Many places on Earth share similar climatic conditions despite being found in geographically different areas. As a result of natural selection, comparable ecosystems have developed in these separated areas. Scientists call these major ecosystem types biomes. The geographical distribution (and productivity) of the various biomes is controlled primarily by the climatic variables precipitation and temperature. The maps in Figures 1 and 2 describe the geographical locations of the thirteen major terrestrial biomes of the world. Because of their scale, these maps ignore the many community variations that are present within each biome category.
Most of the classified biomes are identified by the dominant plants found in their communities. For example, the various types of grasslands are dominated by a variety of annual and perennial species of grass, while deserts are occupied by plant species that require very little water for survival or by plants that have specific adaptations to conserve or acquire water.
The diversity of animal life and subdominant plant forms characteristic of each biome is generally controlled by abiotic environmental conditions and the productivity of the dominant vegetation. In general, species diversity becomes higher with increases in net primary productivity, moisture availability, and temperature.
Adaptation and niche specialization are nicely demonstrated in the biome concept. Organisms that fill similar niches in geographically separated but similar ecosystems usually are different species that have undergone similar adaptation independently, in response to similar environmental pressures. The vegetation of California, Chile, South Africa, South Australia, Southern Italy and Greece display similar morphological and physiological characteristics because of convergent evolution. In these areas, the vegetation consists of drought-resistant, hard-leaved, low growing woody shrubs and trees like eucalyptus, olive, juniper, and mimosa.
The geographical distribution of the tundra biome is roughly poleward of 65° North latitude. In the Southern Hemisphere, the tundra biome has a very limited distribution. Within the tundra biome, temperature, precipitation, and evaporation all tend to be at a minimum. In fact, the tundra is the coldest of all biomes and this environmental factor has played an important role in the evolution of adaptations for plant and animal survival. Most tundra locations, have summer months with an average temperature between 3 and 12° C (37 to 54° F). The average winter monthly temperature is around -34° C (-30° F). Precipitation in the wettest month is usually no greater than 2.5 centimeters (roughly 1 inch). Yet, despite the low levels of precipitation the ground surface of the tundra biome is often waterlogged because of low rates of evapotranspiration and poor drainage.
The tundra biome is characterized by the absence of trees and the presence of low-lying shrubs, mosses, and lichens. Lack of height allows the vegetation to be protected by the insolating properties of snow during the winter season. Perhaps the most characteristic arctic tundra plants are lichens like reindeer moss (Cladonia spp.). In the drier parts of the tundra, grasses are common (Figure 3). Sedges dominate sites that have more moisture. About 400 varieties of flowering plants occur in this biome. Total species diversity of plants in the tundra biome is relatively small numbering about 2000 species. Plants are generally small, are adapted to soil disturbance, and reproduce via budding or other forms of asexual reproduction rather than sexual means. Soils of this biome are usually permanently frozen (permafrost) starting at a depth of a few centimeters to meter or more. The permafrost line is a physical barrier to plant root growth. Thus, there are no deep rooting systems. The presence of permafrost also causes poor drainage and soils are often waterlogged and chemically reduced.
Figure 3: Tundra dominated by flowering arctic cotton grass, Northwest Territories, Canada. (Image Source).
The principal herbivores of the tundra biome include caribou, musk ox, arctic hare, voles, squirrels, and lemmings (Figure 4). Most of the bird species of the tundra have the ability to migrate and live in warmer locations during the cold winter months. The herbivore species support a small number of carnivore species like the arctic fox, snow owl, polar bear, and wolves. Reptiles and amphibians are few or completely absent because of the extremely cold temperatures.
Alpine tundra is quite comparable to arctic tundra but differs in the absence of permafrost, the presence of better drainage, and more extreme annual fluctuations of air temperature. Plants species in the alpine tundra are for the most part similar to the ones found on the arctic tundra. In contrast, alpine tundra animal species tend to be quit different from those individuals that live in the arctic tundra. This takes place because alpine tundra tends to adopt migrating species during the summer months from habitats located at lower elevations.
This moist-cool, transcontinental boreal forests or taiga biome lies largely between 50 and 65° North latitude. The climate of this biome is cool to cold with more precipitation than the tundra. Precipitation here mainly occurs in the summer because this is the season when mid-latitude cyclones move in from the south. The growth season is limited to about 130 days.
