Seasonal Update - September 2019


Autumn has now arrived. There were times last month, however, that you’d have been forgiven for thinking it was already here. Late July saw an unseasonably deep area of low pressure bring strong winds and rain across much of the country, and this was followed by a second and even deeper low that crossed the UK during the second weekend of last month, bringing wind gusts up to126 kmph (78 mph). Worse, for parts of northern England, Wales and Scotland, it brought torrential rain in the wake of severe flooding towards the end of July. After a week of relatively settled weather for Scotland, northern England and Northern Ireland, more wet and windy conditions arrived, ending August on a somewhat soggy note, while southern and eastern England remained under high pressure from the continent, seeing prolonged warm and dry weather.

Modelled path of the polar jet stream, with winds typically touching 180 kmph (110 mph). Fastest winds are depicted in red, slower winds in blue. - Credit: NASA/Goddard Space Flight Center

What’s causing all this weird weather? Climate change is the immediate response from many quarters, but that’s only the overarching reason. More specifically for us here in the northern hemisphere, it’s the changes that the warming of the Earth is having on the jet stream, that fast-moving ribbon of air high in the upper atmosphere that blows west to east, bringing warm water into the North Atlantic from the tropics. As the planet warms, we’re seeing increasingly unpredictable pressure changes which affect both the path and intensity of the jet stream.

The path of the jet stream is dictated by a pressure system in the North Atlantic known as the North Atlantic Oscillation, or NAO. Basically, this is a measure of the pressure between Iceland and the Azores. In the UK and Europe, the default is for a positive NAO - low pressure in the north and high in the south, with high pressure ridges coming up from the south and bringing us settled weather. Since about April, however, we’ve been under a negative NAO - high pressure over Iceland and low pressure over the Azores that pushes the Azores high out towards Bermuda. Ordinarily, this would bring us a cool, wet summer, but during late July the jet stream was “buckled” by a pressure change in the NAO, resulting in warm air being trapped over Greenland and unprecedented heat waves across most of Europe. In Greenland the heat is causing melting of the permafrost, resulting in subsidence of some houses, while the amount of water draining from the glaciers is increasing - 200 cubic-km of ice melted in July alone. This may not mean much, but it’s equivalent to the whole of the UK’s surface being covered in 1.5m (5 ft.) of water.

Staying on the water theme, last month saw the publication of a couple of important reports on the impacts of climate change on Europe. Writing in the journal Nature, a multidisciplinary team of 50 researchers from 35 institutions spread across 24 countries suggest that Britain is likely to experience some of the worst river flooding in Europe in the coming years, with Scotland and northern England bearing the brunt. According to the researchers, central and northwest Europe have seen increased precipitation in recent years, making soils wetter and thus unable to absorb excess water. This means parts of the northern UK could see an 11% increase in flooding levels per decade. Conversely, soils in southern Europe appear to be drying out, which will affect the water cycle in these countries too, potentially reducing their flood risk but affecting the agriculture upon which many countries are dependent.

A new report published in the journal Nature suggests that northern Britain is likely to suffer increased bouts of flooding in future in response to climate change. - Credit: johndal (CC BY-SA 2.0)

Readers in the north of England and Scotland may be grateful not to see some rain for a couple of weeks, having suffered devastating flooding in July and early August, but many countries are suffering through the worst droughts in their history, and the second key report this month suggests we may not be that far behind. The report, published by the thinktank IPPR North, warns that northern England is in danger of becoming water-stressed, with demand outstripping supply by 2035. Apparently, the north accounts for just over 40% of all water abstracted across England, but also relies heavily on surface water. As the climate warms, the report suggests the likelihood of drought will increase while average summer river flows decrease, reducing overall water availability. This may sound contradictory in the face of the Nature study, suggesting increased chances of flooding, but this is all to do with how we capture and manage water supplies. Consequently, even as the risk of flooding increases, most of that water is lost/contaminated and so doesn’t play a part in our water management strategy.

