‘Half the
tree of life’: ecologists’ horror as nature reserves are emptied of insects
A new point
in history has been reached, entomologists say, as climate-led species’
collapse moves up the food chain even in supposedly protected regions free of
pesticides
Tess McClure
Tess McClure
Tue 3 Jun
2025 08.00 BST
Daniel
Janzen only began watching the insects – truly watching them – when his ribcage
was shattered. Nearly half a century ago, the young ecologist had been out
documenting fruit crops in a dense stretch of Costa Rican forest when he fell
in a ravine, landing on his back. The long lens of his camera punched up
through three ribs, snapping the bones into his thorax.
Slowly, he
dragged himself out, crawling nearly two miles back to the research hut. There
were no immediate neighbours, no good roads, no simple solutions for getting to
a hospital.
Selecting a
rocking chair on the porch, Janzen used a bedsheet to strap his torso tightly
to the frame. For a month, he sat, barely moving, waiting for his bones to knit
back together. And he watched.
In front of
him was a world seething with life. Every branch of every tree seemed to host
its own small metropolis of creatures hunting, flying, crawling, eating. The
research facility lay in a patchwork of protected rainforest, dry forest, cloud
forest, mangroves and coastline covering an area the size of New York, and
astonishingly rich in biodiverse life. Here, the bugs gorged, coating the leaf
litter with a thick carpet of droppings.
But the real
show was at night: for two hours each evening, the site got power and a 25-watt
bulb flickered on above the porch. Out of the forest darkness, a tornado of
insects would flock to its glow, spinning and dancing before the light. Lit up,
the side of the house would be “absolutely plastered with moths – tens of
thousands of them”, Janzen says.
Inspired, he
decided to erect a sheet for a light trap with a camera – a common way to
document flying insect numbers and diversity. In that first photograph, taken
in 1978, the lit-up sheet is so thickly studded with moths that in places the
fabric is barely visible, transformed into what looks like densely patterned,
crawling wallpaper.
Scientists
identified an astonishing 3,000 species from that light trap, and the
trajectory of Janzen’s career was transformed, from the study of seeds to a
lifetime specialising in the forest’s barely documented populations of
caterpillars and moths.
Now 86,
Janzen still works in the same research hut in the Guanacaste conservation
area, alongside his longtime collaborator, spouse and fellow ecologist, Winnie
Hallwachs. But in the forest that surrounds them, something has changed. Trees
that once crawled with insects lie uncannily still.
The hum of
wild bees has faded, and leaves that should be chewed to the stem hang whole
and un-nibbled. It is these glossy, untouched leaves that most spook Janzen and
Hallwachs. They are more like a pristine greenhouse than a living ecosystem: a
wilderness that has been fumigated and left sterile. Not a forest, but a
museum.
Over the
decades, Janzen has repeated his light traps, hanging the sheet, watching for
what comes. Today, some moths flutter to the glow, but their numbers are far
fewer.
“It’s the
same sheet, with the same lights, in the same place, looking over the same
vegetation. Same time of year, same time of the moon cycle, everything about it
is identical,” he says. “There’s just no moths on that sheet.”
Crumbling
populations
The declines
witnessed by Janzen – and described by others around the world – are part of
what some ecologists call a “new era” of ecological collapse, where rapid
extinctions occur in regions that have little direct contact with people.
Reports of
falling insect numbers around the world are not new. International reviews have
estimated annual losses globally of between 1% and 2.5% of total biomass every
year.
Widespread
use of pesticides and fertilisers, light and chemical pollution, loss of
habitat and the growth of industrial agriculture have all carved into their
numbers. Often, these were deaths of proximity: insects are sensitive
creatures, and any nearby source of pollution can send their populations
crumbling.
But what
Janzen and Hallwachs are witnessing is a part of a newer phenomenon: the
catastrophic collapse of insect populations in supposedly protected regions of
forest. “In the parts of Costa Rica that are heavily hit by pesticides, the
insects are completely wiped out,” Hallwachs says.
Run that
forward four decades, that’s nearly half the tree of life disappearing in a
lifetime … catastrophic
David
Wagner
“But what we
see here in the preserved areas – that as far as we can tell, are free of even
these destructive insecticides and pesticides – even here, the insect numbers
are going down horrifyingly dramatically,” she says.
