Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 67

For over 100 years an intense debate has been going on among biologists and geologists about the history of the plants and animals on the North Atlantic islands. The question is whether the species survived the last ice age in ice‐free areas, or if they arrived after the last deglaciation. The discussion was opened by the Danish botanist Eugen Warming when he proposed that most vascular plants could have survived the ice age in Greenland (Warming 1888). This was strongly opposed by the Swedish geologist A.G. Nathorst, who believed that most species of vascular plants immigrated after the last ice age (Nathorst 1892).

The debate is still very much alive. Among the botanists who in more recent times have advocated most warmly for survival is Tyge Böcher. Thus he wrote: ‘Is it a too daring assumption that the (tree) birch held its own … in the lowlands on sunny sides of the mountains during the last Glacial Age? If the Greenland birch (Betula pubescens coll.) demonstrably constitutes a special form cycle which is not identical with Icelandic or American races, this will highly strengthen such a survival hypothesis’ (Böcher 1956). Tree birch is one of the most warmth demanding plant species in Greenland, and is only found in the far south of Greenland. If it survived the last ice age in Greenland, this would mean that ice‐free areas were present and that the climate was not much different from the present. Tyge Böcher also proposed that the dwarf birch could be an ice age survivor. Even though dwarf birch is far less warmth demanding than tree birch, it also has a northern range limit in Greenland, and from a Greenland point of view it cannot be considered a cold adapted plant. Betula species are prolific pollen producers and their former arrival at a certain area in Greenland is usually marked by an abrupt increase in pollen values. It is now clear from palaeobotanical studies of lake deposits that all three Greenlandic birch species are Holocene immigrants (Fredskild 1991).

Among zoologists that have advocated for survival on the North Atlantic Islands the Swedish entomologist Carl Lindroth must be mentioned. Originally Lindroth suggested that many beetle species had colonized the Faroe Islands, Iceland and Greenland over a land bridge, which he suggested had existed after the last ice age. When geological investigations in the North Atlantic showed that such a land bridge could not have existed this late, Lindroth proposed that the beetles had colonized the North Atlantic Islands in interglacial time and survived the last ice age (Lindroth 1957, 1973).

Most palaeoecologists have opposed the idea of survival. Johs Iversen, who worked out the first pollen diagram from Greenland, proposed that only the most hardy plants survived (Iversen 1954), a point of view followed by Fredskild who continued Iversen's work (Fredskild 1973) and by Ole Bennike (Figure 15, see Plate section; Bennike 1999). Among zoologists, Russell Coope and Paul Buckland from Britain opposed Lindroth's ideas about survival, as almost all the Greenland beetle species are warmth demanding and could hardly survive the last ice age in Greenland (Coope 1986). Thus they concluded that most beetle species immigrated after the last ice age. In a recent paper, Jens Böcher suggested that the importance of micro‐climate and the number of sun hours during the Arctic summer have not been sufficiently considered when discussing survival contra immigration, and he contended that some hardy beetle species and the seed bug N. groenlandicus could be survivors (Böcher 2012).

Figure 15 Maps of the Earth’s northern parts with arrows that show immigration routes to the North Atlantic islands. The routes are mainly suggested from the modern‐day geographical ranges of the species. However, we know from historical sources that Norse people sailed to the North Atlantic islands from North‐West Europe and hunters travelled with their families to Greenland from Canada as late as the 1860s. In 1937 large flocks of migrating fieldfares Turdus pilaris were blown from Norway to Greenland where they established a small breeding population. Surprisingly many species have colonized the North Atlantic islands from North‐West Europe. The distance between the islands is long, but the islands form stepping stones along the route

Source: From Bennike (1999).

With respect to Iceland, Rundgren and Ingólfsson (1999) suggested that survival was supplemented by renewed immigration during each interglacial period. The interglacial floras and faunas show that rich biota existed in suitable biotopes on the North Atlantic islands during the Quaternary interglacials. However, the interglacial occurrences do not prove continuity, as pointed out by Buckland and Panagiotakopulu (2010). Anyway, it is clear that the old discussion about survival contra immigration is far from dead.