The spatial ecology of the exotic bullfrog (Rana catesbeiana) and its relationship to the distribution of the native herpetofauna in a managed river system

Thesis (M.S.)--Humboldt State University, Natural Resources: Wildlife Management, 2008Prior to western settlement, the Trinity River probably offered little suitable habitat for bullfrogs (Rana catesbieana). Anthropogenic habitat modifications such as mining and homesteading may have resulted in conditions more favorable for bullfrogs. Damming in 1963 led to modifications in riverine habitat, producing a highly modified lotic system which is much different from pre-dam conditions. Research has shown these habitat modifications to negatively impact foothill yellow-legged frogs (Rana boylii). My objectives were to determine the downstream distribution of an exotic ranid frog, the bullfrog, and its relationship to the foothill yellow-legged frog, other native amphibians, and the habitats they occupy along the dam-impacted Trinity River. I used census data from 2004 to determine bullfrog occupancy and data on breeding from 2005 to determine bullfrog breeding habitat on the Trinity River. With the occupancy data, I also examined native species composition in relation to bullfrog habitats and plotted bullfrog and native species distribution along the river. Bullfrog occupancy models were developed for both spring/summer high-flow and late summer low-flow surveys, with the low-flow model producing more accurate information than the high-flow model. Top predictor variables for all occupancy models were: max depth, percentage rooted floating vegetation, and river mile, although individual variables showed some variation in significance between the two flow regimes. The breeding model was more stable and out-performed the bullfrog occupancy models. The breeding model had high predictability and was positively correlated with the variables: percentage lentic, percentage rooted floating vegetation, max depth and river mile. Canopy cover was negatively correlated and water clarity had no relationship to bullfrog breeding sites. Most sites where bullfrogs were found also provided habitat for western pond turtles and western toads. Additionally, rough-skinned newts were commonly found at bullfrog breeding sites. Using the occupancy data set, native species and bullfrog distributions were inversely correlated along the 43 mile river reach below the dam. My results suggested that if one wished to control bullfrog populations on the Trinity River the most successful approach would be to focus management efforts on bullfrog breeding sites. Over half of the bullfrog breeding sites were artifacts of historic mining activities, whereas other breeding sites are a result of dam-induced habitat modifications. Making restoration of those sites a high priority could greatly benefit native species.Trinity River Restoration Program \ud Redwood Sciences Laborator