The role of dugong and turtle grazing in Torres Strait seagrass declines: Exclusion experiments show the role of green turtle and dugong grazing in structuring Torres Strait seagrass meadows

• Seagrass declines in western Torres Strait that occurred in 2019 and 2020 have been recognised as a concern by the local community. Grazing by dugongs and green turtles was identified as a possible cause of these declines. • Exclusion cages were used at two of the affected seagrass meadows (Orman Reefs and Mabuyag Island) to understand how herbivory by green turtles and dugongs (megaherbivores) was impacting seagrass meadows. • Where grazing pressure was removed (inside exclusion cages) the seagrass canopy height and biomass were significantly higher than the open to grazing control plots at both locations by the end of the seven-month experiment. • Grazing pressure is very high at both sites and herbivory is likely to be contributing to the seagrass declines in these meadows. This seems to be driven principally by green turtle grazing. • Based on the declines recorded in the long-term monitoring program at these meadows and the results from this study, it seems likely that megaherbivore grazing may have been a key driver of the declines at the Orman Reefs site, and grazing pressure is continuing to lead to reduced seagrass abundance at both sites. • Studying megaherbivore movements and the changing spatial status of seagrass across the broader region would increase understanding of the dynamics of these plant-animal interactions in the region. This would also help to establish whether grazing is the sole cause of declines at these sites, or part of natural cycles linked to other drivers such as wind, sediment movements or other impacts to seagrasses • This study confirms megaherbivore grazing is a key element in shaping seagrass dynamics in Torres Strait and points to the value of further assessments of megaherbivore and seagrass dynamics in northern Australia