A biome () is a biogeographical unit consisting of a biological community (ecology), community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader term than habitat and 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.

Etymology

The term was suggested in 1916 by Frederic Clements, Clements, originally as a synonym for ''biotic community'' of Karl Möbius, Möbius (1877). Later, it gained its current definition, based on earlier concepts of wiktionary:phytophysiognomy#:~:text=phytophysiognomy (plural phytophysiognomies),physiognomy (appearance) of a plant, phytophysiognomy, formation (vegetation), formation and vegetation (used in opposition to flora), with the inclusion of the animal element and the exclusion of the taxonomic element of species composition. In 1935, Arthur Tansley, Tansley added the climatic and soil aspects to the idea, calling it ''ecosystem''. The International Biological Program (1964–74) projects popularized the concept of biome. However, in some contexts, the term biome is used in a different manner. In German literature, particularly in the Heinrich Walter, 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 Aziz Ab'Sáber, 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.

Classifications

To divide the world into a few ecological zones is difficult, 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 regression, 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 Sunlight, 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 (1947, 1964) life zones

In 1947, the American botanist and climatologist Leslie 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.

Allee (1949) biome-types

The principal biome-types by Allee (1949): * Tundra * Taiga * Deciduous, Deciduous forest * Grasslands * Desert * High plateaus * Tropical forest * Minor terrestrial biomes

Kendeigh (1961) biomes

The principal biomes of the world by Kendeigh (1961): * ''Terrestrial'' ** Temperate deciduous forest ** Taiga, Coniferous forest ** Woodland ** Chaparral ** Tundra ** Grassland ** Desert ** Tropical savanna ** Tropical forest * Marine ** Ocean, Oceanic plankton and nekton ** Balanoid-gastropod-thallophyte ** Pelecypod-annelid ** Coral reef

Whittaker (1962, 1970, 1975) biome-types

Robert Harding Whittaker, 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 had previously compiled a review of biome classifications.

Key definitions for understanding Whittaker's scheme

* Physiognomy: sometimes referring to the plants' appearance; or the biome's apparent characteristics, outward features, or appearance of ecological communities or species - including plants. * 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 (vegetation), 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 a broader method to categorize similar communities.

Whittaker's parameters for classifying biome-types

Whittaker 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.

Biome-types

# Tropical rainforest # Tropical seasonal rainforest #* deciduous #* semideciduous # Temperate giant rainforest # Montane rainforest # Temperate deciduous forest # Temperate evergreen forest #* needleleaf #* sclerophyll # Subarctic-subalpine needle-leaved forests (taiga) # Elfin woodland # Thorn forests and woodlands # Thorn scrub # Temperate woodland # Temperate shrublands #* deciduous #* heath #* sclerophyll #* subalpine-needleleaf #* subalpine-broadleaf # Savanna # Temperate grassland # Alpine grasslands # Tundra # Tropical desert # Warm-temperate desert # Cool temperate desert scrub # Arctic-alpine desert # Bog # Tropical fresh-water swamp forest # Temperate fresh-water swamp forest # Mangrove swamp # Salt marsh # Wetland

Goodall (1974–) ecosystem types

The multi-authored series ''Ecosystems of the World'', edited by David W. Goodall, provides a comprehensive coverage of the major "ecosystem types or biomes" on Earth:

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.

Schultz (1988) eco-zones

Schultz (1988, 2005) defined nine ''ecozones'' (his concept of ecozone is more similar to the concept of biome than to the concept of biogeographic realm, ecozone of BBC):Schultz, J. ''Die Ökozonen der Erde'', 1st ed., Ulmer, Stuttgart, Germany, 1988, 488 pp.; 2nd ed., 1995, 535 pp.; 3rd ed., 2002; 4th ed., 2008; 5th ed., 2016. Transl.: ''The Ecozones of the World: The Ecological Divisions of the Geosphere''. Berlin: Springer-Verlag, 1995; 2nd ed., 2005

# polar/subpolar zone # boreal zone # humid mid-latitudes # dry mid-latitudes # subtropics with winter rain # subtropics with year-round rain # dry tropics and subtropics # tropics with summer rain # tropics with year-round rain

