Biology Reference
In-Depth Information
A
B
C
y
=
-9.16
x
+
35.55
r
=
-0.43,
n
=
165
y
=
-7.17
x
+
30.51
r
= -0.91,
n
=
116
y
=
-8.51
x
+
30.99
r
=
-0.60,
n
=
93
6
4
2
0
3.4
3.6
3.8
3.6
Temperature
-1
(1000/K)
3.4
3.8
3.4
3.6
3.8
Figure 9.14. Effects of mean annual water temperature on species richness of fish (A),
numbers of marine prosobranch snails per latitudinal degree band along the
continental shelves of North and South America (B), and ectoparasite species per host
of marine teleost fish from Antarctica to the tropics (C). From Allen, Brown, and
Gillooly (
2002
). Reprinted by permission of the authors and the American
Association for the Advancement of Science.
In summary, all the latitudinal differences supposedly ''explaining''
gradients in species diversity by assuming that communities are saturated
and in equilibrium can also be the result of and are indeed better explained
by a gradient in ''effective evolutionary time'', i.e., in speed of evolution
and length of geological time over which communities have existed
under more or less the same conditions. However, of course this does
not imply that other factors, such as area, heterogeneity, and local history,
may not play some role as well.
General global patterns in diversity
Latitudinal gradients in diversity are the most conspicuous and the best
studied diversity gradients, but other gradients exist. The overall aim should
be to establish models that describe global patterns in diversity incorporating
latitude, longitude, differences in rainfall, etc. Some recent studies attempt
to do this (e.g., Ricklefs
2004
). At least some of these models can be
reconciled with the hypothesis of effective evolutionary time, even if they
do not specifically consider this. Thus, Zhang and Wu (
2002
) developed a
statistical thermodynamic model of the organizational order of vegetation
(OOV) that can be used to derive vegetation patterns on a large scale. OOV
is defined as ''a thermodynamic measure of the degree of structural and
functional self-organization of natural vegetation in a given environment,
