The numbers are stark, indicating a vast impoverishment of an entire insect universe, writes
Insects are the vital pollinators and recyclers of ecosystems and the base of food webs everywhere.
In the US, scientists recently found the population of monarch butterflies fell by 90% in the last 20 years; the rusty-patched bumblebee, which once lived in 28 states, dropped by 87% over the same period.
A 2017 paper by an obscure German entomological society brought the problem of insect decline into sharp focus. The study found that, measured simply by weight, the overall abundance of flying insects in German nature reserves had decreased by 75% over just 27 years.
If you looked at midsummer population peaks, the drop was 82%. The study would quickly become, according to the website Altmetric, the sixth-most-discussed scientific paper of 2017.
Headlines around the world warned of an “insect Armageddon”. How could something as fundamental as the bugs in the sky just disappear? And what would become of the world without them?
We’ve named and described a million species of insects. There are 12,000 types of ants, nearly 20,000 varieties of bees, almost 400,000 species of beetles.
And yet entomologists estimate that all this amazing, absurd, and understudied variety represents perhaps only 20% of the actual diversity of insects on our planet — that there are millions and millions of species that are entirely unknown to science.
When entomologists began noticing and investigating insect declines, they lamented the absence of solid information from the past in which to ground their experiences of the present.
“We see a hundred of something, and we think we’re fine,” says David Wagner, an entomologist at the University of Connecticut, “but what if there were 100,000 two generations ago?”
Rob Dunn, an ecologist at North Carolina State University, recently searched for studies showing the effect of pesticide spraying on the quantity of insects living in nearby forests.
He was surprised to find that no such studies existed. “We ignored really basic questions,” he said. “It feels like we’ve dropped the ball in some giant collective way.”
If entomologists lacked data, what they did have were some very worrying clues. A 2014 review in Science synthesized the findings of existing studies and found that a majority of monitored species were declining, on average by 45%.
There were studies of other, better-understood species that suggested the insects associated with them might be declining, too. People who studied fish found that the fish had fewer mayflies to eat.
Ornithologists kept finding that birds that rely on insects for food were in trouble: Half of all farmland birds in Europe disappeared in just three decades. At first, many scientists assumed the familiar culprit of habitat destruction was at work, but then they began to wonder if the birds might simply be starving.
The signs were certainly alarming, but they were also just signs, not enough to justify grand pronouncements about the health of insects as a whole or about what might be driving a widespread, cross-species decline.
“There are no quantitative data on insects, so this is just a hypothesis,” said Hans de Kroon, an ecologist at Radboud University in the Netherlands.
Then came the German study. Scientists are still cautious about what the findings might imply about other regions of the world. But the study brought forth exactly the kind of longitudinal data they had been seeking, and it wasn’t specific to just one type of insect.
The numbers were stark, indicating a vast impoverishment of an entire insect universe, even in protected areas where insects ought to be under less stress. The speed and scale of the drop were shocking even to entomologists who were already anxious about bees or fireflies.
The results were surprising in another way too. The long-term details about insect abundance, the kind that no one really thought existed, hadn’t appeared in a particularly prestigious journal and didn’t come from university-affiliated scientists, but from a small society of insect enthusiasts based in the modest German city of Krefeld.
In 2013, Krefeld entomologists confirmed the total number of insects caught in one nature reserve was nearly 80% lower than the same spot in 1989. They had sampled other sites, analysed old data sets, and found similar declines: Where 30 years earlier, they often needed a litre bottle for a week of trapping, now a 500ml bottle usually sufficed.
But it would have taken even highly trained entomologists years of painstaking work to identify all the insects in the bottles. So the society used a standardised method for weighing insects in alcohol, which told a powerful story simply by showing how much the overall mass of insects dropped over time.
The society collaborated with de Kroon and other scientists at Radboud University in the Netherlands, who did a trend analysis of the data that Krefeld provided, controlling for things like the effects of nearby plants, weather, and forest cover on fluctuations in insect populations.
The final study looked at 63 nature preserves, representing almost 17,000 sampling days, and found consistent declines in every kind of habitat they sampled. This suggested, the authors wrote, “that it is not only the vulnerable species but the flying-insect community as a whole that has been decimated over the last few decades.”
The current worldwide loss of biodiversity is popularly known as the sixth extinction: The sixth time in world history that a large number of species have disappeared in unusually rapid succession, caused this time not by asteroids or ice ages but by humans.
What we’re losing is not just the diversity part of biodiversity, but the bio part: Life in sheer quantity. While I was writing this article, scientists learned that the world’s largest king penguin colony shrank by 88% in 35 years, that more than 97% of the bluefin tuna that once lived in the ocean are gone.
Finding reassurance in the survival of a few symbolic standard-bearers ignores the value of abundance, of a natural world that thrives on richness and complexity and interaction.
Tigers still exist, for example, but that doesn’t change the fact that 93% of the land where they used to live is now tigerless. This matters for more than romantic reasons: Large animals, especially top predators like tigers, connect ecosystems to one another and move energy and resources among them simply by walking and eating and defecating and dying.
One result of their loss is what’s known as trophic cascade, the unravelling of an ecosystem’s fabric as prey populations boom and crash and the various levels of the food web no longer keep each other in check.
Scientists have begun to speak of functional extinction. Functionally extinct animals and plants are still present but no longer prevalent enough to affect how an ecosystem works.
Some phrase this as the extinction not of a species but of all its former interactions with its environment — an extinction of seed dispersal and predation and pollination and all the other ecological functions an animal once had, which can be devastating even if some individuals still persist.
