Double threat to UK’s birds and butterflies from climate change and land use

In the UK, rising temperatures are making life increasingly uncomfortable for species of wildlife better adapted to cooler climes. A new study offers the first compelling evidence that how we use land is exacerbating the vulnerability of some popular bird and butterfly species to climate change.

The research is a joint project by researchers at the University of Reading, the NERC Centre for Ecology and Hydrology, the British Trust for Ornithology, Butterfly Conservation and Natural England and is published today in the journal Global Change Biology.

Cold-weather or warm-weather

Most bird and butterfly species prefer either warm or cold conditions. As lead author on the new research, Dr Tom Oliver from the University of Reading, tells Carbon Brief:

“Warm associated species are those that occur mainly in the warmer locations of Europe and, conversely, cold-associated species are likely to be restricted to more northerly countries and upland areas.”

Over time, studies have shown that the proportion of warm-weather species within the UK’s bird and butterfly populations is increasing, while the proportion of cold-weather species is decreasing.

Other scientists have interpreted this as bird and butterfly populations keeping pace with climate change by favouring species that can best tolerate warmer conditions. But the new study says this overall pattern masks much more subtle changes going on.

Using data from the UK Butterfly Monitoring Scheme and the Commons Birds Census, the researchers analysed 114 bird and 63 butterfly species at 613 sites over the past 30 years.

The authors found that both warm and cold-weather bird species have declined in total abundance and species richness over the past three decades. Cold-weather species have seen numbers drop much faster in that time, however.

In other words, the growing proportion of warm-weather birds over the past 30 years is less to do with an increase in those species per se and is more being driven by the loss of cold-weather species, including extinctions in some sites. This suggests a more pessimistic outlook for Britain’s birds in response to climate change than previously thought.

Butterflies appear to be coping with climate change relatively better than birds, showing little change in cold-weather species at the same time as a small rise in those that prefer warmth. The paper suggests:

“Butterflies are thermophilic insects and there are relatively few species in high-latitude countries such as England for which the climate is yet too warm.”

That is a situation that may well change as the climate warms further, in particular with an increase in the frequency of extreme weather, such as droughts, the authors note.

Double hit

The paper take the analysis a step further by looking at how the way we use land influences species’ general responses to climate change over time. On top of rising temperatures, bird and butterfly species are losing their habitat to agriculture and urban development, limiting their access to food and nesting sites.

For birds better suited to warmer temperatures, such as corn bunting, goldfinch and linnet, losing suitable habitat puts a limit on how far they can exploit the more favourable conditions as the climate warms, causing numbers to drop over time rather than grow.

For cold-weather species, such as the willow tit, meadow pipit, willow warbler and bordered fritillary butterfly, the impact is much more pronounced. Losing habitat on which to feed and breed compounds the pressure populations already experience from rising temperatures. Oliver tells Carbon Brief:

“In cold-associated birds, for example, they are being hit with both reductions in habitat quality and a climate that is becoming gradually less suitable.”

The graph below from the paper shows this impact of changing land-use on bird and butterfly abundance. Land use exacerbates the decline in cold-weather birds (top left) and butterflies (bottom left), and prevents warm-weather birds from taking advantage of rising temperatures (top right). Warm-weather butterflies neither increase nor decrease when there is intensive land use around monitoring sites (bottom right).

Relationship between increasingly high land-use intensity (left to right, x-axis) and decreasing abundance (top to bottom, y-axis) for cold-weather birds (top left) and butterflies (bottom left) and warm-weather birds (top right) and butterflies (bottom right) Source: Oliver et al., (2017)

Drivers of change

The study is significant, says Oliver, because it shows the extent to which land use interacts with climate change to affect our bird and butterfly populations in the UK. Susceptibility to the first determines resilience to the second, he tells Carbon Brief:

“[O]ur wildlife is not simply are the mercy of global climate warming driven by international CO2 emissions. The way we manage our landscapes affects the ability of species to cope with climate change.”

Which driver has the bigger impact in the future – climate change or habitat loss – depends on how global CO2 emissions evolve and how we decide to manage our landscape. The latter, Oliver suggests, could depend on policy changes after the UK leaves the European Union.

Given uncertain times ahead, restoring natural habitats – such as woodlands, grasslands, heathlands – would give the UK’s bird and butterfly populations the best chance of adapting to climate change.

A more complete understanding of community changes “should help inform upon ways to reverse detrimental trends, as well as simply documenting them”, the paper concludes.