BIOMES: CHARACTERISTICS AND TYPES - BIOLOGY - 2024

The biome are ecological regions constituted global ecosystem harboring a flora and fauna with structural attributes and similar functional. They receive names allusive to their dominant vegetation type: tropical rainforests, temperate deciduous forests, Mediterranean chaparrals, etc.

In ascending order, the levels of organization of living beings are cell, organism, population, community, ecosystem, landscape, biome and biosphere. Therefore, biomes are the most inclusive category in which ecologists and biogeographers divide life on Earth.

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Biomes are defined based on the physiognomy of the vegetation, without taking into account the taxonomic identity of the organisms. The same type of biome can be found on continents with different flora.

The biome concept assumes that the environment acts both in evolutionary time through natural selection, and in ecological time by filtering species to produce the global pattern of distribution of natural vegetation.

Biome-level approaches allow the formulation of biodiversity conservation strategies and the study of climate change.

Background

In 1874, Augustin de Candolle proposed five latitudinal vegetation zones based on temperature. In 1888, Hermann Wagner and Emil von Sydow recognized ten types of vegetation, such as tundra and desert, currently considered biomes. In 1900, Wladimir Köppen classified the planet's climates based on vegetation.

In 1892, C. Hart Merriam formulated the concept of the life zone, a precursor of the biome since it posed a large-scale relationship between biota and climate.

In 1916, Frederic Clements coined the term biome as a synonym for biotic community. In 1935, Arthur Tansley coined the term ecosystem for the sum of a biotic community and its physical environment.

In 1939, F. Clements and Victor Shelford defined biomes based on their climax vegetation and referred to them at geographic scales larger than ecosystems.

In 1947, Leslie Holdridge created a system to differentiate life zones. In 1948, C. Warren Thornthwaite developed an alternative climate classification to Köppen's.

In 1970, Robert Whittaker added the climatic dimension to the biome concept. In 1975, Heinrich Walter used a special type of graph that he called a climate diagram to classify and characterize the biomes of the planet.

Köppen climate system

The geographical distribution of the vegetation proposed by A. de Candolle served W. Köppen as an objective basis to classify the types of climate and to prepare the first climate maps. Köppen defined five main types of weather designated with letters:

1- A. Humid tropical: every month with an average temperature above 18 ° C; annual rainfall over 1,500 mm. Subdivided into Af (tropical rainy), Am (tropical monsoon) and Aw (tropical dry, or savanna).

2- B. Dry: evapotranspiration higher than annual rainfall. Subdivided into BW (arid, true desert) and BS (semi-arid, steppe).

3- C. Humid temperate, with moderate winters: coldest month with average temperature below 18 ° C and above -3 ° C; warmest month with average temperature above 10 ° C. Subdivided into Cfa (humid subtropical), Cs (Mediterranean) and Cfb (maritime).

4- D. Humid temperate, with extreme winters: warmer month with average temperature above 10 ° C; coldest month with average temperature below -3 ° C. Subdivided into Dw (with dry winters), Ds (with dry summers) and Df (with wet winters and summers).

5- E. Polar: with low temperatures all year round; average temperature of the least cold less than 10 ° C. Subdivided into ET (polar tundra) and EF (glaciers).

Thornthwaite Climate Classification

The original Köppen system continues to be the most widely used, despite the fact that numerous modifications of it have been proposed, such as that of Trewartha (1968) and new climatic classifications, among which that of Thornthwaite stands out.

The same amount of rainfall produces deserts in Africa and splendid forests in Scandinavia. For this reason, Thornthwaite developed the concept of potential evapotranspiration (EP), of great importance in plant ecophysiology, to study the interaction between precipitation and temperature instead of both variables separately.

Thornthwaite proposed a climate classification that, due to its complexity, has been sparsely used and has produced few maps. Based on EP, this author performed somewhat cumbersome calculations of various indices (aridity, humidity, thermal efficiency, seasonality) that produced a kind of kaleidoscope of more than 800 types of climate.

Holdridge Life Zone System

Classify the relationship between climate and vegetation. Widely used due to its empirical simplicity. It allows determining the life zone of a locality based on the logarithms of its biotemperature (BT) and rainfall (P).

