Biology Reference
In-Depth Information
The repeated isolation, like the rise of arches and changes in river drain-
age patterns, followed by reunion of the populations, would have been con-
ducive to the generation of new genotypes and phenotypes. Recall there
were at least eighteen to twenty major climatic cycles during the Pleistocene
(Milankovitch variations), with numerous subcyles (the Younger Dryas, the
Medieval Warm Period, the Little Ice Age, Heinrich and D-O events). Also,
during each of the major cold intervals, for example, at the LGM, sea levels
were lowered by an estimated 120 m and water tables by up to 40 m, con-
tributing to drying in the lowlands. If dry or drying intervals prevailed dur-
ing about 90 percent of the Quaternary, the current habitats supporting cer-
rado, caatingas, and grasslands can be viewed as “reversed refugia” (Gentry
1979, 349) because for most of the interval they (or versions of them) were
the widespread ecosystem in and around the basin. However it is viewed,
the climatic history of Amazonia is proving to be a dynamic one.
After Haffer's proposal, a number of organisms were examined for consis-
tency with the theory (Prance 1982), and the consensus was that in general
it worked. Previously, conventional wisdom held that climatically the low-
land tropics were stable and unchanging, and the CLIMAP and COHMAP
results seemed to support this idea. Many of the major climatic cycles and
subcycles, like Heinrich events and the signifi cant temperature fl uctuations
at and after the LGM, were still undocumented for equatorial regions. But
the idea of a stable and unchanging environment was increasingly diffi cult
to reconcile with evidence for possibly the greatest concentration of species
in the history of the planet, along with the fact that using any reasonable
lapse rate, as van der Hammen observed, temperature changes in the high
Northern Andes must have extended to some degree into the lowlands. A
number of objections have been raised to the Haffer's original theory (Col-
invaux 2007; Colinvaux et al. 1999; Morawetz and Raedig 2007), including
the improbable existence and location of some of the refugia initially pro-
posed, and the simplifi ed causal mechanisms based on the extent and pace
of climatic changes recognized at the time. However, there has also been
some overstatement by opponents about the degree of dryness required
(e.g., “extreme aridity'), when all that is needed or suggested for parceling
is an intervening vegetation drier than lowland rain forest.
Moreover, as discussed in chapter 8 for South America, compelling evi-
dence began to emerge supporting signifi cant cooling in the lowlands by
5°C-6°C (Aeschlbach-Hertig et al. 2000; Webb et al. 1997; Stute et al. 1995)
and for drying (Lichte and Behling 1999; Mayle and Beerling 2004; Muhs
and Zárate 2001; Wüster et al. 2005a, b). Anhuf and colleagues (2006)
suggest that humid forests in the Amazon Basin may have been reduced to
