Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 275

We have discussed northern hemisphere biological response to changes in the
seasons, but how is the northern hemisphere climate changing? Stine et al. (2009)
used meteorological records to determine that in terms of temperature the northern
hemisphere spring advanced by 1.7 days in the period 1954 and 2007. Of course, this
is a hemispheric average and it varies considerably from place to place due to latitude
and circulation patterns. It is minimal near the edge of the tropics and considerably
greater at higher latitudes. This explains why the phenological response of some
species is greater.
6.1.5 Biologicalcommunitiesandspeciesshift
Of course, some species do not change part of their life cycle but will shift their
geographical position, or range, in response to climate change. In the previous chapter
key characteristics and examples of species' response to glacial-interglacial climatic
change were summarised. However, because climate is but one factor (indeed, climate
itself is multi-factorial) of many that determine a species' spatial distribution, species
rarely move uniformly with each other in response to climate change. For example, a
calcareous soil species prefers soils with a high pH and this preference might dominate
that of minor climate change. Figure 6.2 illustrates uniform and non-uniform vertical
and horizontal migration. (See also Chapter 4 and the discussion on biological and
environmental impacts of the last glacial, as well as Figure 4.10.)
Added to the above, and again as noted in the previous chapter, different species
migrate at different rates. Consequently, not only is a period of climate change ecolo-
gically dynamic but it takes time for ecological communities to stabilise after a period
of climate change. Furthermore, again as touched upon in Chapter 4, species at the
leading edge of shifts/migrations tend to migrate faster than those already estab-
lished. As ecologists Walther and colleagues (2002) note, changes in distributions are
often asymmetrical, with species invading faster from lower elevations or latitudes
than resident species receding upslope or poleward. The result is often an increase
in species richness of communities at the leading edge of migrations. However, this
biodiversity 'benefit' is transitory. Furthermore, whereas such biodiversity may be
seen in natural systems (and so found in the palaeontological record), it is less likely
in many of today's systems, which are either managed or bounded by land that is man-
aged by humans. Such managed lands often make for an effective barrier to species
migration. This in turn poses problems for the future of species, let alone ecological
communities; for even, as noted, if old ecological communities are disrupted, species
migration impedes new communities being formed.
An example of all these concerns was described by a largely French team of
biologists led by Romain Bertrand and Jonathan Lenoir in 2011, who compared how
highland and lowland plant species responded to climate change in France over a 44-
year period (1965-2008). They employed a twist on the theory behind species used
as climate proxies. Using living species they established the optimal temperature
for the species to live but the researchers found that, as a result of climate change,
this differed from the temperature of their actual environment. Consequently they
could determine how well the French species were migrating to track climate change.
They found that forest plant communities had responded by 0.54 ◦ C yet the climate
had warmed by 1.07 ◦ C in highland areas (more than 500 m). In lowland regions the