Combining co-introduction with patch-size optimization as a novel strategy to maximize seagrass restoration.

Coastal ecosystem engineers, such as mussels, oysters, salt marsh grasses, and seagrasses, typically shape their environment by ameliorating stressors when they grow beyond a critical population size. In doing so, they not only facilitate themselves but also provide habitat for diverse communities, which in turn, can engage in reciprocal interspecific facilitation. Over the last decades, anthropogenic disturbances have caused rapid degradation of coastal ecosystems, emphasizing the need for their restoration. Although the importance of both inter- and intraspecific positive interactions has been confirmed in small-scale experiments, their combined potential remains to be tested in restoration-scale experiments. Here, we examine whether restoration aimed at simultaneous recovery of both facilitation types can increase restoration yields of the seagrass Zostera marina. We conducted a full factorial experiment by manipulating patch size to test for intraspecific facilitation effects and co-introduced the epiphyte-grazing snails Littorina littorea to investigate interspecific facilitation. We found that the effect of including intraspecific facilitation on restoration yields was highly dependent on the hydrodynamic exposure conditions at the restoration site. Large patches in the most exposed sites showed a 40% increase in seagrass restoration yield compared to small patches, while at sheltered sites, large patches counted up to 60% less surviving plants compared to small ones. Interspecific facilitation, on the other hand, increased success yields by 73% on average. Moreover, in some plots where L. littorea survived through the winter, we counted 20 times more shoots than plots without co-introduction. Our study is the first to demonstrate that co-introduction can serve as a successful large-scale restoration strategy. Additionally, we highlight that integration of both inter- and intraspecific facilitation into restoration designs can be a powerful approach to increase coastal restoration success. However, we emphasize that these strategies should be applied specifically to counter environmental stressors as they may have adverse effects themselves in environments without such stressors.