Holarctic Region(redirected from Holarctic)
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Related to Holarctic: Nearctic
or Holarctica, a floristic and zoogeographic land area. The boundaries of the Holarctic region do not coincide for plants and animals.
The Holarctic floristic region (or floristic kingdom) occupies the extratropical portion of the northern hemisphere; its southern boundary passes approximately along the Tropic of Cancer. It borders on the Neotropical region and the Palaeotropical region. The Holarctic region comprises about one-half of the land area, and if the expanses covered by the continental glacial shields are excluded, even slightly more than one-half of the area available for the growth of terrestrial plants. The floras of the Holarctic region were formed in the late Cretaceous and Paleogene, but were transformed by the overall cooling of the extratropical expanses as well as by the development of the contrasting relief of the earth’s surface beginning with the end of the Paleogene. The basic features of the floras in the more northern parts of the Holarctic were formed in the Neocene and Pleistocene. The ancient features of the floras have survived to a significant degree in the southern parts (particularly in East Asia and, partly, in North America). In the early Paleogene, the climate of the southern borderlands of the Holarctic region was almost tropical; however, in places significant seasonal temperature fluctuations determined the early development of mesophyllous deciduous trees which, in combination with relatively heat-loving conifers and with the presence of an insignificant number of leafy evergreen trees, constituted the basis of the Arctic Tertiary flora. One can note genetic links between the floras of all regions of the Holarctic; these have been caused by the smaller amount of differentiation in the physical geographic conditions during the previous geological eras, the closer contact between the floras of the individual parts of Holarctica, and the presence in the past of the Tethys Ocean, which cut off the Holarctic land area from the tropical continental expanses.
The southern parts of the modern Holarctic region are characterized by a greater specific and, partly, generic richness of the floras, as well as by a greater uniqueness in the specific composition than are the northern parts. The floras are richest in East Asia, southeastern North America, California, and the Mediterranean region. In the USSR, the Transcaucasian flora is the richest. The floras are poor in the Arctic as well as in the Sahara Desert, the Syrian Desert, the Turan, and the deserts of Central Asia. The floras in the mountainous parts are distinguished by the sharp spatial differentiation of their composition.
The floras of the Holarctic region are characterized by such families as Compositae, Gramineae, Cyperaceae, Leguminosae, Ranunculaceae, Liliaceae, Cruciferae, Caryophyllaceae, Rosaceae, Umbelliferae, Scrophu-lariaceae, and Labiatae. However, none of these families is endemic. The woody plants are characterized by representatives of the families Betulaceae, Salicaceae, Juglandaceae, and Rosaceae; the conifers include the Taxodiaceae (basically ancient relict genera with a narrowly limited modern distribution) and particularly the Pinaceae, of which only solitary species have penetrated into the regions that are adjacent to the Holarctic region. The Araceae, Orchidaceae (only terrestrial forms), Euphorbiaceae, Rubiaceae, palms, and Myrtaceae (only near the southern border) have a limited representation. Filicales are represented moderately (relatively abundantly in Eastern Asia and in other wet, moderately warm areas). The role of club mosses and horsetails (which, as everywhere, are limited in number) is relatively greater than in many tropical floras. The proportional amount of mosses and lichens increases (excluding the arid regions) as one moves from the south to the north. Fungi are represented by a significant number of species in all sufficiently wet parts of Holarctica.
The Holarctic region (or kingdom) is divided into a number of floristic subregions (or regions). In the Old World, the following subregions (or regions) can be established: Mediterranean, Middle European, Central Asian, East Asian, Boreal, and Arctic. The last two also encompass the northern parts of North America, while the more southerly parts of North America are divided into two or three independent subregions. Other divisions, including smaller ones (with the isolating of up to 15 and more subregions) are possible. Each subregion is characterized by a more constant set of taxonomic plant groups and relationships between these groups than is found in the Holarctic as a whole, as well as by the presence of endemic genera (few in the Arctic, Boreal, and Middle European regions and many in the East Asian and Mediterranean regions). The most essential features of the floras in the southern areas were basically formed earlier than the younger floras of the temperate zone and particularly earlier than the floras of the northern parts of the Holarctic region. Comparatively recent transformations in the floras are also characteristic for the areas that are arid or have undergone significant uplifting during the alpine folding and after it, as well as those that experienced intensive glaciation in the Anthropogene.
