ECOLOGY

The Arctic tundra is found across northern Alaska, Canada, and Siberia. It is characterized by long cold winters, short cool summers, and low precipitation (less than 10 inches per year) and dry winds. These conditions make the Arctic tundra a desert-like climate. One unique characteristic is permafrost--ground that is permanently frozen. Because the permafrost has no cracks or pores, nothing can penetrate it--neither plant roots nor water.
The surface layer above the permafrost thaws each summer and is called the active layer. Thickness of the active layer depends on its location in the tundra. The more northerly the location, the thinner the active layer is.
Curiously, during the summer Arctic tundra is characterized by large amounts of surface water. Because when snow melts, the water percolates through the active layer but is unable to penetrate the permafrost and since the water has nowhere to go, the active layer becomes saturated and pools of water form on the surface.
Another characteristic of the Arctic tundra is the limited amount of sunlight it receives due to the position of the Sun in the sky. Depending on the latitude, the Sun can remain below the horizon for up to 2 months, leaving the Arctic tundra in darkness. Although the sun remains in the sky 24 hours a day during the summer, it stays close to the horizon and provides only low intensity sunlight.

The taiga is found in the northern hemisphere close to the polar region. This cold biome stretches across the northern portions of North America, Europe, and Asia. A number of large population centers, such as Moscow and Toronto, can be found in the southern portion of this biome, but the northern portion is relatively unpopulated.
There is a wide range of temperatures between winter and summer seasons. The winters being long and cold, and the summers being short and cool. Precipitation is moderately high throughout the year with snow occurring during the winter months. Most of the taiga in North America was once covered with glaciers that have receded, leaving gouges and depressions in the landscape. Since there is moderately high precipitation, these gouges and depressions are frequently filled with water, creating bogs and lakes.
The soil found in the taiga is low in nutrients and high in acid being rocky and covered with non-decayed leaf litter. Patches of permafrost can also be found in areas of the taiga.

Deserts receive less dun 25 cm, (10 in) of rain per year. Temperatures in deserts vary according to latitude and they are not necessarily hot places.
Some deserts are produced and maintained by high mountain ranges that block coastal precipitation. Mountains cause desert conditions by forming rain shadows, as is clearly seen in the American deserts.
Large deserts are found on all continents except Europe and Antarctica All deserts experience dramatic day-night fluctuations in temperature.
Deserts are created along the 300 north and south latitude lines. They are in regions where air cells originating at the equator have lost their moisture.

Plants adapted to desert conditions are called xerophytes. Some become dormant. while others store water.
The annuals germinate after brief rains, quickly producing flowers and seeds before drought conditions return.
A few species, widely distributed, usually dominate a particular desert.
Short grasses, sagebrush, creosote bushes, and cacti are just a few of the plants that can be found in the desert. Plant abundance and variety are determined by the geographic location of the desert. Although short grasses can be found in nearly all desert locations, the saguaro cactus is unique to the Sonoran Desert, and the spiniflex is associated with the Australian Desert. Because of the dry climate, plants have developed a number of different methods of capturing water. Some plants have developed long (20-30 foot) taproots that go deep into the ground and tap into groundwater sources.

Rodents survive by spending daytime in burrows and venturing out at night. Their predators follow the some routine.
Some survive without water through behavioral and physiological strategies. The kangaroo rat rarely drinks, but lives off metabolic water conserved by a highly efficient kidney. The camel drinks huge quantities of water. survives dehydration. and withstands unusual changes in body temperature.
The desert is a relatively simple biome and can be easily disrupted through careless human, acts. It can become productive if water is present. but its exploitation can produce irrevocable changes.

Grasslands exist as huge inland plains in North America. Asia. Australia. South America. and Africa. Grasslands exist in areas where there is a 6 to 8 month wet summer season and a dry winter season. Annual rainfall varies depending on the geographic location from as little as 10 inches of rain annually while others can get as much as 50 inches. The dry season is marked by months of drought and fire which are essential to natural maintenance.. Without the period of drought and fire, it is believed that tropical savannas would eventually change into tropical forests.
Grasslands can result from either climate, soil conditions, animal behavior, or agricultural practices, which limit the occurrence of trees. Humans may also create grasslands by burning vegetation and felling the trees in order to plant crops. Large animals such as elephants can turn a forest into a grassland by stripping the bark from the trees, knocking over trees, and tramping on tree seedlings.
Grasses dominate this biome. Much of the grassland however is utilized for growing grains.
Rainfall is seasonal and limited. Grasses survive by producing deep diffuse root systems and by becoming dormant.
The animals of grasslands produce huge populations and include the largest herbivores. The grasses are well adapted to resist grazing
The food webs of grasslands are complex and relatively stable. Occasionally insect populations explode out of control and devastate this biome. Normally grasslands support the largest populations of animals on earth.

This biome is supported by heavy rainfall and a warm climate. These forests are found in South America. Africa. India. Burma. Central America. Indonesia and the Philippines. The tropical rainforest is a hot, moist biome found near Earth's equator. Tropical rainforests receive from 60 to 160 inches of precipitation that is fairly evenly distributed throughout the year. The combination of constant warmth and abundant moisture makes the tropical rainforest a suitable environment for many plants and animals.
Because these organisms remain active throughout the year, they quickly decompose matter on the forest floor. In other biomes, such as the deciduous forest, the decomposition of leaf litter adds nutrients to the soil. But in the tropical rainforest, plants grow so fast that they rapidly consume the nutrients from the decomposed leaf litter. As a result, most of the nutrients are contained in the trees and other plants rather than in the soil. Most nutrients that are absorbed into the soil are leached out by the abundant rainfall, which leaves the soil infertile and acidic.
The hot and humid conditions make tropical rainforests an ideal environment for bacteria and other microorganisms.
There not any distinctly dominant species of plants or animals, but there are dominant types. The number of animal and plant species is unequaled in other land biomes. Many are arboreal. Tropical rainforests contain the greatest biodiversity in the world. Over 15 million species of plants and animals live within this biome.
The canopy is composed of very tall Vines abound as do epiphytes, which absorb water from the air. Species population density is low, but species diversity is great.
The forest floor is dark and devoid of foliage. Scavenger and reducer populations abound.
The "jungle'' refers to thick growth along rivers or where sunlight reaches the forest floor after disturbances.
The number of animal species is unequaled in other land biomes. Many are arboreal. Tropical rainforests contain the greatest biodiversity in the world. Over 15 million species of plants and animals live within this biome.