Sunshine, skippers and south-facing slopes
2 August 2013 by Jonathan Bennie
Changing weather patterns will affect plants and animals. Jonathan Bennie explains what this could mean for conservation, and how monitoring the impact of tiny microclimates around paths or patches of vegetation could help endangered species cope.
In the next century, our planet's climate system is expected to experience continued man-made warming at a rate not seen in thousands of years. As with past changes in climate, we are likely to see major shifts in the global distribution of plant and animals.
For many species, climate change will cause population declines and ultimately extinction. For others, it may provide opportunities to colonise new habitats and expand into new regions. In many cases the difference between survival and extinction for a species will depend on its ability to disperse to regions of newly-suitable climate. To keep pace with climate change, it must found new populations as those within its old range dwindle and die out.
This situation is a serious challenge to biodiversity conservation in the 21st century; how do we manage habitats and landscapes to encourage species to make the most of emerging opportunities, while still trying to minimise the losses as their historical ranges become climatically less suitable?
The signal of climate change can be swamped by the variation in weather...
The solution is complicated because beneath the global maps and seasonal and regional averages, climate is inherently patchy and variable. The evidence for a global increase in temperature is unequivocal, yet any particular place on Earth may still experience an unusually cold season one year and a heatwave the next.
In the UK, where talking about the weather is a national obsession, statistics showing decades of average warming are easily forgotten after front-page newspaper coverage of a disappointingly cool, wet summer bank holiday or a heavy spring snowfall. For wildlife too, the signal of climate change may seem to be swamped by the variation in weather.
Understanding and managing how species respond to climate change may mean paying more attention to the variability and extremes of weather and microclimates which drive ecological processes. Most species are small; they sample their environment at a fine resolution. They also undergo seasonal cycles of reproduction, development and activity throughout the year, and can be particularly sensitive to climate at specific times of the year.
Certain weather conditions, times of day and seasons of the year can create many variations of microclimate. South-facing slopes may be considerably warmer than north-facing ones; short vegetation is warmer than tall vegetation during the day and cooler at night; regions near the sea, lakes and wetlands may be protected from extremes of temperature, and small depressions and valley bottoms can trap cold air and become several degrees cooler than their surroundings. Even the difference between sunlit and shaded leaves or the southern and northern sides of a tree trunk can provide a contrast of several degrees. Many species use these microclimates at specific stages in their life-cycles to provide the conditions they need to survive.
Climate change - Winners and losers
As an example of how species expand their range in a warming, but highly variable, climate we studied the spread of a butterfly, the silver-spotted skipper, over 27 years at the north-western margin of its range in south-east England. In the early 1980s, habitat fragmentation and changing agricultural practices had restricted this butterfly to a handful of British sites; however, in the three decades since, it has expanded its range considerably, aided by both conservation efforts and warmer summers.
The butterfly spends the winter as an egg, matures as a caterpillar through spring and early summer and flies as an adult butterfly for just a few weeks in August. During this brief adult life, the daytime temperature is critical to allow successful reproduction; butterflies are less active and females lay few eggs when the temperature in their grassland habitat falls below 26°C.
Caerthillian Cove on the Lizard peninsula in Cornwall. The infra-red image shows approximate temperature, with south-facing slopes and bare ground near paths considerably warmer than shaded north-facing slopes.
It's a risky strategy in the unpredictable British climate. If butterflies emerge from their pupae during a period of hot weather they may breed successfully and boost numbers for the following year, but in cool, wet seasons their numbers dwindle. Our study period included the three warmest Augusts on record in England, but interspersed between these summer heatwaves were seasons at or slightly below the 20th-century average temperature.
We modelled the microclimate of individual patches of grassland in each year and the probabilities that each will be colonised by a new population, or that its existing population will go extinct. This let us reproduce the shifting patterns of colonisation and population explosion and limited decline that we observed in field surveys from 1982 to 2009.
Our models and observations of the silver-spotted skipper teach us two important lessons. Firstly, as the species has spread out from refuge populations as the climate becomes warmer, this expansion has mostly happened in exceptionally warm flight seasons. In cooler seasons, more populations go extinct than new ones are established, and some of these advances are lost. The amount of variability - extreme hot seasons above the warming trend and cool seasons below it - determines the pattern of expansion.
Secondly, the newly-established populations that survive are most likely to be in the warmest parts of the landscape - on south- and west-facing slopes capturing the afternoon sun, which can raise the temperature by over ten degrees and significantly boost the chances that a population of butterflies will persist. These local microclimates may act as sources for the next wave of expansion during a warm period, and are critical for rapid expansion.
Insights like these for a single species may be useful in planning for conservation. For example, they could help us choose the best sites to reintroduce plants and animals that have gone locally extinct, or to introduce them into new areas for the first time. They can help us decide how to manage the habitat in places that have not yet been colonised by a species, but may be key staging posts for its future expansion.
Managing for conservation of many species when faced with an uncertain, variable climate is hard. One strategy is to pay particular attention to conserving the range of habitat structures and features in the landscape that create a diversity of microclimates.
Many traditional management practices - coppicing woodlands, grazing pastures, controlled burning of moorland and cutting hay meadows, for example - are already widely used as tools for conservation, as they create varied microclimates and promote species diversity. Can we adapt these practices to meet the new challenges of biodiversity conservation in the 21st century? At a landscape scale, we need to keep space for wildlife along the full range of microclimatic conditions available - from icy mountain tops, snow beds and frost hollows to south-facing sun-traps, from valley-bottom wetlands to steep slopes with drought-prone thin soils. By managing for microclimatic diversity we can protect plants and animals from the worst effects of extreme climate.
Dr Jonathan Bennie is a postdoctoral researcher at the University of Exeter's Environment and Sustainability Institute. firstname.lastname@example.org.
Many ideas were contributed by the organisers and attendees of a NERC-funded workshop 'Managing for Microclimate' held at the Institute for Physics in November 2012, organised by Dr Jenny Hodgson at the University of Liverpool. Dr Andrew Suggitt of the University of York also contributed to the research.