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 (>2,000 km2) urchin barrens have been linked with the reduction of predatory fish stocks as a consequence of the modernization of fishing methods and fleet (Norderhaug et al 2021).
Until recently, it had been thought that urchin grazing did not have a strong structuring role in UK kelp forests (Smale et al 2013), beyond isolated sites e.g. limited suitable habitat on an artificial breakwater in the Isle of Man (Jones & Kain, 1967). In part, the lack of strong top-down control is a result of urchin density being at its greatest towards the lower depth limits of kelp forests (Comely & Ansell, 1988, Smale et al 2013). Recent surveys in NE England and SE Scotland have, however, noted incipient urchin barrens in kelp forests along the UK North Sea coastline. Urchin abundance in these areas is an order of magnitude higher than similar locations around the UK, posing the question 'What is the impact of urchins on North Sea kelp forests and what drives barren formation and their persistence?. To address this question this PhD will seek to address the following objectives: 1. What are the physical and biological drivers of urchin barren formation; 2. How extensive and persistent are urchin barrens in kelp forests along the UK North Sea coastline; 3. At what density of urchins leads to formation of urchin barrens and their recovery to kelp forest conditions?
References:
Comely & Ansell (1988) Estuarine, Coastal and Shelf Science 27: 311-334; Filbee-Dexter & Scheibling (2014) Marine Ecology Progress Series 455: 51-64; Jones & Kain (1967) Helgolander Wiss. Meeresunters 15: 460-466; Krumhanslet al (2016) PNAS 113: 13785-13790; Leclerc et al (2021) Oecologia 196: 441-453; Ling et al (2009) PNAS 106: 22341-22345; Norderhaug et al (2021) Ambio 50: 163-173; Pessarrodona et al 2022 Science Advances 8: eabn2465; Smale et al (2013) Ecology & Evolution 3: 4016-4038; Smale et al (2022) Journal of Ecology doi: 10.1111/1365-2745.13936; Wernberg et al (2019).