Are biodiversity offsetting targets of ecological equivalence feasible for biogenic reef habitats?

Structurally complex habitat is declining across temperate marine environments. This trend has been attributed to changes in land use and increasing coastal development, which are activities likely to continue with governments supporting ongoing economic growth within the marine realm. This can compromise biodiversity, and biodiversity offsetting is increasingly being heralded as a means to reduce the conflict between development and conservation. Offset schemes are often evaluated against targets of ‘ecological equivalence’ or ‘like-for-like’ but these terms can be difficult to define and quantify. Although targets of equivalence have been generally shown to be feasible in terrestrial environments, the complex and dynamic nature of the marine and coastal realms present difficulties when aiming for strict equivalence targets as measures of success. Here, we investigated four intertidal biogenic reef habitats formed by the tube worm Sabellaria alveolata within, and in proximity to, Swansea Bay (Wales, UK). The aim was to identify measurable biodiversity components for S. alveolata reef habitat, and to investigate the natural spatio-temporal variation in these components, to determine whether a target of equivalence was feasible. We also looked to identify the most important drivers of species assemblages within the reefs. Results showed that biodiversity both S. alveolata formation and tube aperture condition showed a significant interaction between site and season, with community composition varying significantly by site only. Site was found to explain the highest variation in community composition, followed by substrate type, and geographical position. These results highlight how widely coastal habitats can vary, in both space and time, and therefore calls into question a strict target of ecological equivalence when planning biodiversity offsets in coastal environments.