This paper reports on the response by amphibians, benthic macroinvertebrates, and zooplankton in naturally fishless alpine lakes to fish introductions and subsequent fish disappearance. We assessed resistance (the degree to which a system is altered when the environment changes) by comparing faunal distribution and abundance in lakes that have never been stocked with fish vs. the distribution and abundance in lakes that have been stocked and still contain fish. We assessed resilience (the degree and rate of a system's return to its previous configuration once the perturbation is removed) by comparing faunal distribution and abundance in lakes that were stocked at one time but have since reverted to a fishless condition (stocked-now-fishless lakes) vs. the distribution and abundance in lakes that have never been stocked. We quantified recovery rates and trajectories by comparing faunal assemblages of stocked-now-fishless lakes that had been fishless for 5-10, 11-20, and Ͼ20 yr.Faunal assemblages in the study lakes had low resistance to fish introductions, but in general showed high resilience. The mountain yellow-legged frog (Rana muscosa), conspicuous benthic macroinvertebrates, and large crustacean zooplankton (Ͼ1 mm) were dramatically reduced in distribution and abundance by fish introductions but generally recovered to predisturbance levels after fish disappearance. Inconspicuous benthic invertebrate taxa, small crustacean zooplankton (Ͻ1 mm), and rotiferan zooplankton (Ͻ0.2 mm) were either unaffected by fish or increased in the presence of fish. For both the benthic macroinvertebrate community and the zooplankton community as a whole, fish disappearance was followed by a steady change away from the configuration characteristic of fishcontaining lakes and toward that of lakes that had never been stocked. Both communities remained markedly different from those in never-stocked lakes 5-10 yr after fish disappearance and converged on the configuration of never-stocked lakes only 11-20 yr after fish disappearance.Recovery was likely facilitated by the winged adult stages of many benthic macroinvertebrates, resting eggs of zooplankton, and nearby source populations of frogs. However, many frog populations have disappeared since the time that lakes in this study reverted to a fishless condition, and the viability of zooplankton egg banks should decline in fishcontaining lakes over time. As a result, faunal resilience may be lower in lakes that revert to a fishless condition today than is suggested by the results of our study. These findings have important implications for the restoration of alpine lake ecosystems.

One of the most puzzling aspects of the worldwide decline of amphibians is their disappearance from within protected areas. Because these areas are ostensibly undisturbed, habitat alterations are generally perceived as unlikely causes. The introduction of non‐native fishes into protected areas, however, is a common practice throughout the world and may exert an important influence on amphibian distributions. We quantified the role of introduced fishes (several species of trout) in the decline of the mountain yellow‐legged frog (  Rana muscosa) in California's Sierra Nevada through surveys openface> 1700 sites in two adjacent and historically fishless protected areas that differed primarily in the distribution of introduced fish. Negative effects of fishes on the distribution of frogs were evident at three spatial scales. At the landscape scale, comparisons between the two protected areas indicated that fish distribution was strongly negatively correlated with the distribution of frogs. At the watershed scale, the percentage of total water‐body surface area occupied by fishes was a highly significant predictor of the percentage of total water‐body surface area occupied by frogs. At the scale of individual water bodies, frogs were three times more likely to be found and six times more abundant in fishless than in fish‐containing waterbodies, after habitat effects were accounted for. The strong effect of introduced fishes on mountain yellow‐legged frogs appears to result from the unique life history of this amphibian which frequently restricts larvae to deeper water bodies, the same habitats into which fishes have most frequently been introduced. Because fish populations in at least some Sierra Nevada lakes can be removed with minimal effort, our results suggest that the decline of the mountain yellow‐legged frog might be relatively easy to reverse.

