Extent of Occurrence calculated by JNCC
The Extent of Occurrence is one way of measuring the range of a
species. It is defined by IUCN as "the area contained
within the shortest continuous imaginary boundary which can be
drawn to encompass all the known, inferred or projected sites of
present occurrence of a species, excluding cases of
for Using the IUCN Red List Categories and Criteria" (IUCN, May
2003) note that Extent of Occurrence can be measured by drawing a
polygon around occupied sites and calculating its area. The
simplest approach to this is to draw a figure known as a "convex
hull" (the smallest polygon in which no internal angle exceeds
Here are examples of convex hulls applied to two species of
Orthoptera from the BRC Grasshopper and Crickets recording scheme:
the Common field grasshopper (Chorthippus brunneus)
and Roesel's bush cricket (Metrioptera roeselii).
It is immediately apparent that there are problems with using
a convex hull to represent Extent of Occurrence. In the first
example of a common and widely distributed species, the convex hull
includes huge areas of sea (an area of "obviously unsuitable
habitat" for this terrestrial animal!) and will therefore greatly
exaggerate the range area.
In the second example, the species has a much more restricted
distribution, but the area of the convex hull in this case is
dominated by a few outlying colonies and again greatly exaggerates
the area of the range. If we were looking at change over time, then
there would potentially be a huge impact if, for example, one of
the outlying colonies like the one on the Welsh coast or the one in
Morecambe Bay was not surveyed in one of the time periods.
The IUCN's guidelines, suggest that the way to resolve these
issues is to use a more complex way of fitting a polygon known as
an "alpha shape". An alpha shape fits around the points more
closely and can result in more than one polygon if there
are gaps in the range.
Here are examples of alpha shapes fitted to the same two species
As these examples show, fitting an alpha shape gives a much
more satisfying description of the range as most people looking at
the map would perceive it.
The parameter α determines how closely the
shape fits. If α is very large then the alpha shape tends towards
the same area as a convex hull. On the other hand, if
α is very small then each point will become a separate
polygon and the area of the alpha shape will tend towards zero. So,
the value used for α is critical when using this method to
calculate the Extent of Occurrence.
A series of case studies suggested that a value of α around
1500-1800 is suitable for British distribution Atlases at 10km
square resolution using this particular formulation for fitting an
alpha shape (Hull
software by Ken Clarkson of the Computing Science
Research Centre, Bell Laboratories, USA).