Consequences of ant thermal tolerance and dominance hierarchies for an ant-plant mutualism

Background/Question/Methods

To explore the consequences of climate change for the functioning of species interactions, we studied the thermal ecology of the mutualism between a Sonoran Desert barrel cactus, Ferocactus wislizeni, and its common ant defenders. We asked: (1) what are the thermal tolerances of the ant mutualists, and (2) is there a tradeoff between thermal tolerance and competitive dominance? To investigate the thermal tolerance of each ant species, we conducted critical thermal maximum (CT_max; the temperature above which ants cannot forage) experiments using a heated circulator and water bath to determine the LD50 temperature. To determine the dominance hierarchy of the ant community, we observed foraging behavior of each species and interspecific interactions on protein baits and assessed dominance using the Colley matrix method. We related the results to relative effectiveness of the ants as mutualists.

Results/Conclusions

Plants tended by Crematogaster opuntiae, Solenopsis aurea, and S. xyloni set more fruits than those tended by Forelius pruinosus. The inferior mutualist Forelius pruinosus exhibited the highest critical thermal maximum (ANOVA, P < 0.0001, Tukey-Kramer HSD). The distribution of ants on plants in response to average plant temperature differed across species, in partial parallel with experimentally determined thermal tolerances (ANOVA, P < 0.0001). F. pruinosus occupied hotter plants than C. opuntiae and S. aurea, but not S. xyloni (Tukey-Kramer HSD). Forelius pruinosus was also the least dominant species, based on Colley dominance rankings, which are highly correlated with the proportion of confrontations won (Spearman Rank Correlation, r=0.9358, P<0.0001). The dominance hierarchy was: S. xyloni > S. aurea > C. opuntiae > F. pruinosus. In this system, the least dominant ant species may be avoiding competition by occupying an extreme thermal niche. Thus, rising temperatures could decrease mutualistic benefit to ant-defended plants by favoring thermally tolerant but competitively and mutualistically inferior ants. Extremely little is known about the thermal ecology of species interactions; further research is essential to understand how they will respond to rising temperatures worldwide.