68
Basic biogeography: estimating biodiversity and mapping nature
birds, one fi nds that a lot more scatter is evident than within similar groups of species, but still the
condition of ‘ abundant and localized ’ is extremely rare, at least within continental avifaunas (Gaston,
1994 ; Brown, 1995 ).
Perhaps the more important question from the perspective of conservation biogeography is what
constitutes a small range and how might the range size/abundance pattern vary in relation to biogeographical context (e.g. continental versus island archipelago contexts) or for taxa of differing
body sizes? As previously noted by Whittaker et al . (2005) , within the North American breeding bird
data set analysed by Brown (1995) , range size varied from c. 10,000 km
2 to over 10 million km
2 and
fewer than 10 species had ranges < 100,000 km
2
. In contrast, the land area of the Canary Islands is
only approximately 7,500 km
2 and that of the Hawaiian archipelago is 16,640 km
2 . Assuming that the
typical pattern of many species with small ranges exists within these essentially self - contained
biogeographical areas or provinces, then many native species have tiny ranges, and even the most
widespread and/or abundant species on these islands will have smaller ranges than the ‘ localized,
low density ’ rarities of the North American data set.
The pragmatic solution used by conservationists has been to defi ne range - restriction – sometimes
called ‘ local endemic ’ species – using an arbitrary threshold of < 50,000 km
2 (from Terborgh & Winter,
1983 ). However, if this were to be universally applied, the entire endemic biota of island archipelagos
such as Hawaii, the Galapagos and the Canaries would be classed as of conservation concern –
which we hope would be unduly pessimistic (e.g. Mart í n, 2009 ; for fuller discussion of island
conservation issues, see Whittaker & Fern á ndez - Palacios, 2007 ).
The 50,000 km
2 threshold has been widely adopted in conservation prioritization analyses (e.g.
Long et al ., 1996 ; Rodrigues et al ., 2004b ), and for particular taxa and contexts it is a reasonable
approximation (e.g. for freshwater fi sh species in North America: Rosenfi eld, 2002 ). Range restriction
is also an integral part of the criteria used by the IUCN to identify and classify species in danger of
global extinction – known as the IUCN Red List (version 2.3; www.redlist.org ). The Red List has nine
categories, ranging from ‘ least concern ’ to ‘ extinct ’ (Figure B4.1b ). From a conservation perspective
the priority species for actions and interventions are those classifi ed as ‘ vulnerable ’ , ‘ endangered ’
or ‘ critically endangered ’ . Interestingly, attribution to these latter categories can default to range size
if other data are lacking, in which case, the thresholds used are 100 km
2 (extent of occurrence) for
critically endangered, 5,000 km
2 for endangered and 20,000 km
2 for vulnerable. This three - point scale
recognizes that, on average, threat increases incrementally as range size reduces.
For conservation prioritization schemes that apply a single range size (whether 50,000 km
2 or
another), it would appear valuable to undertake analyses for different taxa and biogeographical
contexts of the sensitivity of prioritization analyses to the adoption of that threshold (Williams et al .,
2000 ). Moreover, to refi ne the meaning of ‘ range - restricted ’ for conservation prioritization, we need
more empirical data both on range sizes and on how to translate range size distributions into
Figure B4.1b Structure of the IUCN Red List. Re - drawn from www.redlist.org/ version 2.3.
Basic biogeography: estimating biodiversity and mapping nature
birds, one fi nds that a lot more scatter is evident than within similar groups of species, but still the
condition of ‘ abundant and localized ’ is extremely rare, at least within continental avifaunas (Gaston,
1994 ; Brown, 1995 ).
Perhaps the more important question from the perspective of conservation biogeography is what
constitutes a small range and how might the range size/abundance pattern vary in relation to biogeographical context (e.g. continental versus island archipelago contexts) or for taxa of differing
body sizes? As previously noted by Whittaker et al . (2005) , within the North American breeding bird
data set analysed by Brown (1995) , range size varied from c. 10,000 km
2 to over 10 million km
2 and
fewer than 10 species had ranges < 100,000 km
2
. In contrast, the land area of the Canary Islands is
only approximately 7,500 km
2 and that of the Hawaiian archipelago is 16,640 km
2 . Assuming that the
typical pattern of many species with small ranges exists within these essentially self - contained
biogeographical areas or provinces, then many native species have tiny ranges, and even the most
widespread and/or abundant species on these islands will have smaller ranges than the ‘ localized,
low density ’ rarities of the North American data set.
The pragmatic solution used by conservationists has been to defi ne range - restriction – sometimes
called ‘ local endemic ’ species – using an arbitrary threshold of < 50,000 km
2 (from Terborgh & Winter,
1983 ). However, if this were to be universally applied, the entire endemic biota of island archipelagos
such as Hawaii, the Galapagos and the Canaries would be classed as of conservation concern –
which we hope would be unduly pessimistic (e.g. Mart í n, 2009 ; for fuller discussion of island
conservation issues, see Whittaker & Fern á ndez - Palacios, 2007 ).
The 50,000 km
2 threshold has been widely adopted in conservation prioritization analyses (e.g.
Long et al ., 1996 ; Rodrigues et al ., 2004b ), and for particular taxa and contexts it is a reasonable
approximation (e.g. for freshwater fi sh species in North America: Rosenfi eld, 2002 ). Range restriction
is also an integral part of the criteria used by the IUCN to identify and classify species in danger of
global extinction – known as the IUCN Red List (version 2.3; www.redlist.org ). The Red List has nine
categories, ranging from ‘ least concern ’ to ‘ extinct ’ (Figure B4.1b ). From a conservation perspective
the priority species for actions and interventions are those classifi ed as ‘ vulnerable ’ , ‘ endangered ’
or ‘ critically endangered ’ . Interestingly, attribution to these latter categories can default to range size
if other data are lacking, in which case, the thresholds used are 100 km
2 (extent of occurrence) for
critically endangered, 5,000 km
2 for endangered and 20,000 km
2 for vulnerable. This three - point scale
recognizes that, on average, threat increases incrementally as range size reduces.
For conservation prioritization schemes that apply a single range size (whether 50,000 km
2 or
another), it would appear valuable to undertake analyses for different taxa and biogeographical
contexts of the sensitivity of prioritization analyses to the adoption of that threshold (Williams et al .,
2000 ). Moreover, to refi ne the meaning of ‘ range - restricted ’ for conservation prioritization, we need
more empirical data both on range sizes and on how to translate range size distributions into
Figure B4.1b Structure of the IUCN Red List. Re - drawn from www.redlist.org/ version 2.3.