The impact of thermal fluctuations on reaction norms in specialist and generalist parasitic wasps
(2013) Functional Ecology — (2013)
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- Reaction norms depict the environmental effects on phenotypic traits and are used to predict the global change consequences on species distributions. However, studies performed at constant temperatures have limited ecological significance because expressed phenotypes depend on the range and frequency of environmental states. Using Jensen's inequality (i.e. a mathematical property of nonlinear functions), we predicted that the effect of thermal fluctuations on the phenotype depends on the shape of the reaction norm. Thermal fluctuations around the optimal temperature are expected to reduce the phenotypic trait values, especially for specialists because of their narrower reaction norms. This study measured the effects of diel fluctuations in developmental temperature on phenotypic expression of traits related to fitness and energetic resources in two strains of the parasitoid wasp Venturia canescens from different habitats: a thermal generalist strain and a specialist one. In a first experiment, we compared the effect of constant thermal regimes versus fluctuating ones having the same means (20, 25 and 30 °C) on reaction norms of life-history traits and of energetic reserves. In a second experiment, we examined the effects of a natural thermoperiod in the field on these traits. Our results show that the shape of the reaction norm defines the phenotypic changes induced by the development under fluctuating thermal conditions. These results match the predictions of the Jensen's inequality. Moreover, our results emphasize the significance of taking into account several phenotypic life-history traits to study the adaptive value of phenotypic plasticity. We also show that the level of energetic resources depends on the mean developmental temperature and not on the thermal regime. Finally, the field experiment confirms that the phenotype of these parasitoids depends on the temperature variation. Our study highlights the relevance of the Jensen's inequality to predict the effect of thermal fluctuations on fitness of parasitoids with contrasted thermal sensitivities.
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Foray, V., Desouhant, E., & Gibert, P. (2013). The impact of thermal fluctuations on reaction norms in specialist and generalist parasitic wasps. Functional Ecology. Published. https://doi.org/10.1111/1365-2435.12171 (Original work published 2013)