The predominant vegetation of boreal forest biome is cone bearing needle-leaf evergreen variety tree species. Four tree genera are dominant in this biome: spruce (Picea), pine (Pinus), fir (Abies), and larch (Larix). In North America, some common species include: black spruce (Picea mariana), white spruce (Picea glauca), jack pine (Pinus banksiana), tamarack (Larix laricina), and balsam fir (Abies balsamea); with red pine (Pinus resinosa), white pine (Pinus strobus), and hemlock (Tsuga canadensis) limited to an area north and east of the Great Lakes Region. Broad-leaf species, like alder (Alnus), birch (Betula), and aspen (Populus), are common in all areas as an early successional species after disturbance.
Understory vegetation is relatively limited as a result of the low light penetration even during the spring and fall months. Common understory species include orchids, shrubs like rose, blueberry, and cranberry. Mammals common to the boreal forest include moose, bear, deer, wolverine, marten, lynx, wolf, snowshoe hare, vole, chipmunks, shrews, and bats. Reptiles are extremely rare, once again, because of cold temperatures.
Deep litter layers are a common characteristic of boreal forest soils. These deep litter layers accumulate because of slow decomposition rates. Soils of this biome are also acidic and mineral deficient. Mineral deficiency occurs because large amounts of water move down though the profile causing leaching.
Boreal forest soils are characterized by a deep litter layer and slow decomposition. Soils of this biome are also acidic and mineral deficient because of the large movement of water vertically though the profile and subsequent leaching.
Temperate Coniferous Forests
In North America we can find two broad areas of temperate coniferous forests in the more temperate mid-latitudes. In these areas, average annual temperatures range from 20° to 5° C (68° to 41° F). Along the west side of North America and below the boreal forest is one such area. On the wetter sites (up to 400 centimeters or 160 inches annually) that have close proximity to the Pacific Ocean are stands of very tall and productive Douglas fir (Pseudotsuga menziesii), red cedar (Thuja plicata), sitka spruce (Picea sitchensis), and redwood (Sequoia sempervirens). Some of these trees can grow to over 120 meters (390 feet) in height. Beneath the canopy of these trees is a shrub layer that includes various types of berries (Vaccinium spp.), a few herbs, and various ferns. Further inland of this temperate “rain forest” zone precipitation declines significantly, winter temperatures become colder, and summer temperatures become much warmer. This change in climate makes more drought resistant trees like ponderosa pine (Pinus pondersoa), Engelmann spruce (Picea engelmannii), and lodgepole pine (Pinus contorta) dominant.
Another region of temperature coniferous forests occurs in southeastern United States. The species composition of this forest ecosystem does not resemble the coniferous forests found in western North America. Instead, these forests are dominated by pitch pine (Pinus rigida), longleaf pine (Pinus palustris), and slash pine (Pinus elliotti). All of these tree species are adapted to growing on nutrient poor sandy soils and can withstand the effects of fire. Biomass productivity is typically low in this type of temperate coniferous forest.
Outside of North America, the various types of temperate coniferous forest can also be found in northern Japan, and parts of Europe and Asia. In these areas, the plant species are similar in form and ecological function to North American species but not closely related.
Temperate Broadleaf and Mixed Forests
The temperature broadleaf and mixed forests biome (also called temperate deciduous forest) is characterized by a moderate temperate climate and a dominance of broadleaf deciduous trees. This biome once occupied much of the eastern half of the United States, central Europe, Korea, and China. Over the last few centuries, this biome has been very extensively affected by human activity. Much of it has been converted into agricultural fields or urban land-use.
Tree species diversity is this biome is moderate with 5 to 25 dominant trees at a site. Dominant trees include maple (Acer spp.), beech (Fagus spp.), oak (Quercus spp.), hickory (Carya spp.), basswood (Tilia spp.), magnolia (Magnolia spp.), cottonwood (Populus spp.), elm (Ulmus spp.), and willow (Salix spp.). The understory of shrubs, herbs, and ferns in a mature forest are typically well developed and richly diversified. Understory plants in this biome often take advantage of the leafless condition of trees during spring and fall to concentrate their growth.
Many different types of herbivores and carnivores live in the temperate broadleaf and mixed forest. Common fauna include squirrels, rabbits, skunks, birds, deer, mountain lion, bobcat, timber wolf, fox, and bears. Some reptiles and amphibians also exist here.
Nutrient rich brown forest soils characterize the temperate broadleaf and mixed forests biome. Tree cover promotes the accumulation of organic materials in a well-developed humus layer. Surface litter layer in these soils tends to be thin because of rapid decomposition.
Temperate Grasslands, Savannas and Shrublands
In central North America is the temperate grasslands, savannas and shrublands biome (also called prairie). The grassland biome is also found in the continental interior of Eurasia, Australia, and South America. Prior to the arrival of settlers in North America, much of this biome was dominated by species of tall grass known as bluestem (Andropogon spp.). This particular species covered much of the eastern side of this biome forming dense covers 1.5 to 2.0 meters (4 to 6 feet) tall. In the western end of the biome, where precipitation is lower, buffalo grass (Buchloe dactyloides) and other grasses only a few inches above the soil surface are common. Flowering herbs, including many kinds of composites and legumes, are common but much less important than grass species. Trees are found scattered in moist low-lying areas and along a narrow zone adjacent to streams.
