HYDROLOGIC AND GEOMORPHIC FACTORS AFFECTING CONSERVATION OF A RIVER-BREEDING FROG (RANA-BOYLII)

Organisms that live in highly variable environments, such as rivers, r ely on adaptations to withstand and recover from disturbance. These ad aptations include behavioral traits, such as habitat preference and pl asticity of reproductive timing, that minimize the effects of discharg e 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 conserv ation of multispecies assemblages and restoration of biodiversity in d egraded 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 199 2 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. F rogs selected sites over a range of spatial scales and timed their egg -laying to avoid fluctuations in river stage and current velocity asso ciated with changes in discharge. The main sources of mortality were d esiccation and subsequent predation of eggs in a dry year and scour fr om substrate in wet years, both caused by changes in stage and velocit y. At the finest spatial scale, frogs attached eggs to cobbles and bou lders at lower than ambient flow velocities. At larger scales, breedin g sites were near confluences of tributary drainages and were located in wide, shallow reaches. Clutches laid in relatively narrower and dee per channels had poor survival in rainy as well as dry springs. Most b reeding sites were used repeatedly, despite between- and within-year v ariation in spring stage of the river. This pattern of site selection suggests that conservation of Rana boylii may be enhanced by maintaini ng or restoring channels with shapes that provide stable habitat over a range of river stages.