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2 . 5 THE GEOGRAPHIC DISTRIBUTION OF BIOLOGICAL DIVERSITY 35
eates those regions characterised by a constant temperature. Within this
vast domain, moisture patterns differentiate the great pluvial forest,
which is continuously hot and humid throughout the year, from the
savannas, with their characteristic dry season.
How many biomes are identified depends upon the desired degree of
resolution, and different authors distinguish anything from 10 to 100. In
most cases, the physiognomy of the vegetation serves as the basis for
delineating biomes: forests (24 percent of the land surface), savannas
(15 percent), prairies and tundras (15 percent), etc. One should also
mention cultivated areas, which cover over 10 percent of the land
above sea level, as well as the deserts and frozen expanses (30 percent).
2 . 5 . 5 Biogeographic regions as taxonomic units
Numerous attempts have been made to divide the surface of the Earth
into large biogeographic regions by proceeding from the current distribution
of flora and fauna and knowledge about their historical locations
and drawing inferences as to the spatial organisation of biological diversity.
Very broadly, six major terrestrial regions are recognized within
which the flora and fauna share a common history. Corresponding
globally to the major continental shelves, these include three large ‘tropical’
regions: the Afrotropics (Africa), the Neotropics (South America)
and Indo-Malaysia, which are home to over two-thirds of the known
terrestrial species. The Nearctic region (North America), the Palearctic
region (Eurasia; see Table 2.4) and the Australian region (Australia)
cover the temperate to cold zones.
This typological approach can be further refined. Within each of the six
major zones, any number of subsets may be identified depending upon the
Table 2.4
The species present in Europe as compared with the world
Known species
in Europe
Species known
only in Europe
Number of species
known in the world
Freshwater fish 344 200 10 000
Reptiles 198 90 6500
Amphibians 75 56 4000
Nesting birds 520 30 9700
Mammals 270 78 4327
Vascular plants 125 000 3500 270 000