may in¯uence the some taxa (e.g., Moyle and Light 1996a), the generality degree of biotic resistance in some communities. Emer- of these rules remains unclear. For example, previous gent multiple predator effects occur when the combined studies of invasion resistance in California stream ®sh effect of multiple predators on prey differs from that communities have emphasized the importance of the predicted using the predators' separate effects (Sih et physical environment. Moyle and Light (1996b) sug- al. 1998). For example, facilitation among predators, gested that for invading ®shes in California streams, which could enhance invasion resistance, might be pre- ``if abiotic factors are appropriate for an exotic species, dicted where predators utilize different habitats such then that species is likely to successfully invade, re- that prey lack physical refuges. EkloÈv and VanKooten gardless of the biota already present.'' Baltz and Moyle (2001) observed such facilitation in the effect of perch (1993) concluded that the natural hydrologic regime (Perca ¯uviatilis) and northern pike (Esox lucius)on was the key element that prevented invasion of Deer roach (Rutilus rutilus) in pond enclosures containing Creek, California by nonindigenous ®shes. So does a both open water and vegetated habitat. Yellow perch match between the habitat requirements of an invading are more effective predators in open water, while north- species and the receiving system dictate a successful ern pike are more ef®cient than yellow perch in phys- invasion, or might biotic resistance also be signi®cant ically complex habitats. in some circumstances? While patterns of resource use In general, patterns in the success of invasion by by ®shes in Deer Creek did not support the hypothesis stream ®shes in the Eel River of northwestern Cali- of resistance to invasion via competition, Baltz and fornia appear to follow Moyle and Light's (1996b) as- Moyle (1993) noted that predation might be a signif- sertion that, in California streams, correspondence be- icant contributor to invasion resistance. Predation-driv- tween the physical habitat requirements of invading en invasion resistance occurs in other aquatic systems, species and the available habitat determines invasion success. For example, the two nonindigenous species with broad distributions and relatively high densities Manuscript received 6 February 2003; revised 12 June 2003; accepted 13 June 2003. Corresponding Editor: J. E. Havel. in the Eel River drainage are cyprinids native to nearby 1 E-mail: [email protected] drainages offering similar physical conditions (Brown 127 Reports nulse data unpublished ( roach California the both 1997): Moyle and 128 pclddc isjs ptemo h pe ii of limit upper ( the sculpin of coastrange upstream both just lies dace speckled e ie s sebae(ecie nBonand Brown in (described 1997). assemblage depauperate Moyle ®sh the River from poten- risk Eel or predation species signi®cant pike- similar tially Sacramento ecologically nor speckled faced to roach minnow contrast California In pike- neither reach. Sacramento that dace, in although rare km, are 15 minnow River about Duzen Van by the in overlap dace speckled of down- extent and stream pikeminnow of Sacramento extent of upstream distribution speckled The the 2002). that (Moyle substrates prefer also coarse dace with habitats occupy hr yrndas aiet daetdangs the drainages, adjacent ( to a dace native speckled However, also Sacramento means). cyprinid for unknown third 1980 by and both roach pikeminnow, California for about (in 1970 introductions their of years 10 within drainage fseke aei h a ue ie n tasite a at 0.2 and only by River differed Duzen downstream Van km the 20 limit in downstream dace the weekly speckled at maximum of year 1 example, over temperature For mean unrelated factors. Van appears the physical limit in to downstream dace its speckled River, of of Duzen range limit upstream 25-km the current de®nes the barrier physical 1987. a since constant While remained has range species' the of n 9620 B .Hre,J .Wie n .J. R. and White, L. J. Harvey, Nakamoto, C. dur- (B. collections annual 1996±2002 in- in two ing downstream of km