Individual niche trajectories in nesting green turtles on Rocas Atoll, Brazil: an isotopic tool to assess diet shifts over time

Abstract

In this study, multi-tissue (yolk and carapace) stable isotope analysis was used to assess individual isotopic niche trajectories of nesting green turtles on Rocas Atoll, off northeastern Brazil, and to reveal a diet shift in the temporal dimension. The diet trajectories of individual green turtles were highly directional, with a stronger component towards decreasing values of δ15N from carapace to yolk. When the green turtles are in their foraging sites (temporal window measured by the yolk samples), they are more herbivores. Conversely, in a broader temporal window, the green turtles demonstrate a carnivore-omnivore strategy, such as represented by heavier δ15N values in the carapace. This finding confirms a temporal diet shift. This is the first study that applies trophic niche trajectories for sea turtles, adding a new isotopic tool to understand the trophic ecology of these migrant animals.

Stable Isotope Trajectory Analysis (SITA): A new approach to quantify and visualize dynamics in stable isotope studies

“…The recent CTA extension (Sturbois et al, 2021) showed the importance of plotting trajectory metrics in specifically designed figure concepts, adapted for the representation of dynamics. Building on this, and on previous attempts in SI ecology (Agostinho et al, 2021;Cucherousset et al, 2013;Schmidt et al, 2007), we proposed here two new figure concepts specifically devoted to the representation of dynamics at large spatiotemporal scales: the ITM and the TH (Figures 10 and 11). Here, we have chosen the figure concepts that we deemed the most suitable for each ecological application.…”

“…Temporal dynamics in stable isotope ecology are sometimes represented in 1D or 2D δ space by segment or arrows between consecutive surveys (Agostinho et al, 2021; Guzzo et al, 2011). Here, we propose a trajectory diagram (TD) concept to customize ordination diagrams by adding notably SITA metrics.…”

“…By analogy to community ecology, the term and the concept of trajectory have been informally used in stable isotope ecology to characterize dynamics and represent them in spaces of analysis (either δ space, p‐space sensu, Newsome et al, 2007; or spaces based on community‐wide indices, e.g., Rigolet et al, 2015). The analysis and representation of stable isotope trajectories or contrasted patterns requires the use of quantitative geometric properties in 2D δ spaces often complemented by vectors in stable isotope scatter plots and/or circular representation (Agostinho et al, 2021; Black & Armbruster, 2021; Cucherousset et al, 2013; Schmidt et al, 2007). However, scientists are increasingly faced with multivariate datasets (i.e., >2 dimensions) in stable isotope ecology in response to the potential use of (1) other isotopes to complement δ 13 C and δ 15 N (e.g., 34 S ([Connolly et al, 2004], δD [Doucett et al, 2007]) and (2) numerous structural and functional community‐wide metrics or indices (Cucherousset & Villéger, 2015; Layman et al, 2007).…”