Foliar nutritional quality explains patchy browsing damage caused by an invasive mammal

Introduced herbivores frequently inflict significant, yet patchy damage on native ecosystems through selective browsing. However, there are few instances where the underlying cause of this patchy damage has been revealed. We aimed to determine if the nutritional quality of foliage could predict the browsing preferences of an invasive mammalian herbivore, the common brushtail possum (Trichosurus vulpecula), in a temperate forest in New Zealand. We quantified the spatial and temporal variation in four key aspects of the foliar chemistry (total nitrogen, available nitrogen, in vitro dry matter digestibility and tannin effect) of 275 trees representing five native tree species. Simultaneously, we assessed the severity of browsing damage caused by possums on those trees in order to relate selective browsing to foliar nutritional quality. We found significant spatial and temporal variation in nutritional quality among individuals of each tree species examined, as well as among tree species. There was a positive relationship between the available nitrogen concentration of foliage (a measure of in vitro digestible protein) and the severity of damage caused by browsing by possums. This study highlights the importance of nutritional quality, specifically, the foliar available nitrogen concentration of individual trees, in predicting the impact of an invasive mammal. Revealing the underlying cause of patchy browsing by an invasive mammal provides new insights for conservation of native forests and targeted control of invasive herbivores in forest ecosystems

Plant secondary metabolites and primate food choices : a meta-analysis and future directions

The role of plant secondary metabolites (PSMs) in shaping the feeding decisions, habitat suitability, and reproductive success of herbivorous mammals has been a major theme in ecology for decades. Although primatologists were among the first to test these ideas, studies of PSMs in the feeding ecology of non-human primates have lagged in recent years, leading to a recent call for primatologists to reconnect with phytochemists to advance our understanding of the primate nutrition. To further this case, we present a formal meta-analysis of diet choice in response to PSMs based on field studies on wild primates. Our analysis of 155 measurements of primate feeding response to PSMs is drawn from 53 studies across 43 primate species which focussed primarily on the effect of three classes of PSMs tannins, phenolics, and alkaloids. We found a small but significant effect of PSMs on the diet choice of wild primates, which was largely driven by the finding that colobine primates showed a moderate aversion to condensed tannins. Conversely, there was no evidence that PSMs had a significant deterrent effect on food choices of non-colobine primates when all were combined into a single group. Furthermore, within the colobine primates, no other PSMs influenced feeding choices and we found no evidence that foregut anatomy significantly affected food choice with respect to PSMs. We suggest that methodological improvements related to experimental approaches and the adoption of new techniques including metabolomics are needed to advance our understanding of primate diet choice

Landscape-scale analysis of nutritional traits of New Zealand tree foliage using near-infrared spectroscopy

Near-infrared reflectance spectroscopy (NIRS) was used to predict the nutritional quality of tree foliage from a New Zealand forest inhabited by the introduced common brushtail possum (Trichosurus vulpecula) as part of a study to understand the relationship between possum browsing and the nutritional quality of foliage. The traits of interest were the foliar concentrations of total nitrogen, in vitro dry matter digestibility, and available nitrogen (with and without incubation with polyethylene glycol to indicate the effect of tannins). A total of 1399 samples from five plant species were studied, with 650 included in a calibration set and 50 in a separate validation set. We evaluated the performance of classic (GLOBAL) and alternative (LOCAL) calibration techniques. Both were successful, although a GLOBAL equation incorporating all calibration samples was used for the final predictions. Modelling of the spectra enabled the nutritional quality of the foliage samples to be estimated with coefficients of determination (r2) of 0.83-0.99 using modified partial least-squares regression. Nutritional quality varied both within and between five tree species for which possums show varied preferences. The predictive models developed in this study facilitate landscape-scale ecological investigations into the role of plant nutrients on the behaviour and demography of introduced herbivores. This provides new opportunities to test theories on invasiveness and to prioritize landscapes for conservation and possum control

The Use of Polyethylene Glycol in Mammalian Herbivore Diet Studies:What Are We Measuring?