total a 5 recently dividuals most captures downstream, rare of km exception 5±10 the (the With speckled tributary River). major Duzen the one Van to 1997), con®ned Moyle remained has and dace (Brown 1988 before pattern. cli ( sculpin symmetricus hcelsgrandis chocheilus ic t nrdcinit h e ie drainage River Eel the into introduction its Since P LATE .asper C. .Scaet ieinwsimn vrcvruisi h rica tem ht yR .Nakamoto. J. R. by Photo stream. arti®cial the in units cover over swimming pikeminnow Sacramento 1. nulse data unpublished n h armnopkmno ( pikeminnow Sacramento the and ) hncty osculus Rhinichthys .Hwvr h ontemlmtof limit downstream the However, ). .Teentv cliscommonly sculpins native These ). eaewdsra nteEel the in widespread became ) otsaleuticus Cottus ,tedwsra extent downstream the ), ,de o tthis ®t not does ), Њ J .White, L. (J. C n prickly and ) RTC AVYE AL. ET HARVEY C. BRET Lavinia Pty- optto o pc ihrfesupn( drainage. sculpin River rif¯e Eel with the space for in Competition dace speckled dis- of the determining tribution in important be could biotic interactions that suggest observations other River, Duzen Van hne .007 ieadattlae f71m 7.1 of area total a and wide m with stream 0.30±0.75 laboratory channel recirculating a a in 2001 June 6 to ln ihtepten n®hdsrbtosi the in distributions ®sh in patterns the with Along eas t viaiiyi h e ie riaeis drainage River time. Eel and space the in in heterogeneous availability and interest its of likely interactions because is predator±prey it because benthic the experiment, affect arti®cial of the to in availability large factor a the a as included cover in also hypothesis speckled We on this stream. effect tested predator We lead multiple dace. could emergent column, an water benthic to the in occupy pike- sculpins which Sacramento minnow, by by predation predation with combined and habitat, space competi- that for hypothesized tion we observations, Giv- these 2001). Harvey en and White (e.g., 1997, species Moyle benthic and including Brown ®shes of variety wide consuming a Sacra- piscivorous, adults, largely are As pikeminnow been 2002). mento has (Moyle sculpins by frequently piscivory observed and con- (1982), also al. et by sculpin Baltz experiments laboratory Rif¯e during 1982). dace speckled al. sumed et (Baltz stream the margins abun- at water were slower occupied with sculpin dace water speckled rif¯e dant, fast where but occupy substrates, to coarse Deer tended were sculpin dace in rif¯e speckled dace where rare, reach speckled a of In California: use Creek, habitat the in¯uenced f3 msi h arwpr ftecanlad14 main- and was channel temperature the 16 Water at of remainder. tained part the narrow in the cm/s in cm/s 35 velocities of water mean produced pumps entire Electric substratum. the of stream. gravel observation allowed and wall cm inner Plexiglas 45±50 A of depth water with ecnutdteeprmn rm2Jnay2001 January 2 from experiment the conducted We Њ ±17 Њ C. M ETHODS clg,Vl 5 o 1 No. 85, Vol. Ecology, .gulosus C. 2 ) , January 2004 MULTIPLE PREDATORS AND BIOTIC RESISTANCE 129

Arti®cial lights provided 10 h of light per day, aug- ramento pikeminnow were added to the two predator mented by limited natural light from windows along treatments that included them, and before concluding two sides of the stream. each trial on day 3. For 28 trials, we also made ob- The experiment included two levels of three cross- servations of ®sh on the evening of day 2. From these classi®ed factors: Sacramento pikeminnow (absent or observations, we: (1) quanti®ed the use of benthic cov- three individuals, 315±419 mm standard length [SL; er by both speckled dace and sculpins; and (2) gen- from anterior end of head to posterior end of vertebral erated minimum estimates of movement beneath and column]; see Plate 1), sculpin (absent or 10 individuals, among benthic cover units by speckled dace and scul- 59±112 mm SL, ®ve C. asper, and ®ve C. aleuticus), pins while Sacramento pikeminnow were present, by and benthic cover (3 or 30 units constructed of two comparing their distributions on the evening of day 2 black plastic panels 30 ϫ 18 cm, held 2 cm apart by and the morning of day 3. These minimum