Phenotypic variation of toxin levels in a chemically defended amphibian: The ecological and evolutionary significance of tetrodotoxin
The global decline of amphibian populations is well documented, yet researchers have only begun to understand how these declines may affect biological, environmental, ecological, and evolutionary processes. In the first chapter, recently published primary literature is reviewed to describe the complexities of amphibian population declines, with a focus on the interactions of nonnative species, anthropogenic disturbances, and climate change. To begin to understand the potential consequences of the decline of endemic California herpetofauna, the ecological role of a Species of Special Concern, the California newt (Taricha torosa) was evaluated in streams of the Santa Monica Mountains. The California newt possesses a potent neurotoxin, tetrodotoxin (TTX), which is known to elicit antipredator behavior in conspecific larvae. Given these results, potential variation of individual and breeding population TTX levels may affect ecological and evolutionary processes. In chapter 2, TTX levels of adult T. torosa from three breeding localities in the Santa Monica Mountains were quantified to test for variation in amounts of TTX between sites using a novel non-destructive sampling method, a newly developed technique to extract TTX, and an adapted high performance liquid chromatography system with a detection limit of 48 picograms TTX (0.15 picomoles). The amounts of TTX from newts in these three streams suggest fine geographic variation in toxin levels. In chapter 3, the potential ecological effects of newts and TTX were tested using field and laboratory bioassays to determine if newt chemical cues and TTX affect the foraging behavior of macroinvertebrates. Field bioassays indicate that macroinvertebrates move away from point sources of newt chemical cues, while laboratory bioassays show that predatory odonate nymphs exposed to TTX reduce predation rates, mean velocities when striking prey, and mean angular velocities when compared to controls. In the final chapter, male site fidelity and individual TTX levels were tracked in situ over four breeding seasons. Data reveal fluctuations in individual and breeding population TTX levels, suggesting that TTX is not a stable trait. Furthermore, statistical analyses indicate that fidelity to a pool is significantly linked to relative TTX concentrations. Collectively, these results demonstrate a striking ecological role for T. torosa and TTX, and imply that the loss of this amphibian in our study system may have wide-reaching ecological consequences.