The Hebrides. J. M. Boyd

Чтение книги онлайн.

       The Soay Sound, St Kilda, looking from Hirta to Soay with Stac Biorach (73m) in the chasm (Photo J. M. Boyd)

Islands in Natural History

      Islands cast a romantic spell upon people. They possess a mystique from which the pragmatist cannot escape, nor for which the scientist can find ready explanation. Nevertheless, this spell is real in island life, and engenders deep intellectual and physical responses in human beings. In the Hebrides themselves, it is an experience which many share, but which is deeply personal, and indicative of a singular, inner passion for the ultima thule. Charles Darwin knew it. According to Frank Sulloway (1984), Darwin raised the level of mystique of the Galapagos to that of ‘enchanted islands’ (which is the literal translation of galapagos from Spanish), in such unromantic works as biology textbooks and histories of science—so much so, that these islands have become ‘the highly acclaimed symbol of one of the greatest revolutions in Western intellectual thought’. Twenty-four years were to elapse between Darwin’s visit to the Galapagos in 1835, and the Origin of Species (1859). It is clear therefore, that his ‘conversion’ to the evolution theory did not occur in the heroic setting portrayed in the popular history of science. The idea of natural selection did not occur to Darwin until 1837, almost two years after he visited the Galapagos. However, the legend of a supreme, ‘eureka-like’ discovery by the great naturalist coming face to face with evolution in the primeval islands lives on, and has fired the imagination of generations of on-coming biologists.

      The Hebrides do not occupy a grand plinth in scientific history as do the Galapagos, but, like all other archipelagos, they have their own endowment of nature and well-kept secrets to be discovered and enjoyed. The Galapagos are celebrated for their biology, but their geology (Simkin, 1984) is far less illustrious than that of the Hebrides. In studies of evolution and biogeography, the oceanic islands are unmatched by islands, like the Hebrides, that are strung along the continental edges. However, in studies of geology, ecology and animal behaviour, the continental edge is of the greatest interest. For example, the natural environment of the British Isles can be described as ‘maritime’, when compared with continental Europe. In greater detail, the western seaboard, including the Hebrides, when compared with the bulk of mainland Britain, is termed ‘oceanic’, because the communities of plants and animals there thrive in moist, mild conditions, or are greatly affected by the sea. It is this contrast of living conditions and life forms which has broadly attracted biologists to the Hebrides, while the geologists have been attracted to the Pre-cambrian and Tertiary rocks which are poorly represented in Britain south of the Great Glen. There are ample opportunities to observe how the structure of habitats changes from south-east to north-west, and also how each island has acquired its own rock base and complement of living things. Indeed, each island has its own unique and rich potential for the study of natural processes.

Island Races

      Every island has a ‘gene pool’ and, between the islands and the mainland ‘reservoir’, there is a constant but usually small ‘gene flow’. Each island is a unique assembly of species, which have been brought together by natural or man-assisted colonisation over long periods of time. Genetically, it is important to distinguish between ‘relict’ species which were present on the land before it became an island, and the colonisers which arrived after the land became an island. Small founder groups of either category possess fewer alleles of each gene than the large mainland populations from which they derive. When the founder group has grown and becomes established, the island species can have different frequencies of the different morphs than in the parent population. This is the theory anyway—in reality the situation is much more complex.

      In the Hebrides, the islands became colonised from the south as the British Isles emerged from the retreating ice sheet. As time advanced more and more plants and animals arrived. Changes in sea level destroyed ‘land bridges’, thus isolating fragments of erstwhile mainland populations. The flora and fauna resulting from natural colonisation and physical isolation have been further complicated by man-assisted colonisation. Again in theory, many original colonisers of the north of Scotland may have been eliminated from the mainland by species which arrived later but did not reach the islands. The Hebrides, therefore, may possess relict life forms, such as the fossorial bee (Colletes floralis), the arctic charr (Salvelinus alpinus), and the plant Koenigia islandica. The Soay sheep (Ovis aries) of St Kilda is an outstanding example of a domesticated animal introduced by man to Britain in neolithic times, which became extinct as a breed (superseded by improved breeds of sheep) in all areas except the remotest and most inaccessible of islands, Soay at St Kilda.

      The distribution of species in the Hebrides, therefore, begs many questions of when and how they came to be there. Analysis of pollen from peat and the beds of lochs have shown much of the time-scale and species of colonisation of the islands by vegetation; the affinities of most plant species to ‘oceanic’ and ‘continental’ biomes have been described; problems of taxonomy have arisen and identification of rare or key species has been questioned when voucher specimens and satisfactory records were lacking. However, the biogeography, taxonomy, and genetics of the Hebridean flora and fauna is still a wide-open field for research. This work is closely linked to the need for more information on the invertebrate fauna—and, with new techniques such as ‘genetical fingerprinting, in the revision of existing information on the entire biota.