common blue (Polyommatus icarus), the grey mountain carpet (Entephria caesiata), the twin-spot carpet (Perizoma didymata), the mottled beauty (Alcis repandata), and the lesser yellow underwing (Noctua comes). Distinct forms of the bumble bee Bombus jonellus and the dragonfly Sympetrum nigrecens occur in the Hebrides. Amongst birds, the St Kilda wren (Troglodytes t. hirtensis) is distinct from that of the Hebrides (T.t.hebridensis), and in fact more closely resembles the Fair Isle wren (T.t.fridariensis), which in turn is distinct from the Shetland wren (T.t.zetlandicus). Starlings (Sturnus vulgaris) from Shetland and the Outer Hebrides are thought to be distinct from the race occupying the rest of Britain.
Professor R.J. Berry (1979, 1983) has examined the genetical and evolutionary significance of the Hebrides, where in his own words ‘genes and geography meet’. He concludes:
The physical tides that have caressed and pounded the Western Isles have biological parallels: waves of animals and plants have beaten on the islands and formed their biological environment in the same way that the waves of rock, ice and water have determined their geographical limits. And just as the physical waves have laws which must be obeyed, so the interactions of drift, migration, and selection have forged the genetical constitution of the island races; and as the incoming tide cleans the sands and rocks over which it passes, but leaves unexplained features in secluded eddies, so the biological tides have left us with many genetical puzzles. The scientist believes as an article of faith that these eddies can be explained as knowledge accumulates, though some will remain as statistical anomalies of history.
We have used the example of genetical evolution and change to set the islands in the light of scientific discovery. We see the Hebrides not simply as the beautiful physical shapes they are, but as complete little worlds in themselves—each a unique repository of life. But there are also the rocks and the puzzles they hold. We try to interpret the genesis of the Hebrides from the Geological Record and find, in the great span of geological time, that part of the earth’s surface which was destined to become the British Isles, moved northward across the surface of the globe from tropical to temperate latitudes. Having done so, and assumed its present geographical stance, the crustal plates parted and the British Isles were formed. This is a spellbinding story captured forever within the rocks of Ireland and Western Scotland, including the Hebrides. The disentanglement of the rocks on the north-west seaboard of Scotland, which plumb the depths of 3000 million years, is a wonderful achievement, and now part of classical geology of world-wide significance. The dynamic, three-dimensional perception of geological processes over such long periods of time, punctuated as they were by upheavals of the earth’s crust such as the Grenville and Caledonian orogenies and the Moine Thrust, are so complex as almost to defeat lay presentation.
Looking like a living tissue, the Jurassic sandstone at Elgol, Skye has been eroded by the sea into this delicate, pale yellow, lacunary web (Photo J. M. Boyd)
The coral islands of tropical seas display a biological process in which living corals extract lime from the sea water and build enormous reefs which, following changes in sea level, become raised islands or coastal platforms. In the Hebrides, there is at work a similar grand relationship between sea, land, and air, in which marine invertebrates and algae provide a vital link in the accretion of shell sand. Since the end of the last ice age, about 10,000 years ago, vast quantities of lime have been extracted from the sea-water by countless generations of shell-forming animals, whose remains have been ground in the surf and cast up by sea and wind upon the rocky shores. Spacious coastal platforms of dunes and machair (Hebridean maritime grassland) have been formed in the southern Outer Hebrides, Tiree and Coll, enriching both the natural and human ecology of these islands (See here). The whole process is supported by untold numbers of animals and plants of many different kinds. A thimbleful of shell sand, spread and magnified, will reveal the fragmented shells of a host of humble creatures, each of which makes its tiny but vital contribution to the grand scheme.
The Hebrides, therefore, have a potential for research in fundamental, natural processes, and none have been used more than Rum and St Kilda for this purpose. In the 1950s, both of these islands were recognised as outstanding for their unique flora and fauna. They have concise temperate/maritime ecosystems and classical geology, and are laboratories for long-term ecological research. Accordingly, they were made National Nature Reserves in 1957 and have been centres for research ever since.
Studies of the fundamental biology of large herbivores—the red deer on Rum and the Soay sheep on St Kilda—have been central research endeavours, which have provided an understanding not only of the animals themselves, but of the ecology of their whole island. The research on the red deer on Rum has been done in controlled conditions, which would be hard to obtain among wild deer on mainland deer forests. This has revealed the precise structure and dynamics of the deer population, and the behaviour of stags and hinds, through entire life-spans. At St Kilda, the mechanism of natural control of numbers of a free-ranging (unmanaged) population of Soay sheep has been studied over thirty years. These sheep have survived in their island home for probably over 1,000 years, and the mechanism of control of numbers seems to protect them and their habitat from degradation through overpopulation and inbreeding.
Twenty-two species of seabird breed in the Hebrides. St Kilda alone has fifteen species and possibly holds over a million seabirds in summer. The oceanic seabirds—petrels, auks, gannets and kittiwakes—have the mystical beauty of all truly wild creatures. They live most of their lives far upon the face of the wide ocean, and in summer they gather in their thousands for a great carnival of nesting. The beauty and excitement of the birds wheeling and darting in the splendour of sunlit cliff and chasm brings awe and rapture to the dullest of hearts. For those who brave the benighted tops of Rum or the cliff terraces of the outliers there is a contrast equally as moving—the weird, dark world of the night-flying petrels.
The study of the seabirds poses physical as well as intellectual problems. Simple routines of counting the birds and interpreting the census data are difficult to achieve with any degree of consistency between counts. Nonetheless, in the past thirty years, marine ornithologists throughout the world have greatly improved census methods of many species which present different technical problems. For example, gannets and fulmars nesting in the open require different techniques from burrowing puffins, and both are different from night-flying petrels. To detect changes in the size of the populations, a sustained census effort is required over decades, and this needs forward planning and the handing on of the techniques to successive workers.
Figs. 2 a & b Location maps of the Outer and Inner Hebrides
The south-east face of Sula Sgeir showing the northern limits of the gannetry in 1962 (Photo J. M. Boyd)
The seabirds of the Hebrides are a major part of Britain’s heritage of wildlife, requiring study for its own sake. However, the status of the seabird populations can also be an indicator of the health of their environment. Through the food chain which starts with the microscopic life in the sea and passes through invertebrates and fish, the seabirds can become the repository of pollutants such as polychlorinated biphenols (PCB’s) and heavy metals. Such pollutants are likely to affect the breeding performance of the seabirds
