Abstract
Measuring the quality of the nutritional resources available to wild herbivores is critical to understanding trophic regulation processes. However, the direct assessment of dietary nutritional characteristics is usually difficult, which hampers monitoring nutritional constraints in natural populations. The feeding ecology of ruminant herbivores has been often assessed by analyzing fecal nitrogen (FN) concentrations, although this method has been less evaluated in other taxa. This study analyzed the suitability of FN as an indicator of ingesta quality in the European rabbit (Oryctolagus cuniculus), which is a keystone lagomorph species in Mediterranean ecosystems and of great conservation interest. Firstly, domestic O. cuniculus were used to evaluate under experimental conditions the accuracy of total FN and the metabolic FN as diet quality indicators of forages with characteristics similar to those available under natural conditions. Secondly, the accuracy of Near-Infrared Spectroscopy (NIRS) to calculate FN was tested using partial least squares regression. Thirdly, a pilot field study was conducted to monitor FN dynamics from wild O. cuniculus in three different habitats during wet and drought periods. A strong association was found between diet type and total FN and metabolic FN (Pseudo-R2 ≥ 0.89). It was also found that NIRS calibrations were accurate for depicting nitrogen concentrations (R2 > 0.98 between NIRS and chemical results). Finally, the seasonal FN dynamics measured in the field were consistent with current knowledge on vegetation dynamics and forage limitations in the three habitats. The results support the use of NIRS methods and FN indices as a reliable and affordable approach to monitoring the nutritional quality of rabbit habitats. Potential applications include the assessment of the mechanistic relationships between resource limitations and population abundance, e.g., in relation to natural drought cycles and to habitat interventions aimed at reinforcing rabbit populations.
Highlights
Diet quality has been identified as a key regulator of the nutritional status, body condition, survival rate, reproductive success, and, population dynamics of mammalian herbivore populations (e.g. [1])
The present study evaluated the suitability of fecal nitrogen (FN) concentrations, and Near-Infrared Spectroscopy (NIRS), to analyze the diet quality of the European rabbit (Oryctolagus cuniculus) in ecological studies
All Linear mixed models (LMM) had a high goodness of fit: marginal pseudo-R2 was R2m = 0.91 for FN and R2m = 0.89 for metabolic fecal nitrogen (MFN), confirming the strong association between ingested and excreted nitrogen concentrations
Summary
Diet quality has been identified as a key regulator of the nutritional status, body condition, survival rate, reproductive success, and, population dynamics of mammalian herbivore populations (e.g. [1]). The first approach is to analyze protein concentrations in vegetation samples directly collected in the field, under the assumption that these samples represent the vegetation available for consumption [8,9,10] This assumption may not hold if the sampling scheme fails to account for the selective feeding behavior of animals. The second approach consists in analyzing stomach contents, ensuring that the analyzed material was ingested by the individuals [11,12,13] This is an invasive technique that requires animals to be dead or killed and is an unfeasible approach in many species because of ethical and conservation concerns. The third approach is to analyze nitrogen content in herbivore feces, which are easy to collect non-invasively under normal field conditions (see [14] for a review)
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