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Biome

A biome is a biogeographical unit, a distinct geographical region characterized by specific climate conditions, vegetation types, and animal life. It represents a complex ecosystem where a biological community has developed in response to its physical surroundings and prevailing climate. The concept evolved over time; in 1935, Tansley expanded upon it, incorporating climatic and soil factors to define the ecosystem. The term "biome" gained prominence through the International Biological Program during the period of 1964–74.

It's crucial to note that the term "biome" can be interpreted differently. In some German scientific literature, particularly within the framework established by Walter, "biome" is used synonymously with biotope, referring to a specific, localized geographical unit. The definition employed here, however, aligns with an international, non-regional approach. Regardless of continent, an area exhibiting similar characteristics is designated by the same biome name. This aligns with Walter's concepts of "zonobiome" (determined by climate zone), "orobiome" (determined by altitude), and "pedobiome" (determined by soil).

Furthermore, in Brazilian academic contexts, "biome" is sometimes used interchangeably with biogeographic province, which is defined by its species composition. When focusing solely on plant species, the term floristic province is used. It can also be a synonym for the "morphoclimatic and phytogeographical domain" as described by Ab'Sáber, denoting vast geographical areas with similar geomorphological and climatic traits, and a particular form of vegetation. Both these Brazilian usages often encompass multiple biomes as understood in the broader international sense.

Classifications

The challenge of definitively dividing the Earth into distinct ecological zones is compounded by the ubiquitous presence of small-scale variations and the gradual transitions between different biomes. Consequently, the boundaries drawn between them are inherently arbitrary, and their characterization relies on the predominant average conditions within each.

A study conducted in 1978 on North American grasslands revealed a significant logistic correlation between evapotranspiration levels and above-ground net primary production. The findings indicated that precipitation and water availability directly influenced above-ground production, while solar irradiance and temperature were key drivers of below-ground production (root systems). Temperature and water also played a role in determining whether plant growth was favored during cool or warm seasons. These insights were instrumental in shaping classification schemes, such as Holdridge's bioclassification, which were later simplified by Whittaker. The multiplicity of classification schemes and the diverse determinants employed underscore the inherent difficulty in perfectly categorizing biomes.

Holdridge (1947, 1964) life zones

The Holdridge life zones classification scheme, conceptualized by American botanist and climatologist Leslie Holdridge in 1947, categorizes climates based on the biological impact of temperature and rainfall on vegetation. Holdridge posited that these two abiotic factors are the most significant determinants of vegetation types within a given habitat. His scheme utilizes four axes to define 30 "humidity provinces," clearly depicted in his diagram. While acknowledging the importance of soil and sun exposure, Holdridge's primary focus remained on temperature and precipitation.

Allee (1949) biome-types

William C. Allee, in 1949, proposed a classification of principal biome-types:

Kendeigh (1961) biomes

S. Charles Kendeigh's 1961 classification identified the following principal biomes of the world:

Whittaker (1962, 1970, 1975) biome-types

Robert Harding Whittaker's classification, developed between 1962 and 1975, employed precipitation and temperature as the primary abiotic factors for categorizing biomes. His scheme is often considered a more accessible simplification of Holdridge's, though it sacrifices some of the latter's detailed specificity.

Whittaker’s approach was grounded in both theoretical considerations and empirical field sampling. He had previously undertaken a comprehensive review of existing biome classifications.

Key definitions for understanding Whittaker's scheme
  • Physiognomy: This term refers to the apparent characteristics of plants, or more broadly, the outward features and appearance of ecological communities or species.
  • Biome: A biome is defined as a collection of terrestrial ecosystems found on a particular continent that share similar vegetation structure, physiognomy, environmental features, and animal community characteristics.
  • Formation: This term designates a major type of plant community on a given continent.
  • Biome-type: This represents a grouping of convergent biomes or formations from different continents, identified by their shared physiognomy.
  • Formation-type: This refers to a grouping of convergent formations.

Whittaker’s distinction between "biome" and "formation" can be simplified: "formation" is used when the focus is exclusively on plant communities, while "biome" is employed when both plant and animal components are considered. The use of "biome-type" or "formation-type" by Whittaker provided a broader framework for categorizing similar communities across diverse geographical locations.

Whittaker's parameters for classifying biome-types

Whittaker utilized what he termed "gradient analysis" of ecocline patterns to establish relationships between communities and climate on a global scale. He identified four principal ecoclines within the terrestrial realm:

  • Intertidal levels: This gradient pertains to the varying degrees of wetness in areas exposed to alternating water and dryness, influenced by tidal cycles.
  • Climatic moisture gradient: This axis reflects variations in moisture availability across different regions.
  • Temperature gradient by altitude: This gradient illustrates how temperature changes with increasing elevation.
  • Temperature gradient by latitude: This gradient shows the typical decrease in temperature from the equator towards the poles.

