Geoscience Reference
In-Depth Information
supply. It is of little use, for example, if all factors are
favourable but one, e.g. a soil nutrient, is absent or in low
supply. Productivity in this situation will be zero or low,
and is determined by the law of the minimum: 'Growth
is governed by the factor which operates at a minimum.'
competition
will eliminate the less efficient plant, or the
less efficient species. Competition between individuals of
the same species is
intra-specific competition
, and often
occurs at the beginning of successions on fresh, bare
surfaces where colonizers are competing for space and
resources. Competition between species,
inter-specific
competition
, is universal, and results in actual species
ranges that are much narrower than their full tolerance
ranges.
Figure 20.3
shows the viability of four species
with different competitive abilities along an environ-
mental gradient like soil moisture. Species A is not
competitive, and therefore remains a secondary compo-
nent of the final communities. Species B has a wide range
of tolerance but is only moderately competitive, and
therefore dominates at the wet end of the gradient. Species
C is highly aggressive and dominates in its narrow range,
as does species D for most of its wider range. The resulting
structure of the plant communities along the gradient, in
terms of dominant and secondary species, is shown at E.
The pattern of plants which results is thus the result of
two broad influences: first, the
range of tolerance
of the
species of an environmental gradient and, second, the
inter-specific competition between the plants.
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 21,
and, second,
the position of the species along environmental gradients
such as temperature, moisture, soil pH, soil fertility and
Gradients, competition and niches
In the real world environmental conditions exist as
gradients
. There may, for example, be a pH gradient from
a basic igneous rock such as basalt to an acid igneous rock
such as granite. A moisture gradient may go from a wet
valley bog to dry ridge crests in the same valley. The
changes in the performance of a plant species along such
a trend are called an
environmental gradient
(
Figure 20.2
).
There will be upper and lower threshold values on the
gradient beyond which the species cannot survive. These
points are the
upper limit of tolerance
and the
lower limit
of tolerance
.In
Figure 20.2
the tolerance range is shown
as a broad-based normal curve, though in reality it may
be much narrower for a particular species on a particular
gradient. The
ecological optimum
for the species is that
part of the tolerance range where the vigour of the plant
is at a maximum.
In the real world there are two complications in the
concept of tolerance range. First, a species has a separate
range for each environmental factor. Each separate
response includes a different range and optimum. When
all ranges are added together we get the
ecological
amplitude
of the species. This is a multidimensional
'hyperspace' which it is not easy to define or represent.
However, it is a useful concept for summarizing the sum
total of the effects of all environmental factors. The second
complication is that plants differ in their ability to utilize
a resource which is in limiting supply. The process of
A Moderate range, non-
competitive species
B
Wide range (cosmopolitan),
moderately competitive
species
Range of Tolerance
C
Narrow range, highly
competitive species
Ecological
optimum
D
Moderate range, highly
competitive species
E
Species performance
DRY
WET
Environmental
gradient
Environmental Gradient
competitive abilities (A-D). Graph E shows resulting vegeta-
tion along a gradient of wetness.
