Environmental Engineering Reference
In-Depth Information
Fig. 1.3 Numbers of
species identifi ed and
named (dark histo-
grams) and estimates of
unnamed species that
exist (light histograms).
(Modifi ed from
Millennium Ecosystem
Assessment, 2005a.)
Number of species (in millions)
0
1
2
3
4
5
6
7
8
Insects, centipedes and millipedes
Fungi
Spiders, mites, etc.
Algae, amoebas, etc.
Nematodes, worms
Plants
Molluscs
Named species
Unnamed species (estimate)
Crustaceans
Vertebrates
1.2.1 The scale of the
biodiversity problem
To judge t he scale of the problem facing environmental managers it would be useful
to know the total number of species that exist, the rate at which these are going
extinct and how this rate compares with pre-human times. Not surprisingly, there
are considerable uncertainties in our estimates of all these things. For example,
only about 1.8 million species have so far been named, but the real number lies
between 3 and 30 million. Most biodiversity specialists think it is around 10 million
(Figure 1.3).
Palaeontologists estimate that species exist, on average, for between 1 and 10
million years. If we accept this assumption, and taking the total number of species
on earth to be 10 million, we can predict that each century between 100 (if species
last 10 million years) and 1000 species will go extinct (if species last 1 million years).
This represents a 'natural' extinction rate of between 0.001% and 0.01% of species
per century. The current estimate of extinction of birds and mammals, the groups
for which we have the best information, is about 1% per century. In other words,
the current rate may be as much as 100 to 1000 times the 'natural' background rate.
And when we bear in mind the number of species believed to be under threat
(Box 1.1), the future rate of extinction may be more than ten times higher again
(Millennium Ecosystem Assessment, 2005a).
Estimates of extinction rates are beset with diffi culties and most extinctions pass
unnoticed. Another way to gauge the problem is to focus on long-term assessments
of the population sizes of species that have not yet gone extinct. In the case of British
birds it is clear that woodland species and, more particularly, farmland species have
been in decline for many years (Figure 1.4a). Worldwide, amphibians (Figure 1.4b)
and marine and freshwater vertebrates (Figure 1.4c) also show clear signs of wide-
spread population declines.
Consider how instructive it would be to carry out a massive experiment in which
a region is allowed to completely fulfi ll its economic potential while simultaneously
documenting the consequences for biodiversity. This decidedly 'unethical' experi-
ment would give us a glimpse of what the world could be like if unlimited population
growth and development continue indefi nitely everywhere. In fact the 'experiment'
Search WWH ::




Custom Search