o_19ko2dt161ng2j4e1tgnoqv1s45a.pdf

APPLICATION OF GIS TO CONSERVATION ASSESSMENTS AT THE ROYAL BOTANIC GARDENS, KEW
fragmentation, decline or fluctuations). and D (very small population or very restricted
distribution).
IUCN recommends obtaining estimates by counting the number of occupied cells in a uniform
grid that covers the entire range of a taxon, then tallying the total area of all occupied cells
(Fig. 2b). This is a calculation that can be easily automated within a GIS.
One problem which arises with area of occupancy calculations is that the results of this method
are highly influenced by the placement of the grid. For example, if an organism is found at
four localities that are less distant from each other than the distance across a grid cell, the
calculated area of occupancy can vary by a factor of four (White 2004). This problem can be
reduced by searching for a grid position which minimizes the area of occupancy. White (2004)
describes a method for automating this procedure within ArcView and concludes that “the
arbitrary nature of using fixed grid methods should be avoided”. Using an automated method
has the advantage that it results in a consistent grid placement, ensuring consistent results if
the procedure is replicated (Willis et al. 2003).
However, another problem arises with calculating AOO because the “size of the area of
occupancy will be a function of the scale at which is measured, and should be at a scale
appropriate to relevant biological aspects of the taxon, the nature of the threats and the available
data” (IUCN 2001). For example if a species has been rarely sampled, then the distance between
observed locations might reflect a lack of observations rather than a lack of occupied habitat
and a coarser grid may therefore be more appropriate. It is therefore not appropriate to use one
set cell size for a wide range of taxa, but what is an appropriate grid size to use when automating
AOO calculations?
The guidelines for using the IUCN criteria (IUCN 2005) recommend a grid size of 2 km,
recognising that for intensely sampled species, a finer grid of 1 km may be more appropriate,
and for sparsely sampled species, a coarser grid. Grid sizes of more than 3.2 km are not
recommended as they preclude the listing of species as Critically Endangered (CR) because
the AOO threshold for CR is 10 km 2 . A method is described of standardising AOO estimates
by scaling the AOO estimate up or down to the reference scale (a 2 km grid size).
Willis et al. (2003) suggest that a suitable grid cell width/height is one tenth of the maximum
distance between any two points on the extent of occurrence polygon. This effectively scales
AOO to the EOO measurement and has given good results so far. Calculations of AOO using
this ‘sliding scale’ grid width/height are currently adopted by the GIS unit at Kew, although
the grid cell size can be manually set by the user within the application. AOO can also be
calculated when there are only two unique localities; the ‘sliding scale’ technique can be used
where grid cell width/height is one tenth of the distance between the two points.
Number of Sub-populations
IUCN defines sub-populations as “geographically or otherwise distinct groups in the population
between which there is little demographic or genetic exchange” (IUCN 2001). Subpopulations
are used in Criteria B (geographical range size, and fragmentation, decline or fluctuations)
and C (small population size and fragmentation, decline or fluctuations). Two techniques for
estimating the number of subpopulations of a species have been developed: the cell adjacency
250