Biome

A biome /ˈbaɪoʊm/ is a community of plants and animals that have common characteristics for the environment they exist in, and can be found over a range of continents. Spanning 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 as well, but on a much smaller scale. For example, the human microbiome is the collection of bacteria, viruses, and other microorganisms that are present on a human.

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.

Vegetation.png
One way of mapping terrestrial biomes around the world

History of the concept

The term was suggested in 1916 by Clements, originally as a synonym for biotic community of Möbius (1877). 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. In 1935, 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 Walter terminology, the term is used similarly as biotope (a concrete geographical unit), while the biome definition used in this article is used as a 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 determinated 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 includes 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 (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

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 forest
  • Grasslands
  • Desert
  • High plateaus
  • Tropical forest
  • Minor terrestrial biomes

Kendeigh (1961) biomes

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

Whittaker (1962, 1970, 1975) biome-types

PrecipitationTempBiomes.jpg
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 are 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.

  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 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 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 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

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

Schultz (1988) ecozones

Bailey (1989) ecoregions

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

Biogeographic realms (terrestrial and freshwater)

Biogeographic realms (marine)

Biomes (terrestrial)

Biomes (freshwater)

Biomes (marine)

Summary of the scheme

Other biomes

Marine biomes

Anthropogenic biomes

Microbial biomes

Endolithic biomes

Dermal biome

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