Improved estimates of certainty in stable-isotope-based methods for tracking migratory animals

Ecological Applications 01/2008; 18:549-559. DOI: 10.1890/07-0058.1
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    ABSTRACT: Stable isotopes in metabolically inert tissues of migratory animals can be used to infer migratory and dispersal histories. The general approach for estimating geographic origins of migratory animals based on stable isotope values of their keratinous tissues is to develop or calibrate an assignment model based on tissues of known geographic origin. This paper reviews the general forms and evaluates the application of the 3 assignment approaches. Two of these approaches are considered as nominal assignment frameworks because they require prior declaration of named locations as the set of candidate origins. Individual samples can be sorted into the most likely location using a classification tree or a likelihood-based assignment test. The 3rd and more recent approach is considered a continuous assignment framework because it does not require a predetermined list of candidate locations. This approach depends on an underlying mechanistic geographic model of variation in isotope values. Such models can be developed directly from spatially intensive sampling of keratins or by calibrating a spatial model for isotopes in physical (water or soil) or biological (dietary species) resources. Productive approaches to increase spatial resolution of assignment models will use experiments designed to identify specific geographic-based, variance-generating mechanisms, especially if the contributing factors can be quantified for animals that are released back to the wild. Over the past few decades, patterns of stable isotopes in animal keratins including hair, claw, skin, nail, horn, baleen, and feathers have been increasingly useful for studying animal migration (Ben-David and Flaherty 2012; Hobson and Wassenaar 2008). Landmark studies (Best and Schell 1996; Chamberlain et al. 1997; Hobson and Wassenaar 1997) demonstrated the potential for using stable isotopes to track migration by documenting systematic geographic patterning in stable isotopes in tissues from wild animal populations. Since then, stable carbon (13 C) and nitrogen (15 N) isotopes have been widely used to elicit spatiotemporal structure in patterns of mammalian migration in both marine (Best and Schell 1996; Lee et al. 2005; Witteveen et al. 2009) and terrestrial (Cerling et al. 2006) systems. Stable hydrogen (2 H) isotopes have been used to study patterns of migration and connectivity for a broad range of species of birds (Inger and Bearhop 2008) and bats (Britzke et al. 2009; Cryan et al. 2004, 2012; Fraser et al. 2010), and are increasingly finding utility in human forensics applications (Ehleringer et al. 2008, 2009; Fraser et al. 2006). In general, these isotopes have been used because of relatively predictable isotopic patterns over geographic and other ecologically meaningful gradients. Because animal keratin is metabolically inert once formed, the chemical composition of keratin reflects the environmental conditions under which it was developed, even if the animal subsequently moves to a novel environment. If an animal molts in 1 location and then migrates across a relatively steep gradient in any of these isotope landscapes, the isotopic signature held in keratin becomes a migratory tag that can be used to estimate where that animal was when it grew the keratin. At natural abundance levels, stable carbon, hydrogen, and oxygen (18 O) isotopes in plants and water vary geographically along temperature and humidity gradients (West et al. 2006), and radiostable isotope values of strontium (87 Sr/ 86 Sr) vary with age and type of bedrock (Beard and Johnson 2000). One important assumption in linking isotopes in animal tissue to geographic locations is that these geographic patterns from 1st principles are predictably maintained through food webs such that they remain identifiable in animal keratins sampled from across the same geographic gradients. And indeed, the process
    Journal of Mammalogy 01/2012; 93:360-367. · 2.31 Impact Factor
