, Volume 163, Issue 4, pp 985–996 | Cite as

Geographic variation in a facultative mutualism: consequences for local arthropod composition and diversity

  • Jennifer A. RudgersEmail author
  • Amy M. Savage
  • Megan A. Rúa
Community ecology - Original Paper


Geographic variation in the outcome of interspecific interactions may influence not only the evolutionary trajectories of species but also the structure of local communities. We investigated this community consequence of geographic variation for a facultative mutualism between ants and wild cotton (Gossypium thurberi). Ants consume wild cotton extrafloral nectar and can protect plants from herbivores. We chose three sites that differed in interaction outcome, including a mutualism (ants provided the greatest benefits to plant fitness and responded to manipulations of extrafloral nectar), a potential commensalism (ants increased plant fitness but were unresponsive to extrafloral nectar), and a neutral interaction (ants neither affected plant fitness nor responded to extrafloral nectar). At all sites, we manipulated ants and extrafloral nectar in a factorial design and monitored the abundance, diversity, and composition of other arthropods occurring on wild cotton plants. We predicted that the effects of ants and extrafloral nectar on arthropods would be largest in the location with the mutualism and weakest where the interaction was neutral. A non-metric multidimensional scaling analysis revealed that the presence of ants altered arthropod composition, but only at the two sites in which ants increased plant fitness. At the site with the mutualism, ants also suppressed detritivore/scavenger abundance and increased aphids. The presence of extrafloral nectar increased arthropod abundance where mutual benefits were the strongest, whereas both arthropod abundance and morphospecies richness declined with extrafloral nectar availability at the site with the weakest ant–plant interaction. Some responses were geographically invariable: total arthropod richness and evenness declined by approximately 20% on plants with ants, and extrafloral nectar reduced carnivore abundance when ants were excluded from plants. These results demonstrate that a facultative ant–plant mutualism can alter the composition of arthropod assemblages on plants and that these community-level consequences vary across the landscape.


Ant–plant Community structure Conditionality Geographic mosaic Gossypium 



We are indebted to R. Karban, who introduced us to the wild cotton system. Thanks are also extended to P. Ward (UC-Davis) and C. Olson, Associate Curator of the University of Arizona Insect Collection, for assistance with species identifications. J. Hodgen, W. White, C. Stephens, D. Hoyer, A. Stein, J. Howard, J. M. Rudgers, B. Bedard, and P. Murch provided invaluable field assistance. K. Whitney offered advice and help of many kinds. Thanks to S. Chamberlain, A. Dunham, J. Ness, M. Peterson, G. Wimp, and anonymous reviewers for improvements to this manuscript. This work was supported by an EPA S.T.A.R. fellowship and the Godwin Assistant Professorship to J.A.R. and a Wray-Todd fellowship to A.M.S. The research practices used for this study complied with all laws and regulations of the USA.

Supplementary material

442_2010_1584_MOESM1_ESM.doc (114 kb)
Supplementary material 1 (DOC 113 kb)


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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Jennifer A. Rudgers
    • 1
    Email author
  • Amy M. Savage
    • 1
  • Megan A. Rúa
    • 2
  1. 1.Department of Ecology and Evolutionary BiologyRice UniversityHoustonUSA
  2. 2.Curriculum in EcologyUniversity of North Carolina-Chapel HillChapel HillUSA

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