Conservation planning in a changing world
197
Rosenzweig (2004) essentially supports MacArthur and Wilson, reporting that values typically fall between 0.25
and 0.45. By contrast, z - values typically observed for inter - provincial ISARs have a lower margin of z = 0.6 and
range upwards, with most lying around 0.8 – 1.0 (Rosenzweig, 2004 ) and some exceeding 1.0 (Rosenzweig, 1995 ,
2004 ). These observations have very considerable signifi cance for conservation science as humans continue to
alter the extent and connectivity of habitats both locally and globally. It is thus important to test the robustness
and explore the properties of Rosenzweig ’ s species – area pattern model.
A t est of the f orm of i nter - p rovincial ISAR s from s trongly i solated i slands
Archipelagic ISARs have been studied for real and habitat islands for many taxa in numerous studies over several
decades, and their properties are thus reasonably well known. By contrast, generalizations on the form of inter -
provincial ISARs derive from relatively few data sets. Biotic provinces(/regions) can be defi ned as self - contained
areas that, for the target taxon, are relatively independent from an evolutionary perspective (i.e. wherein most
species are endemic to these provinces).
In this analysis, we focus on the form of ISARs for single - island endemics (SIEs), exploring the idea that for
those species restricted to single islands, the islands in question can be regarded as provinces. Using this
approach, and by focusing on these systems, we can study the evolutionary contribution to ISARs and gain
signifi cant insights regarding the patterns that arise when speciation is a signifi cant or the major source of
diversity, as happens in the case of continental biotic provinces/regions.
Our fi rst aim is therefore to explore the consistency in form of SIE – area relationships across taxa and archipelagos of varying proportion of SIEs. Our second aim is to use SIE data to test the robustness of the generalization that inter - provincial ISARs are signifi cantly steeper than archipelagic ISARs, producing z - values typically
falling between 0.8 – 1.0, and 0.25 – 0.45, respectively. We do so by comparing z - values for SIE with those for ‘ all
native species ’ for the same taxa, using 13 different data sets from the Caribbean, Fiji, Hawaii, Canary Islands
and Great African Lakes, and using the power (log – log) model.
For further details of data set properties and methods of analysis, see the source paper. The key fi ndings of
the analyses were as follows:
Eleven of the SIE – area relationships were statistically signifi cant, explaining high proportions of the variance
in SIE numbers (R
2 0.57 – 0.95), the two exceptions being for the Canary Islands, whereby inclusion of the two
oldest islands greatly reduces the fi t of traditional species – area models (see Whittaker et al ., 2008 ). The z - values
of the statistically signifi cant SIE – area relationships ranged from 0.47 to 1.13, with a mean value of 0.80
(SD ± 0.24). All the island systems in which SIE represent > 50 per cent of species exhibited z - values for the
SARs of native species higher than those deemed typical of archipelagic SARs. Moreover, for the three cases
in which the percentage of SIE equals or exceeds 90 per cent, the mean z - value is unity (1.00 ± 0.21). The fi ndings thus approximate the schematic shown in Figure B8.1b .
Hence, the results of the present work provide signifi cant support for Rosenzweig ’ s proposition that z - values
from inter - provincial ISARs should be very high, approaching unity. This should hold not only for the scale of
recognized global biogeographical regions, but also for any system in which speciation is the major process. Of
course, the timescales in which species evolve and go extinct may differ between island systems and global
biogeographical provinces. Nevertheless, it seems that the two system types exhibit analogous patterns of
species accumulation with area.
Therefore, studies of evolutionary dynamics in relation to area, employing data for single island endemics,
would be well worth pursuing in other taxa and regions, as they could be used as model systems to test:
1 variation in critical island sizes below which within - island diversifi cation does not occur;
2 how these thresholds vary with taxa and island groups;
3 how consistent the form of inter - provincial ISARs are;
4 their capability for predicting future diversity;
5 and they may also be used to recognize and examine the infl uence of additional factors such as climate,
productivity, etc.
Such studies can offer great insights into basic questions of conservation biogeography, such as the potential
impact of habitat fragmentation and loss and homogenization on biological diversity.
