Environmental Engineering Reference
In-Depth Information
Chapter
23
Diversity and stability in ecosystems
A natural ecosytem is a self-regulating community of organisms in equilibrium with their
physical environment. Species become adapted through processes of natural selection to
the conditions - biological and non-biological - which exist in the ecosystem. These
processes of natural selection have worked through evolution to give us a world which
can support a rich variety of species. Precisely how many species are known has been
estimated by E. O. Wilson to be 1ยท4 million, including all plants, animals and micro-
organisms. However, the accuracy of this figure is qualified by the fact that biologists
agree that as an estimate it is probably less than a tenth of the number that actually live on
Earth! In fact the true number probably lies somewhere between 10 million and 100
million species.
Table 23.1 shows the number of species of living organisms known at present. Each
group of organisms shows immense variety, which gives our first definition of
diversity
,
namely the genetic diversity of organisms. Implicit in this definition is the idea of
'genetic banks' and 'genetic resources'. An alternative definition of diversity is simply
the number of separate species in a defined geographical area.
Why are there more species in some areas than in others? The question has intrigued
ecologists for some time, as it raises fundamental questions concerning
speciation
(the
rate at which new species are formed),
adaptation
(the process of acquiring structural,
physiological and behavioural characteristics which improve an organism's chances of
survival in a particular habitat) and
evolution
(changes in an organism's genetic make-up
through time).
Biodiversity
has recently become a popular term for the variety of
organisms at all levels (varieties within species, species, genera and families). It is also
used to include the variety of ecosystems at different spatial scales. As biodiversity has
become more prominent in political and economic debates at an international level it has
given rise to the term
biocomplexity
, which is biodiversity within both a natural systems
and a social systems context. Thus biocomplexity would include considerations of native
rights, trade, aid and political-economic issues beyond purely biological features.
The present chapter examines the nature of diversity and the ways it can be measured.
This enables us to compare different ecosystems at the local, field scale or at larger
national and international scales (pp. 463-5). Then the reasons for such large contrasts in
diversity in the world are discussed. Are the contrasts related to the fact that some places
have had more time to evolve new
Table 23.1 Number of living species
Insects
751,000
Other animals
281,000
Higher plants
248,400
