The Tundra Biome provides examples of adaptation to extreme conditions. About a fifth of the land surface of the earth is tundra.

    The tundra biome is found next to the icy zones in the arctic. (If there were land at those latitudes in the southern hemisphere, tundra might be found there, too, but this is not the case on earth right now.) There is also Alpine Tundra high on the slopes of mountains. The first part of this page is about arctic tundra. The two kinds of tundra have many characteristics in common, including a very short growing season and an absence of trees.

     During most of the year, temperatures on the tundra are below freezing, and may sometimes drop to as low as -70 degrees Fahrenheit. There are powerful winds that can blow up to 100 miles an hour. As a result of the cold, the water in the ground freezes: the ground can be frozen to a depth of 2000 feet or more. In some places only a few inches of the top part of the ground thaw out in the summer, in other places several feet may be thawed. The part of the ground that thaws is called the active layer. This layer is very wet, because the water from the melted ice cannot drain away. The frozen ground that never thaws out is in the grip of permafrost, and is called the permafrost layer.

     Although the tundra may get less than five inches of precipitation a year, the area is still very wet in summer. (One inch of precipitation (rain) = about ten inches of snow.)  The water that comes from the melting ice has nowhere to go. During the summer, the whole landscape is one of open, gently rolling ground, covered with many small lakes and ponds. The ground between the ponds is soggy. During this time, the days are close to 24 hours long, so that there is light for the little plants that grow wherever the ground is not under water. On a warm day the temperature may rise to 50 degrees Fahrenheit. 

     The plants in the tundra zone are only about four inches high. Many of them are perennials, building up food reserves in their roots from year to year. The plants consist of grasses, sedges, mosses, little flowering plants, and tiny dwarf willow bushes. They grow in shapes that protect them from the cold, drying winds -- in dense round cushions, or mats that hug the ground. Some of them have reddish leaves to get as much energy as possible from the sun. In the rocky places, lichens grow on the rocks. (Lichens were very early land plants, and are a partnership between an alga and a fungus. They have no roots.) Tundra plants must grow rapidly, because the growing season lasts for only six to ten weeks. Many of the plants reproduce vegetatively, by growing new roots and shoots, rather than by making seeds. Spring starts in June when the ice begins to melt, and winter returns by September.

    The earth in the tundra biome is not really considered to be a true soil. Dead plant material decomposes very slowly because it is so cold. (Chemical changes occur more quickly with more heat.) Water expands when it turns to ice, and the constant melting and freezing of the top of the ground moves the ground around. This results in the ground being full of hollow spaces which serve as tunnels for the lemmings that live in this biome. It also makes it impossible for plants like trees, who have strong permanent root systems, to live there.

Lemmings

     Lemmings are small rodents. They eat plants, weigh between an ounce and four ounces and are three to five inches long. They look rather like hamsters, and may be brownish or dark gray in the summer, when they live in tunnels in the ground. They have short tails and fur on their foot pads to help them to keep warm. In winter lemming fur turns white, and the little creatures make tunnels under the snow and eat the plants that they find.

     The lemmings use the r reproductive strategy, breeding very rapidly, hoping that some members of their group can survive despite predators and difficult weather conditions. After a 20 day pregnancy the mothers produce litters of 6 to 9 babies, and the mothers soon become pregnant again. A lemmiing can have 3 litters a year. More than half of the new lemmings are female, and when they are a month old they can also become pregnant.

Arctic Foxes

   Arctic foxes are year round predators in the tundra biome. These small, dainty little animals are about the size of a cat, weighing six to ten pounds. They have the warmest fur of any mammal, and look fluffy. Their short legs, small ears, and short noses are adaptations to reduce their surface area in the cold climate. They are brownish in the summer and white in the winter. They hunt lemmings, chasing them down in the summer and listening for them under the snow in the winter time. They are very tough little animals and will travel great distances when food is scarce.

