IUCN Red List Guidelines - The IUCN Red List of Threatened Species

Red List Guidelines 34
distance”.) The value of α can be chosen with a required level of resolution in mind. The
smaller the value of α, the finer the resolution of the hull. Experience has shown that an α
value of 2 is a good starting point for some species (however, the value to use for specific
cases of assessing reductions in EOO should be based on a compromise between minimizing
the potential bias associated with incomplete sampling of outlying occurrences and
minimizing the departure from a convex hull). This process results in the deletion of lines
joining points that are relatively distant, and may subdivide the total range into more than
one polygon. The final step is to calculate the extent of occurrence by summing the areas of
all remaining triangles. When this exercise is repeated to estimate EOO from a second
temporal sample of points (and hence assess change in EOO), the same discontinuity
distance between points should be used as a threshold for deleting lines (rather than the same
value of α). This will reduce bias due to variation in sampling effort between the two surveys
and the bias due to changing average line length with more or fewer occurrences.
Extent of occurrence and area of occupancy are measures of the current distribution, i.e. they
should not include areas where the species no longer exists. On the other hand, these
measures should not only include the actually known sites, but also inferred sites. For
instance, sites can be inferred from presence of known appropriate habitat, but where the
species has not yet been searched for. In doing so, it will be important to judge to what extent
the taxon has been looked for. Incorporating inferred sites results in a range of plausible
values, which may give a range of plausible Red List Categories (see sections 3.1 on Data
availability, inference and projection, and 3.2 on Uncertainty).
4.10 Area of occupancy (criteria A, B and D)
“Area of occupancy is defined as the area within its 'extent of occurrence' (see 4.9 above),
which is occupied by a taxon, excluding cases of vagrancy. The measure reflects the fact
that a taxon will not usually occur throughout the area of its extent of occurrence, which may
contain unsuitable or unoccupied habitats. In some cases, (e.g., irreplaceable colonial
nesting sites, crucial feeding sites for migratory taxa) the area of occupancy is the smallest
area essential at any stage to the survival of existing populations of a taxon. The size of the
area of occupancy will be a function of the scale at which it is measured, and should be at a
scale appropriate to relevant biological aspects of the taxon, the nature of threats and the
available data (see below). To avoid inconsistencies and bias in assessments caused by
estimating area of occupancy at different scales, it may be necessary to standardize estimates
by applying a scale-correction factor. It is difficult to give strict guidance on how
standardization should be done because different types of taxa have different scale-area
relationships.” (IUCN 2001)
Area of occupancy (AOO) is a parameter that represents the area of suitable habitat currently
occupied by the taxon. As any area measure, AOO requires a particular scale. In this case,
the scale is determined by the thresholds in the criteria, i.e. valid use of the criteria requires
that AOO is estimated at scales that relate to the thresholds in the criteria. These scales (see
“Problems of scale” below) are intended to result in comparable threat status across taxa;
other scales may be more appropriate for other uses. For example, much finer scales are
appropriate for planning conservation action for plants, and larger scales may be appropriate