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An Emergent Multiple Predator Effect May Enhance Biotic Resistance in a Stream Fish Assemblage

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An Emergent Multiple Predator Effect May Enhance Biotic Resistance in a Stream Fish Assemblage Ecology, 85(1), 2004, pp. 127±133 q 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 #25 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 127 128 BRET C. HARVEY ET AL. Ecology, Vol. 85, No. 1 PLATE 1. Sacramento pikeminnow swimming over cover units in the arti®cial stream. Photo by R. J. Nakamoto. and Moyle 1997): both the California roach (Lavinia Along with the patterns in ®sh distributions in the symmetricus) and the Sacramento pikeminnow (Pty- Van Duzen River, other observations suggest that biotic chocheilus grandis) became widespread in the Eel interactions could be important in determining the dis- drainage within 10 years of their introductions (in about tribution of speckled dace in the Eel River drainage. 1970 for California roach and 1980 for Sacramento Competition for space with rif¯e sculpin (C. gulosus) pikeminnow, both by unknown means). However, a in¯uenced the habitat use of speckled dace in Deer third cyprinid also native to adjacent drainages, the Creek, California: In a reach where rif¯e sculpin were speckled dace (Rhinichthys osculus), does not ®t this rare, speckled dace tended to occupy fast water with pattern. coarse substrates, but where rif¯e sculpin were abun- Since its introduction into the Eel River drainage dant, speckled dace occupied slower water at the stream before 1988 (Brown and Moyle 1997), the speckled margins (Baltz et al. 1982). Rif¯e sculpin also con- dace has remained con®ned to one major tributary (the sumed speckled dace during laboratory experiments by Van Duzen River). With the exception of rare captures Baltz et al. (1982), and piscivory by sculpins has been Reports 5±10 km downstream, most recently a total of two in- observed frequently (Moyle 2002). As adults, Sacra- dividuals 5 km downstream in annual collections dur- mento pikeminnow are largely piscivorous, consuming ing 1996±2002 (B. C. Harvey, J. L. White, and R. J. a wide variety of ®shes including benthic species (e.g., Nakamoto, unpublished data), the downstream extent Brown and Moyle 1997, White and Harvey 2001). Giv- of the species' range has remained constant since 1987. en these observations, we hypothesized that competi- While a physical barrier de®nes the upstream limit of tion for space and predation by sculpins in benthic the current 25-km range of speckled dace in the Van habitat, combined with predation by Sacramento pike- Duzen River, its downstream limit appears unrelated minnow, which occupy the water column, could lead to physical factors. For example, maximum weekly to an emergent multiple predator effect on speckled mean temperature over 1 year at the downstream limit dace. We tested this hypothesis in a large arti®cial of speckled dace in the Van Duzen River and at a site stream. We also included the availability of benthic 20 km downstream differed by only 0.28C (J. L. White, cover as a factor in the experiment, because it is likely unpublished data). However, the downstream limit of to affect the predator±prey interactions of interest and speckled dace lies just upstream of the upper limit of because its availability in the Eel River drainage is both coastrange sculpin (Cottus aleuticus) and prickly heterogeneous in space and time. sculpin (C. asper). These native sculpins commonly occupy habitats with coarse substrates that speckled METHODS dace also prefer (Moyle 2002). The upstream extent of the distribution of Sacramento pikeminnow and down- We conducted the experiment from 2 January 2001 stream extent of speckled dace in the Van Duzen River to 6 June 2001 in a recirculating laboratory stream with overlap by about 15 km, although Sacramento pike- a channel 0.30±0.75 m wide and a total area of 7.1 m2, minnow are rare in that reach. In contrast to speckled with water depth of 45±50 cm and gravel substratum. dace, neither California roach nor Sacramento pike- A Plexiglas inner wall allowed observation of the entire minnow faced ecologically similar species or poten- stream. Electric pumps produced mean water velocities tially signi®cant predation risk from the depauperate of 35 cm/s in the narrow part of the channel and 14 Eel River ®sh assemblage (described in Brown and cm/s in the remainder. Water temperature was main- Moyle 1997). tained at 168±178C. 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 3 18 cm, held 2 cm apart by and the morning of day 3.
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