REFERENCESDiels, L. Botanicheskaia geografiia. Petrograd, 1916.
Pavlov, N. V. Botanicheskaia geografiia SSSR. Alma-Ata, 1948.
Pavlov, N. V. Botanicheskaia geografiia zarubezhnykh stran, parts 1–2. Moscow, 1965.
Alekhin, V. V., L. V. Kudriashov, and V. S. Govorukhin. Geografiia rastenii s osnovami botaniki, 2d ed. Moscow, 1961.
The fauna of the Holarctic region, in spite of the great territory occupied by it and the diversity of natural conditions (from the deserts and tundras of the arctic to the wet subtropics and tropics), is relatively poor both in terms of the absolute number of species as well as in terms of the number of endemic groups. Of the mammals, the only endemic families are the beavers (two species), the mountain “beavers” (one species), moles and jerboas. Of the birds, the endemic families are the Tetraonidae and Gaviidae (the Alciformes are considered to be marine fauna). There are four endemic families of Urodela; of the fish, the endemics include Acipen-seridae, Salmonidae, Esocidae, Umbridae, and Gasteroste-idae. The paucity of the fauna in the Holarctic region is explained by the fact that a significant portion of its territory lies in the higher latitudes, as well as by the relative youth of the fauna of the larger portion of the territory; the profound climatic changes of the Ice Age encompassed the entire northern portion of the Holarctic region. The commonness of the fauna in the Nearctic and Palaearctic parts of the Holarctic region, which are now completely separated, is explainable, aside from the considerable similarity in the climatic and landscape conditions (zonality), by the fact that in the Pliocene and Quaternary period, there arose wide strips of land that connected Alaska and the Chukchi Peninsula as well as areas to the north of them (Beringia or the Beringian continental “bridge”) over which intensive migrations of animals occurred in both directions. Thus, the mountain goat, elk, deer, brown bear, and other animals migrated from Eurasia to America, as well as animals that became extinct there, including the mammoth, yak, and saiga. The reindeer, musk ox, and other animals moved from North America to Eurasia.
The faunas of the Holarctic region are characterized by the following main features. (1) In each portion of the region the fauna in the extreme north is very poor and uniform and becomes richer and more diversified to the south, where with favorable natural conditions (outside the deserts and high mountains) it can become rather rich. This is explained by the severity of modern conditions, the uniformity of landscapes in the north, and the designated historical factors, as well as by the penetration of numerous species from the neighboring tropical regions into the southern part of the Holarctic region. Moreover, a certain number of elements from the preglacial (Tertiary) fauna that was displaced to the Holarctic region survived here in part in the so-called glacial shelters (refuges). (2) The commonness of the fauna in the Holarctic region is most strongly expressed in the north and declines to the south. The fauna in the tundra zone of Eurasia and America shows virtually no differences, and the common traits are also well expressed in the forest zone (particularly the taiga). In the southern parts of the Holarctic region the similarity between the faunas of North America and Eurasia is considerably weaker. The faunas in the individual parts of the southern Holarctic region differ sharply not only on the different continents but even on the same continent. In the south of the Palaearctic there are three sub-regions, and two in the south of the Nearctic. (3) Although a number of species have a very broad distribution in the Hoiarctic region both in Eurasia and in North America (such as the wolf, fox, elk, ermine, and hawk owl), there is a more clearly expressed similarity between the faunas of northeastern Siberia, Alaska, and northwestern North America, which were more closely linked to the previously existing land of Beringia.
The Holarctic region is usually divided into seven subre-gions: the Arctic, Circumboreal, Mediterranean, Central Asian, Sino-Himalayan, Western American, and Eastern American subregions.
REFERENCESGeptner, V. G. Obshchaia zoogeografiia. Moscow-Leningrad, 1936.
Bobrinskii, N. A. Geografiia zhivotnykh (Kurs zoogeografii). Moscow, 1951.
Darlington, P. J. Zoogeografiia. Moscow, 1966. (Translated from English.)
Reinig, W. F. Die Holarktis. Jena, 1937.
V. G. GEPTNER