Invasive bullfrog use of novel ephemeral habitats in the Pacific Northwest

Graduation date: 2012Invasive species are a major threat to global biodiversity. Understanding the mechanisms underlying successful invasions is imperative for developing effective management strategies. Plasticity in physiological, morphological, and behavioral traits could be an indicator of invasion potential in non-native species. The invasive American bullfrog (Lithobates catesbeianus) has recently been observed breeding in ephemeral wetlands in the Willamette Valley, OR, which represents a shift from typical breeding sites in its native range. We hypothesized that plasticity in larval growth and development rates in response to ephemeral hydroperiod conditions has facilitated invasion into this novel habitat. In addition, we hypothesized that invasive bullfrogs would respond with plastic changes in behavior to a novel predator in ephemeral wetlands, the larval long-toed salamander (Ambystoma macrodactylum). First, we tested physiological plasticity in invasive bullfrog larvae with a 6 x 3 factorial mesocosm experiment. We quantified growth and development rates of 6 bullfrog clutches in response to 3 hydroperiod treatments. We found no differences due to hydroperiod treatments, but we found significant differences in growth between clutches. Clutch differences in growth could lead to population-level differences in the ability to invade ephemeral wetlands. We also tested behavioral plasticity in larval bullfrog response to A. macrodactylum chemical cues. We hypothesized that bullfrogs would reduce activity levels in response to predator chemical cues but we found no differences in activity or spatial orientation due to predator treatment. Instead, we found that bullfrog activity and spatial orientation were positively associated with bullfrog size. This indicates bullfrog larvae do not recognize predatory chemical cues from A. macrodactylum. Together, these results indicate that bullfrog populations will vary significantly in their invasion success into ephemeral wetlands. Further study of local variation in growth and development rates and site fidelity to ephemeral wetlands is needed to determine threats to ephemeral wetlands by bullfrog invasion