c. Authenticity/integrity

Natural

The proposed boundary of the extended St Kilda World Heritage Site encompasses the entire terrestrial components of the archipelago (854.6ha) and a significant adjacent sea area (23,346.8ha) for a total of 24,201.4 ha. The terrestrial area of St Kilda (to High Water Mark) is already inscribed on the World Heritage List on the basis of natural criteria iii and iv. In addition to seeking inclusion for St Kilda on the World Heritage List for additional natural criteria (i and ii), this re-nomination proposes to extend the site boundary into the marine area.

Research in the St Kilda area has demonstrated the importance of these sea areas to the globally important breeding seabirds for such activities as feeding, preening, loafing and moulting (see maps on page 27). Research on other Scottish islands has also demonstrated the value of such sea areas to island breeding seabirds.

Under its obligations in respect of the European Union, the United Kingdom is seeking Special Area of Conservation (SAC) and Special Protection Area (SPA) status for the St Kilda archipelago. SAC status will protect the vegetated sea cliffs and the marine communities of the islands. SPA status will further protect the seabirds on the island itself and in the surrounding sea areas.

The proposal to extend the World Heritage Site boundary into the sea area around the islands, and to make the boundaries of the SAC, SPA and WHS identical, will significantly strengthen protection of the natural heritage of the area. The protected area will include the sea areas that are important to the seabirds and also contains important seabed communities. The proposed inclusion of the sea areas in the World Heritage Site will therefore significantly add to the outstanding universal value of the property as a whole.

The proposed boundary of the site encompasses the entire terrestrial and underwater topographic features that are the result of the preferential erosion of an ancient volcano and include the full range of rock types found in the area. The site also comprises the full range of glacial, periglacial and coastal landforms of the archipelago, including much of the remarkable assemblage of submerged coastal features. The 35km long coastline of St Kilda is entirely natural and undeveloped, except for short lengths of coastal defences and two landing facilities adjacent to the historic settlement and MoD Base in Village Bay. The landing facilities comprise a narrow concrete ramp cutting across the boulder beach in the north-west corner of the Bay, and a concrete pier in its north-east corner. Between these, a 60-m length of stacked gabion baskets protects some of the military property from coastal erosion. Further east, two 10-m lengths of baskets also protect a more recent septic tank. Though visually prominent, none of these structures impacts upon the landforms or rocks for which the site is so noteworthy.

Stac Biorach and Soay StacDespite its long history of human occupation, the islands and stacks comprising the St Kilda archipelago have retained their natural integrity. Sheep have grazed much of the archipelago since people first arrived. Other human impacts have been largely confined to the walled area around the shores of Village Bay where some plant communities have been modified by previous traditional agricultural activities, such as cultivation and stock grazing. Some minor changes, probably part of a reversion back to its previous status, are being detected since the evacuation of the islanders in 1930. All domestic blackface sheep were removed from Hirta at the time of the evacuation but this loss of grazing pressure was soon negated by the introduction from the nearby island of Soay of 107 Soay sheep. Not being actively managed, the numbers of this feral flock fluctuate cyclically, apparently in synchrony with the parent wild population left on Soay, and with the primitive blackfaces abandoned on Boreray. It is thought likely to be from a density independent cause, such as climate, rather than the consequence of overgrazing. The sheep have contributed to the diversity of plant communities on the islands that they inhabit notably the interesting lichen swards on Hirta. Dun, on the other hand, where there is no grazing, has less floristic diversity.

Remarkably, with such a long history of human occupation, there have been few alien introductions, mostly a few plants (and probably some small invertebrates) which are confined to the Village area and cultivations. The house and fieldmice might be considered two such introductions but of such antiquity that they have evolved into unique natural interests in their own right. The former is now extinct, surviving only for a few years after the evacuation. Rats, mink and other land mammals are absent; these are important predators on many other seabird colonies and their absence is an important factor in the success of the seabird populations.

