Species associations and community structure in an oceanic elasmobranch assemblage

Elasmobranch assemblages support marine ecosystem health by regulating populations, transferring nutrients across habitats, and sustaining high-level consumers. Shifts in elasmobranch assemblages can reshape ecosystem dynamics through direct and indirect species associations. Here, we analyzed a 26 yr underwater visual survey dataset from Cocos Island National Park in the Eastern Tropical Pacific to examine community-level temporal shifts in the island’s elasmobranch assemblage. We studied 13 species, analyzing changes in network strength, diversity indices, and species associations. We show a shift from a sparse, low-richness network in the early 1990s to a more densely connected assemblage by 2014. The elasmobranch assemblage showed 2 seasonal sub-communities: migratory oceanic species in the wet season and resident coastal species in the dry season. Scalloped hammerhead Sphyrna lewini and whitetip reef sharks Triaenodon obesus were the main contributors to year-to-year diversity changes. Significant association model outputs showed mostly negative associations, suggesting that predator-prey dynamics and intraguild competition are taking place in Cocos Island. We inferred 3 types of association patterns: (1) competition, when negative associations were possibly caused by competitive interactions rather than predator-prey dynamics; (2) predation, inferred when prey abundances were negatively associated with predator covariates; and (3) environmental preference, where shared or opposing thermal or seasonal habitat patterns explained positive or negative associations, respectively. Our findings advance understanding of the evolving Cocos Island elasmobranch community structure and network dynamics. Our study also provides a framework for examining the mechanisms driving interspecific associations in similar marine ecosystems.