Environmental Engineering Reference
In-Depth Information
balanced by emigration to areas outside. This is where the DNA fingerprinting of
individuals and the use of radio tracking can establish the minimal critical size needed to
conserve the population.
Studies of birds in Britain have shown that the number of species found in woodland
does indeed reflect area. However, area seems to be an indicator of species diversity
rather than a cause; a larger area is usually associated with greater habitat diversity in the
form of floristic diversity and canopy height. Smaller woodlands possess fewer kinds of
species than larger areas, and also contain smaller populations of particular species. This
in turn leads to genetic drift, inbreeding and loss of genetic diversity, especially where
habitat islands are physically isolated from each other. The net result is that population
numbers may fall below a critical threshold, become vulnerable to a physical disturbance
and may become locally extinct.
On a global scale the tropical rain forests are being destroyed at a rate of 2 per cent per
year, and it is estimated that the present area of 8 million km
2
is about half that in
immediate postglacial times. The rate of loss is increasing and will reduce the cover to 4
million km
2
by AD 2020. The question arises: what proportion of species will disappear?
The answer will lie between 10 per cent (
z
value 0·15) and 23 per cent (
z
value 0·35).
This elimination of 10-23 per cent represents 5-10 per cent of all species on Earth, at the
most conservative estimate. The species-area equation accounts for most, though not all,
of this loss. Hence many tropical countries try to preserve 'islands' of forest, as in Brazil,
where a government law requires landowners to leave at least 50 per cent of their land
under forest. Analysis of such 'islands' by ecologists shows that diversity decreases more
rapidly the smaller the island. Winds and desiccation reduce shade-loving insects (ants,
butterflies) in plots less than 10 ha in size, as well as amphibians, mammals and birds
which depend on them. Large ground-dwelling mammals migrate quickly but some
species of birds and monkeys flourish around the forest edges.
STABILITY
DEFINITIONS
Stability of ecosystems is not an easy property to define. Indeed, the ecological literature
suffers from confusion; in some cases the same term is used with different meanings, and
in others different terms are used to convey the same meaning. Table 23.4 presents the
most acceptable definitions of stability.
An ecosystem is
stable
if all variables return to the initial equilibrium position
(defined as
K
) after suffering a perturbation or shock which has displaced the variables
from their equilibrium position. How fast the variables return to their equilibrium is the
resilience
. If a system is unable to return to equilibrium it is
unstable
and therefore has no
resilience. A special case is where biological populations do not return to equilibrium but
cycle indefinitely (lemming in the Arctic, lynx in the subarctic, red grouse in Britain).
Figure 23.6 shows the reaction of four ecosystems to a perturbation. System A (a tropical
rain forest) is stable; it does not depart far from equilibrium, and returns rapidly to it.
System B is unstable; it passes beyond the stability domain and collapses. System C (a
boreal coniferous forest) is stable, but less stable than A, owing to its larger displacement
