Traveling through time was as easy as taking the stairs. Darkness swallowed up the world behind us, and darkness waited below; through it I could make out mottled brown walls of stone and hard-packed earth, the timeline of strata, illuminated by the ambient glow shed by our collective headlamps. We were here for a survey of the resident bat population in the final stretch of its annual hibernation. But it was no routine survey: we had to search for a bat-killing fungus that was spreading throughout the northeastern quarter of the United States—an event as significant as these prehistoric layers of earth.
In my final season as a naturalist at one of Wisconsin’s show caves (natural caves in which the owners or government conduct tours for guests), I was invited to observe the annual winter bat survey conducted by the Department of Natural Resources. Caves are one of the few kinds of places left on Earth where people can view wild animals up close. As a naturalist, it had been my job for two summers now (and would be over the next) to facilitate that connection between people and nature. The bats needed to be checked on every winter during hibernation to monitor their continued health.
The cave owner and I wore our regular caving gear: old clothing supplemented by boots, gloves, and headlamps from our caving area, while the DNR team—trying to reduce contaminants to and from this location—barricaded themselves in plastic coveralls, complete with booties and gloves. They bore clipboards and spreadsheets, sampling kits, and handheld black-lights.
As they prepared, conversation turned to the recent spread of white-nose syndrome, the deadly bat disease, through Wisconsin. The leaders of this survey had already witnessed firsthand its ravages.
“We’ve lost two sites this winter. Came to do the survey, and no bats,” one team member said.
“We’ll be doing a site in the southwest part of the county next. This is their second year, might not be any bats there, either,” another continued. The cave owner nodded knowingly. I crept up next to him, alarmed.
“Really? It’s in this county already?” I asked. I hadn’t thought it to be so close.
“Yep. It’ll be here soon,” the owner replied.
Concerned but also excited to get started, the team fell into more cheerful moods upon our descent.
The space around us was wide and open, with a ceiling height of close to fifteen feet. Treated wooden railings, pungent in the damp air, guarded paths between the levels: from the bedroom-size platform that was the first, to the balconies of the second, and down into the maze of the third.
On our descent, I checked the reactions of the team, realizing that none of them had been here recently, and some had never been; I noted with satisfaction their craned necks and sweeping beams. I felt the same emotion I always had with tourists: I wanted them to love this place like I did. Unlike many of the tourists, they adored the bats.
Headlamps as the only light source rendered this familiar place new again. I took some time to scan the stone.
Walls of mud-crusted, tan dolostone surrounded us, climbing to a domed ceiling striped with green-gray illite clay and rusty-red iron ore. Tiny pockets of drusy quartz crystals glittered where our beams fell on the walls, and layers of waxy gray flint stood out in the beige. Every surface glistened with moisture. We had to pick out, in all this brown, the tiny, fluffy specks that were hibernating bats.
Though the team had to talk in whispers to maintain the quiet peace of the cave, there were punctuations of muffled laughter and the rustling of plastic as they gestured animatedly. We started locating bats—more than I had ever seen at once in the cave before—immediately, scattered across the walls and ceiling. The team called out numbers of bats, whether they were individual or in clusters, and their species. A rhythm of spotting, hushed description, and scribbling followed by crunching footsteps settled on us. Before long, it was time for us to head down to the third level—a mile-long maze of tunnels about seventy feet below the surface.
The bedrock beneath this part of the country forms what is known as a karst landscape. Composed of two bands of limey dolostone with a sandwiched layer of sandstone, it is particularly susceptible to dissolution by acidic water, and thus, ideal for the formation of caves.
I reflected on the history of the rock the bats were using. During the Ordovician Period between 488 and 444 million years ago, before the time of the supercontinent Pangaea, before dinosaurs, before reptiles and mammals even, the Midwest sat just south of the Equator, flooded by shallow ocean waters off the shores of the pre-North American continent of Laurentia. The warm seas were rich with life, much of which grew calcium carbonate shells. Organisms like coral, calcareous plankton, and prehistoric snails and shellfish lived and died, and their shells settled in thick layers on the seabed.