Long-term
data for insect populations – particularly less charismatic species – is still
patchy, but Janzen and Hallwachs join a number of scientists that have recorded
huge die-offs of insects in nature reserves around the world.
They include
in Germany, where flying insects across 63 insect reserves dropped 75% in less
than 30 years; the US, where beetle numbers dropped 83% in 45 years; and Puerto
Rico, where insect biomass dropped up to 60-fold since the 1970s. These
declines are occurring in ecosystems that are otherwise protected from direct
human influence.
When David
Wagner stepped out into the US’s southern wilderness this spring, he found
landscapes emptied of life. The entomologist has devoted much of his career to
documenting the vast diversity of US insect life, particularly rare
caterpillars. He traverses the country to find specimens, often on long road
trips searching for caterpillars by day and moths by night.
Now, he
finds himself coming home empty-handed. “I just got back from Texas, and it was
the most unsuccessful trip I’ve ever taken,” he says. “There just wasn’t any
insect life to speak of.”
It was not
only the insects missing, he says, it was everything. “Everything was crispy,
fried; the lizard numbers were down to the lowest numbers I can ever remember.
And then the things that eat lizards were not present – I didn’t see a single
snake the entire time.”
Wagner
recalls when a series of international reviews began hitting headlines in 2019,
saying global insect biomass was declining at a rate of 1% a year (although
some estimates put it as high as 2.5%).
“We
[entomologists] were thinking conservatively,” he says, looking at the data
that has emerged in the five years since then.
“I now think
that that’s too low. Now I would say that 2% is happening in some areas, and
we’re seeing some places threatened by climate change or urbanisation or
agriculture get as high as 5% decline per year.”
Neat lines
in a display case of dead beetles with shiny green abdomens
A few
percentage points a year may not have the ring of disaster. “But if you run
that forward just four decades,” Wagner says, “we’re talking about nearly half
the tree of life disappearing in one human lifetime. That is absolutely
catastrophic.”
Developing a
clear picture of how many insects we have lost is complicated by a lack of
baseline data for many species: while some eye-catching insects, such as
butterflies, have been collected and monitored for decades, others have been
mostly ignored.
And within
the overall declines, the picture is not homogeneous: populations and losses
vary by species, by location, by habitat. The same heat that destroys the
living conditions of one butterfly, for example, could expand the range of a
mosquito or help a cricket species thrive.
“No matter
what we do in nature, there will be winners and losers,” Wagner says. “But we
are seeing a lot of losers.”
And those
who doubt there is sufficient species data to prove the “insectageddon” can now
track it by proxy, Wagner says: via the sharp declines in birds, lizards and
other creatures that depend on them for food.
Two men
crouch down on a canoe on a waterlily-strewn body of water. More people on
another canoe can be seen behind them.
Scientists
in the US, Brazil, Ecuador and Panama have now reported the catastrophic
declines of birds in “untouched” regions – including reserves inside millions
of hectares of pristine forest. In each case, the worst losses were among
insectivorous birds.
At one
research centre – falling within a 22,000-hectare (85 sq mile) stretch of
intact forest in Panama – scientists comparing current bird numbers with the
1970s found 70% of species had declined, and 88% of these had lost more than
half of their population.
When I
arrived here in 1963 the dry season was four months. Today, it is six months
Daniel
Janzen
In 2019,
researchers found that almost a third of US birds – about 3 billion – had
disappeared from the skies since the 1970s. The losses, however, were not
evenly distributed: those birds that ate insects as their main food had
declined by 2.9 billion. Those that didn’t depend on insects had actually
gained, increasing by 26 million.
More recent
research from the US found a decline in three-quarters of nearly 500 bird
species studied – with the steepest downward trend in stronghold areas, where
they once thrived.
In Puerto
Rico’s Luquillo rainforest, scientists in 2018 mapped how the loss of insects
set other dominoes falling: as bugs declined, so too did the populations of
lizards, frogs and birds. Their disappearance, they wrote, had triggered “a
bottom-up trophic cascade and consequent collapse of the forest food web”.
In Costa
Rica, Janzen described the fall in numbers of insectivorous birds in the
reserve as “cratering”. A colony of about 20 nectar-eating bats have long
nested in the dark nooks of Janzen and Hallwachs’ house, but Janzen has noticed
the flowers they used to feed from are now failing to bloom.
