Environmental Engineering Reference
In-Depth Information
Fungi
69,000
Protozoa
30,800
Algae
26,900
Bacteria
4,800
Viruses
1,000
Total
1,412,900
Source
: After Wilson (1992).
species, or to the larger flows of energy in some ecosystems (pp. 465-6)? Do the factors
of environmental stress and its converse, environmental stability, have any influence on
diversity (pp. 467-75)? Or is the size of a particular habitat the important influence?
Ecosystem stability
is a property of ecosystems which has aroused much interest in
recent years. The reasons for the interest have much to do with concern about human
impacts on the world's natural ecosystems, and uncertainty about how these ecosystems
may respond to disturbances. However, the study of stability has proved to be difficult,
partly owing to the different meanings which can be attached to the term 'stability' and
also partly from lack of basic information on many ecosystems. Definitions of stability
are discussed (pp. 470-1) and an example is taken from data about British birds (pp. 471-
3). Many attempts have been made by ecologists to construct a general ecological theory
linking diversity and stability (pp. 473-4). Finally there is a discussion of current interest
in biodiversity from a variety of viewpoints (pp. 475-7).
DIVERSITY
DEFINITIONS
Diversity of ecosystems is not an easy property to define or measure. First it is necessary
to define precisely the limits of the community being described in time and space (the
temporal and spatial bounds). Thus one can talk of the diversity of sea birds on an island
in spring, the diversity of plants in an oak woodland or the diversity of insects in the
whole of the Arctic Tundra biome. Very often the boundaries in space, time and
community are set by the logistics of the sampling programme of a particular field study.
Two major characteristics make up diversity - the number of species in the system
(
species richness
) and the evenness of species within the system (
equitability
of species
abundance
). Consider the data in Table 23.2, which show the relative dominance of five
tree species in three woodlands. Woodland A has perfect evenness and the largest number
of species. Woodland B has fewer species, but is still relatively even. Woodland C (a pine
plantation) has few species and very uneven equitability.
One index of stability is the total number of species, which gives species richness.
Thus for the three woodlands the values of 5, 3 and 2 respectively would reflect the
different diversities. However, that would be a crude, unweighted measure, taking no
account of the relative proportions. A better index of diversity would take into
