A sea lion slumps
on its back, belly and neck exposed. A wildlife worker swabs another’s nose;
curled into a comma, the emaciated animal screeches in protest. In another shot
of Reuters B-roll, humans in hazmat suits shovel a shoreline grave under
colorless skies, sprinkling a red mass of carcasses with chemical powder before
sealing the burial with dark wet sand.

The numbers are
as bleak as the footage: More than 3,400 sea lions sickened and dead of the H5N1
variant of avian influenza in Peru this winter and spring. With the animal
weighing, on average, about 770 pounds, each of those infected corpses threatens to further pulse the pathogen along the Peruvian coastline, according to researchers there.

Their scale—that
of carnage—seems to speak to our modern ecological anxiety, rage, and grief; mass
mortality events are material evidence of anthropogenic apocalypse, an
unignorable and immediate tally of the ravages we’ve sown. And they will likely
become more common as heat waves, droughts, disease outbreaks, storms, fires, and other environmental disturbances grow more frequent and deadlier. News
coverage of die-offs, however, fails to acknowledge all we don’t understand
about mass death. In reality, we’re nowhere close to grasping the repercussions
these cascades of death have on ecosystems. “We’re still a far way away from
having a firm view,” Fey said. “As a field we know very little about these
events.”

Each year,
millions of salmon spawn, stop eating, rot alive, and dissolve away.
When billions of cicadas emerge
from the ground, their remains fertilize that same soil a few weeks later. A
masting beech tree can carPuppy a square meter with 500 seeds, almost none of which will get the
chance to sprout and take life. Nature is full of episodes of mass death; not
all are devastating mass mortality events.

The kind of mass mortality events worrying scientists are anomalous punctuations—they are not an
evolved and recurring dynamic in a creature’s life history (as the examples
above are). They also, for the most part, don’t discriminate. “It’s everything: top
of the food chain all the way down,” University of Arkansas biologist Simon Tye
told me. “It’s just a full-on decimation of the population.” Most
characteristically, these occurrences are dramatic. Experts have described them as single events that
wipe out huge chunks of a population, kill more than a billion individuals, or
leave behind 700 million tons of dead tissue (a mass equivalent to one million
Christ the Redeemer statues).

In the last
decade or so, mass mortality events worldwide have included the loss of 450-plus elephants in Botswana, 18,000-plus migratory birds in India’s Sambhar
Lake, 350 Magellanic penguins in Argentina, hundreds of emaciated gray whales along the
Pacific coast, and 99.9 percent  of Spain’s fan mussels. Five thousand dead red-wing blackbirds rained down in
Arkansas, and 45,000 flying foxes hung from trees and piled
on the ground in Australia. Lightning struck down a huddle of 300
Norwegian reindeer. Billions of starfish melted into goop.

In 2022 alone,
many die-offs made themselves known by washing ashore: 2,500 endangered seals on the Caspian Sea’s
Russian shores, hundreds of seabirds on ice and shoreline in
Newfoundland, more than 2,250 trout and salmon along Ireland’s Glenagannon
River, thousands of crabs and lobsters along
England’s River Tees, 60 dolphins on Bulgaria’s Black Sea coast.

For some, the
culprit was clear. Microscopic algae smothered aquatic life to death in the San
Francisco Bay. Drought killed 512 wildebeests, 430 zebras, 205
elephants, 51 buffaloes, and 12 giraffes in Kenya. Starvation wasted away the hundreds of flightless little
blue penguins that washed up on New Zealand beaches at half their typical
weight.

Compressed and
compiled, these death tolls collapse into senselessness; it’s a familiar
feeling to the modern writer or reader. “While statistical shock and awe is
abundant, we are often unable to grasp the true meaning of such figures—stymied
by basic
innumeracy, the incomprehensible scale of our present crises, and the profound mismatch between hard data and human feeling,”
Eleanor Cummins wrote
of Covid-19 for this magazine in 2020. But a careful look at any mass mortality
event can restore its contours. Take the hundreds
of Cape fur seal carcasses that washed up on Namibia’s coastline early last
year. On a single February day, more than 400 appeared along the water’s edge.
In one photograph taken by biologists during the aftermath, a dark, wet seal
pup lies as flat as a slipper near a tape measure indicating it never grew to
be longer than 75 centimeters long; in another, 14 dead seals are spread on the
beach in a lonely colony of dark, lifeless curves. “It’s been mentally and
emotionally taxing,” a conservationist said at the time. “It truly feels like a war zone
out there.”

