Environmental Engineering Reference
In-Depth Information
THE CONCEPT OF THE ECOLOGICAL NICHE
The way in which the distribution of organisms is influenced by the physical and biotic
factors of the environment is the essence of the concept of the ecological niche . The
ecological niche of a species is defined by, first, the functional role of the species in its
community (i.e. its trophic position, as described in Chapter 22), and, secondly, the
position of the species along environmental gradients such as temperature, moisture, soil
pH, soil fertility and other factors. The American ecologist Hutchinson introduced in the
1940s the concept of the multidimensional hypervolume where the limits of a species'
existence are defined by a large number of variables. He also drew a distinction between
the fundamental niche and the realized niche . The fundamental niche is the maximum
'theoretically inhabited hypervolume' where a species, free from any sort of interference
from another species, can occupy the full range of variables within the community to
which it is adapted, to the outer limits of its tolerances. The realized niche is a smaller
hypervolume which is occupied by the species under interference from the competition of
other species. A niche is thus defined by n variables, and can be defined as 'the limits, for
all important environmental features, within which individuals of a species can survive,
grow and reproduce'.
VEGETATION ZONES OF EARTH
In describing the terrestrial vegetation of the entire Earth, large units of vegetation known
as vegetation formations or biomes are used (see Table 20.3). Different biogeographers
have proposed different schemes, but all agree that formations and biomes are strongly
correlated with regional climate: climate determines what kind of vegetation can be
expected in a particular locality.
The German biogeographer Walter in 1976 recognized nine climatic zones and
corresponding vegetation zones. These were labelled I-IX. Transitional types were given
a double label e.g. I (III). Mountainous regions were designated as X, with a second label,
e.g. X (V) indicating the climatic zone from which the mountains rise (Figure 20.4). The
Equatorial Zone (I) lies between about 10° N and 10° S. The daily variation in
temperature is greater than the annual variation, which is in the range 25°-27° C.
Generally annual rainfall is high, with the maximum occurring at the times of the
equinoxes. Vegetation is classed as the tropical evergreen rain forest zone (1). The
Tropical Zone (II) is located between 10° and 30° N and S approximately. Some seasonal
variation in mean daily temperature is noticeable. Rainfall is at a maximum during the
summer rainy season, and in the cool season there is a dry season that increases in
duration with increasing distance from the equator . Tropical moist forests (2) and dry
Table 20.3 Climate and vegetation zones of Earth
Climatic zone
Vegetation zone
I
Equatorial
1
Tropical evergreen rain forest
II
Tropical
2
Tropical moist forest
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