The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 148

change in association with a transitional refuge had a lasting effect on biodiversity on
a near-continental scale. In this case, which we look at shortly, the example is aquatic
(Joyce et al., 2005).
Of course, in one sense all refugia are transitional in that at a certain latitude an
area of land mass may at one time favour a species, enabling it to survive, while at
another be inhospitable to it. In both hospitable and inhospitable times the land area
(be it a mountains or whatever) physically remains, but during inhospitable times
the refuge disappears. One example is the palaeo-Lake Makgadikgadi that existed in
the lowest part of the Kalahari Plain (present-day northern Botswana) during the last
glacial, which was for part of the mid-southern African watershed a time of rain, or
pluvial. During this time the lake was large and comparable in size to present-day
Lake Victoria further north in eastern Africa. Today little remains other than a mix
of salt pan and swamp: primarily the Makarikari Salt Pan and Okavango Swamp.
The importance of palaeo-Lake Makgadikgadi was discovered because of the rich
biodiversity of cichlid fish species in sub-watersheds 800 km or so apart that would
have once fed into this lake. Conversely, the rivers associated with Lake Victoria, and
the rivers flowing into other large African lakes, have only two widespread riverine
lineages of haplochromine cichlids. This contrasts with the lakes themselves, which
have high cichlid biodiversity. The reason for this, it is supposed, is that these large
lakes provide many opportunities for speciation whereas the rivers do not. So why
are the rivers that flow into the Kalahari Plain species-rich?
The analyses carried out suggest that palaeo-Lake Makgadikgadi, when it existed,
did provide many opportunities for speciation. This species assemblage was physically
forced to move into the various surrounding river systems as the lake gradually dried
out over the period from when the Holocene began to about 2000 years ago. Had
the lake dried out extremely quickly instead, then this movement of populations may
not have happened. The same would have been true if the lake were very small. The
palaeo-lake's large size also enabled there to be a variety of ecological niches that
facilitated speciation. Finally, it is likely that this speciation happened over more than
one glacial-interglacial cycle.
Although important, not all species rely on refugia and with some species the
entire population migrates as the climate changes. So the biological response to
climate change is complex and we will return to this later when considering the
response to possible future climatic change.
Things are rarely simple in biology. Unlike in theoretical physics or chemistry,
where much work includes laboratory experiments where researchers just look at,
and just work with, aspects they wish to study, biological systems are complex. The
above studies look at how individual species respond to climate change, but natural
systems are more complex. For example, not only may a species respond to climate
change but its range migration may well bring it into contact with other species.
These other species may otherwise have been able to ride out the climate change had
it not been for the incoming migratory species with which they then have to compete.
Such an example was provided by a small team of Scandinavian, British and German
ecologists led by Heikki Seppa, Teija Alenius and Richard Bradshaw in 2009.
At the end of the last glacial, Norway spruce ( Picea abies ), one of the dominant
tree species in Eurasia, spread slowly westward in northern Europe, invading eastern
Finland about 6500 years ago, eastern central Sweden about 2700 years ago and