Climate change and invasive species pose significant threats to freshwater ecosystems. We explored the effects of these two stressors on larvae of a widespread native amphibian, the Pacific chorus frog (Pseudacris regilla; PCF) in two laboratory experiments. In Experiment 1, we tested the effects of climate and invasive American bullfrog (Lithobates catesbeiana) larvae on survival, growth, and development of PCF larvae using a 2x2x2 factorial design. Treatments in this experiment were climate (current/future), invasive species (present/absent) and cage (open/closed). The cage treatment allowed us to test for differences between direct and indirect interactions between the bullfrogs and PCF tadpoles. In Experiment 2, we tested the effects of climate (current/future), invasive brook stickleback fish (Culaea inconstans) chemical cues (present/absent), and conspecific density (1 or 2 tadpoles) on PCF larval survival, growth, and development rate using a 2x2x2 design. All data were analyzed using generalized linear models.
Results/Conclusions
In both experiments, the development rate was faster under future climate conditions (Exp 1, p<0.0001; Exp 2, p <0.0001). In Experiment 1, a significant three-way interaction between climate, invasion, and the cage treatment was detected for weight at metamorphosis (Gosner stage 42; p=0.039), such that the effect of the invasive species was less under future climate conditions. In Experiment 2, a significant three way interaction between climate, invasion, and conspecific presence was detected (p= 0.005), where development rate was accelerated when all three stressors were present. Furthermore, fish chemical cues reduced the weight of PCFs at metamorphosis (Gosner stage 42; p=0.005), as did the presence of a conspecific (p<0.001). Weight of PCF larvae was reduced in the future climate treatment without a conspecific present, but no difference in weight between climate treatments was observed in the presence of a conspecific (two-way interaction, p=0.0166). Our findings suggest that a single suite of stressors can have both additive and non-additive effects in the same organisms, depending on the measured outcome. This work highlights the complexity of stressor interactions in natural systems.