Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 272

and 2008. Fatter marmots having to hibernate for shorter periods means reduced
winter mortality and so the marmot population has increased. The population of
marmots at a site in the Upper East River Valley, in Colorado, USA, more than
doubled its 2000-3 mean by 2006-8.
One of the most detailed assessments so far of long-term changes in the seasonal
timing of biological events across marine, freshwater and terrestrial environments
in Britain was coordinated by Stephen Thackeray and Sarah Wanless, with second
author Tim Sparks of the Centre for Ecology and Hydrology. They looked some 25 000
long-term phenology data series for 726 species of plants and animals covering the
period 1976-2005. The range of species covered included plankton, plants, insects,
amphibians, fish, birds and mammals. They found that more than 80% showed earlier
seasonal events. On average, the seasonal timing of reproduction and population
growth became earlier by more than 11 days over the whole period, and more recently
this change has become faster. There were large differences between species in the rate
at which seasonal events have shifted. Changes were most rapid for many organisms
at the bottom of food chains, which means that plants and the animals that feed on
them were not effectively tracking them. Predators showed slower overall changes
in the seasonal timing of their life cycle events, even though the seasonal timing of
reproduction is often matched to the time of year when food supply increases, so that
offspring receive enough food to survive. Species relationships can depend on time
as much as space (see the next section on species' shift in ranges).
Ecologists studying biological responses to environmental change are all too aware
that responses are not always straightforward. Confounding a simple analysis of
species response to climate-driven changes in seasons is that some related species
may respond in markedly different ways. One instance comes from a study of the
phenology of reproductive migrations in 10 amphibian species at a wetland site in
South Carolina, USA, over more than 30 years (1978-2008) (Todd et al., 2010).
It showed that two autumn-breeding amphibians were breeding increasingly later
in recent years, coincident with an estimated 1.2 ◦ C increase in local overnight air
temperatures during the September-to-February prebreeding and breeding periods.
Nothing surprising there, you might think. But they also found that two winter-
breeding species in the same community were breeding increasingly earlier .
The other interesting aspect of the study by Todd et al. (2010) was that four of the
10 species studied shifted their reproductive timing an estimated 15.3-76.4 days in
the 30-year study period. This resulted in rates of phenological change ranging from
5.9 to 37.2 days per decade, so providing examples of some of the greatest rates of
phenological change in ecological events reported up to that time.
However, it is the two amphibian species responding to seasonal change differently
that is a central concern when it comes to long-term future ecological consequences
of climate change. If a species occupies a specific ecological niche at a specific time
in its life cycle then it needs two things. These are to have that niche track climate
change (including that manifested seasonally) in a similar way (so that it has food and
the other environmental resources upon which it relies), and also not to have to face
increased competition from responses by other species to the same climate change.
We will shortly look at an example of the former. However, with regards to the latter
Todd et al. note that owing to the opposing directions of the shifts in reproductive