The growth rate of the acacia tree increases by having sequential associations of several partners to meet changing needs.

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"Trees that sequentially partner with multi-species sets of ants produce more offspring than trees that maintain a lifelong association with any single ant...said Todd Palmer, a UF [University of Florida] biology professor...'Looking at the costs and benefits to the tree, not just at a single moment, but in terms of number of offspring produced over a lifetime, the best possible outcome is obtained not by having what we thought was the 'good mutualist,' but rather by having all four ant species at different life stages...,' [Palmer] said. The key to the new findings is the timing. When a species lives a long time, its needs may change drastically as it grows from young to old, and sequential associations with several partners may help it meet those needs at different times, he said...[A] mutualistic species may require a partner that helps it survive during its vulnerable younger years, even if that partner prevents it from reproducing, Palmer said. Later in life, when large size makes individuals less vulnerable, the ideal partner may be one that enhances reproduction even as it reduces the chances of longer-term survival, he said." (Palmer 2010:1)

Abstracted Design Principle: An entity benefits from a changing partners in mutualistic relationships, depending on its needs as it matures. For example, at first it might need a partner that provides support until it can stand on its own, then it might need a partner that provides nutrients or resources. Next, to grow larger, it might need a partner that helps it become, and finally another partner might help it expand or branch out. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.

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"Determining how multiple partnerships might interactively affect lifetime fitness is a crucial unexplored link in understanding the evolution and maintenance of cooperation. 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." (Palmer et al. 2010:17234)

Journal article
Synergy of multiple partners, including freeloaders, increases host fitness in a multispecies mutualismProceedings of the National Academy of SciencesSeptember 21, 2010
T. M. Palmer, D. F. Doak, M. L. Stanton, J. L. Bronstein, E. T. Kiers, T. P. Young, J. R. Goheen, R. M. Pringle

The Joy of sets: For ants and trees, multiple partners are a boon

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Whistling ThornAcacia drepanolobiumSpecies

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