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In the following, I discuss such an alternative explanation for latitudinal
diversity gradients.
Effective evolutionary time in nonequilibrium ecosystems
Considering the objections to the explanations discussed above and some
other explanations, Rohde ( 1978a , b , 1992 , 1999 ) concluded that none of
the explanations for latitudinal gradients in species diversity that assume
saturation of habitats with species and equilibrium conditions, and higher
''ceilings'' of diversity in the tropics due to some limiting factor, is accep-
table. Hypotheses based solely on evolutionary or ecological time do not
give a satisfactory explanation either, because tropical habitats are not
generally older than cold-temperate ones. Latitudinal diversity gradients
have been in existence for at least 270 million years (Stehli et al. 1969 )or
may be time-invariant altogether (Crame 2001 ); there were marked tem-
perature changes in tropical seas in the geologic past, possibly greater than
in cold-temperate waters; and mass extinctions have occurred in tropical
and cold-water environments (for discussion and references see Rohde
1992 ). Rohde suggested that available data are best explained by the
assumptions that species saturation has not been reached, that higher energy
input accelerates evolutionary speed by shortening generation times,
increasing mutation rates, and speeding up selection, and that evolutionary
speed and the time under which communities have existed under relatively
constant conditions (both determining the ''effective evolutionary time'')
are responsible for the gradients. In other words, more species have
accumulated in the tropics because evolution there is faster. Essential
evidence for this hypothesis is as follows:
(1) diversity has increased over evolutionary time;
(2) extant niche space is not filled, and an increase in diversity is likely
(i.e., nonequilibrium conditions prevail);
(3) higher temperature increases speciation rates by (a) shortening gen-
eration times, (b) increasing mutation rates, and (c) speeding up selec-
tion due to generally accelerated physiological processes (Q 10 ).
According to Jablonski ( 1999 , see also Benton 1995 , 1998 ; Courtillot
and Gaudemer 1996 ; and Jackson and Johnson 2001 ), there was a sharp
rise in diversity in the Cambrian, followed by a Paleozoic plateau inter-
rupted by several mass extinctions, and a sharp rise since the Triassic, also
interrupted by several extinction events. Thus, the fossil findings lend
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