Coastal ecological engineering and habitat restoration: incorporating biologically diverse boulder habitat

Ecological engineering is increasingly being studied and applied in order to reverse declines of biological diversity caused by coastal urbanisation and habitat degradation. As methods become more sophisticated and the theoretical framework more advanced, engineering of more complex and biologically diverse habitat types becomes possible. This review discusses the benefits of incorporating boulder habitat, which provides a unique combination of intermediate stability and high structural complexity, and can be occupied by many rare species. The inclusion of this habitat into engineered coastlines would therefore represent an important outcome for coastal ecological engineering by providing habitat for these species. Some methods are already in use to restore degraded boulder habitat; these methods should strive to closely mimic boulder habitat because semi-natural habitats (e.g. building rubble at bases of seawalls) have not been found to support rare species at this stage. Creation of new boulder habitat is also valuable for important fisheries (e.g. Haliotis spp.). Methods will be improved by focusing on small-scale microhabitats created by boulders and how these microhabitats provide shelter from locally relevant predators. Boulder habitat can reliably stabilise shorelines whereas alternative ecological engineering options based on littoral vegetation (e.g. mangroves, seagrass or saltmarsh) provide stabilisation involving strong spatiotemporal variability. Ecological engineering methods that include highly novel habitats, such as boulders, will achieve valuable biodiversity outcomes by allowing large-scale increases in along-shore distribution of specialist species. Overall, incorporation of boulder habitat in ecological engineering will help ensure coastal habitats include highly diverse assemblages and important ecological functionality as the pressure to modify coastlines increases.