Abstract
In the face of global coral decline, coral reefs in the Gulf of Aqaba (GOA), northern Red Sea are considered a coral reef refuge from global warming and acidification, with echinoid herbivory playing a fundamental role in maintaining their balance. Like many echinoderms, echinoids are known for their 'boom-and-bust' population fluctuations. However, as population fluctuations are influenced by multiple, complex and non-linear processes that operate at various temporal scales, short-term studies may fail to capture the true trajectories of population trends. We explored echinoid population dynamics in the GOA, spanning 15 years (2007-2022), complemented by historical data dating back to the 90s and 70s. We show that while some species oscillated around a steady mean, others collapsed by up to 98%. Consequently, the once most common herbivores on these reefs currently account for only a fraction of their population size from half a century ago. Increased anthropogenic stress attributed to the accelerated regional urbanization, rather than the direct effect of a single environmental variable, is suggested as a key facilitating driver of the observed declines. This study reveals ongoing, alarming declines of key echinoid species, calling for rapid, species-aware management. We highlight the necessity to transition from year-long to decades-long monitoring to facilitate the detection of ongoing, long-term trends.