Bailey (1989) ecoregions

Robert Bailey (geographer), Robert G. Bailey nearly developed a biogeography, 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 four domains (polar, humid temperate, dry, and humid tropical), with further divisions based on other climate characteristics (subarctic, warm temperate, hot temperate, and subtropical; marine and continental; lowland and mountain). * 100 Polar Domain ** 120 Tundra Division (Köppen: Ice Cap Climate, Ft) ** M120 Tundra Division – Mountain Provinces ** 130 Subarctic Division (Köppen: Tundra, E) ** M130 Subarctic Division – Mountain Provinces * 200 Humid Temperate Domain ** 210 Warm Continental Division (Köppen: portion of Humid continental climate, Dcb) ** M210 Warm Continental Division – Mountain Provinces ** 220 Hot Continental Division (Köppen: portion of Humid continental climate, Dca) ** M220 Hot Continental Division – Mountain Provinces ** 230 Subtropical Division (Köppen: portion of Humid subtropical climate, Cf) ** M230 Subtropical Division – Mountain Provinces ** 240 Marine Division (Köppen: Oceanic climate, Do) ** M240 Marine Division – Mountain Provinces ** 250 Prairie Division (Köppen: arid portions of Humid subtropical climate, Cf, Humid continental climate, Dca, Humid continental climate, Dcb) ** 260 Mediterranean Division (Köppen: Mediterranean climate, Cs) ** M260 Mediterranean Division – Mountain Provinces * 300 Dry Domain ** 310 Tropical/Subtropical Steppe Division ** M310 Tropical/Subtropical Steppe Division – Mountain Provinces ** 320 Tropical/Subtropical Desert Division ** 330 Temperate Steppe Division ** 340 Temperate Desert Division * 400 Humid Tropical Domain ** 410 Savanna Division ** 420 Rainforest Division

Olson & Dinerstein (1998) biomes for WWF / Global 200

A team of biologists convened by the World Wide Fund for Nature, 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).Olson, D. M. & E. Dinerstein (1998). The Global 200: A representation approach to conserving the Earth’s most biologically valuable ecoregions. ''Conservation Biol.'' 12:502–515

.Olson, D. M., Dinerstein, E., Wikramanayake, E. D., Burgess, N. D., Powell, G. V. N., Underwood, E. C., D'Amico, J. A., Itoua, I., Strand, H. E., Morrison, J. C., Loucks, C. J., Allnutt, T. F., Ricketts, T. H., Kura, Y., Lamoreux, J. F., Wettengel, W. W., Hedao, P., Kassem, K. R. (2001). Terrestrial ecoregions of the world: a new map of life on Earth. ''Bioscience'' 51(11):933–938

. This classification is used to define the Global 200 list of ecoregions identified by the WWF as priorities for conservation. For the terrestrial ecoregions, there is a specific EcoID, format XXnnNN (XX is the biogeographic realm, nn is the biome number, NN is the individual number).

Biogeographic realm, Biogeographic realms (terrestrial and freshwater)

* NA: Nearctic realm, Nearctic * PA: Palearctic realm, Palearctic * AT: Afrotropical realm, Afrotropic * IM: Indomalayan realm, Indomalaya * AA: Australasian realm, Australasia * NT: Neotropical realm, Neotropic * OC: Oceanian realm, Oceania * AN: Antarctic realm, Antarctic The applicability of the realms scheme above - based on Udvardy (1975)—to most freshwater taxa is unresolved.Abell, R., M. Thieme, C. Revenga, M. Bryer, M. Kottelat, N. Bogutskaya, B. Coad, N. Mandrak, S. Contreras-Balderas, W. Bussing, M. L. J. Stiassny, P. Skelton, G. R. Allen, P. Unmack, A. Naseka, R. Ng, N. Sindorf, J. Robertson, E. Armijo, J. Higgins, T. J. Heibel, E. Wikramanayake, D. Olson, H. L. Lopez, R. E. d. Reis, J. G. Lundberg, M. H. Sabaj Perez, and P. Petry. (2008). Freshwater ecoregions of the world: A new map of biogeographic units for freshwater biodiversity conservation. ''BioScience'' 58:403–414

.

Biogeographic realms (Marine ecoregion, marine)

* Arctic * Temperate Northern Atlantic * Temperate Northern Pacific * Tropical Atlantic * Western Indo-Pacific * Central Indo-Pacific * Eastern Indo-Pacific * Tropical Eastern Pacific * Temperate South America * Temperate Southern Africa * Temperate Australasia * Southern OceanSpalding, M. D. et al. (2007). Marine ecoregions of the world: a bioregionalization of coastal and shelf areas. ''BioScience'' 57: 573–583

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Biomes (terrestrial)