It assumes that: 1) throughout the world, climax plant formations differ into ecologically equivalent physiognomic types; 2) the climate determines the geographical limits of these formations, called life zones.

BT depends on latitude and longitude and is the sum of the monthly positive temperatures divided by 12. P is quantified in millimeters. Based on BT, the potential evapotranspiration (EP) is calculated.

The potential evapotranspiration ratio (EPP) is calculated as EPP = EP / P. EPP and delimits nine provinces (H) of humidity-aridity.

30 life zones are represented as hexagonal cells within a triangular graph, whose sides have scales corresponding to P, EPP and H. To the right and left vertical scales are represented corresponding to six latitudinal regions and six altitudinal floors.

In the graph, the life zone gradations are: P, rain forest to rain tundra; EPP, desert to dry tundra; H, desert to rain forest; top, dry tundra to pluvial tundra.

Whittaker Biomes

Based on the characteristics of the vegetation, Whittaker defined nine types of biome:

- Tropical rain forest

- Seasonal rainforest / savanna

- Subtropical desert

- Sparse / bush forest

- Temperate rain forest

- Temperate seasonal forest;

- Temperate grassland / desert

- Cold boreal forest

- Tundra.

Whittaker illustrated the areas occupied by the biomes on a two-dimensional graph whose horizontal axis represents mean annual temperature (° C) and whose vertical axis represents mean annual rainfall (cm). This minimalist graphic emphasizes the approximate climatic boundaries between biomes.

In the Whittaker graph, the vast majority of locations on the planet are in a triangular area whose vertices correspond to hot / humid (tropical rainforest), hot / dry (subtropical desert) and cold / dry (tundra) climates.

The triangular area of ​​the graph that would correspond to temperate / cold and rainy / very rainy climates appears empty. This is due to the fact that cold regions with very abundant annual rainfall are scarce or non-existent. This is because water does not evaporate easily at low temperatures and cold air holds very little vapor.

Walter's zonobiomes

Unlike Whittaker, Walter first defined climates. He then chose the boundaries between climatic zones by matching them with vegetation types (zonobiomes) equivalent to the Whittaker biomes.

Walter used climad diagrams in which monthly temperatures (T) and rainfall (P) are represented on the same graph using vertical scales adjusted to indicate wet and dry periods. If P is above T, there is no water deficit and plant growth is limited only by T. If P is below T, the water deficit limits said growth.

Walter's zonobiomes are: I) equatorial evergreen forest; II) tropical deciduous forest; III) subtropical desert; IV) Mediterranean chaparral; V) temperate evergreen forest; VI) temperate deciduous forest; VII) temperate grasslands and steppes; VIII) cold boreal forest; IX) tundra.

These zonobiomes are characterized by: I) P and T are not limiting; II) P is limiting in winter; III) P is limiting throughout the year; IV) P is limiting in summer; V) T is limiting (<0 ° C) briefly in winter; VI) T is limiting in winter; VII) P is limiting in summer and T in winter; VIII) T is limiting most of the year; IX) T is limiting practically all year round.

Biome types

The Whittaker and Walter classification of biomes into nine types is the most general possible. There is currently no general consensus on how many types of biome should be distinguished. For example, the WWF (World Wildlife Fund = World Conservation Fund for Nature) distinguishes 14, while some authors claim that there are more than 20.

The ecological and biogeographic characterizations of various types of terrestrial biome presented below are limited to Walter's scheme. It should be noted that this represents a simplification.

Equatorial evergreen forest

It is distributed in lowlands of the tropical regions (10 ° N – 10 ° S) of America (Amazon and Orinoco basins, Atlantic coast of Brazil, Central America), Africa (from the Atlantic coast to the Congo basin, Madagascar), Asia (Vietnam, Thailand, Malaysia) and the Pacific islands ranging from Asia to Australia (Indonesia, the Philippines, New Guinea, Queensland).

Climates are characterized by annual rainfall of at least 2,000mm, with every month exceeding 100mm. The temperature is uniform throughout the year (> 18 ° C) and varies less seasonally than throughout the day.