Elsewhere in the world, the taps ran dry in some South African cities recently and it is estimated that, largely in response to declining rainfall, poor water management, and increasing population, 21 cities in India will run out of groundwater next year and 40% of the population may not have access to drinking water by 2030. More broadly, the World Resources Institute, a U.S. thinktank, warned that, unless serious measures are taken to address climate change, population growth and water management, up to a quarter of the world’s population are at risk of the “Day Zero” that South Africa and parts of Brazil have already experienced – when the taps run dry.

Unless serious measures are taken to address climate change, population growth and water management, up to a quarter of the world’s population are at risk of “Day Zero”, when the taps run dry. - Credit: Julien Harneis (CC BY-SA 2.0)

If you’re interested in becoming more water efficient, check out the Eden Project’s water saving tips and the Water Footprint Calculator. According to the IPPR North report, there’s still work to be done in becoming more water efficient in Britain, with the average person using nearly 7,300 litres per year more than people in Germany, for example.

As September gets underway, the weather models are predicting a change to the northerly airflow, bringing polar maritime air across the UK and making things feel more autumnal. There appears to be a lot of warm air in the northern hemisphere at the moment, however, so it remains to be seen how long the cooler weather will last. If you’re up for getting outside for the start of autumn, as usual, the Wildlife Trusts have a series of nature-themed events up and down the country, as do the RSPB. If you feel like being proactive this month and live near the coast, Surfers Against Sewage is looking for people to organise cleans of their local beaches – details here. The Forestry Commission is running a series of events this month – full list here.

Interested in the wildlife to be found this month? Check out my Wildlife Watching - September page. Elsewhere on the site, a new section discussing badger senses has been added and there’s one on squirrel play behaviour in the works, along with a QA on scent-marking behaviour of foxes. Finally, keep an eye on the site this month as I’ll be starting a new ad hoc blog called “Walks with the Camera”. Those of you who have followed me on Facebook for a few years may have seen some of the descriptive posts covering my early morning walks, largely in the New Forest. These will start appearing on the website shortly, replete with photos from the excursion.

Discoveries of the Month

Cross-species mating may explain black grey squirrels

Most of us are familiar with grey squirrels (Sciurus carolinensis) and, love them or loathe them, they’re an ever-present feature of much of Britain’s landscape. In eastern counties of England, however, there is an increasing population of black squirrels. These are the same species as the grey, just a very dark colour morph. Nobody knows exactly how many black squirrels call England home, although I have seen unsubstantiated estimates of 25,000 in the media, but genetic analysis by researchers at Anglia Ruskin University in East Anglia suggests that the population stems from releases or escapes of black individuals, imported from the US, during the early 20th century.

Melanism appears to be spreading in Britain's Grey squirrel population. New genetic data suggest that it arrived in the Grey genome courtesy of a chance hybridization event with a Fox squirrel. - Credit: Marc Baldwin

Melanism seems to confer some benefits under certain ecological conditions, helping with camouflage and temperature control, for example, so its origins are of great interest to evolutionary biologists. As such, the Anglia Ruskin geneticists recently investigated how this black gene arose in grey squirrels in the first place, and the results were something of a surprise.

A team of researchers, led by Helen McRobie, investigated the genetic basis for melanism in 51 grey squirrels and 42 fox squirrels (Sciurus niger) from the U.S. Fox squirrels are ecologically similar to greys; slightly larger, but occupying the same habitat, feeding on much the same foods and with a partially overlapping range. In their previous studies, the biologists found that melanism in greys was caused by a 24 bp deletion in MC1R. In other words, grey squirrels turned out black because a genetic ‘processing glitch’ cut out a section of one of the genes responsible for pigment production. Based on this they looked for changes in the same gene in their fox squirrel subjects.

