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Distinct biological communities that have formed in response to a shared physical climate
One way of mapping terrestrial biomes around the world
A biome/ˈbaɪoʊm/ is a community of plants and animals that have common characteristics for the environment they exist in. They can be found over a range of continents. Biomes are distinct biological communities that have formed in response to a shared physical climate. "Biome" is a broader term than "habitat"; any biome can comprise a variety of habitats.
While a biome can cover large areas, a microbiome is a mix of organisms that coexist in a defined space on a much smaller scale. For example, the human microbiome is the collection of bacteria, viruses, and other microorganisms that are present on or in a human body.
A 'biota' is the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biotas of the Earth make up the biosphere.
However, in some contexts, the term biome is used in a different manner. In German literature, particularly in the Walter terminology, the term is used similarly as biotope (a concrete geographical unit), while the biome definition used in this article is used as an international, non-regional, terminology - irrespectively of the continent in which an area is present, it takes the same biome name - and corresponds to his "zonobiome", "orobiome" and "pedobiome" (biomes determined by climate zone, altitude or soil).
In Brazilian literature, the term "biome" is sometimes used as synonym of "biogeographic province", an area based on species composition (the term "floristic province" being used when plant species are considered), or also as synonym of the "morphoclimatic and phytogeographical domain" of Ab'Sáber, a geographic space with subcontinental dimensions, with the predominance of similar geomorphologic and climatic characteristics, and of a certain vegetation form. Both include many biomes in fact.
To divide the world in a few ecological zones is a difficult attempt, notably because of the small-scale variations that exist everywhere on earth and because of the gradual changeover from one biome to the other. Their boundaries must therefore be drawn arbitrarily and their characterization made according to the average conditions that predominate in them.
A 1978 study on North American grasslands found a positive logistic correlation between evapotranspiration in mm/yr and above-ground net primary production in g/m2/yr. The general results from the study were that precipitation and water use led to above-ground primary production, while solar irradiation and temperature lead to below-ground primary production (roots), and temperature and water lead to cool and warm season growth habit. These findings help explain the categories used in Holdridge’s bioclassification scheme (see below), which were then later simplified by Whittaker. The number of classification schemes and the variety of determinants used in those schemes, however, should be taken as strong indicators that biomes do not fit perfectly into the classification schemes created.
Holdridge classified climates based on the biological effects of temperature and rainfall on vegetation under the assumption that these two abiotic factors are the largest determinants of the types of vegetation found in a habitat. Holdridge uses the four axes to define 30 so-called "humidity provinces", which are clearly visible in his diagram. While this scheme largely ignores soil and sun exposure, Holdridge acknowledged that these were important.
The distribution of vegetation types as a function of mean annual temperature and precipitation.
Whittaker classified biomes using two abiotic factors: precipitation and temperature. His scheme can be seen as a simplification of Holdridge's; more readily accessible, but missing Holdridge's greater specificity.
Whittaker based his approach on theoretical assertions and empirical sampling. He was in a unique position to make such a holistic assertion because he had previously compiled a review of biome classifications.
Key definitions for understanding Whittaker's scheme
Physiognomy: the apparent characteristics, outward features, or appearance of ecological communities or species.
Biome: a grouping of terrestrial ecosystems on a given continent that is similar in vegetation structure, physiognomy, features of the environment and characteristics of their animal communities.
Formation: a major kind of community of plants on a given continent.
Biome-type: grouping of convergent biomes or formations of different continents, defined by physiognomy.
Formation-type: a grouping of convergent formations.
Whittaker's distinction between biome and formation can be simplified: formation is used when applied to plant communities only, while biome is used when concerned with both plants and animals. Whittaker's convention of biome-type or formation-type is simply a broader method to categorize similar communities.
Whittaker's parameters for classifying biome-types
Whittaker, seeing the need for a simpler way to express the relationship of community structure to the environment, used what he called "gradient analysis" of ecocline patterns to relate communities to climate on a worldwide scale. Whittaker considered four main ecoclines in the terrestrial realm.
Intertidal levels: The wetness gradient of areas that are exposed to alternating water and dryness with intensities that vary by location from high to low tide
Climatic moisture gradient
Temperature gradient by altitude
Temperature gradient by latitude
Along these gradients, Whittaker noted several trends that allowed him to qualitatively establish biome-types:
The gradient runs from favorable to the extreme, with corresponding changes in productivity.
Changes in physiognomic complexity vary with how favorable of an environment exists (decreasing community structure and reduction of stratal differentiation as the environment becomes less favorable).
Trends in the diversity of structure follow trends in species diversity; alpha and beta species diversities decrease from favorable to extreme environments.
Each growth-form (i.e. grasses, shrubs, etc.) has its characteristic place of maximum importance along the ecoclines.
