Mutualisms are more complex than they appear

The African Ant-Acacia system continues to yield valuable ecological insight. PNAS published another excellent paper yesterday by Todd Palmer’s group that combines modeling with empirical data to show how short-term evaluations of mutualisms miss key dynamics:

[excerpted from the abstract] The tropical tree Acacia drepanolobium associates with four symbiotic ant species whose short-term individual effects range from mutualistic to parasitic. Using a long-term dataset, we show that tree fitness is enhanced by partnering sequentially with sets of different ant symbionts over the ontogeny of a tree. These sets include a “sterilization parasite” that prevents reproduction and another that reduces tree survivorship. Trees associating with partner sets that include these “parasites” enhance lifetime fitness by trading off survivorship and fecundity at different life stages. Our results demonstrate the importance of evaluating mutualism within a community context and suggest that lifespan inequalities among mutualists may help cooperation persist in the face of exploitation.

Plants have different needs as they age, with young ones prioritizing growth while older ones favor reproduction. As their four ant symbionts differ in how they affect plant growth versus reproduction, trees that host all four ant partners as they pass through life are more successful than those that are more static in their partnerships.

The most striking finding is that ant species previously thought parasitic for their tendency to temporarily sterilize the host plants actually benefit the system when the 100+ year lifespan of the tree is considered. These supposed parasites favor early growth so that larger, healthier trees are better able to reproduce once they switch over to a non-sterilizing partner. It’s a striking example of nature’s subtlety, and not the only recent reminder that “cheaters” aren’t always what they seem to be.

More coverage at Not Exactly Rocket Science.

source: Palmer, T. M. et al. 2010. Synergy of multiple partners, including freeloaders, increases host fitness in a multispecies mutualism. PNAS online early 10.1073/pnas.1006872107

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