Apparent resource partitioning and trophic structure of large-bodied marine predators in a relatively pristine seagrass ecosystem

Large predators often play important roles in structuring marine communities. To
understand the role that these predators play in ecosystems, it is crucial to have knowledge of
their interactions and the degree to which their trophic roles are complementary or redundant
among species. We used stable isotope analysis to examine the isotopic niche overlap of dolphins
Tursiops cf. aduncus, large sharks (>1.5 m total length), and smaller elasmobranchs (sharks and
batoids) in the relatively pristine seagrass community of Shark Bay, Australia. Dolphins and large
sharks differed in their mean isotopic values for δ13C and δ15N, and each group occupied a relatively
unique area in isotopic niche space. The standard ellipse areas (SEAc; based on bivariate
standard deviations) of dolphins, large sharks, small sharks, and rays did not overlap. Tiger sharks
Galeocerdo cuvier had the highest δ15N values, although the mean δ13C and δ15N values of pigeye
sharks Carcharhinus amboinensis were similar. Other large sharks (e.g. sicklefin lemon sharks
Negaprion acutidens and sandbar sharks Carcharhinus plumbeus) and dolphins appeared to feed
at slightly lower trophic levels than tiger sharks. In this seagrass-dominated ecosystem, seagrassderived
carbon appears to be more important for elasmobranchs than it is for dolphins. Habitat use
patterns did not correlate well with the sources of productivity supporting diets, suggesting that
habitat use patterns may not necessarily be reflective of the resource pools supporting a population
and highlights the importance of detailed datasets on trophic interactions for elucidating the
ecological roles of predators.