**2. Definition of deserts, semi-deserts, and habitats prone to desertification**

Efficient functioning of the Earth's ecosystems is based on the autotrophic plant life, which can make use of the radiation energy of the Sun directly. This plant life needs, besides the light of the Sun, simultaneous availability of minerals in the lithosphere, carbon dioxide in the atmosphere, and liquid water in the hydrosphere. Thus, biosphere appears on the interface of these three spheres. Where these four factors are available in the most efficiently usable form, tropical rainforests can be found. Every other habitat is more or less a "struggle zone" for plants because some of the four above mentioned factors limit the biosphere. Based on the degree of environmental limitations, ecosystems can be arranged along a scale. Desert ecosystems can be found on the opposite endpoint of this scale compared to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

tropical rainforests. Semi-deserts can be found near deserts on this imaginary line, just a little bit nearer the other endpoint of the scale.

• Volcanic areas exposed to regular lava flow or tuff eruption

• Areas exposed to extreme environmental pollution (e.g., mazut lakes)

faces, airports, highways—"anthropogenic deserts")

average)

**4. Climatic approach**

does not depend on the climate only.

climatic variation).

semi-deserts.

• Areas exposed to anthropogenic transformation and disturbance (urban concrete sur-

Introductory Chapter: Global Aspects and Scientific Importance of Desert Ecological Research

http://dx.doi.org/10.5772/intechopen.78368

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• Certain agricultural areas with low productivity (not irrigated, plowed areas with regular disturbance, lying fallow most of the year, semi-desert category in annual

At the borders of the semi-desert category, there are tundra, open grasslands (e.g., rocky, saline, and mountain) and several other habitat types besides classic tropical and temperate

Vegetation is normally able to evapotranspirate water equivalent to 20 mm of precipitation at a temperature of 10°C. A month is considered to be climatically arid if its monthly average temperature exceeds double the monthly amount of precipitation. If each of the 12 months of the year is arid in multi-year average, the area is considered to be a desert from a climatic point of view. If less than 2 months of the year are not arid, we speak about semi-desert climate. Similarly, if the multi-year average of the monthly average temperatures is below 0°C for 12 months, we speak about ice desert from a climatic point of view; if the period with an average temperature above 0°C lasts up to 2 months, the climate is tundra. However, the climatic approach may be misleading because the typical vegetation type of a certain area

Plants and animals have been able to adapt to areas with different environmental (among them climatic) conditions better and better during the evolution (on a historic time scale). In the second part of the Cambrian period (approx. 542–488 million years ago), hot tropical conditions were dominating most of the Earth, however, 100% of land could be considered a desert, only traces of some coastal invertebrates indicate terrestrial life. Also, during the Ordovician (488–443 million years ago), the climate was hot in several areas, the first plants (liver mosses and hyphae) appeared on land, however, this type of vegetation could have reached rather desert than semi-desert level. The first vascular terrestrial plants appeared in the Silurian (443–416 million years ago), however, they began to form vegetation mainly on the waterside, the continents' interior kept being a desert for the most part from a vegetation point of view. During the evolution of terrestrial life, more and more various adaptation modes have appeared in response to the various climatic conditions, and this process is still going on. At the same time, climatic regulation capacity and generally self-regulation capacity, biodiversity, total biomass, and productivity of the biosphere have also shown an increasing tendency (excluding fall-backs caused by
