![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Oregon’s invasive bullfrogs and bluegills linked in their impact on native species
Invasive species in Oregon’s Willamette Valley were the subjects of three WED projects resulting in presentations to a joint meeting of Oregon chapters of the American Fisheries Society and the Wildlife Society in February. Two were mesocosm analyses with broad landscape surveys examining the hypothesis that the bluegill (Lepomis macrochirus), an introduced game fish, is aiding the survival of the bullfrog (Rana catesbeiana), a non-native known to have a negative impact on native species of fish and amphibians. The third was a field survey on bullfrog dispersal and habitat use.
Intermediary in the bluegill-bullfrog relationship is the dragonfly. Experiments showed that dragonfly larvae are voracious predators of bullfrog larvae, reducing bullfrog survival to zero in field enclosures with either native fish or no fish. But when bluegills were added to the enclosures, bullfrog survival increased to 20 percent. Field surveys in Oregon and Washington found bullfrogs more abundant and macroinvertebrate populations lower when non-native fish were present. The authors of one of the studies conclude that non-native fish are facilitating the survival of bullfrogs and suggest that introduced fish should be a greater concern to land managers than bullfrogs.
The second mesocosm study examined the susceptibility of the larvae of three salamanders and three frog species to being devoured by certain native and introduced species of fish in Oregon’s Willamette Valley. They found that the larvae of native rough-skinned newts, which develop toxic skins early in life, were eaten by fish less often than larvae of two native salamanders lacking such defenses. Bullfrog larvae were eaten less often than two species of native frogs. Native frogs, the scientists reported, are palatable to all fish tested. Of all fish species tested, the bluegill showed the strongest preference for tadpoles of native frogs rather than bullfrogs. They found the bluegill among the most harmful of introduced fishes to native amphibians in the Willamette Valley and they recommend mitigation, such as the occasional draining of ponds or the creation of semi-permanent waters for the conservation of wetland-breeding amphibians. (Contact: A.V. Nebeker, 541-754-4884; nebeker.alan@epa.gov)
The third study examined whether alteration of landscape pattern can minimize the spread of bullfrogs. Employing field surveys and radio telemetry, the researchers found that juvenile and adult bullfrogs may travel as much as two kilometers or more over land and utilize various upland habitat. Using that information in a population model designed to evaluate the effect of alternative landscapes on bullfrog distributions, they found that any existing or future wetlands in the Willamette Valley are likely to be invaded by bullfrogs. They learned, however, that vegetation, hydrology, connectivity by canals or ditches, and distance from source populations do affect bullfrog distributions. That knowledge may be useful to wetlands restoration and mitigation projects. (Contact: P.J. Wigington, 541-754-4341; wigington.jim@epa.gov)