Half a century of echinoid population decline in the northern Gulf of Aqaba, Red Sea.

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.