CENOZOIC BIODIVERSITY 157
Fig. 2. Pliocene closure of the Central American
Seaway. Maps redrawn and simplified from Stanley
(1995, fig. 4). Arrows indicate principal current
directions.
there was the tectonic isolation of Antarctica,
then the progressive disruption of the circum-
equatorial current system, and finally the
thermal isolation of the Arctic Ocean by the
formation of NADW. Of course, there may well
have been other contributory factors to global
cooling too, such as the uplift of the Himalayas
and changes in atmospheric CO 2 levels (Raymo
& Ruddiman 1992), but there is an undeniable
link throughout the Cenozoic between changes
in palaeogeography and global cooling.
The net effects of global cooling on
biodiversification
Although an intuitive reaction might be to link
global cooling with a progressive loss of bio-
diversity, this is not necessarily the case. The
reason for this lies in the phenomenon of differ-
entiation diversity, or how many communities
can be packed within a province/region (beta
diversity), and how many provinces can be
packed within the biosphere (delta diversity)
(sensu Magurran 1988). It is thought that,
throughout the Cenozoic Era, a series of thermal
provinces was gradually imposed on the now
widely separated continents. The present-day
north-south barriers formed by a unique con-
figuration of continents and ocean basins has led
to the development of an unprecedented
number of latitude-parallel provincial chains
in both the marine and terrestrial realms
(Valentine et al. 1978; Jablonski et al. 1985;
Niklas et al. 1985). Independent evidence
gleaned from the Palaeozoic fossil record has
recently attested to the strong control of differ-
entiation processes (i.e. beta diversification) on
the formation of global diversity patterns
(Adrain et al. 2000).
However, we also have to consider whether an
increase in provinciality alone would have been
sufficient to account for an order of magnitude
increase in global species diversity. Some have
argued that it would not, and indicated that we
must also take into account significant Cenozoic
increases in alpha (within-habitat) diversity
(Bambach 1977,1990; Niklas et al. 1985; Vermeij
1987). In his seminal study of Palaeozoic diver-
sification, Sepkoski (1988) estimated that alpha
diversity might account for some 50-70% of the
taxa produced in the Ordovician radiations.
There is some evidence to suggest that alpha and
beta diversity co-vary widely in nature; at the
present day increased regional diversity is
accompanied by both increased local diversity
(i.e. alpha) and increased provincialism (i.e.
beta) (Ricklefs & Schluter 1993; Gaston 2000).