estimates two 12-cm-long spacers). The number of Sacramento of movement were generated only from differences in pikeminnow we used produced a density (0.4 ®sh/m2) the number of ®sh beneath speci®c cover units; we did approaching the maximum we have observed in the not identify individual ®sh. We weighed and measured Van Duzen River (0.5 ®sh/m2) for entire mesohabitats speckled dace and sculpins before each trial and (B. C. Harvey, unpublished data). We selected this den- weighed and measured surviving ®sh at the end of each sity in part because our observations of piscivory by trial. We used the change in weight of sculpin to quan- Sacramento pikeminnow indicated they commonly tify their consumption of speckled dace. feed in groups. We used a sculpin density (1.4 ®sh/m2) We analyzed the survival of speckled dace using log- that we estimated was similar to the density in rif¯es transformed data in a three-way ANOVA with Sacra- at the upstream limit of sculpins in the Van Duzen mento pikeminnow, sculpin, and benthic cover as main River. Sculpin densities in rif¯es 5±10 km below their effects. With log-transformed data, the interaction term upstream limit were 0.6±1.2 ®sh/m2 in 2001 (J. L. for the two predators tests the multiplicative model of White, unpublished data). Sculpin density in a short multiple predator effects, in which the predicted effect reach that included both a pool and a rif¯e about 20 of two predators in combination is Pra ϩ Prb Ϫ (PraPrb), 2 km below the upstream limit averaged 0.6 ®sh/m over where Pra and Prb represent the effects of the two pred- 3 yr (Brown et al. 1995). We attempted to use a size ators alone (Soluk and Collins 1988, Sih et al. 1998). Reports distribution of sculpin in each trial that approximately The simpler additive model (where the predicted effect matched ®eld data from multiple sampling sites on the of the two predators ϭ Pra ϩ Prb) is not appropriate in Van Duzen River. The survival of 10 speckled dace this case because that model includes prey that would

(39±70 mm SL) provided the main response variable. be killed twice (the PraPrb term in the multiplicative The entire arti®cial stream was used for each trial, and model) in situations where prey depletion occurs. the order of treatments randomized. We completed ®ve We analyzed the use of cover by speckled dace and replicates of each treatment combination, except for sculpins with two-way ANOVA. For speckled dace, we the two treatments lacking either predator, which were analyzed the effect of predator treatment (Sacramento replicated three times. We obtained ®sh for the exper- pikeminnow or sculpin) and cover availability on the iment from nearby rivers by seining, hand netting, and change in use of cover during the trials, by using as a electro®shing. Both sculpins and speckled dace were response variable the difference between use of cover collected from sites where predators that commonly at the end of each trial and use of cover at the beginning occupy the water column, such as Sacramento pike- of day 2, before Sacramento pikeminnow were added minnow and adult cutthroat trout (Oncorhynchus clar- in trials that received them. The combined predator ki), were rare. Fish were held in laboratory tanks with treatment could not be included in this analysis because the same water temperature as the arti®cial stream for survival of speckled dace in that treatment was not at least 7 d before being used in the experiment. During adequate to yield meaningful data on use of cover. We this period, we fed Sacramento pikeminnow various similarly analyzed use of cover by sculpins, except that cyprinids while sculpins and speckled dace received the analysis could only include the two predator treat- frozen brine shrimp (Artemia spp.), tubi®cid worms, ments that included sculpin. Finally, we inspected the and benthos collected from nearby streams. minimum estimates of movement beneath and among Each trial began with the addition of sculpins and cover units by speckled dace and sculpins while Sac- speckled dace to the arti®cial stream on the morning ramento pikeminnow were present to determine if of day 1, followed by Sacramento pikeminnow on the