Higher temperatures mean lower rewards and poorer defense in a desert protection mutualism

Background/Question/Methods

The outcomes of mutualistic interactions are dependent on how mutualists respond to temporal and spatial variability in both the biotic and abiotic environment. Temperature is a key abiotic factor that can affect species directly (e.g., physiological tolerances, metabolic influences) and indirectly (e.g., via its effects on interactions with consumers, competitors, mutualists). We tested the effects of temperature on reward production for participants in a protection mutualism, in which a cactus provides extrafloral nectar to a set of defensive ant species that vary in antagonist removal effectiveness. To investigate how extrafloral nectar production varies with temperature in Ferocactus wislizeni, plants were housed in a controlled environment chamber and extrafloral nectar quantity was measured at multiple temperatures. We used nonlinear regression to fit functions to extrafloral nectar production and related that to ant activity data under varying temperatures for patterns collected in the field. We then developed mathematical models to predict (i) extrafloral nectar availability to each ant species based on its thermal range, and (ii) the amount of extrafloral nectar each ant species could obtain based on the number of workers attending cacti as a function of temperature.

Results/Conclusions

We report three key results. First, extrafloral nectar production was greatest at intermediate temperatures (T_opt = 28.20.4°C) and declined at both higher and lower temperatures. Second, ant attendance on plants could be predicted by temperature alone. Finally, the model predictions allowed for an analysis of the cost-effectiveness of different qualities of species interactions. Interestingly, Forelius pruinosus, the least-effective ant mutualist, was predicted to receive less extrafloral nectar relative to more-effective ant species because it was active at temperatures at which plants produce less extrafloral nectar, and was less abundant on plants. These data suggest that rising temperatures would likely reduce the amount of extrafloral nectar produced by these plants, which could impact the forces that hold these suites of interactions together. Additionally, global warming may cause temperatures to exceed the thermal tolerance of more-effective ant mutualists, impacting the potential participant pool. How these interactions are influenced by a sensitivity to rewards, or how competition for these rewards alter the network of interactions, is key to understanding the long-term persistence of this mutualism as the environment significantly changes in the coming years.