The same growth forms may be dominant in similar environments in widely different parts of the world.
Whittaker summed the effects of gradients (3) and (4) to get an overall temperature gradient and combined this with a gradient (2), the moisture gradient, to express the above conclusions in what is known as the Whittaker classification scheme. The scheme graphs average annual precipitation (x-axis) versus average annual temperature (y-axis) to classify biome-types.
The multiauthored series Ecosystems of the world, edited by David W. Goodall, provides a comprehensive coverage of the major "ecosystem types or biomes" on earth:
Natural Terrestrial Ecosystems
Wet Coastal Ecosystems
Dry Coastal Ecosystems
Polar and Alpine Tundra
Mires: Swamp, Bog, Fen, and Moor
Temperate Deserts and Semi-Deserts
Temperate Deciduous Forests
Heathlands and Related Shrublands
Temperate Broad-Leaved Evergreen Forests
Hot Deserts and Arid Shrublands
Tropical Rain Forest Ecosystems
Ecosystems of Disturbed Ground
Managed Terrestrial Ecosystems
Field Crop Ecosystems
Tree Crop Ecosystems
Inland Aquatic Ecosystems
River and Stream Ecosystems
Lakes and Reservoirs
Intertidal and Littoral Ecosystems
Estuaries and Enclosed Seas
Ecosystems of the Continental Shelves
Ecosystems of the Deep Ocean
Managed Aquatic Ecosystems
Managed Aquatic Ecosystems
Walter (1976, 2002) zonobiomes
The eponymously-named Heinrich Walter classification scheme considers the seasonality of temperature and precipitation. The system, also assessing precipitation and temperature, finds nine major biome types, with the important climate traits and vegetation types. The boundaries of each biome correlate to the conditions of moisture and cold stress that are strong determinants of plant form, and therefore the vegetation that defines the region. Extreme conditions, such as flooding in a swamp, can create different kinds of communities within the same biome.
Zonal soil type
Zonal vegetation type
ZB I. Equatorial, always moist, little temperature seasonality
ZB IX. Polar, short, cool summers and long, cold winters
Tundra humus soils with solifluction (permafrost soils)
Low, evergreen vegetation, without trees, growing over permanently frozen soils
Schultz (1988) ecozones
Schultz (1988) defined nine ecozones (note that his concept of ecozone is more similar to the concept of biome used in this article than to the concept of ecozone of BBC):
tropical/subtropical arid lands
Bailey (1989) ecoregions
Robert G. Bailey nearly developed a biogeographical classification system of ecoregions for the United States in a map published in 1976. He subsequently expanded the system to include the rest of North America in 1981, and the world in 1989. The Bailey system, based on climate, is divided into seven domains (polar, humid temperate, dry, humid, and humid tropical), with further divisions based on other climate characteristics (subarctic, warm temperate, hot temperate, and subtropical; marine and continental; lowland and mountain).
A team of biologists convened by the World Wildlife Fund (WWF) developed a scheme that divided the world's land area into biogeographic realms (called "ecozones" in a BBC scheme), and these into ecoregions (Olson & Dinerstein, 1998, etc.). Each ecoregion is characterized by a main biome (also called major habitat type).
This classification is used to define the Global 200 list of ecoregions identified by the WWF as priorities for conservation.
Humans have altered global patterns of biodiversity and ecosystem processes. As a result, vegetation forms predicted by conventional biome systems can no longer be observed across much of Earth's land surface as they have been replaced by crop and rangelands or cities. Anthropogenic biomes provide an alternative view of the terrestrial biosphere based on global patterns of sustained direct human interaction with ecosystems, including agriculture, human settlements, urbanization, forestry and other uses of land. Anthropogenic biomes offer a new way forward in ecology and conservation by recognizing the irreversible coupling of human and ecological systems at global scales and moving us toward an understanding of how best to live in and manage our biosphere and the anthropogenic biomes we live in.
The endolithic biome, consisting entirely of microscopic life in rock pores and cracks, kilometers beneath the surface, has only recently been discovered, and does not fit well into most classification schemes.
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WorldBiomes.com is a site covering the 5 principal world biome types: aquatic, desert, forest, grasslands, and tundra.
UWSP's online textbook The Physical Environment: – Earth Biomes
Panda.org's Habitats – describes the 14 major terrestrial habitats, 7 major freshwater habitats, and 5 major marine habitats.
Panda.org's Habitats Simplified – provides simplified explanations for 10 major terrestrial and aquatic habitat types.
UCMP Berkeley's The World's Biomes – provides lists of characteristics for some biomes and measurements of climate statistics.
Gale/Cengage has an excellent Biome Overview of terrestrial, aquatic, and man-made biomes with a particular focus on trees native to each, and has detailed descriptions of desert, rain forest, and wetland biomes.