Climate change can greatly alter long-standing symbioses between species. In some cases these mutualisms are strengthened while others are weakened or transition to new mutualist partners. The resulting behaviors can have a cascading effect throughout their communities. Understanding how these relationships might change is an important step in preparing for and adjusting to climate change. This study specifically looks at the effects of warmer temperatures on the mutualism between ants and aphids in the subalpine zone of the Rocky Mountains.
Ants and aphids are mutualists where aphids receive protection from predators in exchange for honeydew, a sugar-rich excrement eaten by ants. As a result, aphid colonies without ants have reduced growth rates in comparison to ant tended colonies due to increased predation. In order to observe the effects of warmer temperatures on this mutualism we established 30 aphid (Aphis helianthi) colonies on replicate host-plants (Ligusticum porteri) near the Rocky Mountain Biological Laboratory near Crested Butte, CO. We randomly assigned each colony to 1 of 4 treatments; ambient temperature + no ants, ambient temperature + ants, elevated temperature + no ants, and elevated temperature + ants. Elevated temperatures were achieved using open top warming chambers and presented no observable barrier to ant or aphid predators.
Results/Conclusions
The interactive effect between temperature and ant presence on aphid growth rates was significant (F=4.2729 P=0.04882). Aphid colony growth rates were dependent upon ant presence under ambient conditions, but independent of ant presence at elevated temperatures. Ant presence did not differ between warmed and unwarmed plants (F=0.0064, P=0.9367), but ants were less likely to engage in tending behaviors on warmed plants than unwarmed plants(F=4.6027, P=0.04225). Accordingly, predation rates were higher on warmed plants (F=35.533, P=0.0354), measured as rate of parasitoid infected aphids. This suggests the effect of elevated temperature on aphid growth rates supersedes their dependence on ant mutualisms.
Our study has shown how an insect mutualism dissolves during increased temperatures within the range we expect from climate change. The implications of this result and the extent to which other insect mutualisms will be affected are unclear and require further study, yet our results suggest a significant shift in animal behavior will occur as temperatures increase from climate change.