Collins New Naturalist Library. David Cabot
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Название: Collins New Naturalist Library
Автор: David Cabot
Издательство: HarperCollins
Жанр: Природа и животные
isbn: 9780007400423
isbn:
During the next 4,900 years, from 10,000–5,100 years ago, the Irish landscape evolved from open tundra to a country almost totally swaddled by woodlands. Only the mountains, poking above the green canopy, and the rivers, lakes and bogs in the lowlands differed from their surroundings. Temperatures continued to rise, more than doubling the July mean temperature from about 7°C to 15°C, approximately the same as today. Because of these new climatic conditions, Ireland became available for colonisation by the flora and fauna that had survived on the ice-free and warmer European mainland and also possibly in parts of Ireland.
How Ireland acquired its flora and fauna is a continuing and unresolved saga. There are three principal scenarios. Firstly, many plants and animals may have entered the country before the Ice Age or during interglacial periods and survived in ice-free areas. The flora and fauna then colonised the landscape at the end of the Ice Age. Forbes first championed this preglacial survival hypothesis in 1846.9 It received support from Praeger in 1932 and Beirne in 1952.10,11 Secondly, there may have been no Ice Age or interglacial survivors, and Ireland’s flora and fauna mostly arrived during the postglacial period, migrating from Britain and southern Europe when sea levels were some 130 m lower than present. This hypothesis was supported by Charlesworth in 1930, Godwin in 1975 and most recently by Mitchell & Ryan in 1997.2,12,13 Finally, postglacial arrival may have been by aerial dispersion, chance methods, and introduction, deliberate or accidental, by early man. This hypothesis was postulated by Reid in 1899 and more recently by Corbet in 1961.14,15
The most probable explanation is likely to be a combination of the three possibilities. Thus it would seem quite plausible for many species to have survived the Midlandian glaciation in the southern ice-free zones, and possibly earlier episodes of extreme coldness, while many other species may have arrived in Ireland during the postglacial period. The postglacial land bridge migration of flora and fauna has been strongly argued by Mitchell & Ryan. They postulated that land bridges between Ireland and Britain existed when the sea level fell to about 130 m below today’s levels exposing dry ridges of land, thus making it possible to cross dry-shod from the Lancashire-Cumbria area to Dublin, from north Wales to Co. Wicklow and from Cornwall to southeast Ireland. Ireland also had an Atlantic coastal ‘pathway’ linking it with southwest England, France and northern Spain. The sea fell to its lowest level some 15,000 years ago. By 10,000 years ago it was back to present day levels. Despite the appeal of the land bridge routes, the country could well have been repopulated from a reservoir of flora and fauna that had survived in the southern ice-free areas of Limerick, Cork and Waterford.
Late glacial Ireland. Column A gives the timescale for the last 13,000 radiocarbon years. Column B gives the names of the Irish type sites. Column C shows temperature trends, largely based on information from fossil animals and plants rather than instrumental measurements. Column D shows geomorphological and soil developments. Column E outlines vegetational developments. Column F lists mammalian records. From Mitchell & Ryan2.
In support of their land bridge concept, Mitchell & Ryan explain the process whereby the retreat northwards of the large wedge of ice that filled the area now occupied by the Irish Sea created a land bridge which was ‘pulled’ northwards with the withdrawal of the glacier. It is argued that the great weight of the ice depressed the land underneath which was squeezed out laterally and at the front of the glacier. Pushing down a fist into a ball of dough would produce a similar effect with the dough squeezed out laterally and rising up around the edges. The squeezed-out land moved out sideways and in front of the ice as a sort of bow of land as the ice pushed southwards. On the retreat of the ice northwards up the Irish Sea area the forebulge of land also retreated. Mitchell & Ryan, drawing upon a detailed study by Wingfield of the British Geological Survey,16 postulated that the fore-bulge moved or was pushed into the south end of the Irish Sea area about 11,000 years ago, when it provided a land bridge link between Devon and Carnsore Point, Co. Wexford. As the glacier melted and retreated northwards up the Irish Sea so the fore-bulge followed, providing a sort of moving land bridge link, of a continually diminishing height, across which plants and animals were able to migrate into Ireland from west Wales. About 9,500 years ago the bridge was enveloped and submerged by the rising waters of the Irish Sea.
Outline curve to indicate possible course of sea level around Ireland during the last 40,000 years. From Mitchell & Ryan2.
A land bridge also spanned what is now the English Channel, remaining open for business for approximately 2,500 years longer than the Irish-Welsh bridge. It was along this route that the plants and animals almost certainly moved from southern Europe to Ireland. They had about 1,500 years to travel across a ‘dry’ Irish Sea from Britain, having already trekked from the Continent into Britain over a ‘dry’ English Channel. It may seem a long time but in fact the colonisation process was a race against time as the immigration routes were being rapidly cut off by the rising waters, first in the Irish Sea and subsequently in the English Channel. Many species failing to cross the last bridge remained circumscribed to Britain, and the paucity of the Irish flora and fauna today is mainly – albeit not entirely – attributable to that late phenomenon. There remains, however, much conjecture and many difficult unanswered questions about the ways and means by which many animals and plants may have moved back into Ireland over the land bridges.17,18
Many of the plants and animals involved in the migration process would have travelled a distance of at least 1,000 km by direct line, say, for example, from Luxembourg to Dublin. The time allocated for the trip (about 2,500 years including the 1,500 years when the Irish land bridge was open) would have allowed a rate of migration of 400 m per annum, a not unrealistic rate of progress. The pace of settlement was fast indeed as oaks were already growing in the south of Ireland 9,000 years ago while wild boars were being hunted near Coleraine, Co. Derry, by the first men on record. Wild deer too, had already established their presence in the Midlands some 8,400 years ago. However, opponents of the land bridge hypothesis would argue that these species were already present in the country and their populations expanded their range with rising temperatures.
Ten thousand years ago, pollen deposits were being laid down in a raised bog near Littleton, Co. Tipperary, starting an important historical archive and providing one of the most important chronologies of vegetational development in Ireland from 10,000 years ago to the present time.19 It tells the story of rich meadows of grasses, docks and meadowsweet which were quickly replaced by a juniper scrub mixed with willow trees. These low-growing species were subsequently overtaken by the taller downy birch forming the first real woodlands. Hazel then established itself with a patchy distribution of sometimes quite dense stands, while Scots pine also started expanding from about 9,000–8,500 years ago to produce a pine-hazel wooded landscape.
The oak first put in an appearance around 9,000 years ago and quickly expanded its range together with the wych elm to form a high forest. As a result of this woodland expansion, the pine was pushed off the better Midland soils onto the poorer regions of the west. Alder also extended СКАЧАТЬ