6.2. The biosfere like an ecosystem (I). Terrestrial ecosystems.

biosferaThe biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed (apart from solar and cosmic radiation), and self-regulating system. From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, hydrosphere, and atmosphere. The biosphere is postulated to have evolved, beginning through a process of biogenesis or biopoesis, at least some 3,5 billion years ago.

The biosphere is limited by the external part of the litosphere, the hidrosphere until 11 km deep and the first lay of the troposphere.

We can distinguish, according with the fluid in which the organisms are living, between two different ecosystems: terrestrial and aquatic.

TERRESTRIAL ECOSYSTEMS

The different big ecosystems, or biomes, are determined by different ambiental factors or abiotic factors, like the clima or the different kinds of soil.

The clima is determined by the precipitations and the temperature. These factors depends on the latitude, altitude, the winds and the topography of the zone. At the same time, these factors determine the type of vegetation, which condition the fauna.

This is the reason why we find very different ecosystems in areas of the Earth located at a similar latitude, and similar ecosystems at different latitudes. A mountain next to the Equator and another place next to the sea in the north of Europe can have forests with similar plants.

Terrestrial ecosystems occupy 144,150,000 km2, or 28.2%, of Earth’s surface. Although they are comparatively recent in the history of life (the first terrestrial organisms appeared in the Silurian Period, about 425 million years ago) and occupy a much smaller portion of Earth’s surface than marine ecosystems, terrestrial ecosystems have been a major site of adaptive radiation of both plants and animals. Major plant taxa in terrestrial ecosystems are members of the division Magnoliophyta (flowering plants), of which there are about 275,000 species, and the division Pinophyta (conifers), of which there are about 500 species. Members of the division Bryophyta (mosses and liverworts), of which there are about 24,000 species, are also important in some terrestrial ecosystems. Major animal taxa in terrestrial ecosystems include the classes Insecta (insects) with about 900,000 species, Aves (birds) with 8500 species, and Mammalia (mammals) with approximately 4100 species.

TERRESTRIAL BIOMES

Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to asecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a large area, creating a typical ecosystem over that area. Such major ecosystems are termed as biomes. Biomes are defined by factors such as plant structures (such as trees, shrubs, and grasses), leaf types (such as broadleaf and needleleaf), plant spacing (forest, woodland,savanna), and climate. Unlike ecozones, biomes are not defined by genetic, taxonomic, or historical similarities. Biomes are often identified with particular patterns of ecological succession andclimax vegetation (quasiequilibrium state of the local ecosystem). An ecosystem has many biotopes and a biome is a major habitat type. A major habitat type, however, is a compromise, as it has an intrinsic inhomogeneity. Some examples of habitats are ponds, trees, streams, creeks, and burrows in the sand or soil.

The biodiversity characteristic of each extinction, especially the diversity of fauna and subdominant plant forms, is a function of abiotic factors and the biomass productivity of the dominantvegetation. In terrestrial biomes, species diversity tends to correlate positively with net primary productivity, moisture availability, and temperature.

Biomes are often known in English by local names. For example, a temperate grassland or shrubland biome is known commonly as steppe in central Asia, prairie in North America, and pampas in South America. Tropical grasslands are known as savanna in Australia, whereas in southern Africa it is known as certain kinds of veld (from Afrikaans).

Sometimes an entire biome may be targeted for protection, especially under an individual nation’s biodiversity action plan.

Climate is a major factor determining the distribution of terrestrial biomes. Among the important climatic factors are:

  • Latitude: Arctic, boreal, temperate, subtropical, tropical
  • Humidity: humid, semihumid, semiarid, and arid
    • Seasonal variation: Rainfall may be distributed evenly throughout the year or be marked by seasonal variations.
    • Dry summer, wet winter: Most regions of the earth receive most of their rainfall during the summer months; Mediterranean climate regions receive their rainfall during the winter months.
  • Elevation: Increasing elevation causes a distribution of habitat types similar to that of increasing latitude.

According with the WWF we can found these 14 biomes in the world:

Vegetation-no-legend

01 Moist broadlef forest Tropical and subtropical, humid
02 Dry broadleaf forest Tropical and subtropical, semihumid
03 Tropical coniferous forest Tropical and subtropical, semihumid
04 Temperated broadleaf and mixed forest Temperate, humid
05 Temperated coniferous forest Temperate, humid to semihumid
06 Boreal forests/taiga Subartic, humid
07 Tropical and subtropical grasslands, savannas, and shrublands Tropical and subtropical, semiarid
08 Temperate grasslands, savannas, and shrublands Temperate, semiarid
09 Flooded grasslands and savannas Temperate a tropical, aigua dolça o salobre inundades
10 Montane grasslands and shrublands Alpine or montane climate
11 Tundra Arctic
12 Mediterranean forests, woodlands, and scrub or sclerophyll forests Calid Temperate, semihumid to semiarid with winter rainfall
13 Deserts and xeric shrublands Temperate to tropical, arid
14 Mangrove Subtropical and tropical, inundats d’aigua salada

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