Quantifying multi-taxon functional change on tropicalising reefs for conservation

Coral reef ecosystems are undergoing community reassembly due to climate-induced range shifts, thermal stress events and localised disturbances such as coastal development, threatening reefs worldwide. The ecological processes that drive species and community shifts, and the functioning of resultant ecosystems is poorly understood, presenting a challenge for climate-resilient conservation management strategies. Here, I take a functional trait-based approach to understand and quantify functional change on Japanese coral reef ecosystems to inform conservation plans. Urban reefs experience elevated levels of anthropogenic stressors, resulting in turbid, marginal conditions. It is unclear how urban reef ecosystems are structured at the community and functional level, and how they will respond to future disturbance events. Chapter two of this thesis quantifies how the functioning of a tropical urbanised reef has changed between 1975 and 2018 in Nakagusuku Bay, Okinawa, Japan. I identified widespread reef fish and coral genera community turnover, but functional space was maintained, suggesting the communities had retained ecosystem function. Japan’s coastal marine ecosystems form a tropical to temperate transition zone, where many high latitude reefs have undergone tropicalisation, with phase shifts from temperate to tropical species. Determining the winners and losers under further environmental change, and how to incorporate this into management is a key conservation priority. In Chapters three-five, I address this by classifying species into trait-based groups to understand and manage functioning. Chapter three explores how fish functional groups represent the within-group species. Species were found to have similar environmental drivers to that of their respective functional group, suggesting traits determine how species respond to the environment. It is important to consider multiple taxa to understand how range shifts will affect the functioning of the whole ecosystem. Chapter four models the spatial distributions of fish, coral, echinoderm, mollusc and algae functional groups for now, and 2050 with climate change. Groups were found to have distinct tropical and sub-tropical distributions. Future predictions showed mixed responses to environmental change, with some tropical groups shifting poleward, some subtropical groups reducing in abundance, but also subtropical groups that remained stable, resulting in high latitude novel functional communities with enhanced functions. Reserve networks based on current distributions may not remain effective in the future. In Chapter five, I outline a climate resilient framework for prioritising reefs for static and dynamic conservation management I use the predicted multi-taxon group distributions form Chapter four, to identify areas for protection that would maximise ecosystem function, whilst considering range shifts. Overall, this thesis provides an enhanced understanding of the functioning and protection of coastal reefs under ongoing climate change. Methods in this thesis could be applied to other localities along marine biogeographic transition zones, and be adapted for terrestrial ecosystems with latitudinal and altitudinal range shifts, improving evidence-based conservation action in a changing world.