Secondary metabolites in Eucalyptus melliodora : field distribution and laboratory feeding choices by a generalist herbivore, the common brushtail possum

Abstract

We studied the influence of a group of plant secondary compounds, the sideroxylonals, on feeding by the common brushtail possum (Trichosurus vulpecula), a generalist herbivore. Possums were offered synthetic diets either with or without sideroxylonals or, in separate experiments, foliage from 28 individual Eucalyptus melliodora trees. Possums ate less of the synthetic diet at sideroxylonal concentrations of 4 and 7 mg g-1 when offered a choice or no choice, respectively. Possums fed foliage in no-choice protocols ate 12-61 g of dry matter per day. Sideroxylonal concentration was an essential determinant of feeding on foliage but the wide variation, particularly at intermediate sideroxylonal concentrations, suggests that other secondary plant chemicals, e.g. tannins, are important also. The normal distribution of sideroxylonal concentrations (mean = 27.7, s.d. = 7.85 mg sideroxylonals per g dry leaf) in a natural population of 150 E. melliodora trees shows that brushtail possums will rarely encounter highly palatable trees (<10 mg sideroxylonals per g dry leaf) nor highly unpalatable foliage (>40 mg sideroxylonals per g dry leaf). When foraging on E. melliodora, brushtail possums must contend with intermediate sideroxylonal concentrations (20-30 mg per g dry leaf), where variability in food intake is most noticeable.