197
Rosenzweig (2004) essentially supports MacArthur and Wilson, reporting that values typically fall between 0.25
and 0.45. By contrast, z - values typically observed for inter - provincial ISARs have a lower margin of z = 0.6 and
range upwards, with most lying around 0.8 – 1.0 (Rosenzweig, 2004 ) and some exceeding 1.0 (Rosenzweig, 1995 ,
2004 ). These observations have very considerable signifi cance for conservation science as humans continue to
alter the extent and connectivity of habitats both locally and globally. It is thus important to test the robustness
and explore the properties of Rosenzweig ’ s species – area pattern model.
A t est of the f orm of i nter - p rovincial ISAR s from s trongly i solated i slands
Archipelagic ISARs have been studied for real and habitat islands for many taxa in numerous studies over several
decades, and their properties are thus reasonably well known. By contrast, generalizations on the form of inter -
provincial ISARs derive from relatively few data sets. Biotic provinces(/regions) can be defi ned as self - contained
areas that, for the target taxon, are relatively independent from an evolutionary perspective (i.e. wherein most
species are endemic to these provinces).
In this analysis, we focus on the form of ISARs for single - island endemics (SIEs), exploring the idea that for
those species restricted to single islands, the islands in question can be regarded as provinces. Using this
approach, and by focusing on these systems, we can study the evolutionary contribution to ISARs and gain
signifi cant insights regarding the patterns that arise when speciation is a signifi cant or the major source of
diversity, as happens in the case of continental biotic provinces/regions.
Our fi rst aim is therefore to explore the consistency in form of SIE – area relationships across taxa and archipelagos of varying proportion of SIEs. Our second aim is to use SIE data to test the robustness of the generalization that inter - provincial ISARs are signifi cantly steeper than archipelagic ISARs, producing z - values typically
falling between 0.8 – 1.0, and 0.25 – 0.45, respectively. We do so by comparing z - values for SIE with those for ‘ all
native species ’ for the same taxa, using 13 different data sets from the Caribbean, Fiji, Hawaii, Canary Islands
and Great African Lakes, and using the power (log – log) model.
For further details of data set properties and methods of analysis, see the source paper. The key fi ndings of
the analyses were as follows:
Eleven of the SIE – area relationships were statistically signifi cant, explaining high proportions of the variance
in SIE numbers (R
2 0.57 – 0.95), the two exceptions being for the Canary Islands, whereby inclusion of the two
oldest islands greatly reduces the fi t of traditional species – area models (see Whittaker et al ., 2008 ). The z - values
of the statistically signifi cant SIE – area relationships ranged from 0.47 to 1.13, with a mean value of 0.80
(SD ± 0.24). All the island systems in which SIE represent > 50 per cent of species exhibited z - values for the
SARs of native species higher than those deemed typical of archipelagic SARs. Moreover, for the three cases
in which the percentage of SIE equals or exceeds 90 per cent, the mean z - value is unity (1.00 ± 0.21). The fi ndings thus approximate the schematic shown in Figure B8.1b .
Hence, the results of the present work provide signifi cant support for Rosenzweig ’ s proposition that z - values
from inter - provincial ISARs should be very high, approaching unity. This should hold not only for the scale of
recognized global biogeographical regions, but also for any system in which speciation is the major process. Of
course, the timescales in which species evolve and go extinct may differ between island systems and global
biogeographical provinces. Nevertheless, it seems that the two system types exhibit analogous patterns of
species accumulation with area.
Therefore, studies of evolutionary dynamics in relation to area, employing data for single island endemics,
would be well worth pursuing in other taxa and regions, as they could be used as model systems to test:
1 variation in critical island sizes below which within - island diversifi cation does not occur;
2 how these thresholds vary with taxa and island groups;
3 how consistent the form of inter - provincial ISARs are;
4 their capability for predicting future diversity;
5 and they may also be used to recognize and examine the infl uence of additional factors such as climate,
productivity, etc.
Such studies can offer great insights into basic questions of conservation biogeography, such as the potential
impact of habitat fragmentation and loss and homogenization on biological diversity.