     Arctic foxes are very well adapted to the cold. They even have fur on the bottoms of their feet. Their metabolic rate increases when the temperature drops to -50 degrees Celsius: at -70 degrees Celsius they start shivering.

     Arctic foxes may have one or two litters of pups a year, but one litter is usual in the difficult circumstances of the arctic tundra. Litter size is related to the food supply. They may have no pups, or only one or two, when food is scarce, and as many as twenty five when food is abundant. A normal sized litter would be six to twelve pups. However, most of these puppies will die of starvation before they are six months old. The puppies are weaned between two to four weeks of age, and survival on their own is difficult.

     Because of the flexibility of their reproductive patterns, arctic foxes can respond quickly to increases in the number of lemmings. When there are more lemmings the foxes have more pups, and more pups survive. The increase in predators reduces the number of lemmings. Then some of the foxes starve. This is repeated in four year cycles, although mild or severe winters also have an effect on the population numbers.

Snowy Owls

     Snowy Owls are well adapted to tundra life, although they will leave the area to search for food in particularly severe winters.

     They are small predators, weighing two to four pounds. They are about two feet tall and have a wing span that often exceeds five feet. Unlike most owls, they hunt in the daytime as well as at night. This is an adaptation to the 24 hour daylight that the tundra enjoys in summer. They also change color, being brown in the summer and white in the winter.

     Snowy owls prey on lemmings and eat about a dozen lemmings each a day. They nest on the ground on the highest and driest part of the tundra: there are no trees in the tundra biome.

     The female lays 8 - 10 eggs and incubates the eggs for 33 days. The young start developing in the eggs as soon as the eggs are laid, so that the owlets are different sizes. This is a kind of insurance against sudden storms or food shortages that might kill some of the chicks. The young are ready to leave the nest in 16 days and begin to fly at around the age of 52 days. The parents are kept busy feeding the owlets.

Musk Oxen

     Small herds of musk oxen also roam the tundra. Groups of 10 or 12 of these peaceful grazers eat the small plants and lichens. They are very hardy, and covered with thick, soft, very warm fur. They weigh from 500 to 1000 pounds, and are between 3 and 6 feet high at the shoulder. Although the tundra does not support wolves, wolves do attack musk oxen when they wander into the bordering lands. Musk oxen have horns for protection. When they are attacked they form a circle with all the adults facing out and the young calves in the center.

     Musk oxen use only a sixth of the food that cattle need, so they must be processing what they eat very efficiently. They take three years to mature and then bear calves in alternate years. The calves drink milk for nine months, while also eating plants. Contrast this slow reproduction rate with the frenzied reproduction of the lemmings. These animals are using the K Reproduction Strategy.

Other Animals     

        Other animals found on the tundra in summer include reindeer, caribou, arctic hares, and snow shoe rabbits. Reindeer and caribou migrate across the tundra, eating lichen and plants. Arctic voles and mice are also seen. Migrating birds may nest in the tundra areas near the ocean, where there is more choice of food. Ptarmigans are birds that stay in the arctic year round and change their brown feathers to white when winters come. Summer insects include mosquitoes and black flies.

This Fragile Ecosystem

    Notice that this biome has short food chains and only a few species of animals. This is representative of a challenging biome with a fragile ecosystem. Although the systems function smoothly, the balance could be destroyed if a single type of animal were to be wiped out by disease, over-hunting, or predation. The plant systems are also fragile, and the ground bears the marks of human traffic for many years. There is an interesting contrast between the incredible hardiness of the animals and plants that live here and their vulnerability to outside stressors for which they are not prepared.

     Tundra environments also exist in the mountains above the tree line. The high meadows are sprinkled with mountain flowers in the summer. These areas are close to the forest area below, so there are more species of animals present than are found in the Arctic tundra. Animals are not totally dependent on the tundra vegetation, and can get food from lower elevations or migrate down the mountain as winter comes on. Marmots make their homes here, and mountain goats and sheep spend their summers in the rocky crags.