Context dependency of phenotypic divergence and eco-evolutionary feedback: insight from a mesocosm experiment on moor frog tadpoles.

Rapid environmental change is driving global biodiversity declines, challenging species to persist through genetic adaptation and phenotypic plasticity. These responses can also feed back onto ecosystems ecology, a process called eco-evolutionary feedbacks, potentially reshaping both selective environments and ecosystem properties. However, how phenotypic divergence and potential eco-evolutionary feedbacks depend on the environmental context is rarely assessed in ecologically realistic settings. Here, we used an outdoor mesocosm experiment to investigate context-dependent phenotypic divergence and ecological feedbacks in amphibian tadpoles, which are key players in their native ecosystems, and show strong potential for local adaptation and phenotypic plasticity. Specifically, we assessed the extent of i) phenotypic divergence and ii) differential effects on ecosystem properties between two divergent populations of the moor frog (Rana arvalis) in ecologically contrasting conditions. To this end, we conducted a full factorial experiment rearing tadpoles from two contrasting pH populations (acidic versus neutral origin) in two contrasting pH environments (pH 4.3 versus 8.4). To assess the effects of tadpole presence per se, and the relative effects of within species phenotypic divergence on key ecosystem properties, we complemented the design with no-tadpole control mesocosms. In terms of parallel responses to the contrasting environments, both population origins showed substantial phenotypic plasticity. Tadpoles had higher corticosterone levels, developed faster and to a larger metamorphic size in the pH 4.3 than the pH 8.4 treatment. Diet also differed between pH treatments. Regarding phenotypic divergence, acid-origin tadpoles had higher survival in the pH 4.3 treatment and reached a larger metamorphic size than neutral-origin tadpoles (in both treatments). We also found genotype-by-environment interactions in dietary morphology: acid-origin tadpoles had relatively longer guts than neutral-origin tadpoles in the pH 8.4 treatment, suggesting potential for divergence in diet-mediated ecological effects. Finally, several key findings emerged from the ecological effects of tadpoles. Tadpole presence per se (relative to no-tadpole controls) influenced several ecosystem parameters (i.e. light penetration, phyto- and zooplankton abundance). While no population-origin effects were observed in the pH 4.3 treatment, the two populations had different effects on periphyton and phytoplankton abundance and vegetation biomass in the pH 8.4 treatment. These findings highlight the potential for within-species divergence in amphibians to alter ecosystem properties and call for further investigation into the context dependency of eco-evolutionary dynamics in face of the ongoing environmental changes.