A multiscale perspective on how much wetland restoration is needed to achieve targets for ecosystem services

Percentage-based targets for conservation and restoration provide a compelling narrative for enhancing nature and human wellbeing. However, evidence is still lacking for what these percentage targets should be, especially for improving multiple ecosystem services. Furthermore, restoration targets can be challenging to implement across decision-making scales. We explored these challenges in the Ruamahanga Basin in the North Island of Aotearoa New Zealand, where ∼98% of wetlands have been drained. We created restoration scenarios that increase the percentage area of wetland restored at two spatial scales: first, across the entire Ruamahanga Basin, and second, using subcatchments delineated from the contributing areas of individual historical wetlands. These scales allow us to adopt the decision-making perspective of basin-scale managers/planners as well as groups/individuals restoring single wetlands. At each scale, we estimate gains and losses towards plausible targets for nitrogen and phosphorus retention, carbon storage, and agricultural productivity using the high-resolution, ecosystem service modelling tool, LUCI. At the basin scale, ecosystem service changes were incremental, showing linear trends through to full wetland restoration. We found no percentage value at which restoration did not add benefits for nutrient retention and carbon storage and no percentage value at which restoration did not detract from agricultural productivity. Within subcatchments, gains in phosphorus retention were achieved across all restoration percentages. Nitrogen retention targets were mostly met when the percentage of wetlands restored exceeded 60%. Contrasting outcomes at two different scales showed that most of the variability in ecosystem service outcomes is found at fine spatial scales, rather than at the basin scale, which has implications for choice of policy mechanisms and spatially-targeted management of ecosystem services.