Unfortunately,
experts say it’s fair to categorize mass mortality events as a phenomenon on
the historic upswing. “They’re happening more often, and more individuals are
dying each time they happen,” Tye said. In 2015, Fey and his colleagues
reviewed more than 7,000 mass mortality events since 1940 and found that reported die-offs have, indeed, become more
common for birds, fish, and marine invertebrates. The scientists attributed one
in four mass mortality events to disease (like the avian flu), one in five to
human perturbation (like contamination), and about one in six to biotoxicity
(like harmful algae blooms). One in four, they found, was directly influenced
by climate: weather, heat stress, oxygen stress, starvation. Other research on
North American freshwater lakes and the Mediterranean
Sea demonstrates a link between rising environmental
temperatures, extreme heat events, and die-offs.

Often during mass
mortality events, a tangle of stressors will ensnare a vulnerable species in
crisis, experts say. Warming waters, for example, combined with nitrogen pulses
from wastewater pollution or stormwater runoff can fuel harmful algal blooms,
which asphyxiate and suffocate life. Heat waves and droughts set the stage
together for wildfires, which can then leave a landscape vulnerable to
flooding. Warmer conditions can abet pathogenic spread or upset delicate
microorganism-host relationships.

It’s this
cascading effect that has some scientists worried. “One catastrophe makes it
more likely that you’ll suffer a second or third,” said Christopher Harley, a
University of British Columbia zoologist. And every devastation leaves an
ecosystem more vulnerable for the next. “It might take years to get back up
from that loss.”

Welcome to the necrobiome,
the ecosystem of carcasses. In the necrobiome, dead matter is not inert—it is a
base unit of life: Carrion beetles pilfer, maggots feast, seeds scatter,
raccoons scavenge, vultures peck, and microbes bloom.  “If you zoom in to the microbial level, life is
exploding. It’s multiplying, it’s diversifying,” Jeffery Tomberlin, a professor
in entomology at Texas A&M, told me. “It’s a beautiful thing.”

But our
understanding of decomposition largely comes from single-carcass studies. Those
fundamental dynamics, experts warn, might collapse under the sudden appearance
of a thousand times more carrion than an ecosystem is designed to
recycle—as in the case of a mass mortality event. That could lead to carcasses
mummifying instead of breaking down, pathogens seeping into an ecosystem’s soil
and water, and oxygen-starved dead zones. Hypothetically, not certainly. When
it comes to mass mortality events, Harley said, “there’s a lot of blank areas
on our ecological map.”

With good reason: Die-offs can be surprisingly cryptic in nature and exquisitely onerous to
analyze in practice.

Because mass
mortality events are inherently unpredictable in time and space, we often lack
a given site’s baseline demography from before it’s wiped out. (You can’t,
after all, monitor an area anticipating catastrophe.) So we struggle to
accurately capture the exact extent of devastation. An event’s suddenness also
challenges researchers to assemble as quickly as possible to gather postmortem
data. “It’s hard to be in the right place at the right time with the right
equipment,” Tye said. What’s more, some die-offs are impossible to detect at
all. In oceans, dead organisms sink to depths beyond observation; in the
tropics, they more speedily decompose out of sight.

Elsewhere,
scientists might try to understand a phenomenon by replicating it and observing
it in a controlled setting. “At no point are you ever going to do that” for
mass die-offs of wild animals, Tomberlin said. “It’s just not on the table.”
Slaughtering and studying domesticated livestock—arguably, a more practical approach—would
fail
to shed an accurate light on natural decomposition processes, given the
industrial antibiotic regimens that fundamentally alter the animals’ microbiomes,
and therefore their necrobiomes.

Some researchers
have found workarounds. In 2016, for example, a team of wildlife biologists at
Mississippi State University acquired 6,000 pounds of invasive feral swine
carcasses from the Department of Agriculture and other state and federal agencies with removal
programs. They left the carrion in forest plots to rot and observed the
dramatic metamorphosis that ensued. A writhing, six-inch-deep, 30-foot-long stream of blowfly maggots flooded the
carcasses, carrying away scientific equipment and heralding the arrival of
thousands of flies, beetles, spiders, hornets, armadillos, lizards, and
vultures. To collect microbial data during decomposition, researchers waded
through a “muck
and soup and slime” of corpse
wax. The soil’s new chemistry—weighted with excesses of nitrogen, gases,
and acidic body fluids—killed off grasses and trees, opening up the forest
canopy and shedding more sunlight on its floor.