Although the soils are often lateritic and therefore poor in nutrients, the vegetation consists of a continuous canopy of evergreen trees that reach heights of 30–60 m. Under that canopy are several strata made up of smaller trees and shrubs. Lianas and epiphytes abound.

Despite occupying only 6% of the earth's surface, it is the most productive, complex and diverse biome: it is home to half of the planet's plant and animal species.

Tropical deciduous forest

For many current authors and, taking into account Walter's sense, this biome consists of two clearly different sub-biomes: tropical deciduous forest and tropical savanna.

The forest formations of this biome are distributed in lowlands outside the equatorial zone (10–30 ° N and S) in South America, Africa, India and Australia. The climates are warm and characterized by seasonal rainfall of 900–1,500 mm, with marked rainy and dry seasons (eg monsoon climate in India).

In the case of forest formations, the vegetation consists of deciduous trees that lose their leaves during the dry season, with only one or two layers under the canopy, which is discontinuous.

The tropical savanna formations of this biome have the same distribution as the forested ones. In some regions, especially Asia, these savannas are likely derived from deciduous forests degraded by fire and cattle grazing.

In these savannas, the vegetation consists of grasses with scattered trees. In the case of Africa, they are home to the most diverse communities of herbivorous and carnivorous mammals on the planet.

Subtropical desert

It is distributed in the southwestern United States, northern Mexico, South America (primarily Peru, Chile, and Argentina), northern Africa (Sahara), and Australia (30–40 ° N and S). Together with the cold desert biome, it occupies about a fifth of the Earth's surface.

They are called hot deserts because the temperature rarely drops below 0 ° C. Rainfall is low (often less than 250mm per year) and unpredictable.

The vegetation does not form a canopy and consists mainly of shrubs and low trees, often thorny, typically with small, evergreen leaves, separated by bare ground.

Soils are almost completely devoid of organic matter. The fauna, among which reptiles abound, consists of small, behavioral and physiologically specialized species to resist heat and survive water scarcity.

Mediterranean chaparral

It is distributed in southern California, southern Europe in the northern hemisphere, central Chile, the Cape region (South Africa), and southwestern Australia in the southern hemisphere (30–40 ° N and S).

Winters are characterized by moderate temperatures and rain, while summers by drought. Annual rainfall does not exceed 600 mm.

The vegetation consists of dense shrubs 1–3 meters high, evergreen, with small sclerophyllous leaves resistant to desiccation and deep roots. In summer, frequent fires burn aerial biomass, preventing the establishment of trees. Shrubs regenerate after fires and produce fire resistant seeds.

The soils are not peculiar to this type of vegetation. In contrast to the flora, the fauna has few endemic species.

Temperate evergreen forest

It is distributed near the coast in northwestern North America, southern Chile, Tasmania, and New Zealand. It occupies small extensions.

The climate is characterized by mild winters with heavy rainfall and cloudy summers. Rather cold temperatures prevail throughout the year, but are always above 0 ° C. Annual rainfall exceeds 1,500 mm. The vegetation consists of extremely tall evergreen forests.

In North America two conifers stand out, Douglas fir (Pseudotsuga sp.) And redwood (Sequoia sempervirens), which can exceed 100 meters in height. In the southern hemisphere it is necessary to mention the broadleaf trees (Agathis, Eucalyptus, Nothofaugus) and the conifer (Podocarpus).

Due to the permanent humidity, these forests are not affected by fire. The growth of trees is slow, but they reach great size because they are among the longest living beings on the planet.

Temperate deciduous forest

It is mainly distributed wherever there is enough water for the growth of large trees. For this reason it is distributed to southeastern Canada, eastern United States, Europe and eastern Asia. This biome is underdeveloped in the southern hemisphere because there the high ocean / land ratio moderates the climate and prevents winter frosts.

Trees lose their leaves in the fall and regenerate them in the spring. The dominant species have broad leaves. Vegetation includes shrubs and herbaceous plants on the forest floor.

Soils contain abundant organic matter. Fruit and nut bearing trees abound, feeding a diverse fauna including squirrels, deer, wild boar and bears.

