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The Hebrides. J. M. Boyd
Читать онлайн.Название The Hebrides
Год выпуска 0
isbn 9780007406630
Автор произведения J. M. Boyd
Жанр Биология
Серия Collins New Naturalist Library
Издательство HarperCollins
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 and the golden eagles and sea eagles which feed upon them in the Hebrides. In the case of a Chernobyl-like nuclear fall-out in the north-east Atlantic, St Kilda might prove an invaluable nuclear sensor. The great puffineries are rich in marine organic debris gathered from a wide area of ocean. They are grazed heavily by sheep which could become contaminated. The concentration of radio-active material in the individual seabird might be very small, but that in the bone marrow of the lambs may be much greater. Is it too imaginative to see the seabird-sheep islands as future sensors of the marine environment?
What happens to us
Is irrelevant to the world’s geology
But what happens to the world’s geology
Is not irrelevant to us.
We must reconcile ourselves to the stones,
Not the stones to us.
Hugh MacDiarmid
Natural history starts with the elements of fire, earth, air and water all of which long pre-date life on the face of the Earth. No clear understanding of the origins and nature of life can be obtained without knowledge of the rocks, weather and conditions of the seas and freshwaters. It is on the interface between these elements that all life has sprung and been maintained throughout aeons of time, and nowhere is this truth more explicit than in an archipelago. There, among the islands, the grand relationship between land, sea and sky is obvious and makes a deep appeal to the human mind. Islands are a source of inspiration and happiness; their beauty is enshrined in a multitude of native island cultures all over the world and appreciated by historian, artist and scientist alike. The Hebrides are no exception. In them it is possible to trace the connections between these base elements and the lives of the wild creatures and human beings that spring from them, and to see the islands as one large system with its own in-built stops and balances in terrain, weather and ocean. Let us start with the rocks.
The span of geological time represented in the rocks of the Hebrides is almost as great as anywhere in the world. Though we know that planet Earth is some 4,600 million years of age, in human terms, the Lewisian gneiss formed some 3,000 million years ago is as old as time itself, while on the beds of the sea and the deep lochs the rocks of the future are being formed from the erosion products of by-gone glaciers, rivers and the sea. The cycle of regeneration and decay of hard rock seems timeless when compared with the timespan of human life.
In this vast interval of time, that part of the crust upon which the Hebrides now stand underwent a gradual transposition from tropical to temperate latitudes. Some ages of peace and tranquility are marked by the depositions of the sedimentary rocks: the Torridonian sandstone eroded from a range of mountains and deposited in predominantly desert conditions, 1,000 to 800 million years ago; the sandstones, shales and limestones of Cambro-Ordovician/Dalradian age, 600 to 450 million years ago; and the limestones and mudstones of the Jurassic, deposited in shallow lacustrine or estuarine conditions c. 150 million years ago (called not after the island of Jura, but the Jura Mountains in France).
Fig. 3 a & b Geological maps of the Outer and Inner Hebrides (Smith and Fettes, 1979, Craig (ed) 1983)
Between these periods of quiescence there were periods of profound crustal movement as blocks of continental crust fractured, jostled and were transported on plates of underlying crust, though the first of these hardly touches the Hebrides. During the Grenville mountain building about 1,000 million years ago, rocks which were probably the equivalent of the Torridonian strata far to the east of the present outcrops were compressed, deeply buried and heated in the crust, baked and altered to form the schists and metasandstones of the Moine Supergroup. These metamorphic rocks together with unaltered Torridian in turn formed a land surface on which were deposited limestones, shales and sandstones of Cambrian and early Ordovician age.
The second great upheaval was the Caledonian mountain building, 650 to 400 million years ago, when the rocks of the