Along these gradients, Whittaker observed several consistent trends that informed his classification of biome-types:

  • Productivity gradient: Environmental conditions range from highly favorable to extreme, with corresponding shifts in ecological productivity.
  • Physiognomic complexity: The complexity of vegetation structure diminishes as environmental conditions become less favorable, leading to a reduction in community structure and stratification.
  • Diversity trends: Both alpha (within-habitat) and beta (between-habitat) species diversity decrease from favorable to extreme environments.
  • Growth-form dominance: Each plant growth-form (e.g., grasses, shrubs) exhibits a characteristic zone of maximum abundance along these ecoclines.
  • Global convergence: Similar growth forms may dominate in comparable environmental conditions across widely separated parts of the world, indicating convergent evolution.

Whittaker synthesized the influences of the temperature gradients (3 and 4) and the moisture gradient (2) to develop his classification scheme. This scheme graphically represents average annual precipitation (on the x-axis) against average annual temperature (on the y-axis) to delineate biome-types.

Biome-types (Whittaker)

Goodall (1974–) ecosystem types

The multi-volume series Ecosystems of the World, edited by David W. Goodall, offers a comprehensive exploration of the Earth's major "ecosystem types or biomes." This extensive work categorizes environments into a detailed structure:

  • Terrestrial Ecosystems
    • Natural Terrestrial Ecosystems
      • Wet Coastal Ecosystems
      • Dry Coastal Ecosystems
      • Polar and Alpine Tundra
      • Mires: Swamp, Bog, Fen, and Moor
      • Temperate Deserts and Semi-Deserts
      • Coniferous Forests
      • Temperate Deciduous Forests
      • Natural Grasslands
      • Heathlands and Related Shrublands
      • Temperate Broad-Leaved Evergreen Forests
      • Mediterranean-Type Shrublands
      • Hot Deserts and Arid Shrublands
      • Tropical Savannas
      • Tropical Rain Forest Ecosystems
      • Wetland Forests
      • Ecosystems of Disturbed Ground
    • Managed Terrestrial Ecosystems
      • Managed Grasslands
      • Field Crop Ecosystems
      • Tree Crop Ecosystems
      • Greenhouse Ecosystems
      • Bioindustrial Ecosystems
  • Aquatic Ecosystems
    • Inland Aquatic Ecosystems
      • River and Stream Ecosystems
      • Lakes and Reservoirs
    • Marine Ecosystems
      • Intertidal and Littoral Ecosystems
      • Coral Reefs
      • Estuaries and Enclosed Seas
      • Ecosystems of the Continental Shelves
      • Ecosystems of the Deep Ocean
    • Managed Aquatic Ecosystems
      • Managed Aquatic Ecosystems
  • Underground Ecosystems
    • Cave Ecosystems

Walter (1976, 2002) zonobiomes

The classification scheme developed by Heinrich Walter centers on the seasonality of temperature and precipitation. This system, also evaluating precipitation and temperature, delineates nine major biome types, detailing their key climatic traits and associated vegetation types. The boundaries of each biome are closely correlated with the moisture and cold stress conditions, which are critical determinants of plant form and, consequently, the vegetation that defines the region. Even within a biome, extreme conditions, such as those found in swamps, can lead to the development of distinct community types. [5] [18] [19]

Number 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

Jürgen Schultz, in his 1988 work, defined nine ecozones, a concept more akin to biomes than the broader ecozone concept used by the BBC. His classification focuses on broad climatic and vegetation patterns: [20]

  • 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 G. Bailey developed a biogeographical classification system of ecoregions. Initially focused on the United States in 1976, he later expanded it to cover North America in 1981 and subsequently the entire world by 1989. Bailey's system is primarily based on climate and is structured into four domains: polar, humid temperate, dry, and humid tropical. These domains are further subdivided based on other climatic characteristics, including subarctic, warm temperate, hot temperate, and subtropical; marine and continental; and lowland and mountain variations. [21] [22]

