Summary: Journal of Ecology 2009, 97, 8996 doi: 10.1111/j.1365-2745.2008.01446.x
© 2008 The Authors. Journal compilation © 2008 British Ecological Society
Blackwell Publishing Ltd
Optimal defence theory predicts investment in
extrafloral nectar resources in an antplant mutualism
J. Nathaniel Holland,* Scott A. Chamberlain and Katherine C. Horn
Department of Ecology and Evolutionary Biology, Rice University, Houston, TX 77005, USA
1. Nearly all mutualisms entail the production of resources by one species that attract and reward
the species with which it interacts. As such resource production could otherwise be allocated
to growth or reproduction, mutualists are predicted to minimize these investment costs.
Here, we employ optimal defence theory for plant secondary compounds to evaluate plant
production of extrafloral nectar (EFN) resources to attract and reward ants for resistance
2. Through ant exclusion and artificial herbivory experiments, we examined investment in EFN by
both buds and fruits of Pachycereus schottii (senita cacti) in the Sonoran Desert of North America.
We tested predictions of optimal defence theory that plants invest more in high value parts (fruits)
through constitutive EFN, less in low value parts (buds) through induced EFN, and that for a given
plant part (buds or fruits), constitutive and induced EFN are negatively correlated.
3. Constitutive levels of EFN were greater in fruits than in buds. Only buds showed induced EFN