Biome


A biome 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.

History of the concept

The term was suggested in 1916 by Clements, originally as a synonym for biotic community of Möbius. Later, it gained its current definition, based on earlier concepts of phytophysiognomy, formation and vegetation, with the inclusion of the animal element and the exclusion of the taxonomic element of species composition. In 1935, Tansley added the climatic and soil aspects to the idea, calling it ecosystem. The International Biological Program 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 Walter terminology, the term is used similarly as biotope, 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".
In Brazilian literature, the term "biome" is sometimes used as synonym of "biogeographic province", an area based on species composition, 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.

Classifications

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, and temperature and water lead to cool and warm season growth habit. These findings help explain the categories used in Holdridge's bioclassification scheme, 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

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 :
The principal biomes of the world by Kendeigh :
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

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.
  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:
Whittaker summed the effects of gradients and to get an overall temperature gradient and combined this with a gradient, the moisture gradient, to express the above conclusions in what is known as the Whittaker classification scheme. The scheme graphs average annual precipitation versus average annual temperature to classify biome-types.

Biome-types

  1. Tropical rainforest
  2. Tropical seasonal rainforest
  3. * deciduous
  4. * semideciduous
  5. Temperate giant rainforest
  6. Montane rainforest
  7. Temperate deciduous forest
  8. Temperate evergreen forest
  9. * needleleaf
  10. * sclerophyll
  11. Subarctic-subalpin needle-leaved forests
  12. Elfin woodland
  13. Thorn forests and woodlands
  14. Thorn scrub
  15. Temperate woodland
  16. Temperate shrublands
  17. * deciduous
  18. * heath
  19. * sclerophyll
  20. * subalpine-needleleaf
  21. * subalpine-broadleaf
  22. Savanna
  23. Temperate grassland
  24. Alpine grasslands
  25. Tundra
  26. Tropical desert
  27. Warm-temperate desert
  28. Cool temperate desert scrub
  29. Arctic-alpine desert
  30. Bog
  31. Tropical fresh-water swamp forest
  32. Temperate fresh-water swamp forest
  33. Mangrove swamp
  34. Salt marsh
  35. Wetland

    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:

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.
ZonobiomeZonal soil typeZonal vegetation type
ZB I. Equatorial, always moist, little temperature seasonalityEquatorial brown claysEvergreen tropical rainforest
ZB II. Tropical, summer rainy season and cooler “winter” dry seasonRed clays or red earthsTropical seasonal forest, seasonal dry forest, scrub, or savanna
ZB III. Subtropical, highly seasonal, arid climateSerosemes, sierozemesDesert vegetation with considerable exposed surface
ZB IV. Mediterranean, winter rainy season and summer droughtMediterranean brown earthsSclerophyllous, frost-sensitive shrublands and woodlands
ZB V. Warm temperate, occasional frost, often with summer rainfall maximumYellow or red forest soils, slightly podsolic soilsTemperate evergreen forest, somewhat frost-sensitive
ZB VI. Nemoral, moderate climate with winter freezingForest brown earths and grey forest soilsFrost-resistant, deciduous, temperate forest
ZB VII. Continental, arid, with warm or hot summers and cold wintersChernozems to serozemsGrasslands and temperate deserts
ZB VIII. Boreal, cold temperate with cool summers and long wintersPodsolsEvergreen, frost-hardy, needle-leaved forest
ZB IX. Polar, short, cool summers and long, cold wintersTundra humus soils with solifluction Low, evergreen vegetation, without trees, growing over permanently frozen soils

Schultz (1988) ecozones

Schultz defined nine ecozones :
  1. polar/subpolar zone
  2. boreal zone
  3. humid mid-latitudes
  4. arid mid-latitudes
  5. tropical/subtropical arid lands
  6. Mediterranean-type subtropics
  7. seasonal tropics
  8. humid subtropics
  9. humid tropics

    Bailey (1989) ecoregions

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, with further divisions based on other climate characteristics.
A team of biologists convened by the World Wildlife Fund developed a scheme that divided the world's land area into biogeographic realms, and these into ecoregions. Each ecoregion is characterized by a main biome.
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.

Biogeographic realms (terrestrial and freshwater)

The applicability of the realms scheme above - based on Udvardy —to most freshwater taxa is unresolved.

Biogeographic realms (marine)

  1. Tropical and subtropical moist broadleaf forests
  2. Tropical and subtropical dry broadleaf forests
  3. Tropical and subtropical coniferous forests
  4. Temperate broadleaf and mixed forests
  5. Temperate coniferous forests
  6. Boreal forests/taiga
  7. Tropical and subtropical grasslands, savannas, and shrublands
  8. Temperate grasslands, savannas, and shrublands
  9. Flooded grasslands and savannas
  10. Montane grasslands and shrublands
  11. Tundra
  12. Mediterranean forests, woodlands, and scrub or sclerophyll forests
  13. Deserts and xeric shrublands
  14. Mangrove

    Biomes (freshwater)

According to the WWF, the following are classified as freshwater biomes:
Biomes of the coastal and continental shelf areas :
Example:

Marine biomes

Pruvot zones or "systems":
Longhurst biomes:
Other marine habitat types :
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.
Major anthropogenic 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.