Functional consequences of habitat fragmentation in macroalgal forests: A focus on macroinvertebratesReef-associated Biodiversity

Cristina Galobart¹, Trine Bekkby², Francisco R Barboza³, Imtiyaz B Beleem³, Michael T Burrows⁴, Hartvig Christie², Jonne Kotta³, Daniel Martin¹, Siri R Moy², Paola Mura¹, Kristiina Nurkse³, Eli Rinde², Gail Twigg⁴, Antoni Vivó-Pons^1,5, Emma Cebrian¹

¹Centre d’Estudis Avançats de Blanes (CEAB-CSIC), Spain - ²Norwegian Institute for Water Research (NIVA), Norway - ³
University of Tartu (UTartu), Estonia - ⁴Scottish Association for Marine Science (SAMS), UK - ⁵Danmarks Tekniske Universitet (DTU), Denmark

Habitat fragmentation – where a large area of habitat is divided into smaller, more isolated patches – is a major threat to biodiversity and ecosystem functioning. This process affects large coastal regions worldwide, impacting the structural species of the habitat and their associated communities. However, its effects on ecosystem functioning are less understood. Our study investigates how different levels of habitat fragmentation can affect macroinvertebrate assemblages and their functioning in macroalgal forests. We compared macroalgal forests dominated by species from the order Fucales from four temperate sub-regions (North Sea, Celtic Sea, Baltic Sea, and Mediterranean Sea). In each region, we considered three levels of fragmentation (high, moderate and low), sampling three replicated sites per level of fragmentation. At each site, macroinvertebrate assemblages were collected from three 20 x 20 cm quadrats, with the most abundant groups (Crustacea, Mollusca and Polychaeta) sorted and identified. We first analysed the structure and composition of macroinvertebrates using traditional taxonomic indices (total abundance, species richness, and diversity). Then, using a set of 12 functional traits, we characterised the functional spectra of these groups across fragmentation levels and regions. We showed that in some regions, species richness and taxonomic diversity can persist even in highly fragmented forests. However, this does not always ensure the conservation of their functioning. High levels of fragmentation were associated with a decrease in functional diversity and a loss of traits related to species with long life spans, gregarious behaviour and suspension feeding. The loss of these specific traits probably translates into changes in ecosystem functions. Still, we report varying effects depending on the taxa and the scale considered, underscoring the complex nature of ecological modifications induced by habitat fragmentation.