Desert Tortoise (Gopherus agassizii) Dietary Specialization Decreases across a Precipitation Gradient

Ian W Murray,Blair O Wolf,Brock Fenton

doi:10.1371/journal.pone.0066505

Ian W Murray, Blair O Wolf + Show 1 more

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https://doi.org/10.1371/journal.pone.0066505

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Abstract

We studied the plant resource use between and within populations of desert tortoise (Gopherus agassizii) across a precipitation gradient in the Sonoran Desert of Arizona. The carbon and nitrogen stable isotope values in animal tissues are a reflection of the carbon and nitrogen isotope values in diet, and consequently represent a powerful tool to study animal feeding ecology. We measured the δ13C and δ15N values in the growth rings on the shells of tortoises in different populations to characterize dietary specialization and track tortoise use of isotopically distinct C4/CAM versus C3 plant resources. Plants using C3 photosynthesis are generally more nutritious than C4 plants and these trait differences can have important growth and fitness consequences for consumers. We found that dietary specialization decreases in successively drier and less vegetated sites, and that broader population niche widths are accompanied by an increase in the dietary variability between individuals. Our results highlight how individual consumer plant resource use is bounded under a varying regime of precipitation and plant productivity, lending insight into how intra-individual dietary specialization varies over a spatial scale of environmental variability.

Highlights

IntroductionA dietary index near one indicates that tortoises are foraging on a mix of C3 and C4 or CAM plants with keratin values that encompass a more extensive portion of the available (for a given site) spectra of carbon and nitrogen isotope ratios
It has become increasingly clear that significant levels of variation in resource use may occur between individual consumers within a population [3,4,5]
We examine the dietary niche of desert tortoise individuals and populations from several perspectives: 1) we discuss how total trophic niche breadth and diet variability differ among individuals and populations across a precipitation gradient, 2) we examine how resource use varies among males and females, 3) we show how life history stage affects trophic niche breadth and diet variability in different resource environments, 4) we compare insights from our research to those of the conventional dietary literature on desert tortoises

Summary

Introduction

A dietary index near one indicates that tortoises are foraging on a mix of C3 and C4 or CAM plants with keratin values that encompass a more extensive portion of the available (for a given site) spectra of carbon and nitrogen isotope ratios It follows that individuals in a generalist population can be using either the full extent of available plant resources (i.e., generalists within a generalist population; TAi is broad relative to TAp, and there is high overlap between individuals), or using a subset of the available resources with individuals broadly differing in their specialist diet (i.e., specialists within a generalist population; TAi is narrow relative to TAp, and there is low overlap among individuals; [35,36]). We estimate the proportion of the total lifetime diet that is harvested from specific ecosystem compartments (C3 versus C4/CAM plants) across the populations we sample

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