Thermal tolerance of amphibians and their invasive predator in a polluted environment

Abstract

It is clear that many threats such as habitat loss, chemical pollution and global warming are contributing to both local and global amphibian declines. Yet aside from a few examples, we still know very little about the role and magnitude of synergistic effects among these stressors. Demonstrated in marine ectotherms, thermal tolerance is limited by the incapability of an organism’s circulatory and ventilation systems to supply sufficient oxygen to match with its oxygen demand (i.e. aerobic scope). Beyond certain critical temperatures, survival of aquatic ectotherms becomes passive and time-limited, relying heavily on anaerobic metabolism and molecular protection (e.g., induction of heat shock proteins). Detoxification is known to be an energy-demanding process. It is thus likely that an organism under thermal stress will suffer further decrease in aerobic scope and loss of performance in a contaminated environment, which may in turn threaten their survival. Our study thus focused on the relationships between pollution and thermal stress on three lowland wetland amphibian larvae in South China, namely Asian common toad (Bufonidae: Duttaphrynus melanostictus), brown tree frog (Rhacophoridae: Polypedates megacephalus) and marbled pigmy frog (Microhylidae: Microhyla pulchra), and the relative performance of the invasive fish predator Gambusia affinis as opposed to local larval amphibians. This study determined and compared lethal concentrations of two commonly used agricultural pesticides, glyphosate and methomyl, on the target amphibian and fish species, as well as lethal temperatures of these animals. The investigation on the synergistic effects of thermal stress and pesticides is currently being conducted using integrated measurements of physiological end-points and biomarkers including growth rate, oxygen consumption rate, and expression of heat shock protein and lactate dehydrogenase. Our preliminary results showed that pesticide tolerance varied highly between the three amphibian species, with D. melanostictus being the most tolerant, followed by P. megacephalus and M. pulchra, and that the invasive predator G. affinis had a wider thermal tolerance window (37.16oC) than P. megacephalus (33.97oC) and M. pulchra (34.36oC). Further information generated from this study on the synergy between pollution and thermal stress will aid in developing conservation measures to better protect amphibian populations against these rising threats in South China region.