The data show that melanism appears to have evolved twice, independently, in fox squirrels. Of more interest, however, was that they found both species had identical changes to their MC1R gene; exactly the same alleles had been deleted. The authors posit that this may indicate inheritance from a common ancestor or independent evolution in fox and grey squirrels, but suggest that both scenarios were unlikely owing to the deep genetic divergence seen between the species and the observation that the gene mutation in fox and greys was not just similar, but identical. Instead, they suggest that the gene was introgressed from foxes to greys, introgression being the incorporation of genetic material from one species into another. Put simply, at some point in antiquity a black fox squirrel mated with a grey squirrel, passing on their gene for melanism. So, it seems that black squirrels in Britain today ultimately have a chance mating with a different species to thank for their selective advantage, which may make them less vulnerable to predation from birds of prey than their normal-coloured kin in some circumstances.

Reference: McRobie, H.R. et al. (2019). Multiple origins of melanism in two species of North American tree squirrel (Sciurus). BMC Evol. Biol. 19: 140.

Bumblebees face a nectar drought during the summer

Most of us won’t have failed to notice that our pollinating insects are in trouble. According to Bumblebee Conservation, one-third of our bumblebee species have declined by at least 70% since the 1930s; two have become extinct and seven have seen such a significant drop in numbers they’re now UK Biodiversity Action Plan species of conservation concern. The picture behind the decline is a complicated one and there are many interacting factors to consider, including the use of pesticides on agricultural land and the spread of parasites, such as the tracheal mite Locustacarus buchneri.

A honeybee collecting pollen. Planting a selection of flowers that provide nectar throughout the spring, summer and autumn is hugely important to help support our pollinating insects. - Credit: Marc Baldwin

In recent years it has become increasingly apparent that a lack of food in our countryside is also a problem for our bees. It has been estimated that, since World War II, agricultural intensification in Britain resulted in nectar supplies falling by 32% between 1930 and 1978. Despite some attempts to correct the situation with Environmental Stewardship Schemes, nectar production in our countryside remains below the pre-1930 levels. Recent research by scientists and Bristol and Cardiff universities suggests that it’s not only the quantity of nectar available that’s causing problems for our bees, though - it’s also the timing of it.

During 2016 and 2017, a team of researchers led by Thomas Timberlake at Bristol assessed the nectar production across four medium-sized farms (dairy, sheep and arable) in north Somerset. They regularly visited the farms, recorded the number of open flowers per unit area between at least March and November, and estimated the nectar production of each species recorded. This was an amazingly in-depth study, with one farm being surveyed twice per week between late February and early November. For each species, a level of uncertainty was calculated such that they could estimate maximum, expected and minimum nectar production and model them separately. The results were published in the Journal of Applied Ecology earlier this year.

The biologists found that, although 59 species of flowers produced meaningful quantities of nectar (more than 3g sugar per sq-km per day) at some point during the year, three species (Allium ursinum, Cirsium arvense and Trifolium repens) supplied half the available sugar and 80% of sugar was accounted for by only eight species of plant. More interesting still was that these species produced an abundance of seasonal nectar, but only a couple of plants provided anything out of season. Hedera helix, for example, provided over half of all sugar available from mid‐September until the end of the flowering season, while Taraxacum filled the gap from mid‐March until late April. In their paper, the authors suggest that such strong fluctuations in nectar availability through the year may be as important in limiting pollinator diversity as the total amount of nectar produced:

Considering phenology in the design of agri‐environment or restoration schemes is therefore likely to improve their suitability for pollinators. Plant species which flower during periods of resource deficit (early spring and late summer) should be prioritized in schemes which aim to conserve or restore pollinator populations.”

The researchers suggest that it’s important to maintain a variety of semi-natural agricultural habitats (woodland, hedgerows, field margins, and so forth) in order to ensure a constant supply of nectar on farms throughout the time when bees (and butterflies) are on the wing. This finding likely extends to our gardens, and we can help support our local bees and butterflies through this “nectar famine” by planting species that bloom early and late in the year.

Reference: Timberlake, T.P. et al. (2019). Phenology of farmland floral resources reveals seasonal gaps in nectar availability for bumblebees. J. Appl. Ecol. 56: 1585–1596.

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