Biome

A biome is a major, geographically extensive ecosystem, structurally characterized by its dominant life forms. Terrestrial biomes are usually distinguished on the basis of the major components of their mature or climax vegetation, while aquatic biomes, especially marine ones, are often characterized by their dominant animals. Most of the oceans are considered part of a single biome, although areas with particularly unusual or unique physical characteristics or inhabitants may be considered as separate biomes.

Similar biomes can occur in widely divergent places as long as the environmental conditions are appropriate for their development. Some environmental conditions affecting the location of biomes include climate, latitude, topography, and fire. Often, different species having similar, convergent growth forms will dominate at different places within the same biome. For example, the boreal coniferous forest occurs in suitable environments of northern North America and Eurasia. In northeastern North America this biome is dominated by stands of black spruce, while in the northwest white spruce is dominant. Norway spruce is most important in this biome in northwestern Europe, while in parts of Siberia species of pine and larch are dominant. Because biomes are described according to the structural characteristics of their dominant organisms (in this example, coniferous trees growing under a particular climatic regime) all of these different forest types are considered convergent ecosystems within the same biome, the boreal coniferous forest.

Tundra

Tundra is a treeless biome occurring in areas with cold climates and a short growing season. Alpine tundra occurs at high altitudes on mountains, while arctic tundra occurs at high latitudes. Most tundras receive very small inputs of water as precipitation, but nevertheless their soil may be moist or wet because there is little evaporation in such cold climates, and deep drainage may be prevented by frozen soil. The coldest, most northern, high-arctic tundras are very unproductive and dominated by long-lived but short-statured plants, typically less than 1.97-3.94 in (5-10 cm) tall. Low-arctic tundras are dominated by shrubs as tall as 3.28 ft (1 m), while wet sites develop relatively productive meadows of sedge, cotton-grass, and grass. In North America, arctic tundras can support small densities of mammalian herbivores such as caribou and muskox (although during migration these animals can occur in locally large densities), and even smaller numbers of their predators, such as wolves.

Boreal coniferous forest

The boreal coniferous forest, or taiga, is an extensive northern biome occurring in moist climates with cold winters. The boreal forest is dominated by coniferous trees, especially species of fir, larch, pine, and spruce. Some broad-leaved, angiosperm trees are also important in the boreal forest, especially species of aspen, birch, poplar, and willow. Usually, particular stands of boreal forest are dominated by only one or several species of trees. Most regions of boreal forest are subject to periodic events of catastrophic disturbance, most commonly caused by wildfire and sometimes by insects, such as spruce budworm, that kill trees through intensive defoliation. Montane forests, also dominated by conifers and similar in structure to the boreal forest, can occur at sub-alpine altitudes on mountains in southerly latitudes.

Temperate deciduous forest

Forests dominated by species-rich mixtures of broad-leaved trees occur in relatively moist, temperate climates. Because these forests occur in places where the winters can be cold, the foliage of most species is seasonally deciduous, meaning that all leaves are shed each autumn and re-grown in the springtime. Common trees of this forest biome in North America are species of ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, tulip-tree, and walnut, among others. These various tree species segregate into intergrading communities on the basis of site variations of soil moisture, fertility, and air temperature.

Temperate rainforest

Temperate rainforests develop under climatic regimes characterized by mild winters and an abundance of precipitation. Because these systems are too moist to support regular, catastrophic wildfires, they often develop into old-growth forests, dominated by coniferous trees of mixed age and species composition. Individual trees can be extremely large, and in extreme cases can be more than 1,000 years old. Common trees of this biome are species of Douglas fir, hemlock, cedar, redwood, spruce, and yellow cypress. In North America, temperate rainforests are best developed on the humid west coast.

Temperate grassland

These grasslands occur under temperate climatic regimes that are intermediate to those that support forest and desert. In the temperate zones, grasslands typically occur where rainfall is 9.9-24 in (25-60 cm) per year. Grasslands in North America are called prairie (they are often called steppe in Eurasia), and this biome occupies vast regions in the interior. The prairie is often divided into three types according to height of the dominant vegetation—tall-grass, mixed-grass, and short-grass. The once extensive tall-grass prairie is dominated by various Biomes along 87 degrees west longitude and along 0 degrees longitude. Illustration by Hans & Cassidy. Courtesy of Gale Group. species of grasses and herbaceous broad leaved plants such as sunflowers and blazing stars, some as tall as 9.8-13.1 ft (3-4 m). Fire was an important natural factor that prevented much of the tall-grass prairie from developing into an open forest. The tall-grass prairie is now an endangered natural ecosystem, because it has been almost entirely converted to agriculture. The mixed-grass prairie occurs where rainfall is less, and it supports shorter species of grasses and other herbaceous plants. The short-grass prairie has even less precipitation, and is subject to unpredictable years of severe drought.

