Environmental factors such as temperature can affect the distribution and abundance of species directly by altering physiological rates, or indirectly by altering the sign and strength of species interactions. Although the direct effects of environmental conditions are relatively well studied, indirect effects mediated by species interactions have garnered less attention. In this study, we examined the temperature-dependency of intraguild predation (IGP) between native blue crabs (Callinectes sapidus, the IG predator) and invasive green crabs (Carcinus maenas, the IG prey) in order to evaluate how the indirect effects of temperature on competitive and predatory rates may influence the spatial distribution of these species. We conducted outdoor mesocosm experiments to empirically quantify the competitive and predatory interactions between blue and green crabs at three temperatures reflective of those across their range for two different size classes of blue crab. We then used parameter values generated from these experiments (temperature- and size-dependent attack rates and handling times) in a size-structured dynamical IGP model in which we varied IGP attack rates, maturation rate of the blue crab from the non-predatory to predatory size class, and resource carrying capacity at each of the three temperatures.
Results/Conclusions
At low temperatures, green crabs had a competitive advantage and IGP on green crabs was low, so green crabs were likely to exclude blue crabs in the model. At high temperatures, size-matched blue and green crabs were competitively similar, large blue crabs had a competitive advantage, and IGP on green crabs was high, so blue crabs were likely to exclude green crabs in the model. These results depended on the level of resource productivity. While physiological limitations of these species may certainly play a role in delimiting their northern and southern range limits, our results suggest that northward expansion of blue crabs may be inhibited further by competition with green crabs, and southward expansion of green crabs by blue crab predation. This study provides an empirical example of how IGP rates are affected by temperature and the implications this may have for species coexistence, stability, and responses to environmental change.