Contrasting ecological roles and flexible trophic interactions of two estuarine apex predators in the western Gulf of Mexico

Abstract

Top predators play key roles within their respective ecosystems. However, their ability to adjust their behaviors in response to changes in environmental conditions and food web structure, which is important in light of climate change and continued anthropogenic disturbance, is still unclear in many ecosystems. We combined stomach content and stable isotope analyses to study the trophic ecology and plasticity of the dominant top predators—the bull shark Carcharhinus leucas and the alligator gar Atractosteus spatula—in 2 estuaries of the northwestern Gulf of Mexico. Both species showed a high degree of prey preference and predominantly fed on mugilids. Juvenile bull sharks were also demersal foragers with a high relative trophic position (TP ≈ 4), feeding heavily on sea catfishes (Bagre marinus and Ariopsis felis) and red drum Sciaenops ocellatus. Comparatively, alligator gars were ambush foragers with TP ≈ 3 that targeted schools of gulf menhaden Brevoortia patronus. Sharks and gars exhibited overlap in their trophic interactions, indicating potential competition for prey, but our results highlight mechanisms that mitigate competition, including spatial partitioning and intraspecific variability. The diets of both species also appeared highly flexible, with individuals able to adjust to spatiotemporal shifts in prey availability. These findings suggest the bull shark and the alligator gar may be somewhat resilient to the ecological change associated with climate change and anthropogenic disturbances and highlight the utility of combining different approaches to refine our understanding of the ecological roles of top predators.