Inter-specific relationships and their ecological role in an oceanic elasmobranch community

Marine ecosystems support a diverse array of co-occurring species, whose presence and abundance influence the behavior, population dynamics, and distribution of interacting organisms. Elasmobranchs play a central role in marine ecological processes as top and meso-predators across various ecosystems. Previous work has shown that some elasmobranchs are key to ecosystem health and resilience, regulating communities through competitive, predatory, and territorial interactions. However, few studies have evaluated the effect of interspecific interactions on the abundance and diversity of individual populations in marine ecosystems, partly due to the difficulty of obtaining long-term data on species co-occurrence. Our objective was to analyze elasmobranch species co-occurrence in Cocos Island National Park in the Eastern Tropical Pacific and its effect in species abundance, diversity, and interactions within the community. We analyzed a 26-year underwater visual survey dataset through network statistics, diversity indices, and Bayesian species interaction models. Here we show that the elasmobranch community in early 1990s shifted from a sparse, low richness network to a more diverse, densely connected community in the late 2010s, with diversity peaking in 2006. We identified three types of species interactions within the elasmobranch community: competition, predation, and ecosystem preference. The Scalloped hammerhead (Sphyrna lewini) and Whitetip reef (Triaenodon obesus) sharks were the main contributors to year-to-year variations in diversity. In addition, most species had a negative relationship with the co-occurrence of other elasmobranchs, underscoring the importance of potential competitive and predatory interactions. Our findings offer valuable insights for marine conservation efforts, particularly for understanding how interspecific interactions and ecological processes influence ecosystem stability over time. Our study provides a framework for future research on species co-occurrences in marine ecosystems, emphasizing the need for integrated, interaction-focused models that can better predict ecosystem responses to threats like overfishing and environmental change.