Around 20,783 kilometers of worldwide shoreline are occupied by 2,149 barrier islands. Since they are shaped by offshore sedimentation processes and annually occurring storm tide events, the appearance of these islands may vary between years if they are not protected by dykes. Barrier islands have been ignored by classical island biogeographic analysis until now. To close this knowledge gap, we analyzed more than 8,600 taxa across 37 organism groups (including vertebrates, invertebrates and land plants) on the German barrier islands, also referred to as East Frisian Islands. We tested for classical relationships between species and island area (SAR) and for effects of island habitat heterogeneity (SHH), and further island parameters using generalized boosted regression tree models.
Only few taxa such as mammals (27 species) and bugs (277 species) showed a SAR. SHH was more important for small-sized insects such as Aculeata (339 species), Auchenorrhyncha (193 species) or Saltatoria (13 species). All other taxonomic groups responded to particular island parameters such as the degree of island isolation and the amount of annually deposited marine sediments. Surprisingly, the annual sediment deposition – calculated from 1650 A.C. to 2008 A.C. – had strong positive effects across all taxonomic groups from lichens to mammals. Seven of the 11 islands have permanent dykes, thereby protecting islands from sediment loss. Diked islands have the largest amount of species. We conclude that enhanced annual sediment deposition, promoted by dykes, increases overall biodiversity across taxa on barrier islands.