This paper reports on the response by amphibians, benthic macroinvertebrates, and zooplankton in naturally fishless alpine lakes to fish introductions and subsequent fish disappearance. We assessed resistance (the degree to which a system is altered when the environment changes) by comparing faunal distribution and abundance in lakes that have never been stocked with fish vs. the distribution and abundance in lakes that have been stocked and still contain fish. We assessed resilience (the degree and rate of a system's return to its previous configuration once the perturbation is removed) by comparing faunal distribution and abundance in lakes that were stocked at one time but have since reverted to a fishless condition (stocked‐now‐fishless lakes) vs. the distribution and abundance in lakes that have never been stocked. We quantified recovery rates and trajectories by comparing faunal assemblages of stocked‐now‐fishless lakes that had been fishless for 5–10, 11–20, and >20 yr. Faunal assemblages in the study lakes had low resistance to fish introductions, but in general showed high resilience. The mountain yellow‐legged frog (Rana muscosa), conspicuous benthic macroinvertebrates, and large crustacean zooplankton (>1 mm) were dramatically reduced in distribution and abundance by fish introductions but generally recovered to predisturbance levels after fish disappearance. Inconspicuous benthic invertebrate taxa, small crustacean zooplankton (<1 mm), and rotiferan zooplankton (<0.2 mm) were either unaffected by fish or increased in the presence of fish. For both the benthic macroinvertebrate community and the zooplankton community as a whole, fish disappearance was followed by a steady change away from the configuration characteristic of fish‐containing lakes and toward that of lakes that had never been stocked. Both communities remained markedly different from those in never‐stocked lakes 5–10 yr after fish disappearance and converged on the configuration of never‐stocked lakes only 11–20 yr after fish disappearance. Recovery was likely facilitated by the winged adult stages of many benthic macroinvertebrates, resting eggs of zooplankton, and nearby source populations of frogs. However, many frog populations have disappeared since the time that lakes in this study reverted to a fishless condition, and the viability of zooplankton egg banks should decline in fish‐containing lakes over time. As a result, faunal resilience may be lower in lakes that revert to a fishless condition today than is suggested by the results of our study. These findings have important implications for the restoration of alpine lake ecosystems.

Human-caused fragmentation of habitats is threatening an increasing number of animal and plant species, making an understanding of the factors influencing patch occupancy ever more important. The overall goal of the current study was to develop probabilistic models of patch occupancy for the mountain yellow-legged frog (Rana muscosa). This once-common species has declined dramatically, at least in part as a result of habitat fragmentation resulting from the introduction of predatory fish. We first describe a model of frog patch occupancy developed using semiparametric logistic regression that is based on habitat characteristics, fish presence/absence, and a spatial location term (the latter to account for spatial autocorrelation in the data). This model had several limitations including being constrained in its use to only the study area. We therefore developed a more general model that incorporated spatial autocorrelation through the use of an autocovariate term that describes the degree of isolation from neighboring frog populations (autologistic model). After accounting for spatial autocorrelation in patch occupancy, both models indicated that the probability of frog presence was strongly influenced by lake depth, elevation, fish presence/absence, substrate characteristics, and the degree of lake isolation. Based on cross-validation procedures, both models provided good fits to the data, but the autologistic model was more useful in predicting patch occupancy by frogs. We conclude by describing a possible application of this model in assessing the likelihood of persistence by frog populations.

Habitat use by rainbow trout Oncorhynchus mykiss is described for a southern California stream where the summer water temperatures typically exceed the lethal limits for trout (>25°C). During August 1994, water temperature, dissolved oxygen (DO), and trout distribution were monitored in two adjacent pools in Sespe Creek, Ventura County, where summer water temperature reached 28.9° C. Water temperature was an important factor in trout distribution in the two pools. During 1–11 August 1994, water temperatures in pool 1 ranged from 21.5°C at the bottom (4.1 m) to 28.9° C at the surface. After 5 August, trout were no longer found in this pool, suggesting that trout had moved out of the high temperature water or died. In the adjacent, shallower (1.5m) pool 2, surface water temperatures were as high as 27.9° C, but temperatures on the bottom remained cooler (17.5–21° C) than pool 1, presumably due to groundwater seeps. Consistent aggregations of trout were observed in pool 2 throughout the study period. During the day when water temperature was highest, most trout were found in a region of the pool with the lowest water temperature (mean=18.3° C). Conversely, regions with the highest water temperatures had the fewest trout during the day. The seeps may have introduced water with low dissolved oxygen into pool 2, as the DO in many locations on the bottom ranged from <1 mg 1−1 to 5 mg 1−1 over 24 h, while the surface DO ranged from 4.1 to 10.0mg 1−1. Lowest DO occurred from 2400 to 0600 hours. During August, water temperature and DO were positively related. Thus, rainbow trout faced a trade‐off between the relatively cool water temperature with low, possibly lethal levels of DO (e.g. 1.7 to 3.4 mg 1−1 in region 3), and lethally high water temperature but high DO. Seeps may serve as important thermal refugia for trout, and an increased understanding of their role as potential critical refugia in Southern California is necessary.