Climatically, the temperate grasslands, savannas and shrublands biome can be described as being temperate. Summers are hot to warm and winters are cool to cold. Annual precipitation is less than what is received by the adjacent temperate broadleaf and mixed forests biome. Seasonally, precipitation varies from being concentrated during a few months to spread evenly through the year. This biome generally does not receive enough precipitation to support tree growth. In the wetter parts of this biome nutrient rich black chernozemic soils are common. In many parts of the world, these extremely fertile soils now support crop growth. In drier parts of prairies, soils can be influenced by salinization.
Grassland mammals are dominated by smaller burrowing herbivores (prairie dogs, jack rabbits, ground squirrels, and gophers) and larger running herbivores such as bison, pronghorn antelope, and elk. Carnivores include badger, coyote, ferret, wolf, and cougar. The populations of many of these organisms have been drastically reduced because of the conversion of their natural habitat into cropland. Some of these species are on the edge of extinction.
Montane Grasslands and Shrublands
The montane grasslands and shrublands biome is found at high elevations in temperate, subtropical, and tropical climates. This biome is dominated by grass and shrub species and tends to have a high number of endemic plants and animals. Examples of this biome can be found at the Tibetan plateau, Central Range in New Guinea, eastern Andes Mountains in South America, southeastern Africa, and tropical East Africa. A unique feature of many tropical examples of this biome is the presence of giant rosette vegetation belonging to the plant families Lobelia (Africa), Puya (South America), Cyathea (New Guinea), and Argyroxiphium (Hawaii) (Figure 7k-16). All of these plants have unique adaptations that allow them to successfully grow at high elevations.
Deserts and Xeric Shrublands
In its most typical form, the xeric shrublands and desert biome consists of shrub-covered land where the plants are spatially quite dispersed. This biome is geographically found from 25 - 35° North and South latitude, primarily in the interiors of continents. The formation of precipitation in desert and xeric shrublands biome is limited by the subtropical high-pressure system. Many desert areas have less than 3 centimeters (about 1 inch) of precipitation during an average year.
Dominant plants include drought resistant shrubs like the creosote bush (Larrea divaricata) and sagebrush (Artemisia tridentata), water storing succulents like cactus, and many species of short lived annuals that complete their life cycles during infrequent and short rainy periods (Figure 7k-18). Lastly, desert habitats can be completely devoid of vegetation if precipitation is in very short supply. Most desert mammals tend to be nocturnal to avoid the high temperatures. Desert habitats have a rich lizard and snake fauna because high temperatures promote the success of cold-blooded life forms. Because biomass productivity is low, the litter layer is almost nonexistent and organic content of surface soil layers is very low. Finally, evaporation tends to concentrate salts at the soil surface.
Mediterranean Forests, Woodlands and Scrub
The Mediterranean forests, woodlands and scrub biome (also called chaparral) has a very specific spatial distribution. It is found in a narrow zone between 32 and 40° latitude North and South on the west coasts of the continents. This area has a dry climate because of the dominance of the subtropical high pressure zone during the fall, summer, and spring months. Precipitation falls mainly in the winter months because of the seasonal movement of the polar front and associated mid-latitude cyclones. Precipitation varies from about 30 to 75 centimeters (12 to 30 inches) annually and most of this rain falls in a period only 2 to 4 months long.
Despite the fact that this biome is very limited geographically, it contains a high diversity of animal and plant species that are adapted to the stressful conditions of long, hot summers with little rain. The vegetation of this biome consists of many different types of annuals and drought-resistant, evergreen, short woody shrubs and trees. Dominant tree species include olive (Olea europaea), eucalyptus (Eucalyptus spp.), arbutus (Arbutus unedo), acacia (Acacia spp.), maritime pine (Pinus pinaster), and various species of oak (Quercus spp.). As a result of the climate, the vegetation of this biome exhibits a number of adaptations to withstand drought and fire. Plants tend not to drop their leaves during the dry season because of the expense of replacement. The dry climate slows the rate of leaf decomposition and soils tend to be poorly developed.
