Evaluating different spatial scales of forage item availability to determine diet selection of juvenile green turtles (Chelonia mydas)

Diet selection by a species is determined by comparing the consumption (i.e., use) and abundance (i.e., availability) of prey within their area of occupancy. Because individuals commonly use only a portion of habitat available to them (e.g., a 10-ha home range within a 1000-ha foraging habitat), it is important to quantify forage availability within individuals’ home ranges and core areas, and include these availabilities when calculating diet selection. However, studies of diet selection often consider prey availability across the entire foraging habitat of a species and not within individual home ranges/core areas. Here, we explore how spatial variability in prey availability may influence the results of diet selection for juvenile green turtles, Chelonia mydas , foraging in Bimini, Bahamas. Stable isotope analysis was used to determine prey use and satellite telemetry to infer movements and forage availability for each turtle. Forage availability was assessed at three spatial scales: (1) the full extent of the foraging area (2) across each respective individual’s 95% utilization distribution (UD), or home ranges, and (3) across each individual’s 50% UD, or core areas. Further, we compared potential differences in diet selection by using three selection indices (Ivlev’s, Johnson’s and Chesson’s). Diet selection results varied among individuals and were influenced by the spatial scale of forage items available and the index used. Diet selection variability was observed at various spatial scales and in all indices. Our results highlight the need for careful consideration of the diet selection index and the spatial scale at which prey/forage availability is considered when determining a species’ diet selection. Selecting a more sensitive index will help identify priority resources and/or habitats that are important to species, which in turn carries conservation and management implications.