Conserving cross-realm coastal biodiversity when real-world planning and implementation processes split the land and sea

Conservation planning and implementation are typically applied in land and sea areas separately, placing already impacted coastal biodiversity – which spans the divide – at risk of being inadequately managed and conserved. In South Africa, we tested how well existing land-based and marine biodiversity priority areas cover coastal priorities that we identified cross-realm using Marxan with >1000 biodiversity features. Existing priorities covered 83% of coastal priorities, indicating good but incomplete coverage. Proportionately, the seashore (foredunes, shores) and estuaries had greatest selection as coastal priorities, confirming their important biodiversity value. Finally, we developed a map of Critical Biodiversity Areas for the South African seashore and estuaries, with management recommendations. To include coastal biodiversity in real-world planning, we propose: co-developing data and targets for cross-realm features; identifying broad coastal priorities that can be used in land-based and marine plans; and developing dedicated seashore and estuary priorities to seamlessly align land-based and marine prioritisation maps. • Separate land and sea planning puts effective coastal conservation at risk. • Cross-realm coastal priorities are identified using Marxan and >1000 input features. • We compare existing priorities (separate land/sea plans) and our coastal priorities. • Existing priorities had good but not complete coverage of coastal priorities. • We state three ways in which cross-realm considerations can be embedded in planning.