The numbers of flying insects in nature reserves throughout Germany show a staggering decline. Taken on average over the months of April to October between 1989 and 2016 insect numbers declined 76%. In mid-summer measurements show an even more rapid decline, with insect numbers now 82% down compared to just 27 years ago.
This we learn from a study by a group of German and Dutch ecologists of Radboud University and the Entomological Society Krefeld that was published yesterday.
This is what the Anthropocene Extinction looks like, if you express it in a graph: the general decline of flying insect biomass in nature protected areas in Germany. In just 27 years, insect numbers went down by over 75 percent.
Downward general population trends are indicative of (mass) extinction. And in Europe, insects seem to lead the way…
As we’ve seen before in our series, species extinction is accompanied by general species population decline. Therefore the trend for flying insects in Germany is an extremely worrying sign – that also in the insect world the Holocene-Anthropocene Mass Extinction is well underway, at least in Europe.
The researchers point to intensive agriculture as the likely main culprit of the rapid insect decline, and also mention climate change as an increasing pressure that together with habitat loss and habitat fragmentation shares responsibility for similarly observed trends elsewhere in Europe, for instance a decline in bees [see our file on Colony Collapse Disorder (CCD) <– and please note extensive evidence pointing to neonicotinoid insecticides(!)], bumblebees [25% of European bumblebee species threatened with extinction], moths – and a 50 percent decline in European grassland butterflies over the period 1990-2011, citing a special report of the European Environmental Agency from 2013.
The new study, that was published October 18 in PLOS ONE, confirms these downward European flying insect trends and improves the temporal and spatial coverage by adding a dataset of semi-continuous measurements in 96 nature protection areas throughout Germany, that the authors call representative of similar low-altitude protected nature areas in human-dominated landscapes of Western Europe.
Now what causes the German insect decline?
The researchers write that the measured decline in flying insect numbers was uniform across different habitat types, including nutrient-rich and nutrient-poor habitats, suggesting overlying, external stressors.
As part of this series we have a special interest in the effects of climate change, the share of climate change in global biodiversity decline – and luckily the researchers did indeed look into that factor, by comparing their dataset to an established local meteorological dataset:
“Climate change is a well-known factor responsible for insect declines. To test if weather variation could explain the observed decline, we included mean daily temperature, precipitation and wind speed in our analysis, integrating data from 169 weather stations located within 100km to the trap locations.”
“We examined temporal trends in each weather variable over the course of the study period to assess changes in climatic conditions, as a plausible explanation for insect decline. Estimates of each weather variable at the trap locations were obtained by interpolation of each variable from the 169 climate stations.”
Climatic variations could explain seasonal changes in insect biomass. For instance a positive correlation was found with temperature, and a negative one with precipitation [frost days (declining) and winter precipitation (increasing) could not be correlated to insect numbers in the following year]. Together the climatic variables could however not explain the trend.*
[*) The researchers do point to a positive biodiversity-biodiversity feedback (remember biodiversity is protected by biodiversity – so a decline tends to accelerate) from vegetation to insects – as throughout the reserves also plant biodiversity decreased, and insects very often depend on plants. Now plant biodiversity is of course influenced by as many factors as the insect world – including climate change.]
Image to the left: as one would expect insect numbers increase seasonally during the summer halfyear. In mid-summer though the declining trend of flying insects in German protected nature areas is the strongest: total insect biomass is now 82% lower than 27 years ago – indeed a very rapid decline.
Discussing their results the researchers point to agricultural intensification and the limited size of nature reserves in the human-dominated landscape as the root cause of the insect decline:
“The decline in insect biomass, being evident throughout the growing season, and irrespective of habitat type or landscape configuration, suggests large-scale factors must be involved. While some temporal changes in climatic variables in our study area have taken place, these either were not of influence (e.g. wind speed), or changed in a manner that should have increased insect biomass (e.g temperature). However, we have not exhaustively analysed the full range of climatic variables that could potentially impact insect biomass. For example prolonged droughts, or lack of sunshine especially in low temperatures might have had an effect on insect biomass.”
“Agricultural intensification (e.g. pesticide usage, year-round tillage, increased use of fertilizers and frequency of agronomic measures) that we could not incorporate in our analyses, may form a plausible cause. The reserves in which the traps were placed are of limited size in this typical fragmented West-European landscape, and almost all locations (94%) are enclosed by agricultural fields. Part of the explanation could therefore be that the protected areas (serving as insect sources) are affected and drained by the agricultural fields in the broader surroundings (serving as sinks or even as ecological traps)*. Increased agricultural intensification may have aggravated this reduction in insect abundance in the protected areas over the last few decades.”
[*) They're not the first to suggest intensive agriculture acts as an ecological sink – in turn also harming rare wild plants, by luring pollinators away.]
“Whatever the causal factors responsible for the decline, they have a far more devastating effect on total insect biomass than has been appreciated previously. The widespread insect biomass decline is alarming, ever more so as all traps were placed in protected areas that are meant to preserve ecosystem functions and biodiversity. While the gradual decline of rare insect species has been known for quite some time (e.g. specialized butterflies), our results illustrate an ongoing and rapid decline in total amount of airborne insects active in space and time. Agricultural intensification, including the disappearance of field margins and new crop protection methods has been associated with an overall decline of biodiversity in plants, insects, birds and other species in the current landscape.”
“The major and hitherto unrecognized loss of insect biomass that we report here for protected areas, adds a new dimension to this discussion, because it must have cascading effects across trophic levels and numerous other ecosystem effects. There is an urgent need to uncover the causes of this decline, its geographical extent, and to understand the ramifications of the decline for ecosystems and ecosystem services.”
We think it’s absolutely terrible news that deserves broad public and political attention.
Meanwhile in Europe temperatures soar to summer values – and insects are plenty. But they look… unfamiliar
Meanwhile it’s the second half of October 2017 and temperatures throughout much of western Europe are above 20 degrees Celsius, including Germany. That’s caused by a strong southern circulation tugged on by tropical cyclone (Atlantic hurricane) Ophelia that passed straight over Ireland a couple of days ago – because that’s where extra warm Atlantic waters led it to. Indeed, climate change.
This strong southern circulation earlier brought not only smoke from massive Portuguese forest fires [again, climate change] and dust from the Sahara desert as far North as Denmark and beyond – but also very high numbers of flying insects. Insects that were born somewhere on the Iberian Peninsula and were then caught up in the air currents and transported over a thousand kilometres and more. Now if during these extreme Indian Summer days you measure an insect trap somewhere in Germany you are likely to measure an insect biomass increase. But does that count as healthy ecology? Does that have ecological benefits in the long run – or possibly the opposite?
[For those who dislike rhetorical questions by mediocre science journalists, perhaps these are actual questions you may like to ask ecologist Caspar Hallmann of Radboud University, lead author of the new insect decline study.]
We think if a temperature rise leads to an insect increase that’s quite likely also an ecological disturbance, possibly favouring single species over others and therefore also not good (in the long run). The most extreme example is called a plague – a climate plague. These we know from Earth’s paleoclimate, these we now know from the North American pine beetle outbreaks – and these will probably also start manifesting themselves in Europe, as temperature keep rising, other climatic variables are stressed further and further, and for instance Drosophila suzukii has already crossed the Alpes [and yes, that’s Germany on the other side of that mountain range].
But let’s not ignore the main message of the PLOS ONE study – as we’ve seen before in our series: other biodiversity killers can be worse than climate change. And it’s very often direct habitat destruction and intensive agriculture that top that list. Europe is no exception…
© Rolf Schuttenhelm | www.bitsofscience.org