Competition shapes amphibian response to rapid environmental change
Pond-breeding amphibians experience climatic variability primarily in the form of seasonal temperature fluctuations and water availability, two factors that are expected to significantly depart from the norm as climate change progresses. To accurately predict the trajectory of species in response to climate change, we must acknowledge not only the abiotic drivers, but the biological dynamics which directly (or indirectly) influence their response strategies. We experimentally quantified the effects of rapid pond drying on intra- and inter-specific competition, and phenotypic plasticity in larval growth and development, among three co-occurring species from the Cascade Mountain Range: Cascades frog (Rana cascadae), Pacific chorus frog (Pseudacris regilla), and Western toad (Anaxyrus boreas). We utilized a 7 x 2 fully-factorial, blocked mesocosm design with 5 replicates for each of the 7 competition scenarios (intra- and inter-specific interactions) and 2 hydroperiod regimes (permanent and ephemeral). Each replicate contained 30 individuals total, such that competitive interactions were frequency-dependent. Measures of performance for each species included larval biomass and average individual weight (g), larval period (time to emergence), mean daily growth rate, metamorph biomass and average metamorph weight (g), and survival post-emergence. To examine treatment effects on species’ performance, we performed repeated measures analyses using linear mixed effects models and MANOVA.
Across both hydroperiod treatments, the three competitors exhibited competitor-specific staggering of developmental rates, which resulted in temporal partitioning of emergence. In the absence of hydroperiod stress (i.e. permanent hydroperiod), competitive interactions were the primary driver of species larval growth and survival, such that each species exhibited a competitor-specific response in larval growth and metamorph biomass. When a reduction in hydroperiod was imposed, the competitive hierarchies observed in the pairwise competition scenarios of the permanent hydroperiod treatment were not maintained; these relationships appear to shift, and in some cases flip, under the combination of biotic and abiotic stress. The affects of rapid pond drying on each species manifested primarily at the time of emergence. For example, interspecific competition limited the capacity for rapid development in P. regilla in the ephemeral treatment, which resulted in a reduction in the average number of emerging individuals (i.e. biomass of metamorphs) by the end of dry-down even though individuals were able to maintain similar larval body sizes over development. We provide a valuable example of the role of biotic interactions in shaping amphibian species response to rapid environmental change.