Environmental Engineering Reference
In-Depth Information
Box 1.1
Classifi cation
of extinction risk
The
IUCN Red List of Threatened Species
(produced by the World Conservation Union - previously
the International Union for the Conservation of Nature, IUCN) highlights species at greatest risk of
extinction in each taxonomic group in every part of the world. Overseen by expert specialist teams,
plants and animals are defi ned according to criteria related mainly to population size, distributional
range, and whether the population is currently declining. Figure 1.2 illustrates the 'decision tree'
used to categorize species status. Of course, not all have been evaluated - most species remain
to be identifi ed and named! In some cases an evaluation has been attempted but the data are
currently insuffi cient to classify threat (
Data Defi cient
). Some are already considered to be
Extinct
or
Extinct in the Wild
(where individuals remain only in cultivation or captivity).
When there is no reasonable doubt that last individual
has died
When the species is known only to survive in cultivation,
in captivity or as a naturalized population(s) outside
the past range
Extinct (EX)
Extinct in the
Wild (EW)
Critically
Endangered (CR)
Endangered (EN)
(Adequate
data)
(Threatened)
When the species has been assessed against the criteria
and is thought to be facing a high to extremely high risk
of extinction in the wild
Vulnerable (VU)
When a species does not meet the criteria but is close to
qualifying, or likely to qualify, for a threatened category
in the near future
When a species does not meet listing under a higher
category of threat (for widespread and abundant taxa)
When there is inadequate information to make a direct,
or indirect, assessment of the risk of extinction of a
species based on its distribution and /or population status
(Evaluated)
Near
Threatened (NT)
Least Concern (LC)
Data Deficient (DD)
Unknown
extinction
risk
When a species has not yet been evaluated against the
criteria
Not Evaluated (NE)
Fig. 1.2
A 'decision tree' showing the nine IUCN Red List categories in order of increasing
extinction risk. (From Rodrigues et al., 2006.)
Species considered to be under threat of extinction are listed as
Critically Endangered
(more than
a 50% probability of extinction in 10 years or three generations, whichever is longer),
Endangered
(more than a 20% chance of extinction in 20 years or fi ve generations) or
Vulnerable
(greater than
a 10% chance of extinction in 100 years). A further category is
Near Threatened
- species close
to qualifying for a threat category or judged likely to qualify in the near future. Species that do not
meet any of the threat categories are assessed as of
Least Concern
.
An estimated 12% of bird species, 25% of mammals and 30% or more of amphibians are threat-
ened with extinction. The threat classifi cations help conservation managers prioritize their actions.
In addition, conservation plans are supported by the wealth of data collected in the IUCN assess-
ments (Rodrigues et al., 2006).
(genetic diversity within species) and larger than the species (the variety of ecosys-
tem types present - e.g. streams, lakes, grassy glades, mature forest patches).
Human impacts are responsible for driving a multitude of species to such low
numbers that much of the world's biodiversity is under threat. In this section I
consider just how big this problem is (Section 1.2.1) before discussing the conse-
quences of reduced biodiversity for the way that whole ecosystems function - and
for the free 'services' that natural ecosystems provide us (1.2.2). A variety of pro-
cesses are responsible for species extinctions (1.2.3) and the scale of each of these
will be considered in turn: habitat loss (1.2.4), invaders (1.2.5), overexploitation
(1.2.6), habitat degradation (1.2.7) and global climate change (1.2.8).
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