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.
KeywordsGreen turtle Movement ecology Niche partitioning Seagrass Stable isotope analysis
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.
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.
Compliance with ethical standards
Conflicts of interest
The authors declare that they have no conflict of interest.
Supplementary material 2 (MP4 57546 kb)
- Ahasan MS, Waltzek TB, Huerlimann R, Ariel E (2018) Comparative analysis of gut bacterial communities of green turtles (Chelonia mydas) pre-hospitalization and post-rehabilitation by high-throughput sequencing of bacterial 16S rRNA gene. Microbiol Res 207:91–99. https://doi.org/10.1016/j.micres.2017.11.010 CrossRefPubMedGoogle Scholar
- Arias-Ortiz A, Serrano O, Masqué P, Lavery PS, Mueller U, Kendrick GA, Rozaimi M, Esteban A, Fourqurean JW, Marbà N, Mateo MA, Murray K, Rule MJ, Duarte CM (2018) A marine heatwave drives massive losses from the world’s largest seagrass carbon stocks. Nat Clim Chang 8:338–344. https://doi.org/10.1038/s41558-018-0096-y CrossRefGoogle Scholar
- Hückstädt LA, Koch PL, McDonald BI, Goebel ME, Crocker DE, Costa DP (2012) Stable isotope analyses reveal individual variability in the trophic ecology of a top marine predator, the southern elephant seal. Oecologia 169:395–406. http://www.jstor.org/stable/41500040
- Layman CA, Arrington DA, Montaña CG, Post DM (2007) Can stable isotope ratios provide for community-wide measures of trophic structure? Ecology 88:42–48. https://doi.org/10.1890/0012-9658(2007)88[42:CSIRPF]2.0.CO;2 CrossRefPubMedGoogle Scholar
- Limpus CJ, Couper PJ, Read MA (1994) The green turtle, Chelonia mydas, in Queensland: population structure in a warm temperate feeding area. Mem Qld Mus 35:139–154Google Scholar
- Manly BFJ, McDonald LL, Thomas DL, McDonald TL, Erickson WP (2002) Resource selection by animals: statistical design and analysis for field studies, 2nd edn. Kluwer, BostonGoogle Scholar
- Peterson BJ, Fry B (1987) Stable isotopes in ecosystem studies. Annu Rev Ecol Evol Syst 18:293–320. https://doi.org/10.1146/annurev.es.18.110187.001453 CrossRefGoogle Scholar
- Price JT, Paladino FV, Lamont MM, Witherington BE, Bates ST, Soule T (2017) Characterization of the juvenile green turtle (Chelonia mydas) microbiome throughout an ontogenetic shift from pelagic to neritic habitats. PLoS ONE 12:e0177642. https://doi.org/10.1371/jounral.pone.0177642 CrossRefPubMedPubMedCentralGoogle Scholar
- Svanbäck R, Bolnick DI (2005) Intraspecific competition affects the strength of individual specialization: an optimal diet theory method. Evol Ecol Res 7:993–1012Google Scholar
- R Core Team (2016) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. https://www.R-project.org/
- Thomson JA, Burkholder DA, Heithaus MR, Fourqurean JW, Fraser MW, Statton J, Kendrick GA (2015a) Extreme temperatures, foundation species, and abrupt ecosystem change: an example from an iconic seagrass ecosystem. Glob Chang Biol 21:1463–1474. https://doi.org/10.1111/gcb.12694 CrossRefPubMedGoogle Scholar
- Turner Tomaszewicz C, Seminoff J, Avens L, Goshe L, Rguez-Baron J, Peckham S, Kurle C (2018) Expanding the coastal forager paradigm: long-term pelagic habitat use by green turtles (Chelonia mydas) in the eastern Pacific Ocean. Mar Ecol Prog Ser 587:217–234. https://doi.org/10.3354/meps12372 CrossRefGoogle Scholar