Large-Scale Sea Urchin Culling Drives the Reduction of Subtidal Barren Grounds in the Mediterranean Sea

Giuseppe Guarnieri,Stanislao Bevilacqua,Neus Figueras,Laura Tamburello,Simonetta Fraschetti

doi:10.3389/fmars.2020.00519

Abstract

Increasing anthropogenic pressures are causing long-lasting regime shifts from high-diversity ecosystems to low-diversity ones. In the Mediterranean Sea, large extensions of rocky subtidal habitats characterized by high diversity have been completely degraded to barren state due to high grazing pressure exerted by sea urchins, which could persist for long time. In several areas of the world, a positive effect of sea urchin removal on the recovery of overexploited subtidal rocky habitats was found. This study assessed, for the first time in the Mediterranean Sea, the effects of extensive sea urchin culling on the recovery of subtidal reefs from the state of barrens. We tested this approach within a Marine Protected Area where a combination of oligotrophic conditions, general depletion of fish stocks, dramatically high sea urchin densities, and the large extension of barren grounds caused by date mussel fishery hampered the natural recovery of shallow rocky reefs. Culling intervention (through hammering) was carried out in spring 2015, covering an area of 1.2 hectares at about 5 m depth. The effects of sea urchin removal were monitored at regular intervals, for a time span of 3 years, and were compared with two control sites adjacent to the culling area. We documented a progressive reduction of the barren grounds in the fully protected area after the intervention. Also, very low re-colonization of sea urchins was observed during the experiment, so that no additional extensive culling was necessary. Our findings suggested sea urchin culling as a promising practice, also considering the limited costs of the intervention. However, since the reduction of barren grounds was largely driven by turf forming algae, caution is needed in the interpretation of the outcomes in terms of restoration, and results are discussed considering the factors involved in the observed shift and the synergies to be carried out for a full recovery of the system.

Highlights

Habitat degradation, fragmentation, and loss are among the most dramatic consequences of the escalating anthropogenic pressures on natural systems (Tittensor et al, 2014; Haddad et al, 2015; Steffen et al, 2015) and usually imply significant changes in biodiversity and ecosystem functioning (Pereira et al, 2010; Bartlett et al, 2016)
Exacerbated by multiple and synergistic effects of climate change (Bozec and Mumby, 2015), human impacts are increasingly compromising the capacity of marine ecosystems to absorb recurrent perturbations (Nystrom et al, 2000; Gundersen and Pritchard, 2002; Folke et al, 2004), limiting the potential for natural recovery (Scheffer et al, 2001; Lotze et al, 2011)
One of the most frequent and persistent regime shifts is the transition from macroalgaldominated habitats to barren grounds as a result of sea urchin overgrazing (Silliman et al, 2013; Steneck and Johnson, 2013; Filbee-Dexter and Scheibling, 2014; Ling et al, 2015)

Summary

Introduction

Fragmentation, and loss are among the most dramatic consequences of the escalating anthropogenic pressures on natural systems (Tittensor et al, 2014; Haddad et al, 2015; Steffen et al, 2015) and usually imply significant changes in biodiversity and ecosystem functioning (Pereira et al, 2010; Bartlett et al, 2016). One of the most frequent and persistent regime shifts is the transition from macroalgaldominated habitats to barren grounds as a result of sea urchin overgrazing (Silliman et al, 2013; Steneck and Johnson, 2013; Filbee-Dexter and Scheibling, 2014; Ling et al, 2015). Anthropogenic stressors may play a crucial role in triggering and maintaining this shift by eroding the resilience of macroalgal beds while increasing the persistence of sea urchin barrens (Ling et al, 2015). The depletion of populations of sea urchin predators caused by overfishing has been invoked as the main driver leading to kelp bed collapse in different regions of the world [e.g., Hagen and Mann, 1992 (Canada); McClanahan, 2000 (Africa); Shears and Babcock, 2003 (New Zealand); Ling et al, 2009 (Australia); Watson and Estes, 2011 (Alaska)]

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