Drivers and resilience of urchin barrens in UK kelp forests

Rodrigo Muñoz-Cordovez¹, Heather Sugden¹, Catherine Scott², Pip Moore¹

¹Newcastle University, UK - ²Natural England, UK

Kelp forests cover 33% of global coastlines, dominating temperate and subpolar regions, and are considered some of the most productive habitats on Earth. 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. 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. 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. 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 has knock on effects for the goods and services they provide. Urchin abundance and impacts have increased globally due to anthropogenic pressures such as over-fishing and climate induced range shifts. 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 crustacean and abalone. In Norway, the formation of extensive (>2,000 km2) urchin barrens have been linked with the reduction of predatory fish stocks through the modernization of fishing methods and fleet.
Until recently, it had been thought that urchin grazing did not have a strong structuring role in UK kelp forests, beyond isolated sites e.g. limited suitable habitat on an artificial breakwater in the Isle of Man. 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. 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 are 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 there persistence?’. To address this quarrion this study 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?.