Tandem ecosystems: a new perspective on ecological connectivity across the dune-beach interface on sandy shores

Sandy shores are composed of two adjacent but ecologically distinct systems, beaches and dunes, typically functioning as coupled components of the Littoral Active Zone. These systems are linked through multiple abiotic exchanges, including bidirectional sand transport, groundwater discharge from dune aquifers, and salt spray deposition from the surf zone. While they exhibit strong geomorphological and process-based connectivity, their ecological assemblages are entirely distinct, with no resident species in common and only transient faunal movement across the interface. Consequently, beaches and dunes do not fit the meta-ecosystem paradigm. To conceptually capture their tight but largely non-biological linkage, we propose the term tandem ecosystems, reflecting their alignment one behind the other in coastal sand transport and storage. This perspective emphasizes their functional interdependence and carries direct implications for integrated coastal management, conservation, and climate adaptation.