It’s not exactly been a scorching start to the season – less of a "barbeque summer" and more of a "dodge the downpours" kind of summer – so extreme heat and drought is one thing our butterflies and birds haven’t had to cope with.
For the vast majority of UK butterfly species, cold and wet summers result in reduced populations
The weather so far this year has been at the other end of the scale. Spring took a long time to get going with temperatures generally below average and snow falling in April. The start to summer has also been less than typical, with June being a washout for many parts of the UK.
Together with last year’s cold, wet summer it could indicate problems for some of our insect populations.
“The weather has a massive influence on UK butterfly populations from generation to generation, year to year,” says Richard Fox, head of recording for Butterfly Conservation.
During prolonged spells of wet and cold during their flight period, individual butterflies are not able to disperse or feed, set up territories or engage in courtship.
And, most importantly, they aren't able to lay eggs.
So by limiting all these behaviours, explains Fox, extended periods of cold and wet weather can directly reduce the breeding success of our butterflies. However, it can spell double trouble as their caterpillars grow more slowly in colder conditions and are more prone to predation and disease.
“For the vast majority of UK butterfly species, cold and wet summers result in reduced populations,” he says.
There are some species that have evolved to cope with the cooler, wetter climates of northern and upland Britain, such as the mountain ringlet and Scotch argus, where they live amongst the longer vegetation and grassy tussocks. Nevertheless, stresses Fox, they still like some warm and sunny days in order to get out and reproduce.
A summer drought is probably the worst of all weather conditions for butterflies
Another type of adaptation is seen in some of our more widespread species. The green-veined white, for example, where individuals have darker markings in cooler parts of Britain, or may be smaller or have more specialised habitat requirements.
It’s a bit early to tell how this year’s weather will play out for the 18 species of butterfly and two day-flying moths being recorded during this year’s Big Butterfly Count, says Fox, but it’s not looking good for our three species of common whites, common blue, small copper and red admiral. The outlook for brimstones and holly blues, however, is more promising.
Only by taking part in the world’s largest butterfly survey, from 15 July to 7 August, will we know for sure.
Sadly it’s not the cold and wet that’s the biggest threat to our butterflies. “A summer drought is probably the worst of all weather conditions for butterflies,” says Fox. That is because it kills the plants that caterpillars need to eat and impacting the next generation.
An example is the famous butterfly summer of 1976. Those who are old enough will remember the extreme heatwave and severe drought of that year – it caused the populations of many species to crash the following year, some of which have never recovered.
An extreme flooding event
But a lack of water wasn’t something the reintroduced cranes on the Somerset Levels had to deal with back in 2013. It was the exact opposite: unprecedented flooding. Shallow winter water is normal in this part of the country, but this rare event gave researchers from the University of Exeter, the Wildfowl and Wetlands Trust (WWT) and Royal Society for the Protection of Birds (RSPB) an opportunity to assess how these hardy birds coped with extreme weather.
Recently published in the Nature journal Scientific Reports, they found that the deep water forced the cranes out of their usual roosts and feeding sites to spend prolonged periods of time foraging for food at the edge of the flooded areas.
“The flood was quite extraordinary, the most intense in 200 years,” said PhD student and lead researcher Andrea Soriano Redondo.
Species that are already subject to a higher environmental pressure might suffer more
We expected the cranes to be pushed to the edge of the floods, she says, because they are not able to forage in deep waters. “However, previous to the analysis we had no clue on the coping mechanisms, finding out that they had to forage for two extra hours a day was quite surprising.”
And after the water receded the team tracked the cranes returning to forage and roost in their usual territories. What this study shows is that some species, such as the cranes, are able to cope with extreme climatic events, at least to a certain extent. But then cranes are large, resilient and adaptable birds. The adults, Redondo explains, do not have predators and have a wide diet, including plants, invertebrates and small vertebrates.
“Species that are already subject to a higher environmental pressure might suffer more,” she said.
Flooding of this magnitude is thankfully very rare, but can be devastating when it occurs. Any animal response might depend on how specialised a species’ diet is, and the resources available in the surrounding area.
However, a bigger threat to our feathered friends is cold weather in winter, because it is one of the main environmental stresses that bird species face.
Coping with cold
Cold winters can be a significant threat for many of our resident bird populations, particularly small insect-eating birds with high energetic demands that struggle to find food, such as the wren; wetland birds, such as coot and grey heron, because their water freezes reducing access to underwater vegetation and fish; and those birds that rely on soil invertebrates, like lapwing, that struggle because the ground freezes or is covered in snow.
So how do birds cope with the cold?
A small number of species have adapted by trying to store food for the winter, such as crows and tits. Some move to gardens and other areas where there are reliable sources of food. Another strategy is to move across the country, or to the continent, in an attempt to find better conditions.
Wrens are one of our smallest and most widespread songbirds and are not good at coping with cold weather. In fact, winter frosts are the main weather-related feature affecting their survival, making them an ideal species to study adaptations to cold.
A recent study published in the Royal Society journal Open Science found that although all populations of wrens were affected by cold weather, those in the north of the country, particularly Scotland, were better able to cope with long periods of frost than those in the south.
By analysing data collected by bird ringers participating in the Breeding Bird Survey, researchers from the British Trust for Ornithology (BTO) found that body mass was approximately 5% lower in the south west of the country, than in the coldest regions of Scotland. They suggest that larger birds in the colder north are likely to have a thermal advantage, because of their ability to store more body fat. It is a pattern seen more widely across other species.
“We expected to find that wren populations would decline in cold winters (with long periods of frost),” explains Dr James Pearce-Higgins, Director of Science for the BTO and one of the authors of the paper.
“The strength of this effect was a surprise, and strongly suggests that wren populations in northern Scotland are adapted to cope with the colder climate they experience, whilst those in the south are adapted to their warmer climate,” he said.
This research is interesting in light of any changing climate we might experience.
Wrens appear to show adaptations to local conditions and therefore have the potential to adapt to changes in climate, possibly through a micro-evolutionary change in body size. And this is important, explains Pearce-Higgins, because it provides some of the best evidence so far that bird species may have the ability to adapt to changes in temperature through evolution.
“However, what is uncertain I'm afraid is the extent to which the pace of climate change may operate more quickly than the birds are able to adapt.”
A warming climate
Predictions for our future climate are of a warmer one with fewer cold snaps, although this is not yet certain. But if true it would tend to increase survival rates, and lead to an increasing and stable population of wrens.
In fact one of the consequences of our recent warmer winters (with a few notable exceptions) has been larger and more stable populations of many resident species. However, research shows that one of the more negative impacts of a changing climate, explains Dr Pearce-Higgins, could occur by altering interactions between species, such as reducing food resources or increasing numbers of predatory or competitor species.
“So it is possible in the case of the wren that warmer temperatures may eventually cause a reduction in insect food, or lead to competition with other insectivorous species, causing potential population declines,” he said.
But wrens are a very adaptable species, and are likely to be able to cope with any change in climate, compared with other more specialised species.
It’s the same story for our butterflies, explains Richard Fox, because a general increase in temperature is likely to benefit them. This has been demonstrated over recent decades as some species, such as comma, ringlet, marbled white and speckled wood, have spread northwards. On the other hand any increase in extreme climatic events, such as summer droughts, could have very negative consequences.
“However, responses vary between species so it is dangerous to generalise,” he stresses.
So for our butterflies and birds it’s anything but a clear picture for now. But there is still time for a summer heatwave.
Jeremy Coles is a staff writer for BBC Earth. He is @jpcoles on Twitter.
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