Coping with chemical complexity in mammal-plant interactions:\ud Near-infrared spectroscopy as a predictor of Eucalyptus foliar nutrients and of the feeding rates of folivorous marsupials

We investigated the utility of near-infrared reflectance\ud spectroscopy (NIRS) as a means of rapidly assaying\ud chemical constituents of Eucalyptus leaves and of\ud directly predicting the intake of foliage from individual\ud trees by greater gliders (Petauroides volans) and common\ud ringtail possums (Pseudocheirus peregrinus). The\ud concentrations of total nitrogen, neutral detergent fiber,\ud condensed tannins and total phenolics could be predicted\ud accurately by partial least squares regression models relating\ud the near-infrared reflectance spectra of foliage\ud samples to analyses performed using standard laboratory\ud procedures. Coefficients of determination (r2) for all four\ud constituents ranged between 0.88 and 0.98, and standard\ud errors of prediction between 0.80 mg g–1 dry matter\ud (DM) for total nitrogen and 5.14 quebracho equivalents\ud g–1 DM for condensed tannins. Near-infrared spectral-\ud based models of food intake had r2 values of 0.90\ud and 0.95 with a standard error of prediction of 3.4 and\ud 8.3 g DM kg–0.75 day–1 for greater gliders and common\ud ringtail possums respectively. We used the predictive\ud model of food intake for greater gliders to examine the\ud relationship between leaf palatability and documented\ud food preferences of animals in the wild. Ranked differences\ud in leaf palatability across four Eucalyptus species\ud were consistent with documented food preferences of\ud greater gliders in the wild. We conclude that NIRS provides\ud a powerful tool to predict foraging behaviour of\ud herbivores where forage choices are determined by compositional\ud attributes of food

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

A simple, integrative assay to quantify nutritional quality of browses for herbivores

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

Monitoring hunted species of cultural significance: Estimates of trends, population sizes and harvesting rates of flying-fox (Pteropus sp.) in New Caledonia

International audienceAssessing population trends and their underlying factors is critical to propose efficient conservation actions. This assessment can be particularly challenging when dealing with highly mobile, shy and nocturnal animals such as flying-foxes. Here we investigated the dynamics of hunted populations of Pteropus ornatus and P. tonganus in the Northern Province of New Caledonia. First, an ethno-ecological survey involving 219 local experts identified 494 flying-fox roosts. Current status was assessed for 379 of them, among which 125 were no longer occupied, representing a loss of 33% over ca. 40 years. Second, species-specific counts conducted at 35 roosts, and a sample of animals killed by hunters, revealed that the endemic species, P. ornatus, was dominant (68.5%). Between 2010 and 2016, 30 roosts were counted annually during the pre-parturition period. Roosts size averaged 1,425 ± 2,151 individuals (N = 180 counts) and showed high among-year variations (roost-specific CV = 37-162%). If we recorded significant inter-annual variation, we did not detect a significant decline over the 7-yr period, although one roost went possibly extinct. Population size of the two species combined was estimated at 338,000−859,000 individuals distributed over ca. 400 roosts in the Northern Province. Flying-foxes are popular game species and constitute traditional food for all communities of New Caledonia. Annual bags derived from a food survey allowed us to estimate harvesting rates at 5-14%. Such a level of harvesting for species with a 'slow' demography, the occurrence of poaching and illegal trade, suggest the current species use might not be sustainable and further investigations are critically needed. PLOS ONE | https://doi.org/10.1371/journal.pone