“Insects make up about two thirds of all life on Earth. We appear to be making vast tracts of land inhospitable to most forms of life, and are currently on course for ecological Armageddon. On current trajectory, our grandchildren will inherit a profoundly impoverished world.”
- Study co-author Dave Goulson, quoted by Neil Vowles/University of Sussex, Three-quarters of insect population have been lost in nature reserves over three decades December 13, 2017
The above quote leaves me torn between being alarmed and being skeptical. Alarmist language will do that. How to resolve the tension? Ask questions, follow-up answers with more questions, and so on.
This is the exploration process: questions are paths, you keep going down them until you reach a clearing - some light! Then you look around and find another path/question. Repeat. Hope for partial illumination (a clearing!). Yeah, some people think they can achieve total illumination: out of the woods at last! I'm skeptical about the 'total' part, yet ever hopeful another clearing is just down the road. All the while aware that some problems require decisive and timely action, even though the light could be better.
Back to the insects are dying, let's go down the path of the study that inspired the quote. What did the researchers actually find? Measuring insect biomass (weight of dead insects minus fluid content) in various German nature reserves, the researchers found a massive decline in insect biomass that "was apparent regardless of habitat type, while changes in weather, land use, and habitat characteristics cannot explain this overall decline." Check out the nice chart they provided:
To simplify: higher temperature and more arable land were associated with greater insect biomass, more trees and forest with less. And the biggest contributor by far? Unknown. Bottom line: hard to tell what's causing the decline of insect biomass in German nature reserves. That hasn't stopped the lead author, Caspar Hallmann, from having an opinion:
“The research areas are mostly small and enclosed by agricultural areas. These surrounding areas attract flying insects and they cannot survive there. It is possible that these areas act as an ‘ecological trap’ and jeopardize the populations in the nature reserves.”
Professor Hallman may be right, or not. More to the point: a change in Germany’s insect populations over the last 27 years is best explained by a change in something else. If you want to focus on agriculture as the likely culprit, then you need to look at changes in agriculture over the last few decades. For instance, is farm land spreading or shrinking in Germany? Are German farmers planting different crops? Have they changed how they farm? Are they using more potent pesticides?
As it turns out, Hallman et al did look at changes in German land use and found that arable land* decreased from 31.5% in 1989-94 to 20% in 2012-14, while forests increased from 31.4% to 38.1% of the country. Interestingly, they also found that the richness of plant species** (herbs, shrubs, and trees) declined during the same period.
There's no way I'm going to figure out why the insects are dying on German nature reserves. That would entail a lifetime of study. A couple ideas, though. Neonicotinoid pesticides came on the market a couple decades ago. Neonicotinoids have been found in landscapes they were not applied and are implicated in the collapse of bee colonies throughout the US and Europe. These pesticides have been found harmful to other non-target insects as well.
The other idea is related to the apparent loss of plant diversity in Germany. Most herbivorous insects are specialized feeders; that is, they are very particular about what plants they feed on. Are some insect populations dying because their food sources are disappearing? If so, what can be done about it?
Another path, another day.
Next: Birds
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*Per Eurostat, arable land is land worked (ploughed or tilled) regularly, generally under a system of crop rotation
** The authors did note, however, that on a "per plot basis" lower herb diversity was weakly associated with greater insect biomass. Go figure.
References:
Douglas, M. R., Rohr, J. R., Tooker, J. F. and Kaplan, I. (2015), Neonicotinoid insecticide travels through a soil food chain, disrupting biological control of non‐target pests and decreasing soya bean yield. J Appl Ecol, 52: 250-260. doi:10.1111/1365-2664.12372
Goulson, D. and Kleijn, D. (2013) An overview of the environmental risks posed by neonicotinoid insecticides. J Appl Ecol, 50: 977-987. doi:10.1111/1365-2664.12111
Hallmann, CA, M Sorg, et al/PLOS ONE, More than 75 percent decline over 27 years in total flying insect biomass in protected areas 2017
Woodcock, B.A. et al. Country-specific effects of neonicotinoid pesticides on honey bees and wild bees. Science, 356 (6345). 1393-1395. 10.1126/science.aaa1190. 2017 http://science.sciencemag.org/content/356/6345/1393