Drivers and resilience of urchin barrens in UK kelp forests

Kelp forests cover 33% of global coastlines, dominating temperate and subpolar regions (Wernberg et al 2019), and are considered some of the most productive habitats on Earth (Pessarrodona et al 2022). They provide a complex 3D habitat, which provides food and habitat for a diverse range of flora and fauna ranging from invertebrates to marine mammals and birds (Smale et al 2022). They also provide a range of ecosystem services including nutrient and carbon cycling, coastal protection and the provision of food, feed and industrial products that are valued at billions of US dollars per year. Kelp forests are also increasingly threatened by a range of anthropogenic stressors such as ocean warming, eutrophication and over-exploitation, which has resulted in regional losses of kelp forests in 38% of eco-regions globally (Krumhansl et al 2016). Given the importance of kelp forests from a biodiversity and human societal perspective there is an increasing effort to conserve these habitats and, where lost, to restore them.
Globally, sea urchins are considered important consumers of kelp and were found in high abundances can exert strong top-down control affecting kelp distribution, abundance, and composition (Filbee-Dexter & Scheibling 2014). Where urchin densities exceed a threshold, their grazing activities can lead to the formation of extensive barrens, which are less productive, less biodiverse, and less structurally complex habitats, which can have knock on effects for the goods and services they provide (Ling et al 2015). Urchin abundance and impacts have increased globally due to anthropogenic pressures such as over-fishing and climate induced range shifts (Ling et al 2009, Norderhaug et al 2021). For example, in Tasmania, climate mediated range shifts of a sea-urchin and over-fishing has led to the formation of extensive urchin barrens, resulting in reduced commercial landings of crustacea and abalone (Johnson et al 2013). In Norway, the formation of extensive...