Foliar concentration of a single toxin creates habitat patchiness for a marsupial folivore

Abstract

We examined intraspecific variation in susceptibility to herbivory by common ringtail possums (Pseudocheirus peregrinus) for two species of Eucalyptus, (E. polyanthemos and E. sideroxylon) and the chemical basis for that variation. Using a no-choice protocol, we observed dry matter intakes by common ringtail possums ranging from 3.28 to 44 g·(kg body mass)-0.75·d-1 for E. polyanthemos and from 2.4 to 67 g·(kg body mass)-0.75·d-1 for E. sideroxylon. We investigated, using correlative analyses, the relationships between dry matter intake and a range of foliage chemical characteristics, including measures of nutritional quality (total nitrogen, cellulose, hemi-cellulose, lignin, in vitro dry matter digestibility) and plant secondary chemistry (total phenolics, condensed tannins, cyanogenic glycosides, terpenes, and sideroxylonal, a recently identified Eucalyptus toxin). Significant relationships were identified only for terpenes (and 1,8-cineole in particular) and sideroxylonal. Bioassay experiments confirmed that the foliar concentration of sideroxylonal alone was sufficient to explain the variation observed. To undertake a field study of variation in foliar sideroxylonal and dry matter intake by possums of E. polyanthemos, we developed calibrations of both variables against the near infrared spectra of foliage samples. Acceptable calibration equations were developed, and we applied these to samples collected from a number of E. polyanthemos individuals within an area approximating the home range size of common ringtail possums. We found that foliar sideroxylonal varied from nil to 12.6 mg/g, while predicted dry matter intakes by possums ranged from nil to 42.8 g.(kg body mass)-0.75·d-1. We conclude that significant patchiness in nutritional quality of foliage, resulting from variation in foliar concentrations of a single compound, exists at a scale relevant to the feeding decisions of individual animals.