Grassholm / Ynys Gwales

Also within

Dyfed County Arfordir Penfro / Pembrokeshire Coast National Park

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Geology & landscape

Rock Type: Basalt

Landscape processes: Coastal, Volcanic / Igneous

Birds & fish recorded

Birds

Description

Description The site is of special interest for its internationally important breeding population of gannets Morus bassanus, the intertidal communities around the island that are amongst the most wave-and tide-exposed in Wales, and grey seals Halichoerus grypus, which haul out on the rocky foreshore. This small basalt island is located approximately 16 km west of Skomer, with its highest point being a smooth ridge no more than 15 metres above sea-level, falling in shallow steps to the sea on the eastern and southern sides of the island. Soils are composed of mineral material derived from basalt and humus. The seaward fringes are dominated by a dense covering of dabberlocks Alaria esculenta, small mussels Mytilus edulis and an abundance of coralline algae. A dense community of barnacles, small mussels, and associated coralline algae occur above this zone. Limpets Patella spp. and barnacles encrust the rocks further up the shore, over which there is a sparse cover of the exposed form of bladder wrack Fucus vesiculosus var. linearis. Laver Porphyra spp. straddles the gradation from this barnacle and limpet zone to the black tar lichen Verrucaria maura or pygmy lichen Lichina pygmaea zones. Yellow and grey lichens occur on the upper shore around most of the island. Rockpools, mainly small and shallow, occur on the south-eastern side of the island. The two largest pools, located east of South Gut, contain a good cover of encrusting and branching coralline algae with clusters of the anemone Sagartia elegans var. venusta in crevices. The green seaweed Prasiola spp. occurs on the upper shore over guano-enriched areas of black tar lichen in this same area. Approximately 2-3% of the West Wales grey seal population hauls out on the rocky foreshore of the island during the summer months. The gannetry on the north-western half of the island is devoid of vegetation and it has encroached on the maritime grassland in the centre. Red fescue Festuca rubra characterises the grassland, which has been much modified by the activities of seabirds. The grassland covers a thick hummocky layer of humus, which was extensively burrowed by the once immense colony of puffin Fratercula arctica. The large, fleshy-leaved variety of buck’s-horn plantain Plantago coronopus var. maritimus is abundant in the more sheltered nooks between red fescue hummocks and tree-mallow Malva arborea can be found scattered on the more sheltered eastern side of the island. In wet summers, dense stands of Yorkshire fog Holcus lanatus and spear-leaved orache Atriplex prostrata occur on the island. Nearly 17% of the UK’s gannet population and 12.5% of the North Atlantic population breed on the island between January and October, flying in from West Africa. In the early part of the 20th century, only one small corner of the island was used by gannets. The remainder was covered by a huge puffin colony, which is thought to have abandoned the island following the collapse of the turf, undermined as a result of their burrowing. Since then the gannet population has increased in size dramatically, taking over an ever-increasing area of the island such that more than half is now covered in gannets during the breeding season. Non-breeding birds also congregate on the island in large numbers. Other seabirds also nest all around the island, including species such as guillemot Uria aalge, razorbill Alca torda, shag Phalacrocorax aristotelis and kittiwake Rissa tridactyla, as well as herring gull Larus argentatus, lesser black-backed gull Larus fuscus and great black-backed gull Larus marinus. Remarks: The site is owned by the RSPB and lies within Pembrokeshire Coast National Park. The foreshore is owned by the Crown Estate and leased to the National Park.

Species mentioned