Adaptive plasticity in stressful environments: acidity constrains inducible defences in Rana arvalis

Questions: How do environmental stressors affect the expression of adaptive phenotypic plasticity? Is there inter-population variation in these effects? Hypothesis: Acid stress constrains the expression of inducible defences by decreasing investment in defences or by increasing the costs of investment. Organisms originating from neutral environments suffer more from acid stress than organisms originating from acid environments. Organism: Tadpoles of Rana arvalis, originating from two different populations (acid and neutral). This species displays inducible defences in response to insect predators (here dragonfly larvae). Methods: A laboratory experiment with a factorial design crossing two factors: predator presence (present vs. absent) and acidity (neutral vs. acid). We tested the effects of experimental treatment on tadpole morphology as well as age and size at metamorphosis. Results: Tadpoles from the neutral origin population invested less in inducible defences (tail fin depth) in the acid than in the neutral treatment. In contrast, tadpoles from the acid origin population were able to respond equally well to predators in both pH treatments. pH-related costs differed between populations: while tadpoles from the neutral origin population suffered from acid stress in terms of reduced developmental rate, those from the acid origin population seemed to suffer from neutral stress in terms of reduced size at metamorphosis.