|Brittany S. Graham, Paul L. Koch, Seth D. Newsome, Kelton W. McMahon, David Aurioles, Using isoscapes to trace the movements and foraging behavior of top predators in oceanic ecosystems., Isoscapes: Understanding movement, pattern and processes on Earth through isotope mapping, 2009|
The inability to directly observe migratory predators and the vast extent of their pelagic habitat has hindered our understanding of their movements, distribution, and foraging behavior. The stable isotope composition of animal tissues can provide intrinsic tags to study the foraging and migratory ecology of elusive or highly migratory species, such as top marine predators (Hobson 1999; Hobson et al., this volume). This method can be applied to track the movements of juvenile stages of marine vertebrates that are not amenable to current electronic tagging technologies. In addition, stable isotope analysis can provide retrospective information on the movement patterns and foraging ecology in both modern and extant marine predators (Burton et al. 2001). While foraging in specific environments, individuals acquire the isotopic value of their local prey. By comparing the isotope value of the animal to its local prey or the local environmental isotope value (i.e., local primary producer isotopic composition), information can be gained on residency and movements patterns. In other words, an animal’s isotopic composition can be used as a natural “tag” to track their movements through isotopically-distinct habitats. A critical requirement is to construct a map of the geographical distribution of isotope values in the environment on a temporal and spatial scale that is ecologically relevant to movements of the animal of interest. This is especially challenging for isotopic studies examining the movements of marine predators because of their potentially vast foraging ranges. One approach has been to generate isotope maps (i.e., isoscapes) based either on sources that integrate marine production at the base of the food web (e.g., annually integrated phytoplankton, zooplankton, sediment trap, or core top data), or use control taxa at the same trophic level, but with known migratory and habitat preferences. Both approaches require intensive sampling to establish spatial and temporal isotopic patterns, and both entail assumptions about food web structure, animal physiology, and animal behavior. Given these assumptions, however, if an animal has a similar isotope value as the local isotopic baseline, then the animal is a resident, whereas if the animal and baseline isotope values are distinctly different, the animal is an immigrant from another, isotopically distinct region.