HOW LOW CAN YOU GO: SEVERE WINTER CONDITIONS FAVOR OREGON SPOTTED FROGS OVER BULLFROGS

The introduced American Bullfrog (Lithobates catesbeianus) has been implicated in the decline of the federally listed Oregon Spotted Frog (Rana pretiosa). American Bullfrogs have become widely distributed across wetlands of Oregon and Washington whereas Oregon Spotted Frogs have largely disappeared from most of the lowland portions of their historical range. In a few areas, mostly at higher elevations on the east flank of the Cascades, Oregon Spotted Frogs and bullfrogs have coexisted for as much as a century but often remain spatially segregated. In the Deschutes River basin of central Oregon, landscape-scale patterns of frog occurrence suggest that habitat conditions during winter may strongly influence the two species' respective distributions. To examine this hypothesis, we conducted a simple experiment comparing the respective tolerance of both species to cold hypoxic conditions. The results of this experiment revealed that Oregon Spotted Frogs can tolerate cold hypoxic conditions better than bullfrogs. These results may help explain the local coexistence but spatial separation of the two species across the central Oregon landscape.