Why the decline in insect biomass (and diversity)?

The work of the Krefeld Entomological Society surfaced again this week in post on the UK Hoverflies Facebook page, raising the big question ‘why such a decline’? In the absence of fully correlated data it is very difficult to draw any firm conclusions but I suspect we are looking at some form of ‘death by a thousand cuts’.

A lot of blame must lie with post-war agricultural intensification: increased use of pesticides, deep ploughing, amalgamation of fields and the loss of a huge acreage of hedgerows and associated habitat. But, the decline has been ongoing, whilst many of the (seemingly) most damaging actions took place before the Krefeld sampling programme first started. So, although DDT took a terrible toll on insect life, it is not the only culprit. There have been subsequent generations of insecticides, all of which must have some impact, if not quite so serious. Maybe Neonectinoids are to blame for post 1980s crashes? They will have had some impact and it is possible that they will still be found to have had a bigger impact than is currently believed?

I suspect that we are actually looking at a series of hammer blows. Post-war intensification must already have had a massive impact by the time I started to take an interest in insects; yet in my childhood I could still find, with relative ease, the larvae of puss moth, poplar and lime hawkmoth, and a plethora of other lovely animals. Now I cannot, even though I still visit the same place and look on many of the same trees! My garden moth trap is no longer invaded by clouds of garden tiger moths and I cannot recall when I last found its caterpillar in south London!

My local ‘patch’ comprises several square kilometres of open habitat enclosed by urban sprawl. I guess it has become somewhat more isolated over time but, even so, it has not been affected by herbicides and pesticides. It has, however, been affected by increased nitrification and the vegetation seems to me to have lost some of its low-nutrient characteristics, even though most of the characteristic plants still survive. At the same time, we are told that air quality has improved, and so it has if the range of lichens now growing on our roof are a reasonable indicator. We no longer have vast quantities of atmospheric lead from car exhausts, but do we have higher volumes of pm10s.

More recent events

In my lifetime I have witnessed several ‘events’ in which insect numbers have crashed. Each has corresponded to a major drought and period of extreme temperatures. So, in the course of 40 years we have seen a sequence of catastrophic events that will not have helped insect/invertebrate populations. The biggest ones I think were 1976/77, 1990/91 and several years in the early 2000s. Bearing in mind that nine out of the ten hottest years since 1910 have occurred in the past 17 years, climate change is something we have definitely got to look to as a major factor behind insect declines.

It is not, however, individual actions or events that really set the scene. Insect populations are remarkably robust, swinging back and forth as environmental forcing impacts upon them. An obvious example is the rise and fall of the holly blue butterfly whose numbers fall precipitously as parasite levels climb, but once the parasite peaks, its numbers crash and those of the holly blue climb again. Similar patterns can be seen in many species and can also be detected in response to droughts. For example, in 1947 it was remarked that the summer generation of the hoverfly Rhingia campestris failed to materialise. We have seen similar events from time-to-time in response to prevailing arid conditions.

My examples so far are individual species, but what happens when whole assemblages are hit by a particular environmental anomaly? Alan Stubbs always cites the impact of drainage at Wisley Common on its cranefly fauna as an example of how a major land-management change can impact an entire ecosystem. In that case, ditching caused changes in the water table and the cranefly fauna crashed; it never recovered because the change was profound and substantially irreversible. Anecdotally, Dipterists noticed a huge decline in wetland species in the 1990s, especially Sciomyzidae (snail-killing flies). This decline seems to have partially reversed during wetter periods. But, if conditions do not quickly recover towards the ‘norm’ the impact of the change will be longer-lasting and less reversible. Similarly, each autumn, our field meetings are punctuated with a general feeling that cranefly diversity is substantially down; anecdotally perhaps by as much as 30% since the 1970s.

So, why don’t we have the insects that used to occur? It seems to me that we can blame a combination of definable human impacts such as pesticides and intensification/loss of habitat. But, we must also put some blame at the insidious effects of climate change, which is also substantially an anthropogenic effect.

The catastrophic decline in insects highlighted by the Krefeld Entomological Society is simply an expression of the way the earth’s ecosystem is responding to anthropogenic perturbation. If we want to focus blame, we must do so on our entire community. It has become accepted that we will have access to ever-cheaper food, transport and consumer goods. There has to be a cost, and that may well be the demise of current ecosystems that are in fact our own life-support system!