Conservation challenges in urban seascapes: promoting the growth of threatened species on coastal infrastructures

Summary 1. With nearly two-thirds of the human population concentrated along coastlines, coastal development and urbanized seascapes are inevitable. Proliferation of coastal and marine infrastructures, such as breakwaters, ports, seawalls and offshore installations, is associated with loss of natural habitats. This calls for new strategies aimed at elevating the ecological and biological value of coastal infrastructures, while minimizing their ecological footprint. 2. We explored the feasibility of using coastal defence structures as a scaffold for the conservation of threatened marine species. We experimented with fucoids, canopy-forming algae on Mediterranean coasts, in the light of their declared conservation priority. We transplanted juveniles of Cystoseira barbata to a number of breakwaters and natural sites along the Adriatic Sea (Italy) and tested which factors could facilitate or inhibit its successful establishment. 3. Survival of transplanted C. barbata was greater at most artificial and natural sites examined compared to the native sites where severe habitat loss was ongoing. Survival was greater at landward compared to seaward positions on the infrastructure, while no relevant effects of substratum characteristics (horizontal vs. vertical orientation, variable composition and increasing complexity) were observed. Lack of surrounding adult fronds did not impair the survival or growth of the transplants, suggesting a high transplantation potential also on novel infrastructures. 4. Success of transplantation in areas remote from the source population was limited by biotic disturbance, which was more intense on coastal infrastructures in sedimentary environments compared to natural rocky sites. 5. Synthesis and applications. Coastal and marine infrastructures can be harnessed to enhance desired species (such as threatened canopy-forming algae). A comprehensive understanding of the ecological functioning of these urban seascapes compared to natural habitats is required to minimize detrimental impacts, or potentially increase the ecological value, of coastal structures and efficiently incorporate such strategies into management and conservation actions. We investigated the influence of habitat type (including natural and artificial), surface complexity, herbivore exclusion, proximity to established populations and orientation on the transplantation success of threatened algae.