The size of the various populations of waterbirds has special
significance as 1% thresholds are used to designate protected
areas such as Ramsar sites,
Special Protection
Areas (SPAs) and Sites/Areas of Special
Scientific Interest (ASSIs/SSSIs). For example, criterion
6 of the Ramsar convention states that "a wetland should
be considered internationally important if it regularly
supports 1% of the individuals in a population of one species
or subspecies of waterbird". In the application of
this criterion, biogeographic populations as defined by
Wetlands International (2006) are used. The identification
of Special Protection Areas uses similar definitions. In
the UK, the application of Selection Stage 1.2 entails using
1% of relevant biogeographic population to identify Special
Protection Areas. 1% of the Great Britain or All-Ireland
populations are use to identify Special Protection Areas
if the species is listed on Annex 1 of the Council
Directive 79/409/EEC on the conservation of wild birds
or to designate a particular species as a feature of the
SPA if it forms part of the waterbird assemblage that is
used to designate a site under Selection Stage 1.3. The
Great Britain or All-Ireland populations are also used to
identify SSSIs. Any locality which normally contain 1% or
more of the British breeding or non-breeding population
of any native species qualify for selection as an SSSI.
Derivation of populations
The size of the Great Britain non-breeding populations
and UK and GB breeding populations is derived from Baker
et al. (2006) and includes birds in England, Scotland
and Wales, but excludes the Isle of Man and the Channel
Islands. For a small number of species, which occur predominantly
during the migration periods, passage populations are also
listed and are derived from this source. The size of the
All-Ireland population from Crowe et al. (2008),
except for some of the un-listed rarer species for which
the population is listed as being greater than the summed
peak of the latest WeBS
counts from Northern Ireland and I-WeBS
counts. Both Baker et al. and Crowe et al.
(2008) generally rely on WeBS and/or I-WeBS data to
generate population estimates. However, it is important
to note that the populations are not the same as the total
number recorded by WeBS. In calculating population sizes,
considerable work has been done to estimate the proportion
of the population that is actually counted by WeBS. The
European population is derived from BirdLife International
(2004) and the biogeographic population from Wetlands International
(2006). The global population is the sum of all biogeographic
populations listed in Wetlands International (2006).
Population Thresholds
Population thresholds are rounded to the nearest 100 for
species with populations >100,000, to the nearest 10
for species with populations 10,001–100,000, to the
nearest 5 for species with populations 1001–10,000
and to the nearest 1 for species with populations 1–1,000.
In terms of identifying sites of national or international
importance during the non-breeding season, 50 is used as
a minimum threshold and since any site containing more than
20,000 waterbirds is considered internationally important,
this is used as a maximum threshold even if the relevant
population is in excess of 2 million.
References
Baker, H., Stroud, D.A., Aebischer, N.J., Cranswick, P.A.,
Gregory, R.D., McSorley, C.A., Noble, D.G. & Rehfisch,
M.M. (2006) Population estimates of birds in Great Britain
and the United Kingdom. British Birds, 99, 25-44.
BirdLife International (2004) Birds in Europe: population
estimates, trends and conservation status. BirdLIfe
International, Cambridge.
Crowe, O., Austin, G.E., Colhoun, K., Cranswick, P.A.,
Kershaw, M. & Musgrove, A.J. (2008) Estimates and trends
of waterbird numbers wintering in Ireland, 1994/95 - 2003/04.
Bird Study, 55, 66-77.
Wetlands International (2006) Waterbird Population
Estimates. Delany, S. and Scott, D. (eds). Fourth Edition.
Wetlands International, Wageningen, The Netherlands.
