Natural hydrologic regimes are fundamental properties of fluvial systems, shaping geomorphology, physical and chemical gradients, species compositions, and the ecology of riverine communities. Hydrologic alterations due to channelization, regulation by dams, and watershed development are among the most ubiquitous anthropogenic stressors to fluvial ecosystems, yet the unintended consequences for ecosystem processes and species dynamics remain difficult to assess for many systems and species. The decline of many river-breeding amphibians, such as the foothill yellow-legged frog, Rana boylii, have paralleled changes in hydrology and may serve as important sentinel species for river conservation and management. For R. boylii in particular, a combination of short-term field and laboratory experiments, long-term breeding surveys, and comparisons among systems with different forms of altered hydrology, suggest that artificial flood pulses outside the range of natural variability generated by hydroelectric dams negatively affect early life history stages (embryos, tadpoles). To evaluate how such effects influence overall population viability, we developed a stochastic simulation matrix population model for R. boylii. We combined newly collected demographic data from multiple populations and values from the literature to estimate the distributions of fecundity and life-stage specific survival rates for a reference population model, and 20 scenarios that span the range of hypothesized effects of artificial flood pulses.
Results/Conclusions
We find that the 30-year probability of quasi-extinction increases dramatically from the reference model in scenarios with smaller starting population sizes and those with reduced survival of egg masses and tadpoles due to stranding or scouring by artificial pulses. Multivariate sensitivity analysis suggests that adult and tadpole survival rates, and egg mass scour rates are key factors determining variation in overall population growth rates, supporting the hypothesis that artificial flood pulses which reduce the survival of early life-history stages could lead to population-scale declines. These findings suggest that the current re-evaluation of many hydroelectric dam licenses in the U.S. and elsewhere provide opportunities for improved river management.