Temperate grasslands and steppes

It is distributed in North America (Great Basin), South America (pampas), Europe (Ukraine), Central Asia (steppes, Gobi desert) and South Africa (Veld), occupying continental plains (30 ° –60 ° N and S). Geographically and climatically it is found between temperate forests and deserts.

Annual rainfall is 300–850 mm. If the rainfall is less (250–500 mm), the biome is called the cold desert (Great Basin, Gobi). Winter is extreme. The plant growing season (T> 0 ° C) is 120–300 days.

There is a unique stratum of vegetation, dominated by grasses up to 3 m in humid prairies, and up to 0.2 m in cold deserts. Fires are big in late summer.

Due to infrequent rainfall and low temperatures, the debris decomposes slowly. The soils are deep, rich in organic matter and fertile. Natural grasslands, which once occupied 40% of the earth's surface, have been cut in half due to agriculture.

These meadows are home to emblematic animals. In North America, they include the bison, pronghorn, prairie dog (marmot), or coyote. In Europe and Asia they include the tarpan (wild horse), the saiga antelope and the mole rats.

Cold boreal forest

It is often known as taiga. It occupies a wide latitudinal strip centered at 50 ° N in North America and 60 ° N in Europe. At high altitudes, it penetrates the temperate zone. For example, it extends from Canada southward along the Rocky Mountains, continuing to the elevated areas throughout Mexico.

To the north, it is found where summers are short (less than four months with average temperature> 10 ° C; annual average <5 ° C) and winters are long and extreme (down to –60 ° C). In temperate mountains, it is found at heights where frost prevails. Annual rainfall is 400–1,000 mm.

The vegetation is dominated by evergreen conifers (Picea a bias) 10–20 meters high. The canopy is not very dense, so there is an understory of acidity tolerant shrubs, mosses and lichens. Diversity is low.

Due to the low evaporation, the soils are humid and due to the low temperatures, the vegetal debris slowly decomposes and accumulates, forming peat bogs. The taiga is one of the largest reservoirs of organic carbon on the planet. The accumulation of acicular leaves makes the soils acidic and not very fertile.

Tundra

It is found mainly in the northern hemisphere, north of the taiga and south of the polar cap. The alpine tundra is found at high altitudes, just below glaciers, in North America (Rocky Mountains), South America (Andes), Europe (Alps) and, occupying a large area, in Asia (Tibetan plateau).

The climate is more extreme (below 0 ° C for 7-10 months of the year) than that of the taiga. Annual rainfall is less than or much less than 600 mm. Most of the ground is frozen all year round (permafrost). During long summer days, the topsoil (0.5–1 m) thaws, allowing accelerated plant growth.

The vegetation is devoid of trees and consists of dwarf shrubs, grasses. Mosses and lichens are prominent. Primary productivity, plant biomass and biodiversity are lower than those of the other biomes.

Among the herbivorous animals, the caribou, the musk ox, the Dall's sheep or the arctic hare the lemingos stand out. Among the carnivorous animals stand out the brown bears, the wolves and the arctic foxes. On the Tibetan plateau, the yak (similar to cows), the argali (a type of wild sheep) and the snow leopard stand out.

Aquatic biomes

The biome concept was developed for terrestrial ecosystems based on vegetation characteristics. Because they lack vegetation (the primary producers are mainly single-celled algae), aquatic ecosystems do not have biomes in the sense that the term has for terrestrial ecosystems.

Aquatic ecosystems occupy a larger area than terrestrial ones and are structurally and biologically very diverse. Their study and conservation has also made it necessary to group them into biomes.

Aquatic biomes are defined based on characteristics such as their latitude, rainfall, winds, proximity to the coast, depth, temperature, water flow, salinity and concentration of oxygen and nutrients.

The number of recognized aquatic biomes varies. The most general possible categorization would include rivers, lakes, wetlands, estuaries, and oceans.

In more detail, mangroves, salt flats, lentic (lakes and ponds) / lotic (rivers and streams) communities, rocky / sandy / muddy sea coasts, coral reefs, surface / marine pelagic depths, platform / deep ocean benthos can be distinguished.

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