Effects of Flow Regimes Altered by Dams on Survival, Population Declines, and Range‐Wide Losses of California River‐Breeding Frogs

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.

Assessing the flow alteration of temporary streams under current conditions and changing climate by Soil and Water Assessment Tool model

“…In North America, diversion structures caused elimination of plant cover and diminished plant species diversity (Greet et al 2012) and decrease in density of all macroinvertabrates by flow reduction (Baldigo and Smith 2012). Finally, reservoir operation creates flow stabilization resulting in biodiversity decrease (Kupferberg et al 2012), diminishing of riparian forest patch size (Maingi and Marsh 2002), and reduction of fish species richness (De Mérona and Albert 1999).…”

Drought, Deluge and Declines: The Impact of Precipitation Extremes on Amphibians in a Changing Climate

“…Consequently, adult recruitment in this population declined significantly over a 12-year period, with no trend in larval abundance. Flooding, with its associated high water flow and transport of debris (sediment, boulders, large sections of wood and other vegetation) has been linked to declines and extirpations of a variety of other stream and river-dwelling amphibians as well [84,85,86] (but see [87]). For a pond-breeding amphibian, hurricane-related flooding from heavy rainfall prevented breeding in a population of the terrestrial Marbled Salamander ( Ambystoma opacum ) in North Carolina [88].…”

Environmental factors associated with amphibian breeding in streams and springs: effects of habitat and fish occurrence

“…Interactions between stream-dwelling amphibians have received less attention than those among ponddwelling species. Communities of amphibians may be quite complex in river catchment basins (Welsh and Hodgson, 2011) and both pollution and human habitat fragmentation may affect them (Kupferberg et al, 2012). One of the main factors discriminating lotic and lentic habitats is the availability of food: stream amphibian larvae often have little zooplankton or phytoplankton for feeding (Gillespie et al, 2004).…”