Small Isn't Always Bad: Similar Plastic Responses to Elevated Temperature Among Differentially Abundant Brook Trout Populations

The potential influence of population size on the magnitude of phenotypic plasticity, a key factor in adaptation to environmental change, is uncertain for salmonids. Conventionally, small populations might exhibit consistently lower plasticity than large populations if small population habitats are generally poor in quality and if genetic diversity underpinning plasticity is lost as population size is reduced. Alternatively, small populations might exhibit (i) consistently higher plasticity as a response to the increased environmental variation that can accompany habitat fragment size reduction, or (ii) greater variability in plasticity, as fragmentation can increase variability in habitat types. We explored these alternatives by investigating temperature plasticity in a common garden experiment using eight fragmented populations of brook trout varying nearly 50-fold in census size (179-8416) and 10-fold in effective number of breeders (18-135). We found almost no evidence for differences in either the magnitude or variability of plasticity in relation to population size, despite that one temperature represented an extreme climate warming scenario. The documentation of similar plastic responses of small and large populations suggests that phenotypic plasticity is not reduced as population size decreases, and that even very small populations of some salmonids might have the ability to respond to climate change.