Plant colonization, succession and ecosystem development on Surtsey with reference to neighbouring islands

Plant colonization and succession on the volcanic island of Surtsey, formed in 1963, have been closely fol- lowed. In 2013, a total of 69 vascular plant species had been discovered on the island; of these, 59 were present and 39 had established viable populations. Surtsey had more than twice the species of any of the comparable neighbouring is- lands, and all of their common species had established on Surtsey. The first colonizers were dispersed by sea, but, af- ter 1985, bird dispersal became the principal pathway with the formation of a seagull colony on the island and conse- quent site amelioration. This allowed wind-dispersed species to establish after 1990. Since 2007, there has been a net loss of species on the island. A study of plant succession, soil formation and invertebrate communities in permanent plots on Surtsey and on two older neighbouring islands (plants and soil) has revealed that seabirds, through their transfer of nutrients from sea to land, are major drivers of develop- ment of these ecosystems. In the area impacted by seagulls, dense grassland swards have developed and plant cover, species richness, diversity, plant biomass and soil carbon be- come significantly higher than in low-impact areas, which re- mained relatively barren. A similar difference was found for the invertebrate fauna. After 2000, the vegetation of the old- est part of the seagull colony became increasingly dominated by long-lived, rhizomatous grasses (Festuca, Poa, Leymus) with a decline in species richness and diversity. Old grass- lands of the neighbouring islands Elli﷿aey (puffin colony, high nutrient input) and Heimaey (no seabirds, low nutrient input) contrasted sharply. The puffin grassland of Elli﷿aey was very dense and species-poor. It was dominated by Fes- tuca and Poa, and very similar to the seagull grassland devel- oping on Surtsey. The Heimaey grassland was significantly higher in species richness and diversity, and had a more even cover of dominants (Festuca/Agrostis/Ranunculus ). We fore- cast that, with continued erosion of Surtsey, loss of habi- tats and increasing impact from seabirds a lush, species-poor grassland will develop and persist, as on the old neighbouring islands.