Isotopic Tracking of Migrant Wildlife

Abstract

KEITH A. HOBSONIntroductionAnimal movement, including dispersal and migration, is a phenomenon that captures the imagination of scientist and layperson alike. In the Northern Hemisphere, we are perhaps most familiar with the annual movements of birds and much research has been devoted to understanding proximate and ultimate mechanisms that trigger these impressive patterns in nature (Ber-thold et al. 2003). However, tracking animal movements, including those of insects, fi sh, and mammals, is also an endeavor important to both theoretical and conservation disciplines. Unfortunately, we have been severely restricted in our ability to follow vagile organisms using conventional techniques that rely on extrinsic markers. In addition, many organisms of interest are simply too small to hold satellite or radio transmitters for direct monitoring (re-viewed by Hobson 2003). All studies that rely on the initial capture and ul-timate recapture of an individual also are typically subject to sampling bias, especially for those species with widespread distributions, the so-called “nee-dle in a haystack” problem. In contrast, animal tracking methods that rely on intrinsic markers are dependent only on the “recovery” sampling sites. Hobson (2005a) referred to this advantage as “every capture is a recapture”. The measurement of naturally occurring stable isotopes in animal tissues represents one means of assaying intrinsic markers in animals in order to infer information on their origins. Few areas involving the application of stable isotope methods to ecological studies have seen such an explo-sive growth as the fi eld of animal tracking, a consequence of the funda-mental limitations of conventional methods involving extrinsic markers (Hobson 2005b).Rubenstein & Hobson (2004) provided a review of the relative advantages of using intrinsic vs. extrinsic markers to track animal movements. Intrinsic biogeochemical markers include trace elements (Szep et al. 2003) as well as stable isotopes and each have advantages and disadvantages. As with trace elements, the use of stable isotopes is based on the fact that concentrations in animal tissues refl ect those in their foodwebs and that spatial patterns, gradients, or changes in such markers exist in nature. Thus by making iso-topic measurements in animal tissues and knowing how stable isotopic pat-terns in foodwebs change spatially, it is often possible to infer their origins. By combining our knowledge of the behavior of stable isotopes in animal