Tropical and Subtropical Grasslands, Savannas and Shrublands
Vegetation in the tropical and subtropical grasslands, savannas and shrublands biome (also called savanna) consists of a cover of perennial grass species 1 to 2 meters (3 to 6 feet) tall with scattered drought-resistant trees that generally do not exceed 10 meters (32 feet) in height. The savanna biome constitutes extensive areas in eastern Africa, South America, and Australia. Distinct wet and dry seasons and temperatures that are hot all year long characterize the climate of this biome. Annual rainfall varies between 90-150 centimeters (35 to 60 inches).
Tree and shrub species in the savanna usually drop their leaves during the dry season. This adaptation reduces water loss from the plants during the dry winter season. Diversity of plant and animal species tends to be high. Grazing on the grasses and trees are vast herds of hoofed mammals including buffalo, giraffes, eland, impalas, oryx, gazelles, gerenuk, wildebeest, zebra, rhinoceroses, elephants, and warthogs. These herbivores supply food for carnivores like lions, cheetahs, leopards, jackals, and hyenas.
Flooded Grasslands and Savannas
In the tropical and subtropical regions of our planet are large expanses of flooded grasslands and savannas. This biome is slightly different from the savanna biome just described. Because of common flooding, these areas support additional plant and animal species adapted to thrive under this condition. For instance, this biome is home to large numbers of migratory and resident water birds.
Some examples of flooded grasslands and savannas include in the Everglades in Florida, the Sahelian flooded savannas, and the Zambezian flooded savannas. Similar to other tropical biomes, this biome has high species diversity. For example, the Everglades are home to some 11,000 species of seed-bearing plants, 25 species of orchids, 300 bird species, and 150 species of fish.
Tropical and Subtropical Moist Broadleaf Forests
The tropical and subtropical moist broadleaf forests biome (also called moist tropical rain forest) occurs in a zone about 10° of latitude either side of the equator. Annual rainfall generally exceeds 250 centimeters (100 inches) and is evenly distributed throughout the year. Temperature and humidity are relatively high through the year. Flora is highly diverse: a typical hectare (2.5 acres) may contain as many as 300 different tree species as compared to 20 to 30 in the temperate zone. The various trees of the moist tropical rain forests are closely spaced together and form a thick continuous canopy some 25 to 35 meters (80 to 115 feet) tall. Every so often this canopy is interrupted by the presence of very tall emergent trees (up to 40 meters or 130 feet) that have wide buttressed bases for support. Epiphytic orchids and bromeliads, as well as vines (lianas), are very characteristic of the moist tropical rain forest biome. Some other common plant species include ferns and palms. Most plants are evergreen with large, dark green, leathery leaves.
The ground surface of the moist tropical rain forest tends to be dark with only about 1% of the light intensity found above the forest canopy. These light poor conditions cause the understory to be sparsely vegetated. The few plants that grow at ground level do so by being able to tolerate low light levels. The moist tropical rain forest is also home to a great variety of animals. Some scientists believe that 30 to 50% of all of the Earth's animal species may be found in this biome. Most of these organisms are insects.
Decomposition is rapid in the tropical rain forest because of high temperatures and abundant moisture. Because of the frequent and intense rains, tropical soils are subject to extreme chemical weathering and leaching. These environmental conditions make tropical soils acidic and nutrient poor.
Tropical and Subtropical Dry Broadleaf Forests
Tropical and subtropical dry forests (also called seasonal tropical forest or tropical dry forest) are found in southern Mexico, southeastern Africa, central India, Indochina, Madagascar, New Caledonia, eastern Bolivia, central Brazil, the Caribbean, and along the coasts of Peru and Ecuador.This biome exists as a zone that borders the tropical and subtropical moist broadleaf forests biome. Because of its geographical location, the tropical and subtropical dry forest experiences a dry season that lasts several months. This abiotic condition has a great effect on living things in this biome. Many of these species that live here have specific adaptations to help them survive the dry period. Consequently, deciduous trees like teak, mahogany, and mountain ebony dominate these forests. During the seasonal drought these trees loose their leaves to conserve water.The leafless condition also causes more sunlight to reach ground surface and this condition facilitates the growth of thick shrub layer. While less diverse than tropical rain forests, seasonal tropical forests still have a vast assortment of organisms.
Tropical and Subtropical Coniferous Forests
The tropical and subtropical coniferous forests biome is characterized by diverse species of conifer (needle-leaf) trees.This biome has a very limited distribution and is found mainly in Mexico, Central America, and on the islands of Cuba, Dominican Republic, and Haiti where low levels of precipitation and moderate temperature variability occurs. The needle-leaf form of these trees is an adaptation to drought. This biome shares some of the plant and animal species common to tropical and subtropical savanna, dry broadleaf forest, and moist broadleaf forest. Understory vegetation composed of shrubs and small trees is well developed and diverse. Finally, many species of migratory birds and butterflies spend their winter in this biome.