Warm vegetarians? : heat wave effects on aquatic omnivorous ectotherms

Abstract

Freshwater communities are particularly vulnerable to climate change, as they are mainly composed of ectotherm species, whose metabolism is strongly influenced by temperature. This relationship is particularly relevant, as projections show both a global warming and an increase in the frequency, intensity and duration of extreme climatic events such as heat waves. Measurements of imbalanced temperature effects on various metabolic aspects of ectotherms lead to the suggestion that these organisms may optimize energetic intake at higher temperatures by increasing herbivory and avoiding protein-rich animal diets. In this thesis, I tested the effects of different types of heat waves on diet assimilation (and indirectly on diet choice) of five species of freshwater omnivorous ectotherms, belonging to three different phyla. As study models, I used species commonly found in Mediterranean temporary ponds: the tadpoles of three anuran species breeding along a seasonal climatic gradient (Discoglossus galganoi, Hyla arborea and Hyla meridionalis), the gastropod Radix balthica and the invasive crayfish Procambarus clarkii. Individuals were fed animal, plant or mixed diets and exposed to temperature treatments simulating the current short heat waves, as well as long heat waves expected to become more frequent in the future. Treatment responses were recorded in terms of survival and life history traits. In the individuals feeding on the mixed diet, the proportion of animal and plant material assimilated was estimated with stable isotopes. Overall, I found evidence that performance at higher temperatures decreased on animal diets and increased on plant diets, suggesting that diet quality changed with temperature. Although the type of heat wave inducing assimilation shifts varied and the species responded to different extents, heat waves generally increased the assimilation of plant material, and this was adaptive in almost all species. Furthermore, the seasonal climatic gradient in the three anuran species was found to match a gradient in their feeding preferences, the degree of herbivory increasing with minimum temperature experienced in nature. Surprisingly, albeit native to climates warmer than the Iberian Peninsula, the invasive crayfish P. clarkii was negatively affected by the heat waves. However, the results of nutrient assimilation showed that warmer temperatures may increase this species’ impact in freshwater communities through increased herbivory. In conclusion, my results suggest that temperature plays an important role in modulating nutrient acquisition in omnivorous ectotherms. A greater assimilation of plant material or increased herbivory may help ectotherms to cope with the challenges imposed by higher temperatures. However, a reduction in the trophic positions of the omnivores may trigger changes in the functioning and structure of freshwater communities and food webs.