BIODIVERSITY AND CLIMATE 183
Fig. 3. Species diversity plotted against absolute latitude for (a) total non-avian tetrapods, (b) mammals, (c)
reptiles and (d) amphibians.
+ mammals) shows a strong linear (or slightly
curvilinear) trend, with the data from all regions
falling along this latitudinal gradient (Fig. 3a).
There is no apparent hemispheric asymmetry, as
observed in other faunal groups such as marine
bivalves (Crame 2000), and the spatial distri-
bution shows few longitudinal heterogeneities
(Fig. 5). This global trend correlates strongly
with temperature and productivity metrics
(Table 2), consistent with the species-energy
hypothesis. However, regional correlations are
not significant for either Australia or southern
Africa (Table 3), which may reflect the smaller
sample sizes representing these regions and/or
the lack of strong environmental gradients
within their borders (especially within southern
Africa), However, this apparently simple global
relationship belies great variation in the trends
of each taxonomic and physiological com-
ponent, and it is these differences that may be
critical in understanding potential causes.
Reptile species diversity (Fig. 3c) follows a
strongly curvilinear trend, which increases
monotonically from high to low latitudes. There
is no apparent asymmetry between northern and
southern hemisphere groups, with a marked
change in gradient at about 45° latitude. This
pattern most closely approximates to the global
distribution of incident energy (temperature;
Fig. 4a-e), specifically cumulative energy
(cumulative T; Fig. 4d, Table 2). The baseline of
either 0° or 5°C for cumulative T is related to the
typical critical minimum body temperatures
observed for modern reptiles (for crocodilians
this is 4-5°C; Brisbin et al 1982). The geographic
distribution of reptile species diversity (Fig. 6)
shows a similar simplicity, and regionally, reptile
diversity correlates strongly with temperature
for all areas except southern Africa (Table 4).
Amphibian species diversity also shows a
monotonic increase in taxonomic diversity from
high to low latitudes (Fig. 3d). Nevertheless, the
