Print PageMutualism is often a complex interaction among multiple species, each of which may respond differently to abiotic conditions. Temperature can be thought of as an ecological resource, over which mutualist partners may compete. Thus, less dominant ants may avoid competition by occupying extreme thermal niches. We studied thermal ecology of the mutualism between the fishhook barrel cactus, Ferocactus wislizeni, and its common ant defenders, asking (1) how temperature affects ant activity on individual plants, (2) how temperature affects nectar production among plants, and (3) how temperature and competition affect ant mutualist attendance among plants. Plants tended by Crematogaster opuntiae, Solenopsis aurea, and S. xyloni have higher fruit-set than plants tended by Forelius pruinosus. We measured surface temperature and the number of ants foraging (a measure of partner effectiveness) on 12 plants, each occupied by a single ant species. We determined the critical thermal maximum tolerance (CTmax; the temperature at which 50% of the ants die after 10 minutes of exposure) among ant species, and the distribution of ants on plants in response to average plant temperature.
Results/Conclusions
Ant activity on the plant was significantly correlated with surface temperature, with a peak in activity at moderate temperatures and a decline at colder and hotter temperatures. Ant distribution on plants differed across species in response to average plant temperature (ANOVA, P < 0.0001). F. pruinosus occupied hotter plants than C. opuntiae and S. aurea, but not S. xyloni (Tukey-Kramer HSD). F. pruinosus had a higher critical thermal maximum than C. opuntiae, S. aurea, and S. xyloni (ANOVA, P < 0.0001, Tukey-Kramer HSD). F. pruinosus was the least dominant species among these four. High plant surface temperatures experienced in the field, above which no ants are foraging on the plant, corresponded with the critical thermal tolerance of each ant species as determined in the lab. Also, nectar production responded to temperature (Multiple Regression, log transformed y-values, blocked for plant and replicate, R2 = 0.33, P < 0.0001). Nectar quantity decreased but did not shut down at high ambient temperatures (38°C). These results suggest that temperature is an essential factor determining ant activity on the plant. Further, rising temperatures could decrease mutualistic benefit to the plant by affecting ant partner attendance. 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.
