Does ontogeny cause changes in the defensive strategies of the myrmecophyte Cecropia peltata?

Cecropia peltata L. is a myrmecophyte, with a wide distribution in the neotropics, predominantly associated with ants of the genus Azteca. It has been shown that Azteca ants defend Cecropia plants against herbivores, and that the plants provide housing (hollow stems) and food (Müllerian bodies) for the ant colony. In the field, occupation by ants does not take place until plants have reached a minimum colonisable size (ca 1 m height), and defensive ants do not occupy small plants. Therefore, juvenile individuals lack such biotic defence. This constitutes an ontogenetic constraint to biotic defence in these plants. We tested the hypothesis that in the stage previous to colonisation plants of Cecropia peltata in a Mexican tropical forest may exhibit some alternative or complementary defensive mechanism against herbivores. We compared, in pre-colonised and colonised plants: rates of herbivory, concentrations of potentially defensive secondary metabolites (total phenolics and condensed tannins), and trichome density. We also conducted acceptability bioassays with a generalist herbivore (Spodoptera fugiperda). In addition, we measured plant growth to investigate if, by using alternative defence mechanisms, pre-colonised plants experience a reduced performance. Rates of herbivory were higher in pre-colonised individuals. Accordingly, leaf phenolics and tannin concentrations, as well as trichome density, were higher in colonised plants. In addition, acceptability bioassays showed that S. fugiperda preferred the leaves of pre-colonised plants. Relative growth rate was not statistically different between both types of plants. Contrary to our expectation, colonised plants, besides biotic defence by ants, also had higher concentrations of secondary metabolites, higher trichome density and lower herbivory and palatability than pre-colonised plants. This suggests that pre-colonised plants may deal with herbivores by other means and that older, larger plants invest more in all defences rather than shifting defensive mechanisms with ontogeny. Since growth rate of pre-colonised plants was comparable to that of colonised plants (despite the higher levels of herbivory of the former), we suggest that plant tolerance leading to compensation may be used by pre-colonised juveniles of C. peltata.