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    ABSTRACT: Many populations of the American Kestrel (Falco sparverius) are in decline; in northern populations it is unknown whether low recruitment and rates of return are due to mortality, poor detection, or dispersal. Ratios of stable hydrogen isotopes in feathers (δDf) have been used widely to estimate origins of birds, in some cases providing estimates of adults' dispersal between breeding seasons and post-natal dispersal of young. We attempted to use δD in feathers, grown during breeding, to identify returning and immigrant birds and to quantify dispersal. We used birds of known origin to establish the expected local δDf but found a high degree of deuterium enrichment in these individuals relative to the local δD of rainfall (δDp) and a significant difference between δDf of nestlings and adults, which complicated the distinguishing of local birds from immigrants. We subsequently compared the δD of primary feathers and plasma of adults and tested for relationships among the δD of adult feathers and adult mass, body size, and reproductive effort at the time of growth to explore the cause of deuterium enrichment in adult feathers. Adults' feathers were significantly more deuterium-enriched than plasma, and their distribution did not overlap with that of nestlings' feathers. Larger males and females that fledged female nestlings of greater mass had feathers that were more deuterium-enriched, while males whose mates laid clutches of greater volume had less enriched feathers. We discuss our results with respect to the prevailing hypotheses for deuterium enrichment of raptor feathers, particularly the evaporative-cooling hypothesis. Las poblaciones del halcón Falco sparverius están disminuyendo en muchas partes y en aquellas del norte no se sabe si el bajo reclutamiento y las tasas bajas de retorno se deben a mortalidad, a deficiente detección o a dispersión. Las proporciones de isótopos de hidrógeno estables en las plumas (δDf) se han utilizado ampliamente para estimar el origen de las aves, en algunos casos proporcionando estimaciones de dispersión de adultos y jóvenes. Intentamos utilizar el δD que se encuentra en las plumas que crecieron durante la época de cría para identificar a las aves que regresaron y a las que inmigraron, así como para cuantificar la dispersión. Utilizamos aves de origen conocido para establecer la cantidad de δDf local esperada. Sin embargo, encontramos un alto grado de enriquecimiento de deuterio en estos individuos con relación al δD local proveniente de las precipitaciones (δDp) y una diferencia significativa entre el δDf de los pichones y de los adultos, lo que complicó la distinción de las aves locales de las inmigrantes. Posteriormente, comparamos el δD de las plumas primarias y del plasma de adultos y pusimos a prueba la existencia de relaciones entre el δD de las plumas de los adultos y su peso, tamaño corporal y esfuerzo reproductivo en el momento del crecimiento de las plumas para explorar la causa de enriquecimiento de deuterio en las plumas de un adulto. En las plumas de adultos, el δD fue significativamente mayor que en el plasma y su distribución no se superpuso con la de las plumas de los pichones. Las plumas de los machos y de las hembras de mayor tamaño, que produjeron polluelos volantones hembra de mayor peso, estuvieron enriquecidas con deuterio, mientras que los machos cuyas parejas tuvieron puestas de mayor volumen tuvieron plumas menos enriquecidas. Discutimos nuestros resultados en el contexto de las hipótesis vigentes para el enriquecimiento con deuterio de plumas de aves rapaces, con especial énfasis en la hipótesis de evaporación y enfriamiento.
    The Condor 01/2011; 113(3):555-564. · 1.37 Impact Factor
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    ABSTRACT: 1. Stable isotope ratios of H and O (δ2H and δ18O) are intrinsic properties of biological and geological materials, and can be used to constrain the geographic origin and movements of such materials. One of the most widespread uses of such data in ecology is to reconstruct geographic movements of animals by comparing isotope ratios of chemically inert tissues to predictive models (“isoscapes”) of spatial isotopic variation in environmental water.2. Although data analysis for isotope-based geographic assignment is the subject of ongoing research a basic framework for this work has emerged.3. Here we introduce and document a set of data analysis tools, implemented within the IsoMAP (Isoscapes Modeling, Analysis, and Prediction; cyber-GIS system, that support basic analysis of sample data for geographic assignment applications. We highlight important considerations and potential pitfalls associated with use or misuse of the tools.4. In addition to increasing the accessibility of geographic assignment analysis, IsoMAP provides several unique capabilities related to the generation of space- and time-specific water isoscapes that may advance the field of isotope-based assignment. Functionality of the existing toolkit is limited in scope, and although the system will be actively developed in the future it is intended to complement and not supplant more flexible and customizable analytical tools.This article is protected by copyright. All rights reserved.
    Methods in Ecology and Evolution 03/2014; · 5.92 Impact Factor


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