movement might affect the interactions of interest. morning of day 2. We censused the number of surviving speckled dace at the conclusion of each trial on the RESULTS morning of day 3. Fish were haphazardly selected for Sacramento pikeminnow and sculpin consumed sim- use in individual trials; some Sacramento pikeminnow ilar proportions (22±32%) of speckled dace when tested and sculpins were used in multiple trials, but speckled separately in the arti®cial stream. While sculpin out- dace were not. For 29 trials, we observed the distri- numbered Sacramento pikeminnow and were together bution of ®shes on the morning of day 2 before Sac- with speckled dace during the trials for twice as long, Reports e ae nysculpin speck- Only consume dace. to led enough large individuals only two yielded or trial each one in sculpin of distribution size the SE 130 oeegn utpepeao fet ro asindicate bars assuming Error effect. rate predator mortality multiple predicted emergent the no indicate sculpins pikeminnow bars and Sacramento to both with adjacent treatment Arrows the representing mortality controls.) (No predator-free experiment. in stream occurred cover arti®cial benthic an of in levels two availability and treatments predator different fet Fg ,Scaet pikeminnow Sacramento 1, separate that (Fig. their effects than of model effect multiplicative greater a using a predicted and had pikeminnow combination in Sacramento sculpins experiment. the in dace h fet ftepeaos(o h oe fetand effect cover the all The cover, (for involving predators treatment. the interactions cover of in¯uence low effects signi®cantly not the the did cover benthic in of amount dace speckled of s cuidbnhccvra h n ftetil.Less trials. the of end the at cover benthic occupied ®sh hr fteseke aeocpe eti oe be- cover but added, benthic was occupied predator the dace one fore speckled about the treatment, of pikeminnow third Sacramento 2). the (Fig. In treatments among signi®cantly varied sculpin single- treatment. their combined-predator trial in per the dace in trial speckled 0.03 only per but treatment dace the predator speckled in on 2.5 trial consumed, per average, Sculpin dace treatment. speckled single-pred- 7.9 combined-predator their and in trial treatment per ator dace av- speckled on 3.1 consumed, erage, pikeminnow Sacramento single-pred- trials. the ator to compared lower sculpin but by rates pikeminnow predation Sacramento by higher rates from predation resulted treatment combined-predator the All from treatment. trials. sculpins predator-free from 100 combined-predator recovered were of dace the speckled one of only recover pikeminnow: replicates to Sacramento of failed high presence we experienced the sculpins in dace, survival speckled to contrast teraction, . F IG h s fbnhccvrb ohseke aeand dace speckled both by cover benthic of use The in dace speckled of mortality predicted than Greater .Pretg otlt fseke aeudrthree under dace speckled of mortality Percentage 1. . F 1,28 ϭ 5.3, P Ն ϭ 4m Lcnue speckled consumed SL mm 94 .3,cuig9%mortality 94% causing 0.03), Ͼ P 5 ftesurviving the of 95% Ͼ .8.I sharp In 0.28). ϫ RTC AVYE AL. ET HARVEY C. BRET cli in- sculpin ϩ 1 ramn a togefc ntecag nueof ( use dace in speckled change by trials the the on predator during effect cover Thus, strong treatment. a time sculpin had any treatment the at cover of occupied trials dace during speckled of 2% than ϭ ϭ P raeters fpeainfrsupni h ®eld the in sculpin 2001). for Harvey in- predation and strongly of (White risk pikeminnow the Sacramento crease and 2), more pikeminnow (Fig. Sacramento cover of presence occupy the rela- in sculpin frequently predator±prey them: the between by tionship exacerbated in are predators case the of by presence this use the habitat in in column Differences water sculpin. the occupy speckled can while high dace the pikeminnow, even Sacramento from by dace of caused speckled densities probably for that is refuge to effective cover an equal Benthic not combination. prey would in for probably predators risk alone in predators ei- two result of the densities higher of that by ther suggest use predators habitat two al. in the differences et strong (Sih experiment, habitats