In 2019, the MSU
team dumped nearly 15 tons—30,000 pounds, or about
200 bodies—of donated feral hog carcasses in Oklahoma prairie grasslands. They discovered
that grasses in sites where they left a single hog seemed to bounce back,
fueled by the carcass’s recycled nutrients. But in plots where scientists
discarded 10 or more cadavers at once, the effect was poisonous: Flora stayed
brown and dead for months. The soil microbiome lost key fungal and bacterial species;
in some patches, surrounding trees died off.

“If you push a
system, it ultimately will collapse or kick back,” said Richard Kock, a retired
professor of wildlife health and emerging disease at the Royal Veterinary
College in London. But which one will happen is impossible to predict.

The summer of
2021 was a morbid one across North America’s western coast, as temperatures
rose above 100 degrees (and up to a record-breaking 121 degrees
in British Columbia). More than a thousand
people
died. Crops roasted
and withered. Wildfires raged, and glaciers melted. In Seattle, juvenile
terns tried to flee rooftop nests for cooler cover and died, plunging to the burning asphalt pavement
below. Along the shoreline, billions of
marine invertebrates—barnacles, mussels, oysters, clams, gastropods, crabs, sea
stars—cooked alive. On a single, 100-meter stretch of shoreline, the heat struck one million bay mussels dead; on another, it killed 10 million barnacles. “It was
so insufferably hot that things couldn’t handle it,” Harley said. “Anyone who
lived near the coast knew something was going wrong because the breeze off the
shore was awful.”

For more than a
month, Harley and his colleagues functioned as coroners, documenting and
assessing the heat dome’s aquatic victims, many of which remained for weeks physically
attached to the shore where they had (until their deaths) provided housing for
crabs, sea cucumbers, and worms. It was supremely bleak work, he said.

But today,
photographs document what happened to the landscape after the grizzly die-off
receded: Tens of thousands of new mussels and barnacles have sprung up in the
void. “They’re back already,” Harley said. And by carefully studying the mass
mortality event’s scale and scope, scientists hope to bolster the shoreline’s survivability. They’ve concluded that marine protected environments should prioritize north-facing and complex rock faces as well as organisms like mussels and seaweeds, all of which could provide protective shade for sensitive species in future heat waves.

Kazakhstan’s
saiga offer another tale of resilience in the face of massive mortality. During
an uncommonly hot and humid May, more than 200,000 of the critically endangered
antelope—about 60 percent of the world’s population—dropped dead all at once.
The soupy weather conditions had turned a microbe in the animal’s snout, the normally
co-habitating bacterium Pasteurella multocida, virulent and deadly. But
in studying the saiga fatalities, researchers discovered some remaining healthy
herds outside the killing’s envelope; they’ve surpassed a population of one million in the eight years since the die-off. “That small population has reestablished
itself,” said Kock, who studied the mass mortality event and its fallout.
“There is resilience.”

And yet any
resilience, in the context of mass death, feels like a paltry consolation
prize. “Everything’s just kind of hitting the fan at once,” Tye told me. “I’m
honestly just kind of ashamed that we’ve gotten to this point.”

About 70 percent
of Americans are worried about climate change, according to a
2021 Yale and George Mason University survey. Photos and stories of die-offs
confirm that that anxiety is well founded—and tempt news writers with a
tangible, legible manifestation of the biological loss looming all around us.

We see the same
dynamic with the loss of human life; sudden catastrophes like wildfires and
earthquakes get far more coverage than equally deadly threats that are harder
to capture in a single image or headline, like the seven million
lives lost each year to air pollution. How, for example, would we
effectively convey that the banalities of modernity—motor vehicles, industrial
output, urban smog, and indoor smoke—have forced 99 percent of the global
population to breathe in air that dangerously exceeds international guidelines for
pollutants? The threat is too abstract for imagination alone to grasp.

When it comes to
biodiversity, animal mass mortality events offer us the regalement of war
stories in our narrative stalemate with climate change. That’s useful,
perhaps—but we lose nuance as a result. Stories about die-offs, as Tye pointed
out, usually hyperfocus on the gore of specific events. “I wish more [mass mortality event] articles talked about how everything is crumbling,” he told me.