82.
In the global warming context, we compared the thermal tolerance of several populations of the crustacean
Gammarus pulex (Amphipoda: Gammaridae) along a latitudinal thermal gradient in the Rh?ne Valley. To disentangle the effect of regional (North
vs. South) and local (site-specific) factors, the ecophysiological responses of populations were investigated at two levels
of biological organisation: whole organism level considering body size [critical thermal maximum (CTmax), mean speed of locomotion
(MS), time mobile (TM)] and organelle function level [mitochondrial respiratory control ratios (RCRs)]. CTmax and RCRs, but
not MS and TM, revealed a significantly higher thermal tolerance in southern populations compared to northern ones. Nevertheless,
temperatures ≥ 30°C were deleterious for all populations, suggesting that populations located in the warmer limit of the species
distribution will be more threatened by climate change as they live closer to their upper thermal limits. The strong differences
observed between populations indicate that the species-level thermal tolerance used in predictive models may not be informative
enough to study the impact of global warming on species distributions. This work also reveals that an appropriate choice of
indicators is essential to study the consequences of global warming since the response of organisms at the whole body level
can be influenced by local conditions.
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