The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 151

It is also important to note that whereas one place may be a climatic refuge for
some species, it may not be for another. A consequence of this is that a single region
in an interglacial may receive different species from different refugia. For example, at
the end of the last glacial Britain received oaks ( Quercus spp.), shrews ( Sorex spp.),
hedgehogs ( Erinaceus spp.) and bears ( Ursus spp.) from Spain, but grasshoppers
( Chorthippus spp.), alder ( Alnus spp.), beech ( Fagus spp.) and newts ( Triturus spp.)
from the Balkans. There are also co-evolutionary implications if a population is to
spend some of its time with one community and other times with a quite different
community in terms of genetic make-up, if not species make-up.
Matters in Europe and North America differed. In Europe the Alp, Pyrenee and
Carpathian mountains ranges provided latitudinal barriers, as indeed did the Medi-
terranean. In North America the mountain ranges run more north-south and the area
of uninterrupted land is more extensive than in Europe. The species and the genetic
diversity of the south-eastern USA suggest that the area was a refuge for many species
during glacials (Hewitt, 2000).
However, Europe and North America were not totally isolated. As previously men-
tioned (Chapter 3), during glacials the sea level lowered and there was a Beringial
land bridge between present-day Russia and Alaska. Circumpolar species repeatedly
expanded and fragmented with the glacial-interglacial cycles. This repeated
fragmentation led to the formation of subspecies and sometimes new species. Animals
so affected included guillemots ( Uria spp.), dunlins ( Calidris spp.), reindeer/caribou
( Rangifer spp.) and bears ( Ursus spp.; Hewitt, 2000).
Another instance of such population change and fragmentation was provided late
in 2004 by Beth Shapiro from the University of Oxford, who was the lead author of a
paper arising out of an international collaboration of more than 15 institutions. The
team looked at bison ( Bison spp.) and sequenced mtDNA from some 450 individuals
from the present to more than 60 000 years ago. By ascertaining the geographical
distribution of genes, as well as which genes survived to the present and when
others died out, they were able to deduce how the bison population changed. They
showed that the bison population across Beringia declined catastrophically from
42 000 to 35 000 years ago. This was the time in the run-up to the LGM. Unlike with
mammoths (see section 4.3), this was before humans had established a significant
presence in the area. Given that this was a time of declining sea levels (hence an
expanding Beringian land area) the most likely explanation is that the collapse was
climatic. The research also revealed the fragmentation that the bison population
underwent during the LGM. They found that today's North American bison ( Bison
bison ) was separated from the rest of the bison population (including those that
ultimately became the European bison, Bison bonasus ). The North American bison
arose from a subpopulation separated from the rest by the Laurentide ice sheet. These
subpopulations survived in ecological refugia to the south of the major northern
hemisphere ice sheets.
As mentioned, mountains can also provide refugia vertically away from areas that
have become inhospitable. Some tropical mountains especially have been ecologically
important in providing stable refugia where there is vertical migration with climatic
change but, importantly, comparatively little horizontal migration. Species can move
up and down the mountain as the vertical ecological zonation changes with climate