Contrasting patterns in kelp consumption across latitude by two barren forming sea urchin species

Urchin herbivory is a key function in temperate reef ecosystems. Some urchin species overgraze macroalgal forests, leading to their collapse into barren states. In Australia, climate change is enabling the poleward range extension of urchin species, resulting in increased barrens formation at the cool-edge of their distribution. Despite their ecological importance and association with warming, broad-scale effects of temperature on sea-urchin feeding ecology remain unknown. We characterise in-situ feeding rates of two barrens-forming urchin species, one range-extender (Centrostephanus rodgersii), the other range-persistent (Heliocidaris erythrogramma), across a temperature range of 8 °C and 12 degrees of latitude, as well as over seasonal cycles. We assess the extent to which ecological drivers (temperature, macroalgal nutrition, urchin size/weight metrics) explain grazing patterns. We find contrasting patterns in, and drivers of, performance between urchin species. C. rodgersii shows a peak in grazing and abundance at its range-centre, and temperature is shown to be an important driver of grazing rates for this species. For H. erythrogramma, gonad index and macroalgal nutrition are key drivers of grazing rates, which display no significant change across latitude. These contrasting patterns suggest each species occupies different thermal niches, providing key insights into how their ecological impacts may change across their distribution and in response to ocean warming.