Differences in herbivore defense in myrmecophyte and non-myrmecophyte species of Cecropia trees from Peru

Background/Question/Methods Cecropia (Cecropiaceae) is a genus of approximately 60 species of Neotropical trees. These are fast-growing pioneers that colonize disturbed habitats along river edges and in tree fall gaps throughout the Amazon basin and Central America.  Most species are myrmecophytes inhabited by ants. Although several ant species live in the trees, Azteca ants are most common. The relationship between the ants and the trees is mutualistic. Ants defend the trees against herbivores, and the trees provide the ants with domatia in hollow stems and food (Müllerian bodies). A few species of Cecropia are not defended by ants. We have been studying herbivore defense in two of these non-myrmecophytes, C tacuna and C. sciadophylla. Here we compare the chemical and physical characteristics contributing to herbivore defense in C. tacuna and C. sciadophylla with C. membranacea, a myrmecophyte inhabited by Azteca ants. We measured leaf toughness for young and mature leaves from each species. We also extracted leaf samples for alkaloids, cyanogens, condensed tannnins, total phenolic compounds, and protein precipatable phenolics. To estimate potential herbivore pressure at our collecting sites we surveyed insect diversity with sweep samples at four locations along the eastern slopes of the Peruvian Andes at elevations ranging from 300 – 2900 m. 

Results/Conclusions Leaf toughness was higher in mature leaves than in young leaves, and mature leaves of C. sciadophylla were significantly tougher than the other species.  We found no evidence for the presence of either alkaloids or cyanogens, and we conclude that these compounds are not important in herbivore defense. Young leaves of C. sciadophylla were more heavily defended than mature leaves, and they had the highest levels of both condensed tannins and total phenolic compounds compared to the other species. However, we found no significant differences between levels of either condensed tannins or total phenolics between the young and mature leaves in either C. tacuna or C. membranacea. Our sweep sample surveys during the dry season revealed more insect diversity, and possibly more potential herbivores, at mid elevation in the cloud forest where we collected C. tacuna. We will offer explanations for these differences based on trade-offs in resource allocation between young and mature leaves of the myrmecophyte and the two non-myrmecophytes.