Understanding the additive or interactive threats of habitat transformation and invasive species is critical for conservation, especially where climate change is expected to increase the severity or frequency of drought. In the arid southwestern USA, this combination of stressors has caused widespread declines of native aquatic and semi-aquatic species. Achieving resilience to drought and other effects of climate change may depend upon continued management, so understanding the combined effects of stressors is important. We used Bayesian hierarchical models fitted with 10-yrs of pond-based monitoring surveys for the federally-endangered Sonoran Tiger Salamander (Ambystoma mavortium stebbinsi) and invasive predators (fishes and American Bullfrogs, Lithobates catesbeianus) that threaten native species. We estimated trends in occupancy of salamanders and invasive predators while accounting for hydrological dynamics of ponds, then used a two-species interaction model to directly estimate how invasive predators affected salamander occupancy. We also tested a conceptual model that predicted that drought, by limiting the distribution of invasive predators, could ultimately benefit native species. Even though occupancy of invasive predators was stationary and their presence in a pond reduced the probability of salamander presence by 23%, occupancy of Sonoran Tiger Salamanders increased, annually, by 2.2%. Occupancy of salamanders and invasive predators both declined dramatically following the 5 th consecutive year of drought. Salamander occupancy recovered quickly after return to non-drought conditions, while occupancy of invasive predators remained suppressed. Models that incorporated three time-lagged periods (1 to 4 yrs) of local moisture conditions confirmed that salamanders and invasive predators responded differently to drought, reflecting how life-history strategies shape responses to disturbances. The positive 10-yr trend in salamander occupancy and their rapid recovery after drought provided partial support for the 3 hypothesis of drought-mediated coexistence with invasive predators. These results also suggest management opportunities for conservation of the Sonoran Tiger Salamander and other imperiled organisms in human-transformed landscapes.

Reintroductions of the endangered razorback sucker Xyrauchen texanus and Colorado squawfish Ptychocheilus lucius into the Verde River, Arizona, have failed to establish self‐sustaining populations. The failure of these reintroductions is often attributed to predation and competition by nonnative fishes and habitat loss, but parasite infestations (particularly a copepod, Lernaea cyprinacea) also have been implicated. We assessed the relationships among fish health, eukaryotic parasites, and bacterial and viral infections of these endangered fishes collected from two locations (Perkinsville and Childs) on the Verde River, Arizona, during February and June 1996. We also examined surrogate nonendangered native fishes (Sonora sucker Catostomus insignis, desert sucker Catostomus clarki, and roundtail chub Gila robusta) and nonnative species (common carp Cyprinus carpio and smallmouth bass Micropterus dolomieu) to determine if there were any systemwide patterns in parasite infestation and health of fishes. All tests for viruses were negative. We identified 19 bacteria (7 known fish pathogens) and 14 parasites from examined fishes. Six of the seven dominant (>10% prevalence) parasite taxa were more prevalent at Perkinsville than at Childs; Lernaea occurrence was not related to site. Time of year did not have a consistent effect on parasite prevalence across fish species, and Lernaea occurrence was not related to time of year. Lernaea cyprinacea were present on 17.3% of the 313 fish examined; 67% of the parasitized fish were infected with a single L. cyprinacea. Regression analyses on the calculated health assessment index, hematocrit, and leukocrit indicated that month, site, Trichodina, Ichthyophthirius (ich), Ornithodiplostomum and Posthodiplostomum (white grubs), and the cestode Isoglaridacris hexacotyle affected the health of fishes. However, all fishes examined were considered “healthy,” suggesting that parasites were not seriously affecting Verde River fishes. A suite of factors is likely responsible for the failure of these endangered species to reestablish.

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The Kanab ambersnail (Oxyloma haydeni kanabensis) is a federally endangered mollusc currently known to reside in two locations in the southwestern U.S.A. To determine the extent of within- and between-population genetic variation of this taxon, the amplified fragment length polymorphism (AFLP) technique was used to generate 110 genetic markers among individuals sampled from the two Kanab ambersnail populations and from the only two known southwestern populations of the Niobrara ambersnail (Oxyloma haydeni haydeni) in Utah and northern Arizona. Additional information was obtained from sequence data of cytochrome b and cytochrome oxidase I gene fragments. Results suggest high levels of differentiation among populations, as evidenced through the application of UPGMA (unweighted pair-group method with arthimetic averaging) clustering, F statistics, and Fisher's exact test. Various levels of within-population genetic diversity were observed among populations. Expected heterozygosities ranged from 0.239 to 0.086 under a model assuming Hardy-Weinberg genotypic proportions and ranged from 0.205 to 0.061 under an obligate-selfing completely homozygous model. Results from cluster analyses showed that one Kanab ambersnail population and one Niobrara ambersnail population were more similar than the two Kanab ambersnail populations studied (supported by >80% of bootstrap replicates). These findings were further supported through the phylogenetic analysis of both mito chondrial gene fragments. The data suggest that taxonomic designations need revision, an act that will likely affect the protected status of some of the populations.

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