Oh such wonderful weather - or is it?

The press are loving the recent hot spell, as are many of the public. Memories flood back to the ‘wonderful’ summer of 1976, and I expect that in due course another generation will look back to the summer of 2018 as one of those ‘exquisite’ summers. As a biologist I sit and fret about the heat and its effects on our insect fauna.

The 1976 drought had a devastating effect on a lot of insect populations and some, such as the craneflies, probably have never fully recovered. I’m pretty convinced that this event and other exceptional droughts, such as the one in the early 1990s and again in the early 2000s, have been a major factor in the catastrophic decline in our insect life and in corresponding declines in many bird species. Most insects actually dislike the heat! Why might this be so?

We need to think of adult insects as the dispersal stage, but for most of the time insects are either eggs, larvae or pupae/puparia. Most of these stages are poorly adapted to excess heat and to drought. Those in the UK, where the climate is normally mild and comparatively wet, are especially poorly adapted to periods of prolonged drought. They occur in the UK because it is mild and wet. If our fauna was adapted to hot and dry, we would have one more akin to the Mediterranean!

Our fauna IS changing. We have seen the arrival of numerous species from Europe in recent years, and some species at the edge of range have expanded their range markedly. Last night I watched a colony of perhaps 200 burrows of the bee-wolf Philanthus triangulum in Mitcham Common. Yet, I recall that when the first insect Red Data Book was published in 1987 there was concern that it was on the brink of extinction on its few localities on the Isle of Wight. How things have changed! A similar situation obtains with the spectacular hoverfly Volucella zonaria. Once it was the prized find for a few entomologists based in the London area and the south coast. Now it has almost reached Scotland!

Tracking species declines is far harder than watching species expansions. Yet there is no doubt that species are disappearing from many parts of lowland southern England. In Mitcham I noticed that the last record I have for the common and easily recognised Leucozona lucorum was in 2002. I have 6 records for the 1980s and 2 from the 1990s when I probably did very little recording in Mitcham but lots elsewhere in Surrey. Now I work Mitcham Common almost daily when at home, but L. lucorum has gone! The habitat has not changed in any wrong way, but we have had some horrendous droughts in those 35 years.

Drought can have unforeseen consequences. For example, the hoverfly Rhingia campestris lays its eggs on grass blades overhanging cow pats. When the larvae emerge, they drop off the grass blade and into the nice moist cowpat. But during extreme heat, the cowpat develops a hard crust far more quickly and the larvae fail to penetrate this. They die, and there is a failure of the second generation of the hoverfly that year. Numbers quickly pick up, however, once wetter conditions are re-established.

The problem comes when there are repeated events before populations can recover. I think this is the key reason why we have seen such serious declines in insects. Yes, land management has changed, and we have lost a lot of suitable habitat, but the biggest impact comes from droughts. The most devastating land management changes happened between the 1950s and 1970s and yet we cannot show its effects because there was very little biological recording. Since the mid-1970s, biological recording has been revolutionised and we are witnessing the impact of a range of factors. Even so, data for the 1980s is far less robust than data for the current day, so we have to rely on occupancy models to explore the changes. Sadly, I think we greatly under-estimate the simple impact of drought, higher temperatures and a more Mediterranean climate.

The one 'upside' of this latest drought is that with luck we will have a lot more data than in the past, so perhaps we will be in a better position to explore the impact of drought!