Concomitant effects of algae invasion and sea urchin mass mortality drive the shift from barrens to turf grounds

Abstract

Ecosystem regime shifts have received significant attention both from the perspective of the processes governing them and the consequences they entail. Most of the time, the focus is placed on habitat degradation with major transitions happening from healthy habitat states to depauperate conditions. In marine systems, well-known examples include coral reef collapse into macroalgal-dominated habitats and the conversion of kelp forests into sea urchin barrens. In Madeira Island (Macaronesia. NE Atlantic Ocean), the current invasion of the brown macroalgae Rugulopteryx okamurae poses a major threat to native coastal communities with major socioeconomic implications. This species was first detected in the archipelago of Madeira in December 2021 and has since been expanding along the coast of Madeira Island. Later in 2022, a mass mortality event (MME) of the main grazer species in the region, the sea urchin Diadema africanum, was detected. Here, we investigate the direct effects of the synchrony of these two events in time on a major regime shift in the coastal rocky systems along the southern coast of Madeira Island. Benthic surveys were conducted at four sites and two depths (5 m and 15 m) during two years (2021 and 2023), covering both spring and autumn. Results revealed major ecological changes: D. africanum densities dropped to zero, and previously barren areas transitioned into turf- and erect-algae-dominated habitats. including widespread proliferation of R. okamurae. At two sites, R. okamurae became the dominant species across both depths. Species richness and Shannon diversity increased at 15 m in 2023, while patterns at 5 m varied among sites. Our findings highlight the importance of temporal synchrony between ecological disturbances—in this case, invasion and grazer collapse—in driving regime shifts. While the long-term stability of this new state remains uncertain, it adds to growing evidence that algal turfs can act as alternative stable states to both kelp forests and barrens. Given the expected continued spread of R. okamurae in Madeira and across the NE Atlantic and Mediterranean, understanding the mechanisms behind its establishment and ecological impact is essential. Although this shift increased local biodiversitym, it does not signify the recovery of the original marine forests that historically characterized Madeira's reefs.