Hallwachs
began to find their small, emaciated bodies lying on the floor. “Over a period
of five days, I found three of these bats dead,” she says. Researchers at
another site 20 miles away told her they were witnessing the same thing.
Out of
sync
Behind the
steepening declines, a clear culprit is beginning to emerge: global heating. A
tropical forest ecosystem is “a finely tuned Swiss watch”, Hallwachs says –
perfectly engineered to sustain a vastly biodiverse system of creatures.
Each element
is delicately tuned and interlocks with the rest: the heat, the humidity, the
rainfall, the unfolding of leaves, the length of the seasons, the start and
stop of the life cycles of insects and animals.
With each
incremental turn of one cog, the rest of the system responds. Insects and
animals have evolved to time their hibernations and breeding times precisely to
small signals from the system: a change in humidity, a lengthening of the light
hours of the day, a small rise or fall in temperature.
But now, the
system has one gear spinning wildly out of time: the climate.
“When I
arrived here in 1963 the dry season was four months. Today, it is six months,”
Janzen says. Insects that typically spend four months underground, waiting for
the rains, are now forced to try to survive another two months of hot, dry
weather. Many are not succeeding.
The major
drivers of biodiversity losses were land degradation and habitat loss … Now
climate change is by far exceeding that
David
Wagner
Alongside
the changing seasons are other shifts, such as in rainfall or humidity. “It’s
just a general disruption of all the little cues and synchronies that would be
out there,” Janzen says. Across the entire clock of the forest, plants and
creatures are falling out of sync. In the background, the temperature is
rising.
“The killer
– the cause that’s pulling the trigger – is actually water,” says Wagner. For
insects, staying hydrated is a unique physiological challenge: rather than
lungs, their bodies are riddled with holes, called spiracles, that carry oxygen
directly into the tissue.
“They’re all
surface area,” says Wagner. “Insects can’t hold water.” Even a brief drought
lasting just a few days can wipe out millions of humidity-dependent insects.
Some
ecologists now believe these declines could mark a new era in which the
changing climate overtakes other forms of human damage as the biggest driver of
extinction.
“We’re at
a new point in human history,” Wagner says. Up until the last decade, “the
major drivers of biodiversity losses around the planet were really land
degradation and land loss, habitat loss. But I think now that climate change is
by far exceeding that.”
Losing
hope
Last month,
the journal BioScience published new research examining how the five biggest
drivers of biodiversity loss were affecting the US’s endangered creatures. For
the first time – albeit by a very slim margin – the climate crisis emerged in
front, driving the decline of 91% of imperilled species.
Heat-driven
declines could have repercussions far beyond their immediate surroundings. In
the past, even if pesticides wiped out insects over an agricultural region, as
long as healthy populations remained elsewhere, species could return if the
spraying stopped.
“Climate
change is impacting all those different little spots at the same time. It
doesn’t just affect one particular spot that gets a pesticide dose or gets a
tree cut down,” Janzen says. “If the insect population collapses and it happens
everywhere, you don’t have a residual population.”
Today, as
well as being an ecologist Wagner feels he has taken on a second role – as an
elegist for disappearing forms of life.
“I’m an
optimist, in the sense that I think we will build a sustainable future,” Wagner
says. “But it’s going to take 30 or 40 years, and by then, it’s going to be too
late for a lot of the creatures that I love. I want to do what I can with my
last decade to chronicle the last days for many of these creatures.”
Decades on
from his months spent bound to the rocking chair, Janzen still watches. He
records the yearly data, the shifts in dominant species. But today, there is so
much less to see. Once, when he and Hallwachs would type up their notes in the
night, they would pitch a tent in the living room to protect their computers
from thousands of moths that flocked to the blue glow. Now, they work with the
house open to the forest air. “I find myself saying, ‘Winnie! A moth has
arrived at the light on my laptop,’” Janzen says. “One moth.”
Elsewhere in
their profession, some scientists are starting to look away. “We know quite a
number of entomologists who have experience dating back to the 70s, 80s or
90s,” Hallwachs says. “One of our very good friends – he now does not have the
emotional courage to hang up a sheet to collect moths at night. It is too
devastating to see how few there are.”
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