Environmental Engineering Reference
In-Depth Information
THREATS TO GLOBAL BIODIVERSITY
The destruction of the world's vegetation is recognized as one of the most serious of
human impacts. An immediate result is the extinction of plants and animals and the loss
of habitat. It is estimated that between 1990 and 2015 between 2 per cent and 10 per cent
of the flora of tropical forests will become extinct. Another threatened habitat is oceanic
islands, very often with their own endemic species which are jeopardized by the
introduction of competitive foreign species. It is estimated that 30 per cent of plants
under threat of extinction are island endemics. About 1000 plant species are known to
have become extinct in the past 2000 years, and about 25,000 are threatened. However,
these estimates are likely to be on the low side, given the lack of data for many regions.
The International Union for the Conservation of Nature (IUCN) issues Red Data
Topics of threatened species. In 1990 4500 animal species were listed as threatened; this
figure represents 12 per cent of mammals, 11 per cent of birds, 4 per cent of fish, but only
0ยท1 per cent of insects. The major threats to both plants and animals are as follows:
1 Loss or fragmentation of habitat by cultivation, forestry, grazing, settlement.
2 Over-exploitation for commercial gain.
3 Deliberate or accidental introduction of competitive species.
4 Deliberate eradication of pest species.
5 Disease.
Most of the threatened mammal and bird species live in tropical countries or on oceanic
islands. In the latter case, flightless birds have suffered very badly from introduced
vermin (especially rats). By contrast, many threatened reptiles, amphibians and fish live
in temperate latitudes.
A special concern has been to protect entire ecosystems rather than to concentrate on
one or two species within those ecosystems. About 20 per cent of all plants are classed as
endemics
, i.e. plants with a very restricted range and confined to a specific region (see
box, p. 469). The island of Madagascar, one of the most isolated of large islands, has
10,000 plant species, 80 per cent of which are endemic, i.e. found nowhere else. It also
has fifty species of lemurs which are endemic too. The Mediterranean biome in
California contains 25 per cent of all plant species found in North America, of which
about 50 per cent are found nowhere else in the world. On the basis of regions which
have unique species and which are threatened by extinction, Myers listed eighteen hot
spots considered to have the highest conservation priority (Figure 23.11). Of the eighteen
habitats, fourteen are tropical forests and four are Mediterranean ecosystems. The list is
likely to be the very minimum, representing only those areas which are well documented.
There are likely to be many more 'hot spots', especially in oceans, lakes and rivers,
which will be added in the near future. The IUCN also recognizes 250
Centres of Plant
Diversity
(CPDs) which are