this different in Eklo 1998, forage and predators where re- example occupy for predator-speci®c con¯ict, prey appropriate by sponses where effect predator expected multiple emergent is ef- of separate form predators' This the fects. on to based compared risk stream expected arti®cial the the in risk dace predation speckled of for enhancement an in resulted minnow 30%. averaged in which pikeminnow, sculpin Sacramento without for pike- trials estimates Sacramento of movement cover, presence paralleled the high minnow in the esti- sculpin in for movement so mates These did sculpin treatment. of combined-predator 37% pres- least were at pikeminnow ent; Sacramento while cover, units low among cover the or beneath in cover moved sculpin treatment high of combined-predator with 27% 40% least At and availability. availability cover low 10% with was treatment pikeminnow Sacramento the speckled in for dace estimate movement The present. pikemin- were Sacramento now while units cover benthic among units. cover individual cupied hne nueo oe yseke ae( dace speckled by cover of use in changes viaiiy(rdtrtreatment (predator availability ie rdtrtetetwt o oe availability cover low treatment with (predator treatment predator com- the bined in particularly trials, during increased cover of i o eetbyafc rdto ae norex- our in rates predation affect detectably not did uididvda oe nt.Wieueo oe av- cover of oc- use simultaneously While eraged dace units. cover speckled individual eight cupied as many As h obnto fsupn n armnopike- Sacramento and sculpins of combination The hl 10 a While ohseke aeadsupn oe eet and beneath moved sculpins and dace speckled Both Ͻ .3.A aya v cli iutnosyoc- simultaneously sculpin ®ve as many As 0.03). .1;cvraalblt ln loddntaffect not did also alone availability cover 0.41); .01.Ti atr a needn fcover of independent was pattern This 0.0001). Ͼ 0 taltmsfrsupn(i.2,teuse the 2), (Fig. sculpin for times all at 50% vadVnotn20) o h pce in species the For 2001). VanKooten and Èv ϫ ifrnei h viaiiyo cover of availability the in difference ϫ D oe interaction, cover ISCUSSION ϫ clg,Vl 5 o 1 No. 85, Vol. Ecology, oe interaction, cover F F 1,14 1,16 P ϭ ϭ ϭ 5.8, 0.62). 59.5, P P January 2004 MULTIPLE PREDATORS AND BIOTIC RESISTANCE 131

FIG. 2. Use of cover by speckled dace and sculpins under different predator treatments and two levels of benthic cover Reports availability in an arti®cial stream experiment. Data re¯ect the percentage of surviving ®sh using cover. Day-2 observations (top) show use of cover before the addition of Sacramento pikeminnow; day-3 observations (bottom) indicate use of cover one day after the addition of Sacramento pikeminnow (except for the sculpin-only predator treatment). Error bars indicate ϩ1 SE. Numbers above bars indicate sample sizes (number of trials). No day-3 data are presented for speckled dace in the combined predator treatment because more than two ®sh survived in only 1 of 10 trials.

periment, we cannot conclude that the amount of cover of speckled dace distribution, particularly in areas of is irrelevant in the natural system. For example, cover low water velocity. reduces the risk of predation by Sacramento pike- Discussion of differences between the arti®cial minnow for sculpins (White and Harvey 2001); Sac- stream and natural systems raises additional questions ramento pikeminnow would be expected to greatly about the relevance of this experiment to the distri- reduce the abundances of both sculpins and speckled bution of speckled dace in the Van Duzen River. For dace in the absence of any cover. At the other extreme, example, the per capita risk for speckled dace in the abundant cover across mesohabitats might provide arti®cial stream probably exceeded any natural system speckled dace with sculpin-free refuges from preda- in which predators have alternative prey. The arti®cial tion by Sacramento pikeminnow. Speckled dace ap- stream also did not provide mesohabitats found in the parently coexisted with relatively high densities of natural system that might be relevant to the interac- rif¯e sculpin in Deer Creek, California, by shifting to tions studied. The Van Duzen River has large, shallow