Climate-driven trans-Tasman population increase of the habitat-modifying, range extending sea urchin Centrostephanus rodgersii.

Global climate change is shifting thermal gradients in the world's oceans, resulting in the redistribution of species and thermophilisation of reefs. In the Southwest Pacific, warming has underpinned the range extension and population increase of the habitat-modifying sea urchin, Centrostephanus rodgersii. Eastern Tasmania and Northeastern Aotearoa New Zealand (NZ) lie at the forefront of these changes, with increases in C. rodgersii driving declines in kelp in these regions. However, the extent of C. rodgersii increases in both regions remains unclear, although given well-established thermal limits of C. rodgersii there appears greater potential for increases in the warmer waters of NZ than in the cooler waters of Tasmania. Here we leverage a combination of broad spatial scale data and region-specific depth-stratified data to examine regional C. rodgersii abundance changes in recent decades and their relation to minimum sea surface temperature (SST). The abundance of C. rodgersii increased 1.7-fold between 2001/02 and 2016/17 for Tasmania and 3.3-fold between 2012 and 2024 for NZ. Larger C. rodgersii abundance changes in NZ align with predictions based on their modelled abundance against SST. Moreover, modelled estimates suggest C. rodgersii abundance in NZ have the potential to increase further into the future (by ∼2.7-fold at 8.1 m). While the increase in Tasmania has been well documented, these findings demonstrate a greater increase in population density in Northeastern NZ and potential for further increases. Ultimately, increase in C. rodgersii abundance in both regions call for the establishment, or ramping up, of management programs to curb population increases.