Tropical grassland and savanna

Tropical grasslands can occur in regions with as much as 47.2 in (120 cm) of rainfall per year, but under highly seasonal conditions with a pronounced dry season. Savannas are dominated by grasses and other herbaceous plants, but they also have scattered shrub and tree-sized woody plants, which form a very open canopy. Tropical grasslands and savannas can support a great seasonal abundance of large, migratory mammals, as well as substantial populations of resident animals. This is especially true of savannas in Africa, where this biome supports a very diverse assemblage of large mammals, including gazelles and other antelopes, rhinos, elephant, hippopotamus, and buffalo, and some of their predators, such as lion, cheetah, wild dog, and hyena.

Chaparral

Chaparral is a temperate biome that develops in environments with seasonally extreme moisture gradients, characterized by a so-called Mediterranean climate with winter rains and summer drought. The chaparral biome is typically composed of dwarf forest and shrubs, and interspersed herbaceous vegetation. Chaparral is highly prone to events of catastrophic wildfire. In North America, chaparral is best developed in parts of the southwest, especially coastal southern California.

Desert

Desert is a temperate or tropical biome, commonly occurring in the center of continents, and in the rain shadows of mountains. The distribution of this biome is determined by the availability of water, generally occurring where there is less than 9.9 in (25 cm) of precipitation per year. Not surprisingly, the productivity of desert ecosystems is strongly influenced by the availability of water. The driest deserts support almost no plant productivity, while less-dry situations may support communities of herbaceous, succulent, and annual plants, and somewhat moister places will allow a shrub-dominated ecosystem to develop.

Semi-evergreen tropical forest

This type of tropical forest develops when there is a seasonality of water availability due to the occurrence of pronounced wet and drier seasons during the year. Because of this seasonality, most of the trees and shrubs of this biome are seasonally deciduous, meaning that they shed their foliage in anticipation of the drier season. This biome supports a great richness of plant and animal species, though somewhat less than in tropical rainforests.

Evergreen tropical rainforest

This biome occurs under tropical climates with abundant precipitation and no seasonal drought. Because wildfire and other types of catastrophic disturbance are uncommon in this sort of climate regime, tropical rainforests usually develop into old-growth forests. As such, they contain a diverse size range of trees, a great richness of species of trees and other plants, as well as an extraordinary diversity of animals and microorganisms. Many ecologists consider this biome to represent the epitome of ecosystem development on land, because of the enormous variety of species that are supported under relatively benign climatic conditions in old-growth tropical rainforests.

Lotic

The character of this running-water biome is determined by physical factors, especially the quantity, velocity, and seasonal variations of water flow. These hydrologic characteristics influence other important characteristics of lotic ecosystems. For example, the bottom tends to be muddy in places with calm water flows where silt is deposited, and rocky in more vigorous places where fine particles are selectively eroded from the bottom. Similarly, turbidity is great during high water flows, and this interferes with the penetration of light, restricting plant productivity. Although they sustain some primary productivity of aquatic plants, the common lotic ecosystems such as rivers, streams, and brooks are not usually self-supporting in terms of fixed energy. These ecosystems typically rely on inputs of organic matter from their surrounding, terrestrial watershed, or from upstream lakes to support much of their productivity of aquatic invertebrates and fish.

Wetlands

Freshwater wetlands (or mires) occur in shallow waters, usually having pronounced seasonal variations of water depth, sometimes including dry periods during which water does not occur at the surface. The four major wetland types are: marsh, swamp forest, bog, and fen. Marshes are the most productive wetlands, and are typically dominated by relatively tall, emergent species of angiosperm plants such as reed, cat-tail, and bulrush, and by floating-leaved plants such as water lily and lotus. Swamps are forested wetlands, seasonally or permanently flooded, and in North America, dominated by tree-sized plants such as bald cypress or silvermaple. Bogs are acidic, unproductive wetlands that develop in relatively cool but wet climates. Bogs depend on atmospheric inputs for their supply of nutrients, and are typically dominated by species of sphagnum moss. Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs, and are consequently less acidic and more productive.

Open ocean

The character of the open-water, or pelagic oceanic biome is determined by physical and chemical environmental factors, particularly waves, tides, currents, salinity, temperature, light intensity, and nutrient concentration. Primary productivity of this biome is small, and comparable to some of the least productive terrestrial biomes, such as deserts. Primary production in the open ocean is carried out by phytoplankton of diverse species, ranging in size from extremely small photosynthetic bacteria, to larger but microscopic unicellular and colonial algae. The phytoplankton are grazed by small crustaceans known as zooplankton, and these are eaten in turn by small fish. At the top of the pelagic food web are very large predators such as bluefin tuna, sharks, squid, and whales. The deep benthic ecosystems of this biome are supported by a sparse rain of dead biomass from its surface waters. The benthic ecosystems are not well known, but they appear to be extremely stable, rich in species, and low in productivity.