habitats with slower water velocities and coarse sub- pools that commonly contain ®lamentous algae in the strate (Baltz et al. 1982). We hypothesize that, in the dry season. Sculpins and post-young-of-the-year Sac- high cover, combined-predator treatment in the arti- ramento pikeminnow rarely occupy these pools, which ®cial stream, movement among refuges by sculpin, therefore might provide habitat for speckled dace. Our and their subsequent displacement of speckled dace experiment also did not address genetic constraints, from those refuges, enhanced predation on speckled which may in¯uence the current distribution of speck- dace by Sacramento pikeminnow even when the avail- led dace in the Van Duzen River. While the origin and ability of cover appeared excessive. Movement by initial size of the population in the Van Duzen River sculpin in a natural system with benthic cover across is unknown, a small number of founders seems likely. a range of water velocities is probably not extensive Reduced genetic diversity may inhibit invading pop- enough to have the same effect. However, because of ulations through inbreeding depression or by limiting high ®ne-sediment loads, benthic cover is extremely the ability to evolve (Sakai et al. 2001). Finally, the rare in the Van Duzen River near the downstream limit absence of examples of predation limiting the distri- Reports njhn atr ( darters inactivity observed johnny For (1988) in Stein benthic, competitors. and and Rahel and column example, water piscivores the in refuge-occupying piscivores of tion River. Duzen Van the in dace speckled distribution of the limits in alone caution predation suggest that concluding further to 1988), Wellborn rates and (Rob- predation species inson invading high an of very distribution the of and in¯uence systems, failure aquatic apparent in the species introduced of bution 132 atr ntepeec fbt mlmuhbs and bass smallmouth ( both cray®sh of refuge-occupying presence johnny the by in cover benthic darters of use lesser and rates control neo rys ( cray®sh of ence otn seto h iuto esuidi h order the is studied we situation im- the of potentially aspect other portant speckled and One species on pikeminnow. sculpin effect either Sacramento of greater combination a a than to pike- dace lead Sacramento with might combination minnow in the that sculpins possibility system the two another raise 1999) in Harvey species and two (White hab- these in differences between 1995), use al. itat in et habitat (Brown While River similar Eel (3) use the sculpin range. coastrange sculpins' and the prickly of near limit individuals re- large upstream not of the did abundance used relative we the distribution ¯ect size scul- from underesti- the dace because probably speckled pin for experiment risk this predation our the in mated ®shes fact, probably small In River, for reach. Duzen risk Van predation the the in increasing dace speckled downstream of just reach in- stream the size occupy relatively body sculpin Thus, large 1995). that al. is et (Brown River upstream Eel creases two the the in of conse- species histories One sculpin life (2) amphidromous 1982); them the al. with of et quence compete (Baltz and cover Sculpin dace benthic an (1) for speckled system: is on natural resistance prey the both biotic of indeed aspect if important dace, speckled expansion range by to resistance biotic to contribute may ecologically consequences. has important predators or predators competitors benthic water-column and con- between of array Further facilitation what under studied. ditions determine been to needed not is conse- study for have and observed populations they rates, changes natural behavioral predation the of on quences interactions of consequences detect these not did studies laboratory two ru ( trout neo icvru mlmuhbs ( bass smallmouth piscivorous of ence 20)sgetdta rdto ikfrLtl Colo- ( Little al. for spinedace risk et rado predation Bryan that streams, on suggested arti®cial (2002) based in Similarly, to observations bass. darters their smallmouth johnny by of susceptibility predation behav- the in change increased this ior that hypothesized Stein and hel dolomieu