Holarctic

Holarctic
source
Name	Holarctic
Area	~54,000,000 km²
Countries	Canada, United States, Russia, Greenland, Iceland, Norway, Sweden, Finland, China, Mongolia, Japan
Biome	Tundra, Taiga, Temperate broadleaf and mixed forest, Temperate grasslands, Mediterranean climate

Holarctic. The Holarctic is a vast biogeographic realm encompassing most of the Northern Hemisphere's terrestrial ecosystems north of the Tropic of Cancer. It unifies the Nearctic region, covering North America north of the Mexican Plateau, and the Palearctic region, spanning Eurasia north of the Himalayas and Sahara. This realm is defined by shared evolutionary history and the presence of numerous related plant and animal taxa across continents, largely due to past land connections like the Bering land bridge.

Definition and extent

The realm's boundaries are primarily defined by climatic and biological barriers rather than strict political borders. Its southern limit in North America generally follows the Mexican Plateau, while in Eurasia, it is bounded by the arid belt of the Sahara and the high elevations of the Himalayas and the Hengduan Mountains. The realm includes the entire Arctic region, extending south to encompass the temperate deciduous forests of Eastern North America, the steppes of Central Asia, and the Mediterranean Basin scrublands. Major landmasses within it include Greenland, Iceland, the British Isles, and the Japanese archipelago. Key delineating features are the Isthmus of Panama, which separates it from the Neotropical realm, and the Wallace Line in the Indonesian archipelago, marking a transition to the Australasian realm.

Biogeographic significance

The primary significance lies in its demonstration of continental drift and the profound effects of Pleistocene glaciations on species distribution. The historical connection between Asia and North America via Beringia allowed for extensive faunal interchange, leading to closely related species on different continents, such as the brown bear and the American black bear. This realm is a classic example for studying vicariance and dispersal biogeography. The concept was heavily influenced by the work of early biogeographers like Alfred Russel Wallace and later refined through studies of molecular phylogenetics. It provides a framework for understanding global biodiversity patterns and is fundamental to conservation planning by organizations like the World Wildlife Fund.

Flora and fauna

Characteristic flora includes widespread boreal coniferous families like the Pinaceae (pines, spruces) and genera such as Betula (birches) and Salix (willows). Iconic fauna showcases this transcontinental kinship, with families like the Cervidae (deer), Ursidae (bears), Canidae (wolves, foxes), and Castoridae (beavers) found across the realm. Notable species pairs include the Eurasian lynx and Canadian lynx, the moose (known as elk in Eurasia), and the red fox. The Arctic regions support specialists like the polar bear, Arctic fox, lemmings, and migratory birds such as the snow goose. Endemic groups are fewer but include the Aegithalidae (long-tailed tits) and the plant family Crossosomataceae.

Geological and climatic history

The realm's unity is a legacy of the ancient supercontinent Laurasia, which began fragmenting in the Mesozoic Era. The opening of the North Atlantic Ocean separated Europe and North America, while the intermittent emergence of the Bering land bridge during periods of low sea level, such as the Last Glacial Maximum, maintained biotic connections. Repeated Quaternary glaciation cycles sculpted its ecosystems, with ice sheets scouring Scandinavia and the Laurentide Ice Sheet covering much of Canada. These events forced species into refugia in areas like Beringia, the Caucasus, and the Appalachian Mountains, driving speciation and shaping modern genetic diversity. Post-glacial recolonization patterns are a major focus of phylogeography.

Subdivisions and ecoregions

The realm is subdivided into two major regions: the Nearctic and the Palearctic. These are further broken into numerous biomes and ecoregions as classified by the World Wildlife Fund. Major Nearctic biomes include the Canadian Shield taiga, the Great Plains temperate grasslands, and the Appalachian temperate rainforest. The Palearctic encompasses the Siberian taiga, the European Atlantic mixed forests, the Central Asian steppe, and the East Siberian taiga. Significant transitional zones include the Mexican transition zone in the south and the Sino-Japanese region, which harbors exceptional floral diversity. Other notable ecoregions are the Scandinavian coastal conifer forests, the Ussuri broadleaf and mixed forests, and the Rocky Mountain montane forests. Category:Biogeography Category:Ecoregions Category:Realms