J. N. Am. Benthol. Soc., 2008, 27(3):772–782Ó 2008 by The North American Benthological SocietyDOI: 10.1899/07-136.1Published online: 22 July 2008Effects of grazing minnows and crayfish on stream ecosystemstructure and functionJustin R. Bengtson1, Michelle A. Evans-White2, AND Keith B. Gido3Division of Biology, Kansas State University, Manhattan, Kansas 66502 USAAbstract. We compared the effects of 2 common grazers, southern redbelly dace (Phoxinus erythrogaster)and crayfish (Orconectes spp.), on ecosystem structure and function in experimental streams with pool andriffle habitats. Our goal was to identify potentially overlapping roles of these grazers in these systems.Measures of ecosystem structure included algal filament length, particulate organic matter (POM), densitiesof invertebrate taxa, and algal biomass. Ecosystem function was measured as gross primary productivity(GPP). Biomass-dependent effects of crayfish and dace on ecosystem properties were compared in autumn2005 when mean water temperature was 12.98C (range 7.6–27.98C). Increasing crayfish biomass did notinfluence ecosystem properties, but increasing dace biomass negatively affected algal filament length andchironomid abundance and positively affected chydorid abundance. Effects of moderately high biomasses ofdace and crayfish were compared in spring 2006 when mean water temperature was 21.48C (range 17.5–29.98C). Algal filament lengths were generally low relative to values obtained in autumn 2005 in both daceand crayfish treatments. In addition, algal filament length was shorter and chironomid density was lower incrayfish than in dace streams. The contrasting effects of dace and crayfish across sampling days, seasons,and habitats led us to hypothesize that physiological and behavioral traits of these species might limit theredundancy of their effects on ecosystems across broad spatial and temporal scales.Key words: primary production, context-dependent effects, experimental streams, prairie stream, peri-phyton, temperature, physiology, foraging behavior.Aquatic ecosystems are under significant anthropo-genic threats (Dudgeon et al. 2006). However, theeffects of losing aquatic species on ecosystem functionare difficult to predict because of the diverse forms oforganisms and complex interactions among species(Giller et al. 2004). The ecosystem effects of manyspecies might be predicted by grouping species intofunctional groups (e.g., trophic levels or feedingguilds) and assuming that all species within functionalgroups have similar effects. Such groupings imply thatloss of a species within a functional group should havelittle effect on an ecosystem because other members ofthe group can compensate for the loss (i.e., species areredundant within groups; Lawton and Brown 1993).However, individual species can facilitate resource useamong species within functional groups (e.g., Cardi-nale et al. 2002), or species might partition resourcesover space or time (e.g., Loreau 2000, Cardinale et al.2004). Thus, each species in a functional group mightprovide a unique contribution to the functioning of anecosystem. The roles of individual species andfunctional groups and the context within whichspecies in a community alter ecosystems must becharacterized to understand fully the consequences ofbiodiversity losses (e.g., Wardle and Zackrisson 2005).Aquatic grazers are an important group that canstrongly influence stream ecosystem functioning (Fe-minella and Hawkins 1995, Steinman 1996, Hillebrand2002). Grazers consume primary producers directly,and they affect primary producer species abundanceand diversity, productivity, nutrient uptake, andmineralization in stream ecosystems. Grazers are likelyto have overlapping effects in stream ecosystems, butthey differ greatly in morphology, physiology, andbehavior. Grazer effectiveness is dependent on grazermorphology and algal growth form (e.g., scraping andrasping grazers are more able than other types ofgrazers to feed on understory algae; Steinman 1996).1Present address: 5184 E. McReynolds Rd., Gypsum,Kansas 67448 USA. E-mail: [email protected] address: Department of Biological Sciences,601 Science Engineering Building, Fayetteville, Arkansas72701 USA. E-mail: [email protected] whom correspondence should be addr essed. E-mail:[email protected] size of grazers also is important. For example,smaller-bodied snails probably are better able to cropsmall adnate algal taxa than are larger-bodied grazers,such as fishes (e.g., Vaughn et al. 1993). Grazers candiffer greatly in reproductive turnover and physiology,and seasonal changes in grazer communities probablyare associated with thermal tolerance and timing ofreproduction. Therefore, species with similar ecologi-cal roles might not overlap in space or time (Fig. 1A–D), and the loss of a single species might affectecosystemfunctionwhenconsideredoverbroadspatial or temporal scales.Aquatic biodiversity in prairie streams is threatenedby numerous human activities (Cross and Moss 1987,Haslouer et al. 2005). Prairie streams are not heavilycanopied, and they generally have high primaryproduction (P) relative to respiration (R) (P/R ’ 1;Mulholland et al. 2001). Grazers in these streams mightprovide particularly important links between primaryproducers and ecosystem function. In North Americanprairie streams, grazing minnows and crayfish canoccur in high densities and potentially exert top-down(directly through grazing) and bottom-up (indirectlythrough nutrient recycling) control on primary pro-duction (Evans-White et al. 2003, Franssen et al. 2006,Bertrand and Gido 2007).Crayfish and grazing minnows are dominantgrazers in prairie streams and potentially haveoverlapping roles in these ecosystems. Both taxa canalter standing crops of attached algae, particle size ofparticulate organic matter (POM), and invertebratecommunity structure (Power et al. 1985, Gelwick andMatthews 1992, Vaughn et al. 1993, Creed 1994,Charlebois and Lamberti 1996, Bertrand 2007, Ber-trand and Gido 2007). Crayfish also can have directnegative effects on invertebrates through predation(Momot 1995, Nystro¨m et al. 1996, Stenroth andNystro¨m 2003, Usio and Townsend 2004). Morpholog-ical differences between grazing minnows and crayfishmight cause different effects on primary producers atsmall spatial scales. Grazing minnows ingest periphy-ton directly, whereas crayfish have shredding mouth-FIG. 1. Conceptual diagram illustrating the potential