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848 JULIAN D. OLDEN ET AL. Ecology, Vol. 89, No. 3 TABLE 1. Native fishes of the Lower Colorado River Basin, USA, examined in our study. Scientific name Common name Catostomidae Catostomus clarkiit desert sucker Catostomus discobolus bluehead sucker Catostomus insignist Sonora sucker Catostomus latipinnis flannelmouth sucker Xyrauchen texanus razorback sucker Cyprinidae Gila cypha humpback chub Gila elegans bonytail Gila intermediat Gila chub Gila nigrat headwater chub Gila robusta roundtail chub Lepidomeda mollispinist Virgin River spinedace Lepidomeda vittatat Little Colorado River spinedace Meda fulgidat spikedace Plagopterus argentissimust woundfin Ptychocheilus Lucius Colorado pikeminnow Agosia chrysogaster longfin dace Rhinichthys osculus speckled dace Rhinichthys cobitist loach minnow Cyprinodontidae Cyprinodon maculariust desert pupfish Poeciliidae Poeciliopsis occidentalis Gila topminnow Salmonidae Oncorhynchus gilae apachet Apache trout Oncorhynchus gilae gilaet Gila trout t Species endemic to the Lower Colorado River Basin with invasive species may differ (Duncan and Young 2000, Owens and Bennett 2000, Olden et al. 2007). In the present study, we provide the first investigation of how trait synergisms may predispose fish species to risk of extinction through their effects on rarity and local extirpation and their interactions with different ecological mechanisms that underlie the extinction process. We characterize a suite of ecological and life-history traits for native fishes of the Lower Colorado River Basin, and ask if synergistic trait combinations provide greater predictive insight into the overall extinction process compared to the additive effects of individual traits considered in isolation. In doing so we have the opportunity to examine the widely held, yet rarely examined, assumption that attributes of species that contribute to their rarity will further predispose them to greater risk of local extirpation and ultimately global extinction (Pimm et al. 1988, Gaston 1994, Johnson 1998). We then assess the degree to which trait combinations can be used to distinguish the relative contribution to extinction risk arising from anthropogenic habitat alteration vs. interactions with nonindigenous fishes. By addressing these objectives we aim to improve our understanding of the linkages between fish species' extinction, biological traits, and specific drivers of environmental change in the highly- modified Lower Colorado River Basin. MATERIALS AND METHODS Ecological and life-history traits The Lower Colorado River Basin is home to a unique ichthyofauna that exhibits a distinct suite of behavioral, morphological, and life-history characteristics. Our study focuses on 22 native fish species (out of the 28 species present in the lower basin) for which reliable trait data were available (Table 1). Of these species, 12 are endemic to the lower basin and six are endemic to the Colorado River Basin. We collated data for 10 traits, including (1) maximum total body length (cm); (2) swim factor, the ratio of minimum depth of the caudal peduncle to the maximum depth of the caudal fin, where small factors are indicative of strong swimmers (following Webb 1984); (3) trophic guild, the adult feeding mode based on published diet analyses and classified as herbivore-detritivore (->25% plant mat- ter), omnivore (-<5% plant matter), invertivore, or invertivore-piscivore; (4) diet breadth, the total number of major diet items consumed at any time during a fish's lifetime, including inorganic material, vegetative mate- rial, plankton, aquatic/terrestrial insects, oligochaetes/ crustaceans/molluscs, fish/fish eggs, and amphibians/ mammals/birds (range 1-7); (5) longevity, the maximum potential life span (years); (6) female age at maturation (years); (7) female length at maturation (cm); (8) fecundity, the total number of eggs or offspring per breeding season; (9) parental care, a metric representing the total energetic contribution of parents to their offspring (following Winemiller 1989); and (10) repro- ductive guild, described as non-guarders (open substra- Previous studies have shed considerable insight into the ecological and life-history correlates of extinction risk for freshwater fishes (e.g., Angermeier 1995, Parent and Schriml 1995, Reynolds et al. 2005, Olden et al. 2006, 2007). However, an important knowledge gap remains with respect to the specific pathways that biological traits may operate to predispose species to extinction. We believe that our understanding and prediction of fish species extinctions would be improved by explicitly considering how biological traits collectively predispose species to the primary components of the extinction process: rarity, risk of local extirpation, and vulnerability to global extinction. Unfortunately, progress in this research area has been limited for at least two reasons. First, previous studies across many taxonomic groups have principally examined the independent effects of individual traits, not interactions among traits that may influence a species' extinction risk. Ecological theory supports the importance of such trait interactions, or synergisms, for species extinction (Lawton 1994), but there is little empirical support (Davies et al. 2004, Cardillo et al. 2005). Second, past research has concentrated primarily on establishing trait linkages with estimated risk to global extinction, without considering the specific mechanisms by which traits may confer risk. For example, traits associated with rarity and local extirpation or with vulnerability to extinction from habitat loss or interactions