The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 175

Having looked at the climate forcing factors and change of the Holocene with just
broad reference to biosystems in this section, we now look in detail at the biological
and ecological response to these changes in climate.
4.6.4 Biologicalresponsetothelastglacial,LGMandHolocenetransition
The biological response to glacial and Holocene interglacial climate change is, in the
main, manifested in the form of species migration (see also section 4.5.4). However,
fixing one's self in the landscape, an observer would see any one location undergo
considerable ecological change as species come and go. For example, the Beringian
land bridge, the aforementioned area that was ecologically important before the
Quaternary, re-appeared during glacials with their lower sea levels. Due to marine
geological cores containing pollen, plant macrofossils and insect remains, it is pos-
sible to determine how the Beringian environment changed over a glacial-interglacial
cycle. Close to the LGM Beringia's ecology was one of predominately heathland with
some shrubs and the occasional birch tree ( Betula spp.). During the Younger Dryas
birch increased, as did grasses, and there was a patchwork of ponds filled with weeds.
By the time of the early Holocene, Beringia's open ground - which was being reduced
by sea-level rise - was that of mesic tundra, half way between polar tundra and a
wetter temperate biome, where summer temperatures were warmer than present-day
Alaskan northern slopes (Elias et al., 1996).
The transformation between glacial and interglacial conditions was arguably the
greatest in North America as, at the LGM, its glacial ice sheets were at least 50%
greater than those in Europe and Asia combined. As such, the ice sheet was as large
as that of Antarctica, covering all of Canada and the north of the USA (see Figures
4.3 and 4.4). The Canadian ecologist Chris Pielou (1991) has reviewed many of the
biological changes and some of these are summarised here along with other research
observations.
A key problem facing species riding out climatic change is where can they live? The
concept of 'refugia' has been introduced in previous sections and so, just as there were
refugia for present-day temperate species during the last glacial, so there must have
been refugia for peri-Arctic species when the ice sheets were at their greatest extent.
It is likely that during the LGM exposed mountains peaks rising above the glacial ice
sheet (nunataks) would have provided a possible refuge for Arctic stony species such
as the woolly louse-wort ( Pedicularis lanata ) and Ross's sandwort ( Minuartia rossii ),
although Beringia would have undoubtedly been an important refuge for such species
(and this would explain their presence in both present-day Siberia as well as Canada
and Greenland). Nunataks (that is, mountain tops appearing through an ice sheet)
nearer the coast would have seen a comparatively milder climate (although still very
cold) and so possibly be refugia for species such as the green spleenwort ( Asplenium
viride ) and the mountain holly fern ( Polystichum lonchitis ), the present-day ranges
of which cover much of the Rockies at or near the tree line, as well as parts of
Greenland.
Long-distance horizontal, as opposed to short-distance vertical, migration was an
important mechanism for many species to survive the glacial-interglacial climatic