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<br />Ecology, 89(3), 2008, pp. 847-856
<br />© 2008 by the Ecological Society of America
<br />TRAIT SYNERGISMS AND THE RARITY, EXTIRPATION, AND
<br />EXTINCTION RISK OF DESERT FISHES
<br />JULIAN D. OLDEN, 1,4 N. LEROY POFF,2 AND KEVIN R. BESTGEN3
<br />T56J
<br />'School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, Washington 98195 USA
<br />ZGraduate Degree Program in Ecology, Department of Biology, Colorado State University, Fort Collins, Colorado 80523 USA
<br />3Larval Fish Laboratory, Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado 80523 USA
<br />Abstract. Understanding the causes and consequences of species extinctions is a central
<br />goal in ecology. Faced with the difficult task of identifying those species with the greatest need
<br />for conservation, ecologists have turned to using predictive suites of ecological and life-history
<br />traits to provide reasonable estimates of species extinction risk. Previous studies have linked
<br />individual traits to extinction risk, yet the nonadditive contribution of multiple traits to the
<br />entire extinction process, from species rarity to local extirpation to global extinction, has not
<br />been examined. This study asks whether trait synergisms predispose native fishes of the Lower
<br />Colorado River Basin (USA) to risk of extinction through their effects on rarity and local
<br />extirpation and their vulnerability to different sources of threat. Fish species with "slow" life
<br />histories (e.g., large body size, long life, and delayed maturity), minimal parental care to
<br />offspring, and specialized feeding behaviors are associated with smaller geographic
<br />distribution, greater frequency of local extirpation, and higher perceived extinction risk than
<br />that expected by simple additive effects of traits in combination. This supports the notion that
<br />trait synergisms increase the susceptibility of native fishes to multiple stages of the extinction
<br />process, thus making them prone to the multiple jeopardies resulting from a combination of
<br />fewer individuals, narrow environmental tolerances, and long recovery times following
<br />environmental change. Given that particular traits, some acting in concert, may differentially
<br />predispose native fishes to rarity, extirpation, and extinction, we suggest that management
<br />efforts in the Lower Colorado River Basin should be congruent with the life-history
<br />requirements of multiple species over large sjiatial and temporal scales.
<br />Key words: dams; functional diversity; life histlory; Lower Colorado River Basin (USA); native fishes;
<br />river regulation; species attributes; species invasions.
<br />INTRODUCTION
<br />The escalation of anthropogenic pressures confront-
<br />ing natural ecosystems underscores the urgency with
<br />which we must identify and protect species facing the
<br />greatest risk of extinction (Pimm and Jenkins 2005).
<br />This is a challenging task, in large part, because we lack
<br />direct estimates of extinction risk for most species.
<br />Consequently, ecologists have increasingly turned to
<br />using predictive suites of ecological and life-history traits
<br />as rule-of-thumb proxies to triage vulnerable species
<br />(O'Grady et al. 2004). Such rules are based on a wealth
<br />of empirical evidence from a diverse set of taxa which
<br />suggests that a number of intrinsic biological traits can
<br />render species more vulnerable to extinction (reviewed
<br />by McKinney 1997). The identification of such "extinc-
<br />tion-prone traits" has helped to prioritize management
<br />strategies aimed at safeguarding native species with the
<br />greatest risk of extinction (Purvis et al. 2000).
<br />By integrating the landscape activities of humans,
<br />fresh waters are subjected to a panoply of anthropogenic
<br />Manuscript received 8 November 2006; revised 30 July 2007;
<br />accepted 9 August 2007. Corresponding Editor: K. O.
<br />Winemiller.
<br />4 E-mail: olden@u.washingtoTi.edu
<br />threats, including the pervasive effects of hydrological
<br />alteration, agricultural and urban land-use, invasive
<br />species, and climate change (Dudgeon et al. 2006). The
<br />nexus of these threats are well documented in the Lower
<br />Colorado River Basin, USA, where the century-long
<br />exploitation of water resources for human consumption,
<br />irrigation, and hydroelectricity has resulted in one of the
<br />most controlled rivers in the world (Fradkin 1981,
<br />Carlson and Muth 1989). Historically, the Lower
<br />Colorado River was characterized by extreme seasonal
<br />variations in flow, flash flooding, and warm, turbid
<br />waters in which native fishes responded over evolution-
<br />ary time by developing particular behavioral patterns,
<br />morphologies, and life-history traits (Deacon and
<br />Minckley 1974, Smith 1981). However, since European
<br />settlement, hydrologic alteration from extensive dam
<br />building and irrigation and the deleterious effects of
<br />invasive species have caused the precipitous decline of
<br />many native fishes (Minckley and Deacon 1968, 1991,
<br />Fagan et al. 2005b, Olden and Poff 2005). Conservation
<br />of native biodiversity in the Lower Colorado River
<br />Basin will require management strategies that focus on
<br />identifying and conserving those fish species that are
<br />considered to be at the greatest risk to extinction.
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