Environmental Engineering Reference
In-Depth Information
1.2.3 Drivers of
biodiversity loss -
the extinction vortex
Extinction may be caused by one of a number of 'drivers', including habitat loss
(Section 1.2.4), invasive species (Section 1.2.5), overexploitation (Section 1.2.6) and
habitat degradation (including pollution and intensifi cation of agriculture - Section
1.2.7). The relative importance of different drivers for global bird biodiversity is
illustrated in Figure 1.8. During the past 500 years bird extinctions can be attrib-
uted, in roughly equal measure, to the effects of invasive species, overexploitation
by hunters and habitat loss. But now habitat loss is the biggest problem facing
threatened species (whether they are classed as critically endangered, endangered
or vulnerable). And in the case of 'near threatened' bird species, the ones that man-
agers will increasingly need to attend to in future, habitat loss to agriculture is
overwhelmingly the most important driver.
In reality, it seems likely that more than one driver will have played a role in the
extinction of any given animal or plant. Thus, a species may be driven to a very
small population size by habitat loss/degradation and/or the effect of an invader
and/or overexploitation. Then, when numbers become very small there is an
increased chance of matings among relatives that produce deleterious effects due to
inbreeding depression, causing the population to become smaller still - the so-called
extinction vortex (for more detail see Chapter 5, Box 5.1). And a further driver now
needs to be added to the list - the global climate change that is predicted to occur
over the next century (Section 1.2.8).
Changes to the relative importance of different drivers for all species in various
ecosystem types are illustrated in Figure 1.9. Climate change and pollution are pre-
dicted to become progressively more important causes of biodiversity loss across all
ecosystem types. Habitat change is also set to increase in importance, except in
temperate forest, desert, island and mountain ecosystems. Invaders are expected to
Fig. 1.8 Relative
importance of different
'drivers' responsible for
the loss or endanger-
ment of bird biodiver-
sity. Patterns are shown
for fi ve categories of
extinction threat (refer
to Box 1.1). The values
above each histogram
are the numbers of
species in each threat
category globally. In
comparison to the past,
habitat loss/degradation
poses a much bigger
risk now (compare
histogram for extinct
birds with histograms
for endangered and
vulnerable categories)
and this is set to
increase in future, in
particular via agricul-
tural expansion
(histogram for near
threatened species).
(Modifi ed from
Balmford & Bond,
2005, where further
references can be
found.)
129
182
326
684
731
100
75
50
25
0
Habitat loss/degradation
(other)
Habitat loss/degradation
(agriculture)
Other
Invaders
Overexploitation
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