Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 278

ameliorating factors, it will be the dominating factor, and species will respond in the
direction it determines.
The problem of synchrony of biological community species migration as a response
to climatic change is loosely analogous to the synchrony problem for migratory bird
pairs overwintering in different locations (see the end of the previous subsection). The
big difference, though, is that whereas successful breeding pairs are fundamentally
necessary to a species' continued existence - and even a synchronous response
across species is necessary for the continued existence of a biological community -
the continued existence of a biological community is not always necessary for the
continuity of a species. Species may continue, but in new biological communities, as
represented in Figure 6.2b. Here the community of species B and C is disrupted by
climate change and a new community of species A and B is created.
Let's consider a theoretical case of regional warming of an arbitrary (but fixed)
amount over a certain time period. As noted, if species shift at different rates other
species in the ecological community on which they rely may not track the climate
change in the same way. In 2008 Vincent Devictor, Romain Julliard, Denis Couvet
and Frederic Jiguet expressed concern that the French Breeding Bird Survey results
were suggesting that while French March-August climate change over two decades
corresponded to a 273 km northward shift in temperature, bird communities had
only shifted 91 km northwards. They suggested that bird composition was most
likely changing at its maximal possible rate, which is insufficient to catch up with
the accumulated delay. This discrepancy, the researchers feel, may have profound
consequences on the ability of species to cope with climate change in the long run.
However, a contrary view - given that, of all classes of species, birds are among
the most mobile - is that the French bird populations surveyed are simply tracking
the shifting of their food species in response to climate change and not the climate
change itself. Nonetheless, the concern is potentially germane: species track climate at
different rates and so established relationships can get disrupted. The important thing,
if traditional ecological communities are valued, is that the ecological communities
as a whole remain intact. Here, providing their coherence is maintained, it may not
matter that they do not track climate change exactly, or if their tracking lags behind
that of climate change. Alternatively, it may be that it is species survival that is of
greater value, in which case it matters less that ecological communities may change
with warming.
Another factor to take into account when considering plant species' shift in
response to climate is climate buffering: some species at the warmer end of their
spatial range lag in their tracking of a warming climate even if at the cooler end of
their spatial range there is better tracking. At the warmer end of their range (the lower
altitudes or southern end of northern hemisphere ranges), while some aspects of a
species' life cycle may be impaired due to warming stress, the individual plants still
continue to grow. So it can be that, for example, seed production or flowering may
be reduced, but the plant still survives. This potential probably arose due to the evol-
utionary pressure of those plants being able to outride warmer years arising from the
(non-climate change) vagaries of annual weather change (these being driven by cli-
matic cycles). Then, when conditions returned to those more optimal for the species at
that location, they were then in place and able to reproduce. What this means in terms