Polyethylene glycol (PEG) has been used to study the intake and digestion of tannin-rich plants by mammalian herbivores because it preferentially binds to tannins. However, it is not clear whether the responses of herbivores to dietary PEG is due to increased protein availability from the release of tannin-bound protein, amelioration of tannin effects, or whether PEG also may bind to other compounds and change their activity in the gut. We used three native New Zealand tree species to measure the effect of PEG on the amount of foliage eaten by invasive common brushtail possums (Trichosurus vulpecula) and on in vitro digestible nitrogen (available N). The addition of PEG increased the in vitro available N content of Weinmannia racemosa foliage, and possums ate significantly more PEG-treated foliage than untreated foliage. However, possums also ate more PEG-treated Fuchsia excorticata foliage, even though PEG did not increase in vitro available N in this species. Possums ate very little Melicytus ramiflorus, regardless of PEG treatment, even though M. ramiflorus contained the highest concentration of in vitro available N. These results prompted us to use PEG and a protein supplement, casein, to manipulate the available N concentration of diets containing ground eucalypt foliage, a well-studied food species for possums. Again, the response of possums to PEG was independent of changes in in vitro available N. In addition, altering the protein content of the diet via the addition of casein did not affect how much food the possums consumed. We conclude that the effects of PEG on dry matter intake by mammalian herbivores are not due solely to the release of tannin-bound protein. There is need for a better understanding of PEG-tannin interactions in order to ensure that the use of PEG in nutritional studies does not outstrip an understanding of its mechanisms of action

A faecal index of diet quality that predicts reproductive success in a marsupial folivore

Estimating the nutritional value of a herbivore's diet is difficult because it requires knowing what the animal eats, the relative quality of each component and how these components interact in relation to animal physiology. Current methods are cumbersom

A faecal index of diet quality that predicts reproductive success in a marsupial folivore

Estimating the nutritional value of a herbivore's diet is difficult because it requires knowing what the animal eats, the relative quality of each component and how these components interact in relation to animal physiology. Current methods are cumbersome and rely on many assumptions that are hard to evaluate. We describe a new method for estimating relative diet quality directly from faeces that avoids the problems inherent in other methods. We combine this method with near infrared reflectance spectroscopy (NIRS) to analyse many samples and thus provide a technique with immense value in ecological studies. The method stems from the correlation between the concentrations of dietary and faecal nitrogen in herbivores eating a tannin-free diet, but a weaker relationship in browsers that ingest substantial amounts of tannins, which form complexes with proteins. These complexes reduce the availability of nitrogen and may increase faecal nitrogen concentrations. Using the tannin-binding compound, polyethylene glycol, we showed that tannin-bound nitrogen is a significant and variable part of faecal nitrogen in wild common brushtail possums (Trichosurus vulpecula). We developed a technique to measure faecal available nitrogen and found that it predicted the reproductive success of female brushtail possums in northern Australia. Faecal available nitrogen combined with NIRS provides a powerful tool for estimating the relative nutritional value of the diets of browsing herbivores in many ecological systems. It is a better indicator of diet quality than other commonly used single-nutrient measures such as faecal nitrogen and foliage analysis paired with observed feeding behaviour

Probability of severity of browsing by possums on kamahi.

<p>Modelled probability of severity of browsing by possums on kamahi foliage in response to foliar available nitrogen concentration at Line 2, Tararua Mountain Range. The dotted line represents the probability of no browsing by possums occurring, the short dash is light browse, long dash is moderate browse and solid line is heavy browsing. The figure shows the model for reference levels (Spring 2010, non-control zone; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0155216#pone.0155216.t004" target="_blank">Table 4</a>). Grey boxes denote values beyond the range of available nitrogen observed for kamahi foliage at line 2 in the study (0.20%–0.76%).</p

Observations of browsing by possums on kamahi.

<p>Proportion of kamahi observations within each browse category for each season in non-control (a) and control (b) zones in the Tararua Mountain Range. The black bar represents the proportion of trees in the heavy browse category, dark grey represents moderate browsing by possums, light grey is light browsing and white represents the proportion of trees with no browsing observed. Also shown, average bite mark index, BMI (filled circles) ± SE (error bars), as a proxy for possum abundance in each season and each control zone. Browse data presented is for 63 kamahi trees at Line 2. BMI data presented is the average BMI per season from ten wax tag transects (20 wax tags per transect) positioned along Line 2.</p

Summary of model coefficients.

<p>Summary of model coefficients.</p