Human impact outwith the Village area and Gleann Mor has been largely limited to the construction of cleitean and other structures and the cutting of turf for fuel (long since ceased), together with the exploitation of the seabirds for food. Undoubtedly this would have impacted on numbers but it is apparent that this harvesting was undertaken in a highly sustainable manner so that only one species became extinct – the great auk around 1840 – at exactly the same time as it was wiped out in the rest of its range. The housemouse became extinct a few years after the evacuation of the human residents. A few feral cats were left behind for a year or two, before they were shot out. They no doubt hastened the demise of the housemouse, but the fieldmice thrived and have expanded their range to include the village and the modern MoD Base. There are few if any instances of housemice and fieldmice surviving alongside one another on the same small island, without the presence of humans. Biologists have always regretted not transferring housemice to Boreray where they might have had a chance of survival in the absence of any fieldmice. Certainly the prospects for the housemice would have improved had they persisted until the MoD Base was established two decades later.

The shores and sea bed around St Kilda are more-or-less devoid of any human impact. The area adjacent to the historic settlement in Village Bay and current MoD Base has been altered to a small degree with the construction of a pier and of some localised shore defences. The remainder of the intertidal area is essentially unaffected by human intervention and the populations of animals and plants present there can be regarded as entirely natural. There is currently only very limited, sporadic fishing activity within the general area during the summer months, restricted largely to the setting of creels that have little direct impact on the seabed sediments and the associated benthos other than the target species.

The site also includes the main sea area used by the breeding seabirds for critical activities such as preening, loafing and moulting.

Seabirds breeding on St Kilda forage over a wide area of the North Atlantic. Species such as the auks might fly more than 30km daily towards the west coast of the Outer Hebrides to forage while the storm-petrels and Manx shearwaters roam over much larger distances off the continental shelf to search for food. The larger gull species probably fly only short distances to forage and possibly also scavenge, and species such as the European shag and the black guillemot have an inshore distribution at all times and exploit small fish only short distances from the St Kilda coast. However, for the most part, the foraging areas of St Kilda seabirds are extremely large and disjunct, defying accurate identification.

Most species do feed immediately around St Kilda but the waters here are used, perhaps on a daily basis, primarily for purposes other than feeding. These inshore areas are used mainly for display, courtship, bathing and preening. They are also used by non-breeding birds for maintenance activities such as washing and preening. Such areas are more discrete and more readily identifiable than more distant feeding locations.

There are few data that indicate exactly where seabirds occur in those parts of the sea immediately adjacent to St Kilda. Analyses of the available at-sea survey data do suggest core areas of use for various species (see Figures 2.1 a-c).

Similar, more detailed analyses of the first ever small-scale surveys at sea very close to seabird colonies around Britain confirm a widespread pattern of use of the waters next to colonies by seabirds. In contrast to the distribution of feeding birds around colonies, which is likely to be dependent on the specific physical or oceanographic characteristics of each colony or its adjacent waters, the distributions of birds engaged in other activities such as feather maintenance and courtship are not so site-specific; the pattern of use of these waters for these purposes is remarkably consistent among important British seabird colonies. Highest densities and numbers of the three auk species (Atlantic puffin, common guillemot and razorbill) engaged in non site-specific behaviour were observed within 1 km of the colony shore and highest densities of northern gannets were recorded within 2 km of the shore. In addition, greatest use of the sea around colonies by the northern fulmar, the black-legged kittiwake and the European shag also occurred within 1 km of the colonies. Figure 2.2 shows the typical pattern of occurrence of common guillemots in relation to distance from the colony shore at the Isle of May, off the east coast of Scotland.


Common guillemots at the Isle of May
 
 
 
Figure 2.2. Mean interpolated common guillemot density (birds.km–2) in 200 m distance bands off a typical colony (in this case the Isle of May, east Scotland from four at-sea surveys in June 2001).

 

Thus, and in contrast to feeding, seabird use of the sea around colonies for body maintenance and courtship probably pertains to all species and all breeding individuals (as well as many non-breeders), rendering these waters of global ecological importance. Consequently, such areas, including those around St Kilda, are currently being proposed as marine Special Protection Areas under the EU Birds Directive.