Experimental warming disrupts an ant-aphid mutualism

Background/Question/Methods

The ant-aphid mutualism is a classic example of a mutualistic interaction between species, where ants protect aphids from predators because aphids produce honey due that ants consume. This interaction occurs across many ant-aphid communities, including corn leaf aphids (Rhopalosiphum maidis), their lady beetle predators (Harmonia axyridis), and two ant species on corn plants in Wisconsin, USA. Prenolepis imparis is an aggressive defender of aphids, but reduces its activity when temperatures are high. At the other end of the spectrum, Lasius alienus is unaffected by warm temperatures, but is less defensive of aphids and sometimes aphidophagous. We conducted a series of manipulative experiments to test how climate warming affects the relative abundances of these two ant species and indirectly affects corn leaf aphid abundance.

Our design crossed temperature (ambient or warmed), ant identity (P. imparis, L. alienus, or no ants), and predators (H. axyridis added or excluded) and replicated each combination six times (72 corn plants). Each corn plant was enclosed in a fine mesh bag to prevent aphids and predators from immigrating or emigrating. Warmed treatments were also surrounded by thin plastic sheeting to create a greenhouse effect that elevated temperatures approximately 5°C.  We added two H. axyridis larvae to predator-present treatments at the initiation of the experiment. At one-week intervals we surveyed the plants, recording aphid and ant abundances and added two H. axyridis larvae to maintain predator-present treatments.

Results/Conclusions

Our results show that warming decreased the mutualistic benefit of ants on aphids. In ambient temperature treatments, P. imparis was abundant and had a significant positive effect on aphids by reducing predation. In contrast, L. alienus did not reduce H. axyridis predation and likely consumed some aphids. Warming decreased the abundance of P. imparis to nearly zero. Consequently, under warmed conditions aphids were tended only by L. alienus and suffered high predation rates by H. axyridis. These results demonstrate that climate change can alter species composition and generate significant indirect effects on ecological communities.