Seabirds and seals as drivers of plant succession on Surtsey

Abstract

Plant colonization and succession on Surtsey have been monitored since 1965. In 2019, the 75th species of vascular plants was detected on the island, 62 species were present and about 40 species had established viable populations. Over the last decade colonization has slowed down and the number of present species not increased. The rising number of seagulls breeding on the island after 1985 had a great impact on plant colonization and vegetation development. While most parts of the island remained barren, a grassland area (13 ha in 2018) developed in the main seagull breeding colony on the southern part of the island. This development is attributed to transfer of nutrients from sea to land by the seagulls. In recent years a dense patch of vegetation, 2 ha in 2018, has also developed on the low, northern spit of the island, where a few pairs of seagulls breed in the spring and grey seals haul out and breed in the fall in considerable numbers. In a survey conducted during the grey seal pupping period in 2019, the seal abundance and spatial distribution was mapped accurately for the first time. The results show that the dense vegetation of the spit and seal distribution are clearly overlapping. The continuous shrinking of the island and its spit has led to an increasing concentration of the seals in their breeding area. Based on a literature survey we estimated the nitrogen (N) input from sea to land by the grey seals as 9-13 kg N/ha in 2019. This compares to an estimated input of 5-30 kg N/ha/yr by the seagulls breeding in the same area during 2015-2019. Within the grey seal and seagull breeding area on the spit of the island, a distinct community of shore plants has developed. Measurements of plant cover and biomass in permanent plots on Surtsey in 2018 and 2019 show that development on part of the spit is reaching a similar state as in the old gull colony on the southern part of the island. This suggests that the grey seals along with the seagulls are important drivers of plant succession on the northern spit. Further research on the effects of the seals on nutrient transfer from sea to land and ecosystem development on Surtsey is recommended.