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Wand plant architecture in the Fynbos: Testing the rodent herbivory hypothesis

Bailey, L.A., Potts, A.J., Cowling, R.M., Whitfield, M.C., Smit, B.
South African journal of botany 2019 v.124 pp. 564-572
flowers, fynbos, herbivores, leaves, mice, perennials, phylogeny, plant architecture, pollination, predation, risk, seeds, stems, temperature, water use efficiency
Throughout the Cape Floristic Region, in a range of local environments, can be found a distinctive growth form: “wand” plants. This curious plant architecture comprises perennial plants which have slender (wand-like) stems that extend high above the matrix vegetation. We explore whether the evolution of wand-plants may have been driven by plant–herbivore interactions with rodents, where such architecture reduces access to nutrient rich flowers and seeds. To test this idea, we determined if (i) wand-plants were more flexible than non-wand congeners, and (ii) a stabilised wand plant was favoured for climbing (by laboratory mice) over a free-standing wand plant in a laboratory setting. Under a phylogenetic independent contrast framework, wand-plants were not more flexible (across a range of diameters) than non-wand congeners. Also, laboratory mice showed no differences in preference for, or success on, free-standing and stabilised wand-plants. We discuss a range of alternative ideas, including that the architecture may be a coevolutionary relic, to increase pollination visitation, to increase leaf-level water-use efficiency or to maintain cooler leaf temperatures in warm and/or windless environments. However, we suggest that our study may be flawed as it did not include the “landscape of fear” — i.e. the predation risk associated with exposure along a bare stem — and that rodent herbivory may nonetheless be the dominant evolutionary driver. Although we do not find any support for the hypothesis of an arms race between wand plants and rodent herbivores, this fascinating architecture remains unexplained and we hope that our study will stimulate further exploration of the topic.