The more interactions are lost, the more disordered the ecosystem becomes. A 2013 paper in Nature, which modelled both natural and computer-generated food webs, suggested that a loss of even 30% of a species’ abundance can be so destabilising that other species start going fully, numerically extinct.
In addition to extinction (the complete loss of a species) and extirpation (a localised extinction), scientists now speak of defaunation: The loss of individuals, the loss of abundance, the loss of a place’s absolute animalness.
In a 2014 article in Science, researchers argued that the word should become as familiar, and influential, as the concept of deforestation. In 2017, another paper reported that major population and range losses extended even to species considered to be at low risk for extinction.
They predicted “negative cascading consequences on ecosystem functioning and services vital to sustaining civilisation” and offered another term for the widespread loss of the world’s wild fauna: “Biological annihilation.”
Scientists have tried to calculate the benefits that insects provide. Trillions of bugs flitting from flower to flower pollinate some three-quarters of our food crops, a service worth as much as €500bn every year. (This doesn’t count the 80% of wild flowering plants, the foundation blocks of life everywhere, that rely on insects for pollination.)
If monetary calculations like that sound strange, consider the Maoxian Valley in China, where shortages of insect pollinators have led farmers to hire human workers, at a cost of up to $19 per worker per day, to replace bees. Each person covers five to 10 trees a day, pollinating apple blossoms by hand.
By eating and being eaten, insects turn plants into protein and power the growth of all the uncountable species — including freshwater fish and a majority of birds — that rely on them for food, not to mention all the creatures that eat those creatures.
We worry about saving the grizzly bear, says the insect ecologist Scott Hoffman Black, but where is the grizzly without the bee that pollinates the berries it eats or the flies that sustain baby salmon? Where, for that matter, are we?
Bugs are vital to the decomposition that keeps nutrients cycling, soil healthy, plants growing, and ecosystems running. When asked to imagine what would happen if insects were to disappear completely, scientists find words like chaos, collapse, Armageddon.
Wagner describes a flowerless world with silent forests, a world of dung and old leaves and rotting carcasses accumulating in cities and roadsides, a world of “collapse or decay and erosion and loss that would spread through ecosystems” — spiralling from predators to plants.
EO Wilson has written of an insect-free world, a place where most plants and land animals become extinct; where fungi explodes, for a while, thriving on death and rot; and where “the human species survives, able to fall back on wind-pollinated grains and marine fishing” despite mass starvation and resource wars.
In October, an entomologist sent me an email with the subject line, “Holy [expletive]!” and an attachment: A study just out from Proceedings of the National Academy of Sciences that he labelled, “Krefeld comes to Puerto Rico.”
The study included data from the 1970s and from the early 2010s, when a tropical ecologist named Brad Lister returned to the rain forest where he had studied lizards — and, crucially, their prey — 40 years earlier.
Lister set out sticky traps and swept nets across foliage in the same places he had in the 1970s, but this time he and his co-author, Andres Garcia, caught much, much less: Tenfold to sixtyfold less arthropod biomass than before.
Even scarier were the ways the losses were already moving through the ecosystem, with serious declines in the numbers of lizards, birds, and frogs. The paper reported “a bottom-up trophic cascade and consequent collapse of the forest food web”.
Like other species, insects are responding to what Chris Thomas, an insect ecologist at the University of York, has called “the transformation of the world”: not just a changing
climate but also the widespread conversion, via urbanisation, agricultural intensification, and so on, of natural spaces into human ones, with fewer and fewer resources “left over” for non-human creatures to live on. What resources remain are often contaminated.
Hans de Kroon characterises the life of many modern insects as trying to survive from one dwindling oasis to the next but with “a desert in between, and at worst it’s a poisonous desert”.
Of particular concern are neonicotinoids, neurotoxins that were thought to affect only treated crops but turned out to accumulate in the landscape and to be consumed by all kinds of nontargeted bugs. People talk about the “loss” of bees to colony collapse disorder, and that appears to be the right word: Affected hives aren’t full of dead bees, but simply mysteriously empty. A leading theory is that exposure to neurotoxins leaves bees unable to find their way home.
Even hives exposed to low levels of neonicotinoids have been shown to collect less pollen and produce fewer eggs and far fewer queens. Some recent studies found bees doing better in cities than in the supposed countryside.
Since the Krefeld data emerged, there have been hearings about protecting insect biodiversity in the German Bundestag and the European Parliament. EU member states voted to extend a ban on neonicotinoid
pesticides and have begun to put money towards further studies of how abundance is changing, what is causing those changes and what can be done.
Stemming insect declines will require much more, however. The EU already had some measures in place to help pollinators — including more strictly regulating pesticides than the US does and paying farmers to create insect habitats by leaving fields fallow and allowing for wild edges alongside cultivation — but insect populations dropped anyway.
New reports call for national governments to collaborate; for more creative approaches such as integrating insect habitats into the design of roads, power lines, railroads and other infrastructure; and, as always, for more studies.
The necessary changes, like the causes, may be profound. “It’s just another indication that we’re destroying the life-support system of the planet,” Lister says of the Puerto Rico study. “Nature’s resilient, but we’re pushing her to such extremes that eventually it will cause a collapse of the system.”
Scientists hope that insects will have a chance to embody that resilience. A ghost moth in Australia was once recorded laying 29,100 eggs, and she still had 15,000 in her ovaries. The fecund abundance that is insects’ singular trait should enable them to recover, but only if they are given the space and the opportunity to do so.