Biome

A biome /ˈbaɪoʊm/ is a distinct biogeographical unit consisting of a biological community that has formed in response to a shared regional climate.[1][2] Biomes may span more than one continent. Biome is a broader term than habitat and can comprise a variety of habitats.

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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.[3]

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.

History of the concept

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The term was suggested in 1916 by Clements, originally as a synonym for biotic community of Möbius (1877).[4] Later, it gained its current definition, based on earlier concepts of phytophysiognomy, 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.[5][6] In 1935, Tansley added the climatic and soil aspects to the idea, calling it ecosystem.[7][8] The International Biological Program (1964–74) projects popularized the concept of biome.[9]

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).[10]

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.[5][11][12]

Classifications

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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.[13]

A 1978 study on North American grasslands[14] 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.[15] 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):[16]

• Tundra
• Taiga
• Deciduous forest
• Grasslands
• Desert
• High plateaus
• Tropical forest
• Minor terrestrial biomes

Kendeigh (1961) biomes

The principal biomes of the world by Kendeigh (1961):[17]

• Terrestrial
  • Temperate deciduous forest
  • Coniferous forest
  • Woodland
  • Chaparral
  • Tundra
  • Grassland
  • Desert
  • Tropical savanna
  • Tropical forest
• Marine
  • Oceanic plankton and nekton
  • Balanoid-gastropod-thallophyte
  • Pelecypod-annelid
  • Coral reef

Whittaker (1962, 1970, 1975) biome-types

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.[18]

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 a broader method to categorize similar communities.[19]

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.[19]

1. 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
2. Climatic moisture gradient
3. Temperature gradient by altitude
4. 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

Goodall (1974–) ecosystem 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:[21]

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.[10][22][23]

Zonobiome	Zonal soil type	Zonal vegetation type
ZB I. Equatorial, always moist, little temperature seasonality	Equatorial brown clays	Evergreen tropical rainforest
ZB II. Tropical, summer rainy season and cooler “winter” dry season	Red clays or red earths	Tropical seasonal forest, seasonal dry forest, scrub, or savanna
ZB III. Subtropical, highly seasonal, arid climate	Serosemes, sierozemes	Desert vegetation with considerable exposed surface
ZB IV. Mediterranean, winter rainy season and summer drought	Mediterranean brown earths	Sclerophyllous (drought-adapted), frost-sensitive shrublands and woodlands
ZB V. Warm temperate, occasional frost, often with summer rainfall maximum	Yellow or red forest soils, slightly podsolic soils	Temperate evergreen forest, somewhat frost-sensitive
ZB VI. Nemoral, moderate climate with winter freezing	Forest brown earths and grey forest soils	Frost-resistant, deciduous, temperate forest
ZB VII. Continental, arid, with warm or hot summers and cold winters	Chernozems to serozems	Grasslands and temperate deserts
ZB VIII. Boreal, cold temperate with cool summers and long winters	Podsols	Evergreen, frost-hardy, needle-leaved forest (taiga)
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, 2005) 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):[24]

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).[25][26]

• 100 Polar Domain
  • 120 Tundra Division (Köppen: Ft)
  • M120 Tundra Division – Mountain Provinces
  • 130 Subarctic Division (Köppen: E)
  • M130 Subarctic Division – Mountain Provinces
• 200 Humid Temperate Domain
  • 210 Warm Continental Division (Köppen: portion of Dcb)
  • M210 Warm Continental Division – Mountain Provinces
  • 220 Hot Continental Division (Köppen: portion of Dca)
  • M220 Hot Continental Division – Mountain Provinces
  • 230 Subtropical Division (Köppen: portion of Cf)
  • M230 Subtropical Division – Mountain Provinces
  • 240 Marine Division (Köppen: Do)
  • M240 Marine Division – Mountain Provinces
  • 250 Prairie Division (Köppen: arid portions of Cf, Dca, Dcb)
  • 260 Mediterranean Division (Köppen: 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 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).[27][28]

This classification is used to define the Global 200 list of ecoregions identified by the WWF as priorities for conservation.[27]

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 realms (terrestrial and freshwater)

The applicability of the realms scheme above - based on Udvardy (1975)—to most freshwater taxa is unresolved.[29]

Biogeographic realms (marine)

Biomes (terrestrial)

1. Tropical and subtropical moist broadleaf forests (tropical and subtropical, humid)
2. Tropical and subtropical dry broadleaf forests (tropical and subtropical, semihumid)
3. Tropical and subtropical coniferous forests (tropical and subtropical, semihumid)
4. Temperate broadleaf and mixed forests (temperate, humid)
5. Temperate coniferous forests (temperate, humid to semihumid)
6. Boreal forests/taiga (subarctic, humid)
7. Tropical and subtropical grasslands, savannas, and shrublands (tropical and subtropical, semiarid)
8. Temperate grasslands, savannas, and shrublands (temperate, semiarid)
9. Flooded grasslands and savannas (temperate to tropical, fresh or brackish water inundated)
10. Montane grasslands and shrublands (alpine or montane climate)
11. Tundra (Arctic)
12. Mediterranean forests, woodlands, and scrub or sclerophyll forests (temperate warm, semihumid to semiarid with winter rainfall)
13. Deserts and xeric shrublands (temperate to tropical, arid)
14. Mangrove (subtropical and tropical, salt water inundated)[28]

Biomes (freshwater)

According to the WWF, the following are classified as freshwater biomes:[31]

Biomes (marine)

Biomes of the coastal and continental shelf areas (neritic zone):

• Polar
• Temperate shelves and sea
• Temperate upwelling
• Tropical upwelling
• Tropical coral[32]

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)
    • (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. (Plant list)
          • Plant: Silver fir (Abies alba)

Other biomes

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Marine biomes

Pruvot (1896) zones or "systems":[33]

• Littoral zone
• Pelagic zone
• Abyssal zone

Longhurst (1998) biomes:[34]

• Coastal
• Polar
• Trade wind
• Westerly

Other marine habitat types (not covered yet by the Global 200/WWF scheme):^{[citation needed]}

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 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[35]

Microbial biomes

Endolithic biomes

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.[36]

See also

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References

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