Effects of sea urchin and herbivorous gastropod removal, coupled with transplantation, on seaweed forest restoration

This study aims to investigate the relative effects of urchin removal, non-urchin gastropod herbivores removal, and transplantation on macroalgal forest restora-tion using an additive manipulation design. A field experi-ment was conducted in subtidal urchin barren rocks in the eastern coast of South Korea from August 2017 to June 2020 with three experimental treatments: no urchins (NU), NU plus no herbivorous gastropods (NH), NH plus Ecklonia bicyclis transplant (NHT). Six months after experiment initiation, seaweed abundance rapidly increased in all three experimental treatments. The highest peak was found in the spring season of the first year (2018). The year-to-year vari-ations became smaller throughout the survey period. The results of comparing NU and control site indicated that ur-chin removal had an exclusive effect on algal recovery while additional removal of herbivorous gastropods did not affect algal recovery quantitatively. With the successful estab-lishment of E. bicyclis, the community assemblages of the three treatments became more distinct in the subsequent years, showing diverse dominance in NU, red algal domi-nance in NH, and dominance of E. bicyclis canopy and un-derstory groups in NHT. This study provides evidence that urchin removal on its own from barren grounds can lead to rapid restoration of seaweed forest in subtidal habitats.