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852 JULIAN D. OLDEN ET AL. Extirpation = 0.54 n=22 _ Length at maturity c 32.5 cm Yes No Extirpation :-0 47 Extirpation = 0.85 n= 1n=4 Fecundity !:-: 457 eggs Yes No Extirpation = 0.72 Extirpation -- 0.41 n=3 n=15 B Yes Diet breadth :5 15 items Extirpation = 048 n=7 Parental care 1.5 Yes No Extirpation 0:52 Extirpation 0.38 n=5 n=2 C D E FIG. 2. Regression tree discriminating among native species of the Lower Colorado River Basin according to empirical estimates of local extirpation frequency (values between 0 and 1). Letters A-F indicate terminal nodes; n is the number of species. In the right-hand branch, species with more parental care (surrogate split: all reproductive guilds excluding non-guarders spawning on open substrates) that mature at relatively younger ages and smaller body sizes were identified as having no or low risk of extinction (node F). In contrast, species with high parental care but delayed maturity (i.e., long-lived species that mature at older ages and at larger body sizes) were classified as having a high threat of extinction from biological invasions (node G). In the left-hand branch, species with low parental care (surrogate split: non-guarders spawning on open substrates) and herbivory/detritivory were classified to be at minimal risk of extinction (node E). Of the remaining species, body morphology discrim- inated between fishes subjected to different levels and sources of extinction risk. Species with body shapes indicative of strong swimming ability (i.e., small swim factor) were consider to be threatened from biological invasions (node C), whereas weaker swimming species (i.e., large swim factor) were primarily under high risk of extinction from environmental alteration (node D). A No Ecology, Vol. 89, No. 3 Extirpation = 0.36 n=8 Longevity J-5 18.5 years Yes No Extirpation =- 0.30 Extirpation 0.46 n=5 n=3 , An examination of model predictions (Appendix C) shows that fish species were misclassified according to either their level or source of extinction risk, but never both. Gila cypha (humpback chub) and G. elegans (bonytail) were predicted to be under moderate extinc- tion threat from non-native species (node C) rather than the high threat assessed by local experts, whereas Lepidomeda vittata (Little Colorado River spinedace) was correctly classified to its level of extinction risk, but the source of threat was misclassified as environmental alteration (node D). DISCUSSION For the highly endemic fish fauna of the Lower Colorado River Basin, our study points to the impor- tance of trait synergisms for increasing the susceptibility of native fishes to multiple stages of the extinction process. Body size, an essential organismal trait corre- lated with many other life-history characteristics (Peters 1983) and important to the functioning of aquatic ecosystems (Layman et al. 2005), showed a strong