Indirect effects of UV-B radiation on larval amphibian competition across an elevation gradient

Direct effects of UV-B radiation on amphibian larvae can lead to indirect impacts on other members of the community, including competing amphibian species. If sympatric amphibian species differ in susceptibility to damage from UV-B radiation, stress-induced changes in larval growth and development rates could indirectly favor UV-B resistant species. We tested the effects of UV-B radiation on competing larval amphibian species at both high and low elevations. Hyla regilla are sympatric with two species of Rana: Rana aurora at low elevations and Rana cascadae at high elevations. Hyla regilla larvae are relatively resistant to UV-B damage but are inferior competitors relative to R. aurora and R. cascadae in lab experiments. R. aurora larvae are susceptible to UV-B damage, while R. cascadae larvae are not. Therefore, in UV-B exposed environments, the competitive outcome is predicted to shift away from R. aurora to favor Hyla, while R. cascadae are predicted to out-compete Hyla regardless of UV-B. Using mesocosms and a 2x3 factorial design, we quantified growth and development in amphibian larvae in experiments conducted at high and low elevation sites. Treatments included two UV-B exposure regimes (present, absent) and three species combinations (Hyla alone, Rana alone, both species). In the low elevation experiment, Hyla out-competed R. aurora larvae, regardless of UV-B exposure.  In the high elevation treatment however, the competitive outcome was reversed with R. cascadae out-competing Hyla regardless of UV-B exposure. This divergence in competitive ability between Rana species may be a result of historical UV-B exposure rates.