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The demographic consequences of mutualism: ants increase host-plant fruit production but not population growth

  • Plant-microbe-animal interactions - Original research
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Abstract

The impact of mutualists on a partner’s demography depends on how they affect the partner’s multiple vital rates and how those vital rates, in turn, affect population growth. However, mutualism studies rarely measure effects on multiple vital rates or integrate them to assess the ultimate impact on population growth. We used vital rate data, population models and simulations of long-term population dynamics to quantify the demographic impact of a guild of ant species on the plant Ferocactus wislizeni. The ants feed at the plant’s extrafloral nectaries and attack herbivores attempting to consume reproductive organs. Ant-guarded plants produced significantly more fruit, but ants had no significant effect on individual growth or survival. After integrating ant effects across these vital rates, we found that projected population growth was not significantly different between unguarded and ant-guarded plants because population growth was only weakly influenced by differences in fruit production (though strongly influenced by differences in individual growth and survival). However, simulations showed that ants could positively affect long-term plant population dynamics through services provided during rare but important events (herbivore outbreaks that reduce survival or years of high seedling recruitment associated with abundant precipitation). Thus, in this seemingly clear example of mutualism, the interaction may actually yield no clear benefit to plant population growth, or if it does, may only do so through the actions of the ants during rare events. These insights demonstrate the value of taking a demographic approach to studying the consequences of mutualism.

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Acknowledgments

We thank K. Bressmer, C. Gibson, M. Lanan, H. McNelis, A. Reed and L. Sturdivant for assistance in the field. We also thank G. Davidowitz and M. Lanan for their insight into the insect community that interacts with F. wislizeni. Thank you to the reviewers and editors for helpful suggestions. This work was supported by a National Science Foundation Graduate Research Fellowship (DGE-0718124) and a grant from the Seattle chapter of the ARCS Foundation to K. R. F., and by National Science Foundation grant DEB-0716433 to W. F. M.

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The authors declare no conflict of interest.

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Authors and Affiliations

  1. Department of Biology, University of Washington, Box 351800, Seattle, WA, 98195, USA

    Kevin R. Ford

  2. Department of Biology, Skidmore College, Saratoga Springs, NY, 12866, USA

    Joshua H. Ness

  3. Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA

    Judith L. Bronstein

  4. Department of Biology, Duke University, Box 90338, Durham, NC, 27708, USA

    William F. Morris

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  1. Kevin R. Ford
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Correspondence to Kevin R. Ford.

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Communicated by Candace Galen.

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Ford, K.R., Ness, J.H., Bronstein, J.L. et al. The demographic consequences of mutualism: ants increase host-plant fruit production but not population growth. Oecologia 179, 435–446 (2015). https://doi.org/10.1007/s00442-015-3341-3

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