# Tropical and subtropical moist broadleaf forests (tropical and subtropical, humid) # Tropical and subtropical dry broadleaf forests (tropical and subtropical, semihumid) # Tropical and subtropical coniferous forests (tropical and subtropical, semihumid) # Temperate broadleaf and mixed forests (temperate, humid) # Temperate coniferous forests (temperate, humid to semihumid) # Taiga, Boreal forests/taiga (subarctic, humid) # Tropical and subtropical grasslands, savannas, and shrublands (tropical and subtropical, semiarid) # Temperate grasslands, savannas, and shrublands (temperate, semiarid) # Flooded grasslands and savannas (temperate to tropical, fresh or brackish water inundated) # Montane grasslands and shrublands (alpine or montane climate) # Tundra (Arctic) # Mediterranean forests, woodlands, and scrub or sclerophyll forests (temperate warm, semihumid to semiarid with winter rainfall) # Deserts and xeric shrublands (temperate to tropical, arid) # Mangrove (subtropical and tropical, salt water inundated)

Biomes (freshwater)

According to the WWF, the following are classified as freshwater biomes: * Large lakes * Large river deltas * Polar freshwaters * Montane freshwaters * Temperate Upland and lowland (freshwater ecology), coastal rivers * Temperate floodplain rivers and wetlands * Temperate Upland and lowland (freshwater ecology), upland rivers * Tropical and subtropical Upland and lowland (freshwater ecology), coastal rivers * Tropical and subtropical floodplain rivers and wetlands * Tropical and subtropical Upland and lowland (freshwater ecology), upland rivers * Xeric freshwaters and endorheic basins * Oceanic islands

Biomes (marine)

Biomes of the coastal and continental shelf areas (neritic zone): * Polar * Temperate shelves and sea * Temperate upwelling * Tropical upwelling * Coral reef, Tropical coral

Summary of the scheme

* Biosphere ** Biogeographic realms (terrestrial) (8) *** Ecoregions (867), each characterized by a main biome type (14) **** Ecosystems (biotopes) * Biosphere ** Biogeographic realms (freshwater) (8) *** Ecoregions (426), each characterized by a main biome type (12) **** Ecosystems (biotopes) * Biosphere ** Biogeographic realms (marine) (12) *** (Ecoregion#Marine, Marine provinces) (62) **** Ecoregions (232), each characterized by a main biome type (5) ***** Ecosystems (biotopes) Example: * Biosphere ** Biogeographic realm: Palearctic *** Ecoregion: Dinaric Mountains mixed forests (PA0418); biome type: temperate broadleaf and mixed forests **** Ecosystem: Orjen, vegetation belt between 1,100 and 1,450 m, Oromediterranean zone, nemoral zone (temperate zone) ***** Biotope: ''Oreoherzogio-Abietetum illyricae'' Fuk. (Dinaric calcareous block fir forest#Plant list, Plant list) ****** Plant: Silver fir (''Abies alba'')

Other biomes

Marine biomes

Pruvot (1896) zones or "systems": * Littoral, Littoral zone * Pelagic zone * Abyssal zone Longhurst code, Longhurst (1998) biomes: * Coastal * Polar * Trade wind * Westerly Other marine habitat types (not covered yet by the Global 200/WWF scheme): * Open sea * Deep sea * Hydrothermal vents * Cold seeps * Benthic zone * Pelagic zone (trades and westerlies) * Abyssal * Hadal (ocean trench) * Littoral zone, Littoral/Intertidal zone * Salt marsh * Estuary, Estuaries * Coastal lagoons/Atoll lagoons * Kelp forest * Pack ice

Anthropogenic biomes

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 land use, uses of land. Anthropogenic biomes offer a way to recognize the irreversible coupling of human and ecological systems at global scales and manage Earth's biosphere and anthropogenic biomes. Major anthropogenic biomes: * Dense settlements * Croplands * Rangelands * Forested * Indoor

Microbial biomes

Endolithic biomes

The endolithic biome, consisting entirely of microscopic life in rock porosity, pores and cracks, kilometers beneath the surface, has only recently been discovered, and does not fit well into most classification schemes.

Effects of climate change

Anthropogenic climate change has the potential to greatly alter the distribution of Earth's biomes. Meaning, biomes around the world could change so much that they would be at risk of becoming new biomes entirely. General frequency models have been a staple in finding out the impact climate change could have on biomes. More specifically, 54% and 22% of global land area will experience climates that correspond to other biomes. 3.6% of land area will experience climates that are completely new or unusual. Average temperatures have risen more than twice the usual amount in both arctic and mountainous biomes, which leads to the conclusion that arctic and mountainous biomes are currently the most vulnerable to climate change. The current reasoning surrounding as to why this is the case are based around the fact that colder environments tend to reflect more sunlight, as a result of the snow and ice covering the ground. Since the annual average temperatures are rising, ice and snow is melting. As a result, albedo is lowered. Keeping a keen eye on terrestrial biomes is important, as they play a crucial role in climate regulation. South American terrestrial biomes have been predicted to go through the same temperature trends as arctic and mountainous biomes. With its annual average temperature continuing to increase, the moisture currently located in forest biomes will dry up.