, Volume 179, Issue 2, pp 435–446

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

  • Kevin R. Ford
  • Joshua H. Ness
  • Judith L. Bronstein
  • William F. Morris
Plant-microbe-animal interactions - Original research

DOI: 10.1007/s00442-015-3341-3

Cite this article as:
Ford, K.R., Ness, J.H., Bronstein, J.L. et al. Oecologia (2015) 179: 435. doi:10.1007/s00442-015-3341-3


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.


Ant plant Cactus demography Integral projection model Protection mutualism Sonoran Desert 

Supplementary material

442_2015_3341_MOESM1_ESM.pdf (82 kb)
Supplementary material 1 (PDF 83 kb)
442_2015_3341_MOESM2_ESM.pdf (83 kb)
Supplementary material 2 (PDF 83 kb)
442_2015_3341_MOESM3_ESM.pdf (191 kb)
Supplementary material 3 (PDF 192 kb)
442_2015_3341_MOESM4_ESM.pdf (240 kb)
Supplementary material 4 (PDF 241 kb)
442_2015_3341_MOESM5_ESM.pdf (7 kb)
Supplementary material 5 (PDF 7 kb)

Funding information

Funder NameGrant NumberFunding Note
National Science Foundation
  • DGE-0718124
Achievement Rewards for College Scientists Foundation
    National Science Foundation
    • DEB-0716433

    Copyright information

    © Springer-Verlag Berlin Heidelberg (outside the USA) 2015

    Authors and Affiliations

    • Kevin R. Ford
      • 1
    • Joshua H. Ness
      • 2
    • Judith L. Bronstein
      • 3
    • William F. Morris
      • 4
    1. 1.Department of BiologyUniversity of WashingtonSeattleUSA
    2. 2.Department of BiologySkidmore CollegeSaratoga SpringsUSA
    3. 3.Department of Ecology and Evolutionary BiologyUniversity of ArizonaTucsonUSA
    4. 4.Department of BiologyDuke UniversityDurhamUSA

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