The Arctic and subarctic regions Compared with other biomes, the tundra biome is relatively young, having its origin in the Pleistocene (1,600,000 to 10,000 years ago). Individual plant and animal species of the tundra, however, probably first appeared in the Late Miocene (11.2 to 5.3 million years ago) or Early Pliocene (5.3 to 3.4 million years ago). Coniferous forests were present on Ellesmere Island and in northern Greenland, the northernmost land areas, in the mid-Pliocene (2.5 million years ago). Most paleoecologists believe that tundra flora evolved from plants of the coniferous forests and alpine areas as continents drifted into higher and cooler latitudes during the Miocene (23.7 to 5.3 million years ago). The Antarctic region Antarctica has been isolated from other continental landmasses by broad expanses of ocean since early in the Tertiary Period, about 60 to 40 million years ago. Prior to its separation it existed, along with Australia, South America, peninsular India, and Africa, as part of the landmass known as Gondwanaland. This long separation has impeded the establishment and development of land-based flora and fauna in the Antarctic. Other significant factors that have hampered terrestrial biotic evolution are the harsh climate, the ice cover that completely engulfed the continent during the Pleistocene glaciations, and the present limited number of ice-free land areas, which are restricted primarily to the coastal fringes and nunataks (mountain peaks surrounded by the ice cap). As a consequence, the terrestrial flora and fauna of Antarctica are few. The Antarctic Peninsula, which extends to 63| S, is the location of virtually all floral development of the Antarctic. The Antarctic, however, encompasses not only the continent itself but also those islands lying within the Antarctic Convergence, where northward-flowing cold surface waters meet warmer subantarctic waters. Most Antarctic islands, because of their position beyond the seasonal pack ice, are under much stronger maritime influence than comparable Arctic islands. The flora and fauna of these islands are poorly developed, largely because of their isolation from potential sources of terrestrial biota. South Georgia, the largest of these islands, lies 2,000 kilometres (1,240 miles) east of Tierra del Fuego at 54| to 55| S and encompasses an area of 3,756 square kilometres (1,450 square miles). Heavy precipitation and high mountains account for perennial snow cover at higher elevations and extensive glaciers. Nevertheless, near sea level where soil formation has been possible, a unique Antarctic tundra vegetation has developed, dominated by tussock-forming grasses and mosses. Only 26 species of vascular plants are present, and there are no native land mammals, although reindeer introduced by Norwegian whalers in the early 1900s have established feral populations. Grazing by reindeer has altered plant communities by reduction or local elimination of the most preferred species. Polar environments Characteristics of polar environments--the climate, substrates, elevation above sea level, slope, exposure, and proximity to other landmasses--determine the complex of plant and animal life present in the polar regions. It is the interplay of these factors during summer that determines whether temperatures are warm enough and moisture is sufficient to allow plants to grow. The High Arctic is distinguished from the Low Arctic based on ecological criteria, which include a shorter growing season, cooler summers, and a marked reduction in species of flora and fauna. The southern limit of the tundra zone in the Northern Hemisphere may extend from 55| N at the southern tip of Hudson Bay in Canada along the northern Bering Sea coast of Alaska and the Russian Far East to above 70| N on the lower Mackenzie River of Canada, along the Khatanga River of central Siberia, and across northern Scandinavia. This limit generally coincides with the isoline of annual net solar radiation of 75 to 80 kilojoules per square centimetre, which closely parallels the 10| C (50| F) July isotherm. However, local variation in this boundary occurs in North America and Eurasia where influences of mountain ranges or warm ocean currents allow forests to penetrate northward to areas with as little as 67 kilojoules per square centimetre of radiation. Daily summer temperatures may average only 2| to 5| C (35| to 41| F) in typical tundra and polar barrens, and the plant growth season is usually less than 100 days. Although summers are brief at high latitudes, the days of summer are long. At latitudes above the Arctic Circle (66|33' N) for at least a portion of the summer the Sun is above the horizon for 24 hours each day. This constant sunlight enables plants to optimize photosynthesis without the usual nighttime cost of respiration. Total annual precipitation is low in the tundra and polar barrens, generally ranging between 100 and 1,000 millimetres (4 to 40 inches) per year. Precipitation is usually greatest near the coasts and at high altitudes. For example, in Greenland and Antarctica and at higher elevations on the Canadian Arctic Archipelago, permanent ice caps form. The low precipitation characteristic of tundra regions is comparable to that of deserts at lower latitudes ;however, these regions are unlike true deserts in that the low temperatures in both winter and summer limit evaporation. Thus, a greater portion of the limited moisture from snowmelt and summer rains is available for plant growth. In addition, most tundra and polar barrens are underlain by soils that are permanently frozen (permafrost) except for a thin surface zone (active layer) that thaws each summer. The permafrost limits drainage and retains moisture for plant growth within the active layer. This can result in wetland formation, as occurs