A simple, integrative assay to quantify nutritional quality of browses for herbivores
J. L. DeGabriel, I. R. Wallis, W. J. Foley - School of Botany and Zoology, The Australian National University, Canberra, ACT 0200, Australia
J. L. DeGabriel, B. D. Moore - School of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia
B. D. Moore - The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
J. L. DeGabriel - School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK
Many regard the concentrations of nitrogen (N), tannins and plant cell wall constituents (ﬁbre) as key indicators of food quality and habitat suitability for browsing herbivores; yet there is no method for measuring their combined effects. We have developed a simple in vitro assay for measuring the effects of tannins and ﬁbre on N availability in browse. We determined the effects of tannins by measuring the polyethylene glycol (PEG)binding capacity (PEG-BC) of Eucalyptus leaf samples, followed by a two-stage in vitro digestion with pepsin and cellulase to determine the digestibility of dry matter and N. There was a signiﬁcant relationship between concentrations of digestible N and the PEG-BC of the leaves. Furthermore, adding PEG signiﬁcantly improved the digestibility of N. Our results concur with in vivo observations from several mammalian species. This suggests that our method is effective for measuring the nutritional quality of browse and the beneﬁts of adding PEG, providing some index of the detrimental effects of tannins. We further simpliﬁed the assay by removing the PEG step, allowing us to quickly analyse samples in bulk. Nevertheless, this simpliﬁed method is still not practical for analysing the many samples necessary to compare the nutritional values of different tracts of forest. We used near-infrared reﬂectance spectroscopy to produce calibration equations and predicted total and digestible N in 322 trees at eleven sites. Both within and between sites, we found a wide variation in concentrations of digestible N but a much lower variation in total N, with either no relationship or poor relationships between the two measures. This conﬁrms the variability in the nutritional quality of eucalypt forests, which may explain the distribution and abundance of mammalian herbivores. Thus, our assay provides a useful tool for understanding how food resources inﬂuence herbivore populations at different scales.