Biome


A biome is a distinct geographical region with specific climate, vegetation, animal life, and an ecosystem. It consists of a biological community that has formed in response to its physical environment and regional climate. 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 the Brazilian literature, the term biome is sometimes used as a 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 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 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

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 :
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 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: 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.
  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.
  • 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 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 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-subalpine needle-leaved forests
  12. Elfin woodland
  13. Thorn forest
  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 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.
NumberZonobiomeZonal soil typeZonal vegetation type
ZB IEquatorial, always moist, little temperature seasonalityEquatorial brown claysEvergreen tropical rainforest
ZB IITropical, summer rainy season and cooler "winter" dry seasonRed clays or red earthsTropical seasonal forest, seasonal dry forest, scrub, or savanna
ZB IIISubtropical, highly seasonal, arid climateSerosemes, sierozemesDesert vegetation with considerable exposed surface
ZB IVMediterranean, winter rainy season and summer droughtMediterranean brown earthsSclerophyllous, frost-sensitive shrublands and woodlands
ZB VWarm temperate, occasional frost, often with summer rainfall maximumYellow or red forest soils, slightly podsolic soilsTemperate evergreen forest, somewhat frost-sensitive
ZB VINemoral, moderate climate with winter freezingForest brown earths and grey forest soilsFrost-resistant, deciduous, temperate forest
ZB VIIContinental, arid, with warm or hot summers and cold wintersChernozems to serozemsGrasslands and temperate deserts
ZB VIIIBoreal, cold temperate with cool summers and long wintersPodsolsEvergreen, frost-hardy, needle-leaved forest
ZB IXPolar, short, cool summers and long, cold wintersTundra humus soils with solifluction Low, evergreen vegetation, without trees, growing over permanently frozen soils

Schultz (1988) eco-zones

Schultz defined nine ecozones :
  1. polar/subpolar zone
  2. boreal zone
  3. humid mid-latitudes
  4. dry mid-latitudes
  5. subtropics with winter rain
  6. subtropics with year-round rain
  7. dry tropics and subtropics
  8. tropics with summer rain
  9. tropics with year-round rain