Breaking down seagrass fragmentation in a marine heatwave impacted World Heritage Area

Abstract Marine heatwaves, and other extreme climatic events, are driving mass mortality of habitat‐forming species and substantial ecological change worldwide. However, habitat fragmentation is rarely considered despite its role in structuring seascapes and potential to exacerbate the negative impacts of habitat loss. Here, we quantify fragmentation of globally significant seagrass meadows within the Shark Bay World Heritage Area before and after an unprecedented marine heatwave impacting the Western Australian coastline over the austral summer of 2010/11. We use a spatial pattern index to quantify seagrass fragmentation from satellite‐derived habitat maps (2002, 2010, 2014 and 2016), assess potential predictors of fragmentation and investigate seascape dynamics defined by relationships between seagrass fragmentation and cover change. Our spatiotemporal analysis illustrates widespread fragmentation of seagrass following the marine heatwave, contributing to a dramatic alteration of seascape structure across the World Heritage Area. Fragmentation immediately following the marine heatwave coincided with widespread seagrass loss and was best explained by interactions between a heat stress metric (i.e. degree heating weeks) and depth. Based on the relationship between fragmentation and seagrass cover change, we revealed near‐ubiquitous fragmentation from 2014 to 2016 represents a mixture of long‐term seagrass degradation and evidence of early, patchy recovery. Fragmentation effects are expected to compound the ecological impacts of seagrass mortality following the marine heatwave and prolong recovery. As sea temperatures and the threat of marine heatwaves continue to rise globally, our results highlight the importance of considering fragmentation effects alongside the negative impacts of habitat loss. Our seascape dynamic framework provides a novel approach to define the response of habitat‐forming species to disturbances, including marine heatwaves, that integrates the processes of fragmentation and cover change. This framework provides the opportunity to consider these important processes across a range of threatened ecosystems and identify areas of vulnerability, stability and recovery.