When Sea Urchins Dine: The Roles of Friends, Food and Fear and Implications to Kelp Forest Conservation

Kelp forests are vital "blue carbon" sinks and biodiversity hotspots, yet increasingly threatened by sea urchin overgrazing, which can convert them into barren seascapes. Understanding the behavioral drivers of urchin feeding—such as group dynamics, food availability, and predation risk—is therefore critical for effective conservation and ecosystem-based management.This study investigates the behavioral drivers behind overgrazing by the sea urchin Strongylocentrotus intermedius through controlled laboratory experiments, focusing on the roles of group size, diet composition, and predator cues. Results indicate that solitary urchins exhibit lower overall consumption and a preference for drift kelp, whereas grouped urchins show significantly higher feeding rates and shift toward live kelp, intensifying grazing pressure on kelp forests. Diet composition further modulates foraging behavior, with drift kelp abundance sustaining urchin populations even when live kelp is limited. Predator cues suppress total feeding but do not alter the preference for live kelp. The interplay between food availability and predation risk highlights that grazing dynamics are shaped by both bottom-up and top-down factors. The findings underscore the need for integrated conservation strategies that manage urchin densities, prevent aggregation, maintain kelp bed integrity, and consider predator roles to mitigate kelp forest degradation.