Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 277

change. Finally, lowland landscapes in developed nations are highly fragmented and
this further impedes species range migration.
All of this means that it is virtually certain that in a warmer world we will see
new assemblages of species. This provides a challenge for those whose task it is to
manage ecosystems, be it for wildlife conservation, amenity or commercial purposes.
Ecosystem managers are going to have to deal with new ecological communities
involving a new mix, or combination, of species: some call it 'recombinant ecology'.
Given that species shift in response to climate at different rates, could it be that some
might even move in the wrong direction with global warming? This is counterintuitive,
but remember that not all places on Earth track global changes simultaneously or even
in the same way, the vagaries of short-term weather and climate cycles and, finally,
that temperature is just one dimension to the climate. In 2011, Shawn Crimmins
and colleagues published a paper that subsequently spawned considerable comment 2 .
They noted that whereas uphill shifts of species' distributions in response to historical
warming were well documented, and that this leads to the expectation of continued
uphill shifts under future warming, downhill shifts are considered anomalous and so
unrelated to climate change. However, by comparing the altitudinal distributions of
64 plant species between the 1930s and 2010 in California they showed that climate
changes have resulted in a significant downward shift in species' optimum elevations.
They emphasised that this downhill shift is counter to what would be expected given
20th-century warming but is readily explained by species' niche tracking of regional
changes in climatic water balance rather than temperature. So, they said, similar
downhill shifts can be expected to occur where future climate change scenarios
project increases in water availability that outpace evaporative demand.
Another example comes from a study of treelines. In 2009 Melanie Harsch, from
the Bio-Protection Centre in New Zealand, and colleagues conducted a meta-analysis
using a global data set of 166 treeline sites with temperature data taken from the
closest climate station to each site, to determine treeline change throughout the 20th
century. They found that 87 of the 166 site (52%) had changed in the direction as
expected, 77 (47%) remained stable, but two (less than 1%) had moved in the other
direction. The conclusion they drew was that not all species universally respond in the
same way to climate change. They also noted that at the two sites, where treelines had
moved the other way, there were signs of disturbance. This study provides a sound
lesson that other human-generated factors can confound expectations.
Now, assuming that the methodologies for this and the Crimmins study were sound
(some debated the Crimmins results), it is not entirely without precedent to have
system change go the other way. In section 6.1.3 we noted Parmesan and Yohe's
2003 result of 87% of species responding to climate change in the direction one
would expect with warming, so could mechanisms such as the one suggested by
Shawn Crimmins' team explain why the minority of 13% did not? Of course, at the
beginning of a period of climate warming, even if a minority of species do not respond
in the direction expected, as the climate change continues, even in the presence of

2

See the series of comments from various contributors and the study authors' response in Science , 2011,
334, 177.