Continental shelf waters

Near-shore waters of the oceans are relatively shallow, because they overlie continental shelves. Compared with the open ocean, waters over continental shelves are relatively warm, and are well supplied with nutrients. The nutrients originate with inputs from rivers, and from occasional movements of deeper, more fertile waters to the surface, stirred from the bottom by turbulence caused by storms. Mostly as a result of the nutrient inputs, the phytoplankton of these waters are relatively productive, and they support a larger biomass of animals than occurs in the open ocean. Some of the world's most important pelagic and benthic fisheries are supported by the continental shelf biome, for example, those in the North Sea and Barents Sea of western Europe, the Grand Banks and other shallow waters of northeastern North America, the Gulf of Mexico, and inshore waters of much of western North America.

Upwelling regions

In certain places or regions, oceanographic conditions favor upwellings to the surface of relatively deep, nutrient rich waters. Because of the enhanced nutrient supply, upwelling areas are relatively fertile, and they sustain this highly productive, open-ocean biome. Because of their large foundation of primary production, upwelling regions support sizable populations of animals, including large species of fish and shark, marine mammals, and seabirds. Some of Earth's most productive fisheries occur in upwelling areas, such as that off the west coast of Peru and elsewhere off South America, and large regions of the Antarctic Ocean.

Estuaries

Estuaries are a complex group of coastal ecosystems that are semi-enclosed, but open to the sea. Estuaries display characteristics of both marine and freshwater biomes, because they typically have substantial inflows of fresh water from the nearby land, along with large fluctuations of salt water resulting from tidal cycles. Examples of estuaries include coastal bays, sounds, river mouths, salt marshes, and tropical mangrove forests. Because their large water-borne inputs of terrestrial nutrients are partially retained by their semi-enclosed water circulation, estuaries are highly productive ecosystems. Estuaries provide important habitat for juvenile stages of many commercially important species of fish, shellfish, and crustaceans, and they are often characterized as "nursery" habitat for these species.

Seashores

The seashore biome is a complex of ecosystems occurring at the interface of terrestrial and oceanic biomes. The local character of the seashore biome is determined by environmental factors, such as the intensity of wave action, the frequency of major events of disturbance, and bottom type. In temperate waters, hard-rock and cobble bottoms often develop ecosystems dominated by large species of macroalgae, broadly known as seaweeds, or kelp. In some cases, so-called kelp "forests" can develop. These are highly productive ecosystems, which maintain large quantities of biomass, mostly of macroalgae. In situations characterized by softer bottoms of sand or mud, ecosystems are typically dominated by benthic invertebrates, especially mollusks, echinoderms, crustaceans, and marine worms.

Coral reefs

Coral reefs are a distinctive marine biome of tropical seas, occurring locally in shallow but relatively infertile areas close to land. The physical structure of coral reefs is provided by the calcium carbonate exoskeletons of dead coral polyps. This structure supports a species-rich population of living coral, crustose algae, invertebrates, and fish. This biome is dominated by corals, a diverse group of coelenterate animals, living in symbiosis with unicellular algae. Because this symbiosis is highly efficient in the acquisition and recycling of nutrients, coral reefs typically sustain a high productivity, even though they occur in nutrient-poor waters.

Urban-industrial techno-ecosystems

This anthropogenic biome consists of large metropolitan districts, dominated by humans, human dwellings, businesses, factories, and other types of infrastructure. This biome supports many species in addition to humans, but, with few exceptions, these are non-native plants and animals that have been introduced from other places, and that cannot live independently outside of this biome, unless they are returned to their native biome.

Rural techno-ecosystems

This is another anthropogenic biome, occurring outside of intensively built-up areas, and consisting of certain components of the extensive technological infrastructure of civilization. This biome is comprised of transportation corridors (such as highways, railways, transmission corridors, and aqueducts), small towns, and industries involved in the extraction, processing, and manufacturing of products from natural resources. Typically, this biome supports mixtures of introduced species and those native species that are tolerant of the disturbances and other stresses associated with human activities.

Agroecosystems

This biome consists of ecosystems that are managed and harvested for human use. The components of this biome are uneven in their anthropogenic influence. The most intensively managed agroecosystems typically involve monocultures of non-native crop species of plants or animals in agriculture, aquaculture, or forestry, and are not favorable to native wildlife. The management objective is to cultivate the economically valuable species under conditions that ensure optimal growth. Less-intensively managed agroecosystems involve mixtures of species, or polycultures, and these may provide habitat for some native wildlife species.