Weather or climate? The lasting effects of a hot summer

Emerald Damselfly by Ross Hoddinott/2020Vision.

In his second blog on the subject, our Chief Executive Brian Eversham discusses the long-term effects the long hot summer could have on our wildlife.

In my previous blog, I looked at the effects of several weeks of drought and very high temperatures on wildlife, especially insects. Having discussed the short term effects, I thought it would be useful to consider the longer-term impacts of one-off extreme weather events, and how they fit into the expected pattern caused by climate change.

Especially when weather patterns affect a large part of mainland Europe too, species may start to move longer distances than usual. Major influxes of migrant butterflies such as painted lady and clouded yellow often start in February or March with a warm wet lush early spring in Morocco or Spain, producing an unusually large first generation of the butterflies, which then fly north… British arrivals are sometimes from this first wave (if the winds are right), but otherwise, we are reached by the second or third generation of painted ladies moving across the continent. A few have arrived in Cambridgeshire in the last couple of weeks. Hopefully these will lay eggs, allowing us to enjoy the caterpillars and then some freshly-emerged butterflies in this country. Sadly, neither painted lady nor clouded yellow are able to hibernate to survive the British winter. Instead, painted ladies have been shown to migrate south again, at higher altitudes and greater speeds than their northward movement, so spend the winter in southern Spain or north Africa. Although this means the butterflies continue their species, it also means that one boom year in Britain has no effect on the next year’s population of migrant butterflies.

For other insects, and for birds, this is not necessarily so. This summer, there has been been an influx of rare migrant or colonising insects, especially dragonflies: the best year so far for Southern Migrant Hawker (aka Blue-eyed Hawker), with at least one in Bedfordshire already, a good year for Lesser Emperor, and there have been a scatter of the rare Darter species. Careful observations in Kent and Essex have confirmed that the Blue-eyed Hawker is now breeding in the south-east England. Lesser Emperor has bred in Britain on at least one occasion, and may become regular if larger numbers reach here.

A red-veined darter dragonf;y at rest on a small twig

A red-veined darter dragonfly by Brian Eversham

The emerald damselflies are a great example of what is possible as a result of climate change. Back in 1996 when I was co-author of the first Atlas of dragonflies of Britain and Ireland, we had two emerald species, the common emerald, widespread throughout the country, and the scarce emerald which had been thought extinct in the early 1980s, but was found still to be present on coastal and estuarine wetlands on the south-east coast. Other species were extremely rare wanderers to Britain, with no evidence of them breeding. Since then, the willow emerald (recognised by the white rectangular cell, the pterostigma,on the front edge of the wing a little way from the tip), arrived in East Anglia and rapidly became established. It’s spread into our area and is an established breeding species. Unusually, it lays its eggs in willow stems over ponds and ditches, and these hatch the following spring and the young larvae develop rapidly. It can thus survive in water bodies which dry up each summer (females are content to lay in willows over apparently dry ground), so it is well adapted to summers like 2018. Another species, the southern emerald (with strikingly two-tone pterostigmas) is now breeding just over the Bucks border, and scarce emerald is apparently spreading in Norfolk/Suffolk Breckland so could recolonise Cambridgeshire in the next few years (excuse borrowed photos). So, we could have several more dragonfly and damselfly species established in our three counties within a couple of years.

Hot summers can have less benign longterm effects. The late Oliver Rackham documented how the drought of 1976 caused a dying back of the canopy of many oak trees, leaving them ‘stag-headed’ (with dead branches protruding beyond the leaves). This he put down to the drought having killed the peripheral roots of the tree, which subsequently adjusted its above-ground parts to what the surviving roots could support. So, oaks which appeared sick or dying twenty years later might still be suffering from that one severe hot summer.

At present, some of our most important tree species are also suffering from disease: ash die-back, sudden oak decline, and ongoing bouts of Dutch elm disease. Trees suffering water stress are likely to be more severely affected, and perhaps to die sooner of these diseases.

And finally… within a couple of days of my writing about the drought, the rains came. So far, not enough to make much difference, and with more hot weather forecast, the drought may not be over yet. But when more prolonged rains do come, I’d hope they will trigger a good showing of some of the scarcer and interesting fungi such as the morels. And if this happens to be a mild and wet autumn, following this hottest of summers, it could be a season of fungus forays to remember.

Save the date.. some fungi events coming up this autumn