2407 Ecology, 84(9), 2003, pp. 2407–2418  2003 by the Ecological Society of America PREDATOR IDENTITY AND ECOLOGICAL IMPACTS: FUNCTIONAL REDUNDANCY OR FUNCTIONAL DIVERSITY? DAVID R. CHALCRAFT 1,3 AND WILLIAM J. RESETARITS,JR. 2 1 Department of Ecology, Ethology and Evolution, 515 Morrill Hall, University of Illinois, Urbana, Illinois 61801 USA 2 Department of Biological Sciences, Old Dominion University, Norfolk, Virginia 23529 USA Abstract. Different species occupy similar trophic positions in natural communities. However, ecologists have often overlooked the consequences of this variation for local communities by assuming that species occupying similar trophic positions are functionally similar. There have been few experimental tests of this important assumption. We tested the assumption of functional similarity by comparing the effect of six different predators (three fish and three salamander species) on an ensemble of larval anuran prey. Our experiment identified substantial variation in the impact of different predators on a variety of responses. Differences among predators in their selection of prey caused the structure of the larval anuran ensemble to vary continuously as opposed to producing discrete alternative states. Predators also differed in their ability to suppress either the total number or biomass of anurans. Thus, performance of larval anurans was dependent upon the identity of the predator. Moreover, the identity of predators judged similar and their degree of similarity depends on the specific response variable. In our study we found no predominance of weak or strong interactors, as predators appear to be evenly distributed along a gradient of interaction strengths. Knowledge of the identity of the species occupying a particular trophic position is crucial to predicting the impact of that trophic position on a community. Ecologists should not simply assume that species are functionally similar. The differing effects of different predators suggest that variation in their distribution across complex landscapes influences prey number, biomass, population dynamics, distribution, and ultimately local and regional species diversity. Key words: beta diversity; community structure; fish; food web; keystone predation; larval an- urans; ponds; predator–prey interactions; salamanders; species turnover; trophic level. INTRODUCTION Interactions among species influence individual fit- ness, population dynamics, and community structure (see reviews by Schoener 1983, Sih et al. 1985, Gur- evitch et al. 1992, 2000, Bertness and Callaway 1994). An individual species, however, may co-occur with very different sets of species at different locations with- in its range. As a result, the occurrence and intensity of different types of species interactions (e.g., preda- tion, competition, and mutualism) likely changes among locations. MacArthur and Wilson (1967) and Whittaker (1972) recognized this temporal and spatial change in species composition of local communities, calling it ‘‘species turnover’’ and ‘‘beta diversity,’’re- spectively. Inherent in these ideas and much of what we call broadly ‘‘niche theory’’ is the notion that as species composition changes, so too does the precise nature of the species interactions. Manuscript received 9 September 2002; revised 28 December 2002; accepted 17 January 2003. Corresponding Editor: S. P. Lawler. 3 Present address: Department of Biological Sciences, Tex- as Tech University, Lubbock, Texas 79409-3131 USA. E-mail: chalcraft@nceas.ucsb.edu An increasingly common practice in ecology, how- ever, is to group species into ‘‘kinds of organisms’’ (e.g., Cohen 1978, Briand 1983, Sugihara et al. 1989, 1997), functional groups (e.g., Faber 1991, Walker 1991, Ko ¨rner 1993, Hooper and Vitousek 1997, 1998, Smith et al. 1997, Chapin et al. 1998, Symstad et al. 2000), or trophic levels (e.g., Hairston et al. 1960, Men- ge and Sutherland 1976, 1987, Fretwell 1977, Oksanen et al. 1981, McQueen et al. 1986, Ginzberg and Ak- c ¸akaya 1992). This approach assumes that all species so grouped act or respond similarly. Lacking this as- sumption, the grouping of species is meaningless in a predictive sense, and only serves to satisfy our need to simplify and categorize. The practice of grouping may contribute to the fact that the consequences of spatial and temporal variation in the species compo- sition of local communities remain poorly understood. Many authors have challenged this practice of group- ing species for some time (e.g., Glasser 1983, May 1983, Cousins 1987, Paine 1988, Sugihara et al. 1989, Polis 1991, Polis and Strong 1996). Failure to consider turnover in species composition within trophic levels might produce the conflicting results that support op- posing models of trophic structure (i.e., bottom-up vs. top-down effects; see Leibold 1989, 1996, Hunter and Price 1992, Leibold et al. 1997). Furthermore, much