PS 10-117 - Leaf Rolling as a Defense against Herbivory in Tropical Herbaceous Monocots

Abstract: Many species of monocots in the understories of tropical forests have terminal leaves that are fully elongated but tightly rolled, a trait that is hypothesized to defend the youngest leaves against herbivory. Tightly rolled leaves lack a leaf margin necessary for some herbivores to attach before chewing and also present potential herbivores with a thicker stack of layered leaves that may be harder to cut through. We evaluated the importance of this trait as a deterrent to herbivory for several large herbaceous species in a lowland tropical rain forest in Yasuní National Park, Ecuador, using observational and experimental approaches. We surveyed background rates of herbivory and experimentally manipulated leaf offerings to leaf cutter ants, an herbivore that accounts for up to 25% of all herbivory in Neotropical forests. We hypothesized that: (1) Background rates of standing herbivory in these species is dominated by herbivory that occurred when the leaf was unrolled rather than rolled, and (2) Leaf cutter ants will preferentially choose unrolled leaf material and avoid rolled leaves. We quantified the area lost to herbivory on up to 8 leaves of 30 individuals for four species in the Zingiberales order (Heliconia, Renealmia, and Costus). We classified the leaf area loss as having occurred when the leaf was rolled or unrolled, and asked whether herbivory was more likely to occur during either state, and whether total area lost to herbivory when unrolled differed from when rolled. We then experimentally manipulated young and old leaves of three species in all combinations of age and rolled or unrolled status, and conducted replicated feeding trials at multiple leaf cutter ant nests. We recorded the amount of leaf material harvested by the ants in a standard period of time. Our findings support the defense benefit of leaf rolling. We found herbivory was less likely to occur when the leaf was rolled than when unrolled. However, when herbivory did occur on rolled leaves, the total area lost was similar to what was lost on unrolled leaves. Leaf cutter ants preferentially harvested unrolled leaf material and avoided rolled leaves in all species, no matter the leaf age or whether the rolling or unrolling was an experimental manipulation. Given leaf cutter ants’ high herbivory rates, plant traits that deter their harvest can provide a significant benefit to leaf tissue over the lifetime of the life, despite the temporary reduction in photosynthetic area that occurs from leaf rolling.