rswtr®hsmycmol aeacombina- a face commonly may ®shes Freshwater eea pc® etrso h iuto estudied we situation the of features speci®c Several nohnhsmykiss Oncorhynchus ,btjhn atratvt qa o1%of 19% to equal activity darter johnny but ), eioeavittata Lepidomeda roetsvirilis Orconectes tesoanigrum Etheostoma roetsrusticus Orconectes sehne ytepres- the by enhanced is ) .Hwvr these However, ). rmrainbow from ) ntepres- the in ) Micropterus RTC AVYE AL. ET HARVEY C. BRET .Ra- ). ml nti ae osntgetywae Moyle weaken greatly (1996 not Light's does and paper ex- this tentative in The determined. ample be prevent to interactions remains biotic invasion where situations and fects 2001). (Sim- Ricciardi common 1999, Holle be Von can and species berloff facilita- invading that among observations tion to resident contrast the in has of community, species invasion to introduced resistance an biotic where enhanced Sacra- example an and be This sculpins may distribution. their of limit could effect pikeminnow mento combined drainage the the of before parts sculpin-free had reached dace have might speckled they pikeminnow, if Sacramento before dace, introduced been by expansion range pre- vent could alone pikeminnow Sacramento neither nor that sculpins Assuming species. introduced of arrival of at,D . n .B ol.19.Ivso eitneto resistance Invasion 1993. Moyle. B. P. and M., D. Baltz, at,D . .B ol,adN .Kih.18.Compet- 1982. Knight. J. N. and Moyle, B. P. M., D. Baltz, rw n .E il rvddueu omnso drafts on comments useful paper. provided the Lisle of E. T. and Brown rw,L . .A aen n .B ol.19.Com- 1995. Moyle. B. P. and Matern, A. S. R., L. Brown, iua eut a ehgl otneto h or- the on (Lawton 1999). contingent consideration under highly ecosystems and be par- ganisms may 2002), Chesson results understanding and ticular for (Shea framework invasions useful biological a theory this ecology provide of community may Results while that, 2000). suggest al. study et Mack and 1998, (Ricciardi ecosystems Rasmussen on in- effects successful their and communities, vaders, to invaded ability the readily in- need predict limit managers not Resource will ®shes. resistance vading biotic general in streams rw,L . n .B ol.19.Ivdn pce in species Invading 1997. Moyle. B. P. and R., L. Brown, yr,J .20.Pyia aia trbt eitsbiotic mediates attribute habitat Physical 2002. E. J. Byers, 2002. Sweetser. G. M. and Robinson, T. A. D., S. Bryan, atn .H 99 r hr eea asi clg?Oikos ecology? in laws general there Are 1999. H. J. Lawton, Eklo ak .N,D ibrof .M osae .Eas M. Evans, H. Lonsdale, M. W. Simberloff, D. N., R. Mack, h omneso mretmlil rdtref- predator multiple emergent of commonness The nrdcdseisb aieasmlg fCalifornia of assemblage Applications Ecological native ®shes. a stream by species introduced tv neatosbtenbnhcsra se,rfescul- rif¯e ®shes, pin, stream benthic between interactions itive 39 aaieeooyo rcl sculpins, prickly of ecology parative .R acifasse nte®l n aoaoy .R. L. laboratory. and ®eld the in assisted Ratcliff R. D. i.EvrnetlBooyo Fishes of Biology Environmental nia. hp ihrsdn pce.EvrnetlBooyo Fish- of es Biology relation- Environmental species. and resident failures, with successes, ships California: River, Eel the cvru rdtr:efcso ryhbttue Ecology use. habitat prey of 82 effects predators: scivorous Oecologia invasion. species 130 non-indigenous to resistance Fishes in- of multiple Biology 49±56. to Environmental ®sh native predators. small troduced a of responses Behavioral 84 lu,adF .Bza.20.Boi nain:causes, invasions: Biotic 2000. Bazzaz. A. F. and Clout, lus osrnesculpin, coastrange :1502±1511. v . n .Vnotn 01 aiiainaogpi- among Facilitation 2001. VanKooten. T. and P., Èv, aainJunlo ihre n qai Sciences Aquatic and Fisheries of Journal Canadian . :2486±2494. :177±192. 49 otsgulosus Cottus :146±156. :271±291. A L CKNOWLEDGMENTS n pclddace, speckled and , b ITERATURE .aleuticus C. rdcinta o California for that prediction ) C nteElRvr Califor- River, Eel the in , ITED clg,Vl 5 o 1 No. 85, Vol. Ecology, 3 42 :246±255. otsasper Cottus hncty oscu- Rhinichthys :329±343. and , 63 : January 2004 MULTIPLE PREDATORS AND BIOTIC RESISTANCE 133

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