Development and structure of populations and communities

The low species diversity of both plants and animals in polar regions contributes to the lack of complexity that characterizes Arctic and Antarctic ecosystems. The short summer season during which plants can grow and insects and other invertebrates can be active contributes to the lower productivity and relative simplicity of these ecosystems. In addition, the cooler temperatures limit the rate at which soil nutrients essential for plant growth are released through decomposition of organic material, breakdown of the parent rock, and fixation of nitrogen by soil microbes.

A consequence of ecosystem simplicity is a lack of stability. Animals tend to undergo wide population fluctuations in the tundra and polar barrens. These fluctuations are stimulated by periodic extremes of weather and imbalances in herbivore-plant and predator-prey relationships. Each species plays a much more dominant role in the trophic dynamics of ecosystems in the Arctic than do species in the highly complex temperate and tropical ecosystems. The extreme abundance of lemmings during the peak of their population cycle (as many as 200 per hectare) is accompanied by high reproductive success and rapid increase of their predators—Arctic foxes, ermines, snowy owls, and other species. Conversely, when lemming numbers are low (fewer than 1 per hectare) breeding among their predators ceases and predator populations plummet.

In the High Arctic, animal populations live close to their biological limitations. Peary caribou or musk oxen may be so affected by periods of extreme weather that they become locally extinct. Reestablishment of the species may not occur until favourable conditions allow adjacent populations to build up, an event that may take decades. Indeed, in northernmost Greenland, although caribou have not been present in recent history, antlers and bones indicate their periodic presence at least 7,000 years ago.

Snow plays an important role in determining the characteristics and distribution of plant and animal communities in tundra and polar barrens. Winters are long, and the limited snow that falls usually accumulates without melting throughout the entire season. The snow cover, however, is not stable. The strong winds that characterize these treeless landscapes redistribute the snow, removing it from landscape convexities and depositing it as drifts in concavities, in patches of shrubs, or leeward of ridge tops and boulders. The wind also compacts snow, increasing its density, thus enabling animals to move more easily over its surface while hampering their ability to dig through to find food below. Snow, therefore, influences the location, food selection, and energy expenditure of large herbivores that must move across the winter landscape to feed. Areas of deeper snow cover appeal to small rodents who feed and construct winter nests at the ground’s surface under the insulating layer of snow.

The pattern of distribution of snow on the tundra and polar barrens is also a major determinant in the distribution of vegetation. Most plants require snow cover to protect them from the extreme cold and drying conditions of winter. Areas in which the snow cover has been blown away are the first to initiate plant growth in summer, whereas those in which snowbanks are slow to melt support only those plants adapted to the shorter snow-free season. The winter accumulation of snow, however, is often the primary source of moisture for the summer growth of plants. Consequently, windy exposures with little snow cover often suffer summer drought that limits plant growth, whereas snow-bed plant communities are well watered throughout summer.

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Ecosystems

Biological productivity

In the Low Arctic, vegetation covers 80 to 100 percent of the land area. The rapid growth of vascular plants under the continuous daylight of the brief Arctic summer is the basis for the relatively high productivity of these plant communities at such high altitudes. Because all aboveground productivity of plants is close to ground level in the tundra in contrast to forest biomes, it is readily available to vertebrate herbivores. As a consequence large concentrations of caribou and geese graze locally in the Low Arctic in summer but migrate out of the Arctic to winter when the quality of forage declines and its availability is limited by the wind-packed snow cover. Although tundra systems experience a burst of plant and animal productivity during the brief Arctic summer, only a few animal species remain active in the Arctic throughout the long winter. Herbivores are dependent on plant production of the previous summer, and carnivores depend on their herbivore prey. Total annual productivity in the tundra and polar barrens is several orders of magnitude less than it is in most temperate or tropical ecosystems. (For further information on productivity see biosphere: The organism and the environment: Resources of the biosphere: The flow of energy.)

Vertebrate herbivory may result in changes in tundra vegetation through selective feeding on preferred plant species, trampling, and recycling of soil nutrients through excretion. Where caribou, musk oxen, lemmings, and geese concentrate their grazing activity they may actually increase production of tundra vegetation. This results from removal of much of the annual vegetative growth, exposing the underlying new growth of plants to the rays of the sun and the soil to increased heating, and from speeding up the recycling of organic material through digestion and excretion that releases nutrients to the soil.