in level areas of the coastal plains of the Arctic, which contain extensive wetlands that are home to aquatic vegetation, invertebrate fauna, waterfowl, and shorebirds. Circulation of air masses around the Earth transports industrial pollutants from temperate regions into the Arctic and Antarctic. Pollutants are becoming concentrated in these regions, and concern is growing over the possible consequences for life at high latitudes. Thinning of the ozone layer in the polar regions resulting from the industrial release of chlorofluorocarbons into the atmosphere may have long-term consequences for plant and animal life. Ultraviolet radiation, previously intercepted by the ozone layer, is detrimental to most life because it inhibits photosynthesis and increases mutation rates in DNA and damages epidermal tissues in plants and animals. Biota of tundra and polar barrens Biota of the Arctic A transition zone exists at the northern limit of trees where coniferous forest interdigitates with treeless tundra vegetation. In North America, white and black spruce (Picea glauca and P. mariana) interface with tundra, whereas in Siberia and northern Europe larch (Larix) is the primary tree line species. Cottonwoods (Populus species) often penetrate the tundra landscape in the Low Arctic along major rivers. Major vegetation types of the Low Arctic include low-shrub tundra, dominated by species of willow (Salix) and dwarf birch (Betula) ;tall-shrub tundra, dominated by species of willow, shrub birch, and alder (Alnus) ;and combinations of sedges and dwarf shrubs, such as species of Labrador tea (Ledum), blueberry and cranberry (Vaccinium), crowberry (Empetrum), and Arctic heather (Cassiope), in wetter sites. Cushion plants (Dryas and Saxifraga species) are common on windswept uplands. Lichens and mosses are important components of the ground cover in some areas. In the Low Arctic, most land surfaces are fully vegetated, with the exception of rock outcrops, dry ridge tops, river gravel bars, and scree slopes (those slopes that have an accumulation of rocky debris at the angle of repose). The vegetation of the High Arctic is less rich than that of the Low Arctic, containing only about half the vascular plant species found in the Low Arctic. For example, more than 600 species of plants are found in the Low Arctic of North America, but in the extreme High Arctic of northern Ellesmere Island and Greenland--north of 83| N--fewer than 100 species of vascular plants grow. The shorter growing season, cooler summers, and drier conditions, as well as the distance of these landmasses from continental flora, account for this difference. More than 40 percent of vascular plant species of the Arctic are circumpolar in distribution. Mosses increase in importance in High Arctic plant communities, and shrub species decrease markedly, with only a few prostrate willows, dwarf birch, and other dwarf shrubs remaining. Prostrate willows, however, remain important components of plant communities that retain some winter snow cover, even in the northernmost land areas. Sedge-moss meadows occur on limited wet sites in valley bottoms watered by melting snows. Upland sites are drier and have a more sparse ground cover that merges into polar desert at higher elevations or where insufficient moisture is available for plant growth. Grasses, occasional prostrate willows, and mat-forming dryas occur in patches in the uplands and are the dominant vegetation in the polar barrens. The true polar desert generally occurs on coastal areas fringing the Arctic Ocean and on areas of a few hundred metres elevation in the extreme High Arctic where soils have not developed and the frost-free period and soil moisture are insufficient for most plant growth. The occasional plants growing there often become established in frost cracks that capture blowing snow and finer windblown soil material. Plants adapted to these conditions include species of the Arctic poppy (Papaver), some rushes (Juncus), small saxifrages (Saxifraga), and a few other rosette-forming herbaceous species. The Arctic poppy and a few of the other flowering herbs adapted to the High Arctic have flowers that are solartropic (turning in response to the Sun). Their parabolic-shaped blossoms track daily movements of the Sun, thereby concentrating solar heat on the developing ovary, warming pollinating insects that land there, and speeding the growth of embryonic seeds. Arctic ecosystems lack the diversity and richness of species that characterize temperate and tropical ecosystems. Animal as well as plant species decline in number with increasing latitude in both polar regions. Vertebrate species of the Arctic tundra and polar barrens are limited to mammals and birds ;no amphibians or reptiles occur there. About 20 species of mammals and more than 100 species of birds are present throughout the Arctic. Most are circumpolar in their distribution as single species or closely related species ;for example, the caribou of North America and the domestic and wild reindeer of Eurasia belong to the same species, Rangifer tarandus, whereas the lemmings of the Eurasian Arctic are a closely related but distinct species from those of northern North America and Greenland. This similarity in Arctic mammalian fauna is a result of the lower sea levels of the Pleistocene glaciations, when a broad land connection, known as the Bering Land Bridge, connected present-day Alaska and Siberia. Some Arctic mammalian fauna--primarily herbivores such as caribou and reindeer, muskox (Ovibos moschatus ;see photograph), and Arctic fox (Alopex lagopus ;see photograph), and species of Arctic hare (Lepus) and collared and brown lemmings (Dicrostonyx and Lemmus)--rarely occur outside the Arctic and are adapted to life in this environment. Other fauna such as species of ground squirrel