Judie Bronstein

University Distinguished Professor,
Dept. Ecology & Evolutionary Biology,
University of Arizona, Tucson, AZ

Ecology and evolution of the costs of mutualism

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When we think about mutualism, our attention is almost always drawn to the benefits it provides. Even when we explicitly acknowledge and measure costs (e.g., costs of reward production and of destructive side effects inflicted by partners), we simply treat these to be factors that lower benefits. I highlight here that the costs of mutualism are themselves under selection, and have their own evolutionary trajectories; these are interesting and sometimes counterintuitive, and are worthy of theoretical and empirical attention. I will present a variety of examples to illustrate this point, but focus particularly on the interaction between Datura and the sphinx moth Manduca sexta. Manduca is a specialized and highly effective pollinator of Datura, but females lay eggs on Datura leaves and their offspring are voracious herbivores. To understand this interaction, it is critical to separate the actions of the moth by life cycle stage: the adults confer benefits to plants, and the juveniles inflict costs. I will review traits have apparently evolved to reduce these costs. Can selection operate independently to lower the costs of mutualism while maintaining (or raising) the benefits of mutualism? This is not necessarily the case: for example, reduction of the costs of mutualism might unavoidably reduce benefits of the interaction as well (e.g., in the Datura-Manduca example, because a less palatable plant would be avoided by the pollinators). How will mutualism evolve as a consequence of simultaneous selection to reduce costs and raise benefits? We do not yet know the answers to these questions. However, simply posing them may open new ways of thinking about mutualism ecology and evolution, and suggest new research directions.

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