Environmental variability generates sources of resilience in seagrasses

When we ‘look harder’ to quantify and understand the differences among individuals and populations that produce in variation in resilience within species we achieve better outcomes; successes from medical and agricultural industries are testament to this. Such successes have not yet been achieved for many ecosystems as the variation in resilience within species remains largely unexplored. This knowledge gap is acute for seagrasses which are key determinants of coastal ecosystem structure and function. The overarching goal of this research was to assess the role of environmental variability for influencing the resilience within species of seagrasses and to test these in the context of improving predictions under climate change. Using a combination of field and controlled experiments, this work examined how resilience differs among populations and across life history stages of multiple colonising seagrass species in temperate estuaries of Western Australia. The biological response of Halophila ovalis populations was compared along an estuarine-marine gradient to an extreme rainfall event in a ‘natural’ experiment and to salinity changes in a subsequent controlled experiment. I examined the impacts of abiotic and biotic factors on the response of Ruppia polycarpa over the complete lifecycle in a field manipulative experiment and experimentally tested the effects of salinity on seed dormancy release, germination and seedling establishment. Resilience to hyposalinity varied among H. ovalis populations along the estuarine-marine gradient. Estuarine populations were more resilient to low salinity stress than marine populations consistent with generally being exposed to a more variable salinity regime. In the field, upper-estuary populations recovered to historical baseline conditions whereas lower-estuary populations did not. These differences in recovery were verified by the experimental results which showed that upperestuary populations had greater survival and growth compared to lower-estuary populations. The upper-estuary populations represent an important source of adaptive capacity for the species. These results confirmed that environmental variability associated with the salinity gradient exerts strong selective pressure on seagrass populations and influences their resilience. The response of individual life history stages of R. polycarpa to abiotic and biotic factors varied. Salinity shifts from high to low followed by gradual increases promoted seed germination. Increasing temperature positively impacted seedlings but after a point, caused declines in adults. Swan grazing had minimal impact across life history stages but benefited seedling recruitment. Bet-hedging strategies, including the presence of dormant seeds, were also identified. These results indicated that species persistence results from a combination of environment dependent selection that ‘tailors’ individual life history stages to the conditions they are most likely to be exposed to and strategies that reduce the risk of complete mortality associated with a highly variable habitat. Management of habitats should reflect an understanding of the requirements of each and every life history stage and move away from an emphasis on adults. Overall, the findings of this research indicate that environmental variability and life history stages can generate variation in resilience within seagrass species. This variation matters when predicting the vulnerability of species under climate change. Populations that are naturally exposed to variable conditions are likely to be more resilient to emerging disturbances and reduce the overall vulnerability for the species. In habitats where conditions are highly seasonal, individual life history stages may have greater resilience to disturbances than others and be key modifiers of species’ vulnerability to climate change. These populations and/ or life history stages represent important sources of resilience that may have been underestimated in previous predictions. To move forward, conservation ecologists and managers need to consider the variation in resilience within seagrass and begin testing approaches that leverage this knowledge to reinforce these important species for future climate change