DNA metabarcoding reveals that Galapagos pinnipeds might not be competing as thought

Interspecific competition is a key process shaping species distributions and biodiversity. In the western bioregion of the Galapagos archipelago, two endemic and endangered pinniped species coexist: the Galapagos sea lion (GSL) and the Galapagos fur seal (GFS). Previous studies reported marked dietary differences between these species and proposed niche partitioning as the mechanism allowing their coexistence. However, these works neither precisely identified prey at the species level, nor compared diets under conditions of coexistence versus non-coexistence, leaving trophic partitioning by interspecific competition largely untested. Here, we used DNA metabarcoding to analyze GSL and GFS diets across five settings encompassing coexistence and non-coexistence, and multiple Galapagos bioregions. Although we confirmed dietary differences between GSL ( N = 22 to 50 depending on setting) and GFS ( N = 12), our results provide little evidence that these differences arise from interspecific competition. Contrary to expectations under a competition-driven setting, GSL occurring alone did not exploit prey typically consumed by GFS when these coexisted. Instead, GSL diets remained diverse and similar across settings, regardless of GFS presence. Thus, rather than competition, spatial variation in the archipelago’s oceanographic conditions better explain the dietary patterns of both endemic pinnipeds.