Tree use by koalas in a chemically complex landscape

Although defence against herbivores is often argued to be the main action of plant secondary metabolites (PSMs)1, very few examples have demonstrated that intraspecific variation in PSM concentrations influences foraging by wild vertebrate herbivores2,3. Experiments with captive animals often indicate that PSM concentrations influence how much herbivores eat from individual plants3–7, but these experiments do not replicate the subtle tradeoffs in diet selection faced by wild animals, which must avoid predators and extremes of weather, interact with conspecifics, and achieve a balanced, nutritious diet, while avoiding intoxication by PSMs. We characterized the foliar chemistry of every tree from two Eucalyptus species available to a population of koalas (Phascolarctos cinereus) and considered rates of tree visitation over a ten-year period. We show that visitation rate was most strongly influenced by tree size, but that koalas also visited trees less frequently if the foliage contained either high concentrations of deterrent PSMs known as formylated phloroglucinol compounds,or low concentrations of nitrogen. Consequently, plant chemistry restricts the use of trees by this herbivore, and thus limits the food available to koalas and potentially influences koala populations

A metabolomic approach to identifying chemical mediators of mammal-plant interactions

Different folivorous marsupials select their food from different subgenera of Eucalyptus, but the choices cannot be explained by known antifeedants, such as formylated phloroglucinol compounds or tannins, or by nutritional quality. Eucalypts contain a wide variety of plant secondary metabolites so it is difficult to use traditional methods to identify the chemicals that determine food selection. Therefore, we used a metabolomic approach in which we employed (1)H nuclear magnetic resonance spectroscopy to compare chemical structures of representatives from the two subgenera and to identify chemicals that consistently differ between them. We found that dichloromethane extracts of leaves from most species in the subgenus Eucalyptus differ from those in Symphyomyrtus by the presence of free flavanones, having no substitution in Ring B. Although flavanoids are known to deter feeding by certain insects, their effects on marsupials have not been established and must be tested with controlled feeding studies