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will be struck by the enormous differences in species diversity. Numerous
studies deal with such latitudinal gradients in species diversity and there
can be no doubt that such gradients exist for many taxa and habitats, but
there are exceptions (for recent reviews and important papers see Rohde
1992 , 1999 ; Willig 2001 ; Willig et al. 2003 ; Hillebrand 2004 ). Also, a
gradient in species richness does not always lead to a gradient in commu-
nity richness (Rohde and Heap 1998 ). Fossil evidence indicates that the
existence of the gradients is time-invariant, although both latitudinal and
longitudinal gradients in diversity have strongly increased through the
Cenozoic, i.e., during the last 65 million years (Crame 2001 ). A con-
sensus has not been reached about the causes, although a considerable
number of studies have shown that latitudinal gradients in temperature (or
other measures of solar energy input, such as annual potential evapotran-
spiration, solar radiation, actual evapotranspiration) are best correlated
with latitudinal diversity gradients of many taxa, and in many habitats
(e.g., Currie 1991 ; Currie and Paquin 1987 , further references in Rohde
1992 , 1999 ). For angiosperms, Francis and Currie ( 2003 ) recently
demonstrated the primary importance of mean annual temperature (or
annual potential evapotranspiration) and annual water deficit, and their
interaction for global diversity gradients. It is likely that - in view of the
generality of the gradients - some primary cause or causes must be
involved; this may be solar energy input (temperature) (e.g., Rohde
1978a , b , 1992 , 1999 ). However, several other factors, whose relative
importance differs between taxa and habitats, contribute as well.
Among the other important factors that have been suggested are area
(e.g., Rosenzweig 1995 ), heterogeneity of the habitats (e.g., Rahbeck
and Graves 2001 ), productivity (e.g., Rosenzweig 1995 ), and narrower
niches in the tropics (e.g., Ben-Eliahu and Safriel 1982 ). I will give some
examples that show that at least some of these other factors are indeed
important.
Area
Rosenzweig ( 1995 ), following Terborgh ( 1973 ), claimed that greater
species diversity is a consequence of the larger areas in the tropics. That
area is important in determining species richness is, for instance, shown by
the recent study of Valdovinos et al.( 2003 ). Analyzing the distribution of
629 species of molluscs along the Pacific coast of South America, they
demonstrated that richness increases sharply south of 42 o S where shelf
area also increases sharply (Figure 9.9 ) . This increase is not the result of
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