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