Before and after a disease outbreak: Tracking a keystone species recovery from a mass mortality event

Disease Outbreak in a Keystone Grazer Population Brings Hope to the Recovery of Macroalgal Forests in a Barren Dominated Island

“…These phase-shifts often occur with variation in sea urchin grazing intensity (Melis et al, 2019) as their feeding activity can play a major role in the stability, biodiversity, production and functioning of these ecosystems (Duffy and Hay, 1990;Poore et al, 2009;Korpinen et al, 2010;Kraufvelin, 2017). Sea urchins can thrive at very high population densities (Lawrence, 1975;Alves et al, 2001;Gizzi et al, 2020), possess limited mobility and a powerful excavating mouth providing them the ability to regulate the distribution, abundance, and diversity of benthic marine algae communities, influencing the establishment, spread and persistence of algal species (e.g., Steneck, 2013;Filbee-Dexter and Scheibling, 2014;Friedlander et al, 2017;Melis et al, 2019). Numerous studies have unequivocally demonstrated an inverse relationship between urchin density and algal biomass as well as their role in promoting a shift of habitats with complex macroalgae forests into urchin barrens (e.g., Filbee-Dexter and Scheibling, 2014;Ling et al, 2015;Hernández, 2017;Melis et al, 2019;Pinna et al, 2020).…”

“…In the northeastern Atlantic archipelagos of Madeira and Canary Islands (Macaronesian region), populations of the longspined sea urchin Diadema africanum with average densities of 10 individuals/m 2 have been responsible for the persistence of vast urchin barrens (Alves et al, 2001;Hernández et al, 2013). Several ecological and physical processes are involved in the structure and abundance of local D. africanum populations, including water turbulence (Alves et al, 2001), environmental parameters (e.g., nutrient and temperature; Hernández et al, 2006a,b), abundance of top-predators (Tuya et al, 2004b), substrate complexity (Tuya et al, 2004a) and mass mortality events (Clemente et al, 2014;Gizzi et al, 2020), but evidence suggests a general stability in the barren state of shallow coastal habitats across multiple islands of these archipelagos.…”

“…At that time, the lack of long term monitoring and historical data series have hampered dedicated studies on how it may have triggered an increase of algae cover and a phase shift from barren to algae dominated bottoms during the following months. In the summer of 2018 (early August, late September), a second mass mortality event was recorded in Madeira, with a major drop of approximately 90% in population density (described in detail in Gizzi et al, 2020). A disease outbreak that had been initially detected in the Canary Islands during February 2018 (Hernández et al, 2020), took more than 4 months to reach Madeira (where mortality was first detected in late July) but disrupted the local population of D. africanum across the entire archipelago.…”

Uncommon southwest swells trigger sea urchin disease outbreaks in Eastern Atlantic archipelagos

“…In February 2018, we detected another large mass mortality event affecting the Eastern Atlantic archipelagos (Figure 2a). This time, the mortality event was first detected on the eastern part of the archipelago (Canary Islands), and a few weeks later it spread to Lanzarote, Porto Santo and Madeira (Claudia Ribeiro and João Canning Clode, personal communication) (see Figure 1 for dates), and extending during spring and summer months (Gizzi et al 2020). On Tenerife, there was an overall average population reduction of 93.2% compared with pre-mortality densities (before 2016 vs after 2018; F= 183.81, p<0.01; Figure 2a).…”

“…It is important to note that populations on La Palma never recovered after the first sea urchin mortality event; densities after the event always remained lower than before the first mortality event. For the Madeira archipelago, a rapid recovery after the mortality have been described (Gizzi et al 2020).…”