Individual specialization in a migratory grazer reflects long-term diet selectivity on a foraging ground: implications for isotope-based tracking


Stable isotope analysis (SIA) can be a useful tool for tracking the long-distance movements of migratory taxa. However, local-scale sources of isotopic variation, such as differences in habitat use or foraging patterns, may complicate these efforts. Few studies have evaluated the implications of local-scale foraging specializations for broad-scale isotope-based tracking. Here, we use > 300 h of animal-borne video footage from green turtles (Chelonia mydas) paired with SIA of multiple tissues, as well as fine-scale Fastloc-GPS satellite tracking, to show that dietary specialization at a single foraging location (Shark Bay, Western Australia) drives a high level of among-individual δ13C variability (δ13C range = 13.2‰). Green turtles in Shark Bay were highly omnivorous and fed selectively, with individuals specializing on different mixtures of seagrasses, macroalgae and invertebrates. Furthermore, green turtle skin δ13C and δ15N dispersion within this feeding area (total isotopic niche area = 41.6) was comparable to that from a well-studied rookery at Tortuguero, Costa Rica, where isotopic dispersion (total isotopic niche area = 44.9) is known to result from large-scale (> 1500 km) differences in foraging site selection. Thus, we provide an important reminder that two different behavioral dynamics, operating at very different spatial scales, can produce similar levels of isotopic variability. We urge an added degree of caution when interpreting isotope data for migratory species with complex foraging strategies. For green turtles specifically, a greater appreciation of trophic complexity is needed to better understand functional roles, resilience to natural and anthropogenic disturbances, and to improve management strategies.

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JT was supported by National Science Foundation Grant #OCE0746164 to MRH. Fieldwork was conducted under Western Australia Department of Parks and Wildlife permit #SF007813 and subsequent renewals. Tissue samples were imported into the US under CITES permit #12US75172A/9. K. Gastrich, R. Nowicki and numerous volunteer assistants helped with camera deployments. D. Holley, S. Locke, G. Griffin and K. Cross (Department of Biodiversity, Conservation and Attractions) assisted with satellite tag deployments. We thank J. Huisman for his help with algal identifications. This is contribution #85 from the Shark Bay Ecosystem Research Project and contribution #99 from the Center for Coastal Oceans Research in the Institute of Water and Environment at Florida International University.

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JT conceived and designed the study with input from MH. JT collected video and isotope data in the field. EW processed and analyzed scute samples. GH and JT deployed satellite tags. MGR and JT extracted macrophyte availability data. AB and ME identified macroalgae and benthic invertebrates, respectively. JT analyzed the data and led the writing with contributions from all authors.

Correspondence to Jordan A. Thomson.

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Communicated by Helene Marsh.

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Thomson, J.A., Whitman, E.R., Garcia-Rojas, M.I. et al. Individual specialization in a migratory grazer reflects long-term diet selectivity on a foraging ground: implications for isotope-based tracking. Oecologia 188, 429–439 (2018).

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  • Green turtle
  • Movement ecology
  • Niche partitioning
  • Seagrass
  • Stable isotope analysis