No abstract

Widespread alteration of natural hydrologic patterns by large dams combined with peak demands for power and water delivery during summer months have resulted in frequent aseasonal flow pulses in rivers of western North America. Native species in these ecosystems have evolved with predictable annual flood-drought cycles; thus, their likelihood of persistence may decrease in response to disruption of the seasonal synchrony between stable low-flow conditions and reproduction. We evaluated whether altered flow regimes affected 2 native frogs in California and Oregon (U.S.A.) at 4 spatial and temporal extents. We examined changes in species distribution over approximately 50 years, current population density in 11 regulated and 16 unregulated rivers, temporal trends in abundance among populations occupying rivers with different hydrologic histories, and within-year patterns of survival relative to seasonal hydrology. The foothill yellow-legged frog (Rana boylii), which breeds only in flowing water, is more likely to be absent downstream of large dams than in free-flowing rivers, and breeding populations are on average 5 times smaller in regulated rivers than in unregulated rivers. Time series data (range = 8 - 19 years) from 5 populations of yellow-legged frogs and 2 populations of California red-legged frogs (R. draytonii) across a gradient of natural to highly artificial timing and magnitude of flooding indicate that variability of flows in spring and summer is strongly correlated with high mortality of early life stages and subsequent decreases in densities of adult females. Flow management that better mimics natural flow timing is likely to promote persistence of these species and others with similar phenology.

Organisms that live in highly variable environments, such as rivers, rely on adaptations to withstand and recover from disturbance. These adaptations include behavioral traits, such as habitat preference and plasticity of reproductive timing, that minimize the effects of discharge fluctuation. Studies linking hydrologic regime, habitat preference, and population processes, however, are predominantly limited to fish. Information on other sensitive taxa is necessary to facilitate conservation of multispecies assemblages and restoration of biodiversity in degraded river channels. I studied the functional relationship between physical habitat and reproduction of the foothills yellow‐legged frog (Rana boylii), a California State Species of Special Concern. From 1992 to 1994, I mapped breeding sites along 5.3 km of the South Fork Eel River in northern California and monitored egg survival to hatching. Frogs selected sites over a range of spatial scales and timed their egg‐laying to avoid fluctuations in river stage and current velocity associated with changes in discharge. The main sources of mortality were desiccation and subsequent predation of eggs in a dry year and scour from substrate in wet years, both caused by changes in stage and velocity. At the finest spatial scale, frogs attached eggs to cobbles and boulders at lower than ambient flow velocities. At larger scales, breeding sites were near confluences of tributary drainages and were located in wide, shallow reaches. Clutches laid in relatively narrower and deeper channels had poor survival in rainy as well as dry springs. Most breeding sites were used repeatedly, despite between‐ and within‐year variation in spring stage of the river. This pattern of site selection suggests that conservation of Rana boylii may be enhanced by maintaining or restoring channels with shapes that provide stable habitat over a range of river stages.

I studied the invasion of Rana catesbeiana (the bullfrog) into a northern California river system where bullfrogs are not native. Native yellow-legged frogs, Rana boylii, a species of special concern, were almost an order of magnitude less abundant in reaches where bullfrogs were well established. I assessed the potential role of larval competition in contributing to this displacement in a series of field manipulations of tadpole density and species composition. The impact of R. catesbeiana on native tadpoles in the natural community agreed with the outcome of more artificial experiments testing pairwise and three-way interactions. In 2-m 2 enclosures with ambient densities of tadpoles and natural river biota, bullfrog tadpoles caused a 48% reduction in survivorship of R. boylii, and a 24% decline in mass at metamorphosis. Bullfrog larvae had smaller impacts on Pacific treefrogs, Hyla regilla, causing 16% reduction in metamorph size, and no significant effect on survivorship. Bullfrog tadpoles significantly affected benthic algae, although effects varied across sites. Responses to bullfrogs in field settings were similar qualitatively to results seen in smaller-scale experiments designed to study size-structured competition among disparate age/size classes of species pairs and trios. Competition from large overwintering bullfrog larvae significantly decreased survivorship and growth of native tadpoles. Competition from recently hatched bullfrog larvae also decreased survivorship of R. boylii and H. regilla. Native species competed weakly, both interspecifically and intraspecifically. The only suggestion of a negative impact of a native species on bullfrogs was a weak effect of H. regilla on recent hatchlings. Competition appeared to be mediated by algal resources, and there was no evidence for behavioral or chemical interference. These results indicate that, through larval interactions, bullfrogs can exert differential effects on native frogs and perturb aquatic community structure.