To avoid extinction from global warming, organisms that cannot track changing climates by shifting their distribution or phenology must either acclimate or their populations must evolve as the climate warms. Thermal acclimation is a type of phenotypic plasticity and thus occurs within an individual organism, whereas evolution represents a change in a biological trait across generations. To date, most studies investigating species responses to warming expose individuals to warmer temperatures over short periods and thus only capture the process of acclimation. We performed an experiment that utilized a series of geothermally heated streams (5° – 20°C) to determine how multi-generational exposure to temperature affected metabolic rate and movement in a keystone freshwater grazer, the snail, Radix balthica. We measured metabolic rate at each of the stream temperatures and tracked snail movements within their home streams. We hypothesized that metabolic rate would increase with temperature and body speed and distance moved would increase with temperature. We predicted that movement traits that are less constrained by neurophysiological processes, such trajectory shape and time spent moving, might be less strongly tried to temperature, as they incorporate decision-making.
Results/Conclusions
As expected, metabolic rate increased with stream temperature, but this was higher than expected. This increased metabolic rate enabled snails to increase their body speed with temperature more than has been reported elsewhere and to move greater distances at warmer temperatures. This increase in speed and distance moved should increase encounter rates with resources, offsetting higher energy demands that occur with higher metabolic rates. Plant-herbivore interaction strengths often increase with temperature, and this is likely driven by increased locomotor performance at warmer temperatures. Snails had straighter trajectories in warmer streams. This highlights a trade-off between speed and maneuverability and higher speeds are favored at warmer temperatures. Finally, while snail trajectories were slightly straighter in warmer streams, the overall shape of the trajectories did not change with temperature. Trajectories were primarily straight across all temperatures and straight trajectories are the optimal shape given the resource-free arenas in which the snails were tracked. Long-term adaptation to warmer environments may change our predictions of how organisms respond to global warming than predictions that are just based on physiological change alone, requiring greater consideration of how species are exposed to temperature in global change research.