o_19ko2dt161ng2j4e1tgnoqv1s45a.pdf

PETER K. L. NG & DARREN C. J. YEO (2007)
Although the criterion of population size is an important consideration in ascertaining a species’
threat level, this is almost impossible to determine for the freshwater crab species treated here.
The necessary quantifications simply have not been done. Many species are also very secretive
in habits, and several have not been rediscovered since they were first collected. In particular,
species, which are obligate cave dwellers, deep forest terrestrial species, tree-climbers or that
otherwise have very specialised niches, cannot be effectively sampled. As such, the only
objective data-sets of use are of the presence/absence type. Even so, when a species is
supposedly absent from an area, this observation must be considered with regards to its known
habits and behaviour. In many cases, the habitats and habits of a species can be predicted on
the basis of its carapace physiognomy, leg structure and proportions, eye form as well as
colour.
Nevertheless, in general, the presence/absence criterion at least allows the geographic range
to be predicted using either the Extent of Occurrence (i.e. area contained within the shortest
continuous imaginary boundary encompassing known sites of occurrence), or the Area of
Occupancy (i.e. the area within its Extent of Occurrence which is actually occupied by the
taxon). Given that most tropical habitats are very heterogeneous in structure, and aquatic
habitats (including swamp forest structure and underground water-tables) fluctuate substantially
depending on the time of the year; and that some species have small and highly localised
populations; the Area of Occupancy (i.e. the available aquatic habitat particular to the species)
criterion is too subjective to be very useful. The Extent of Occurrence is thus the preferred
criterion for estimates used here for geographic range.
As such, the CR, EN and VU outcomes resulted from evaluation against criteria B1(a) and
(b)(iii) in those categories. Continuing decline in Extent of Occurrence and/or quality of habitat
was inferred if the habitat was not a protected area, or if it was a protected area subject to
anthropogenic impacts such as pollution or encroachment.
A taxon is CR if its Extent of Occurrence is estimated to be less than 100 km 2 (B1) and its
habitat is severely fragmented or it is known to exist at only one location (B1(a)); and there is
a continuing decline in the area, extent and/or quality of its habitat (b)(iii).
It is EN if its Extent of Occurrence is estimated to be less than 5,000 km 2 (B1) and its habitat
is severely fragmented or it is known to exist at no more than five locations (B1(a)); and there
is a continuing decline in the area, extent and/or quality of its habitat (b)(iii).
It is VU if its Extent of Occurrence is estimated to be less than 20,000 km 2 (B1) and its habitat
is severely fragmented or it is known to exist at no more than 10 locations (B1(a)); and there
is a continuing decline in the area, extent and/or quality of its habitat (b)(iii). VU status was
also applied to taxa that have an Area of Occupancy estimated to be less than 20 km 2 ; and are
known from only a single population which is at least partly in a protected area, but is “prone
to the effects of human activities or stochastic events within a very short time period in an
uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a
very short time period” (D2).
NT status was awarded to taxa that were evaluated against the criteria but did not qualify for
CR, EN or VU at present, but likely to qualify for such a category in the near future.
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