  • 100 Polar Domain
    • 120 Tundra Division (Köppen climate classification: Ft)
      • M120 Tundra Division – Mountain Provinces
    • 130 Subarctic Division (Köppen climate classification: E)
      • M130 Subarctic Division – Mountain Provinces
  • 200 Humid Temperate Domain
    • 210 Warm Continental Division (Köppen climate classification: portion of Dcb)
      • M210 Warm Continental Division – Mountain Provinces
    • 220 Hot Continental Division (Köppen climate classification: portion of Dca)
      • M220 Hot Continental Division – Mountain Provinces
    • 230 Subtropical Division (Köppen climate classification: portion of Cf)
      • M230 Subtropical Division – Mountain Provinces
    • 240 Marine Division (Köppen climate classification: Do)
      • M240 Marine Division – Mountain Provinces
    • 250 Prairie Division (Köppen climate classification: arid portions of Cf, Dca, Dcb)
    • 260 Mediterranean Division (Köppen climate classification: 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 classification scheme developed by a team of biologists convened by the World Wildlife Fund (WWF) divides the Earth's land area into biogeographic realms (termed "ecozones" in some classifications), which are further subdivided into ecoregions. Each ecoregion is characterized by a primary biome, also referred to as a major habitat type. This classification forms the basis for the Global 200 list, which identifies ecoregions prioritized by the WWF for conservation efforts. [23] [24]

For terrestrial ecoregions, a specific EcoID format, XXnnNN, is used, where XX denotes the biogeographic realm, nn represents the biome number, and NN indicates the individual ecoregion number.

Terrestrial biomes identified by Olson & Dinerstein et al.:

Biogeographic realms (terrestrial and freshwater):

The western Palearctic terrestrial ecozone encompasses nine of the fourteen biomes listed by Olson & Dinerstein et al.

The major terrestrial biogeographic realms are designated as:

The applicability of this realm scheme, based on Udvardy (1975), to most freshwater taxa remains an area of ongoing research. [25]

Biogeographic realms (marine):

Biomes (terrestrial):

The WWF classification identifies the following terrestrial biomes:

Biomes (freshwater):

According to the WWF, the following are classified as freshwater biomes: [27]

Biomes (marine):

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

Summary of the scheme (WWF)

The WWF's hierarchical classification system organizes life on Earth as follows:

Example:

Other biomes

Marine biomes:

Further information can be found under Marine habitats.

Other marine habitat types not yet fully integrated into the Global 200/WWF scheme include: [ citation needed ]

Anthropogenic biomes

Further information: Anthropogenic biome

Humanity's impact has dramatically reshaped global patterns of biodiversity and ecosystem processes. Consequently, the vegetation types predicted by traditional biome systems are no longer representative of vast areas of Earth's land surface, having been replaced by agricultural lands, rangelands, or urban environments. Anthropogenic biomes offer an alternative perspective on the terrestrial biosphere, based on global patterns of sustained, direct human interaction with ecosystems. This includes activities such as agriculture, the development of human settlements, urbanization, forestry, and various other uses of land. Anthropogenic biomes provide a framework for recognizing the inseparable coupling of human and ecological systems on a global scale, thereby informing the management of Earth's biosphere and its human-altered components.

There are notable similarities between the 14 terrestrial bioregions identified by Olson & Dinerstein et al. and the 17 land cover classes of the International Geosphere-Biosphere Programme. The latter includes 11 natural vegetation classes, 3 developed and mosaicked land classes, and 3 non-vegetated land classes, as detected by satellite imagery. [31]

Major land cover classes (IGBP):

Major anthropogenic biomes:

Microbial biomes

Main article: Microbiome Further information: Habitat § Microhabitats

Endolithic biomes:

The endolithic biome, comprising exclusively microscopic lifeforms inhabiting the pores and cracks within rocks, often kilometers beneath the Earth's surface, has only recently been discovered. It does not fit neatly into most existing biome classification schemes. [33]

Effects of climate change

Anthropogenic climate change poses a significant threat to the distribution of Earth's biomes. [34] [35] It is projected that biomes globally could undergo such profound transformations that they risk evolving into entirely new biome types. [36] Specifically, analyses suggest that between 54% and 22% of the Earth's land area will experience climatic conditions corresponding to different biomes. [34] Approximately 3.6% of land area is predicted to encounter climates that are entirely novel or highly unusual. [37] [38] An example of such a biome shift is woody plant encroachment, where grasslands can transition into shrub savannas. [39]

Arctic and mountainous biomes are experiencing warming at rates more than double the global average, [40] [41] [42] making them the most vulnerable to climate change. Projections indicate that terrestrial biomes in South America will undergo similar temperature trends. [43] [44] As average annual temperatures continue to rise, moisture within forest biomes is expected to diminish. [43] [45]

Climate change is actively altering biomes, negatively impacting both terrestrial and marine ecosystems. [47] [48] Climate change refers to long-term shifts in temperature and average weather patterns, [49] [50] leading to a marked increase in the frequency and intensity of extreme weather events. [51] As a region's climate changes, its flora and fauna inevitably follow suit. [52] For instance, the IPCC Sixth Assessment Report found that out of 4000 species studied, half had shifted their geographic distribution towards higher latitudes or elevations in response to climate change. [53]

See also