Over time—millions of years—the layers built, deeper layers were compressed by the weight above, and the calcium-rich mud hardened to solid rock. At one point, the waters receded and sand grains from the shore built up instead of the shells. The ocean returned, but after long enough a time that a band of gritty, thin-layered sandstone built up between sections of limestone. This limestone slowly transformed into magnesium-rich dolostone as groundwater introduced magnesium into the calcium carbonate molecules. These three layers—the Oneota Dolomite, New Richmond Sandstone, and Shakopee Formation—make up the Prairie du Chien Group of bedrock found in Wisconsin, Minnesota, Illinois, Michigan, and Indiana.
All told, it’s a chronicle of about forty million years and over fifty feet of thickness, waiting beneath the earth. The long story of the Ordovician Period ocean isn’t particularly exciting—at least not until the end—but it is geologic in magnitude: something we can study and parse out in cliff sides and cave depths in the upper Midwest.
The DNR team passed the story by, intent on its own present task. As this was a show cave, our route into the third level was made up of wooden stairs and carpeted ramps, and the path on the level itself was mostly pea gravel pockmarked with puddles. This far below the ground, the temperature was fifty degrees Fahrenheit despite the chillier early-March weather. Steady drips of acidic water accompanied us, glistening on pale flowstone and tiny stalactites in the upper margins of the hallway-size passages.
The team searched for four species of cave bats known to populate the area: little brown bats (Myotis lucifugus), tricolored bats (Perimyotis subflavus, also called Eastern pipistrelles), northern long-eared bats (Myotis septentrionalis), and big brown bats (Eptesicus fuscus). The researchers had cute nicknames for them like “lucy,” “pip,” and “seps,” when we spotted them. Little browns were the most common; we found them frequently in furry clusters on the ceiling. Tricoloreds were frequent tiny puffs squeezed into cracks and pockets in the rock. At only about two to three inches long, they are the region’s smallest bat. It was unlikely we’d find any big brown bats: larger than the others, they can tolerate colder temperatures and so usually remain closer to the entrances of caves (where the temperature fluctuates according to the wider environment rather than remaining constant).
Other northern bat species are travelers, migrating south every winter like many birds do, but these four are more like other mammals in their winter behavior: they hibernate. They eat insects throughout most of the year (up to a few thousand per night) and build up a plentiful storage of body fat. As the weather cools with autumn, they find winter hibernacula like caves and abandoned mines where temperatures remain relatively constant and, after finding safe perches, slow down their metabolisms. To survive all winter on only body fat, they lower their heart rates, breathing rates, and body temperatures.
It is this last that leaves them vulnerable to the recent fungal scourge of hibernating bats in North America: white-nose syndrome (WNS). It pressed at the back of my mind as we searched, but it didn’t seem immediate. No one found the telltale fuzzy white patches on noses or wings. The team continued.
Other animals once called this place home, and like the bats, we could still find them here—albeit as fossils. We made our way from the hallway into a new room, differentiated from the others first by the spurs of rock that jutted from the ceiling at head level, but more obviously by the poster resting on a ledge on one end of the room. An artist’s rendition of the Ordovician Period graced the poster: seabed dotted by invertebrate life just vaguely familiar enough to feel like déjà vu. I made out the naturalists’ fossil examples: a large chunk of a squid-like, predatory cephalopod’s cone-shaped shell, about a foot long and six inches wide. I saw our pill bug-like trilobite buried in shadow below the poster. Tiny rings and pencil-shaped shafts in some of the rocks belonged to the stems of crinoids, creatures that look like spindly flowers but are actually echinoderms, the group which boasts sea stars. I also spotted the tight spirals of gastropods, prehistoric snail relatives, gleaming from the floor and ceiling.
It was fascinating to think that the two varieties of creatures that had lived in this place were so different: marine invertebrates and aerial mammals, divided by a vast gulf of time. I was responsible for helping people learn about both. I had seen bats frequently in this very room, hiding on the ceiling above the fossils. I didn’t have time to point out every bat in every room, so in the room of fossils, bats had always been little secrets of the naturalists.
Life and death, bats and fossils, contrasting—or so I thought.
The team followed the loop of the hallway and eventually entered the largest room of the cave, a wide oval with a vaulted ceiling about fifteen feet above our heads. A few alcoves led off from the room, as well as a tunnel that led back to the stairs. The DNR survey was nearly complete. One of the team members left with the cave owner to start preparations for surveying another cave on the property.
I meandered around the wide, hollow space, searching for bats. This was always the room where the naturalists found the most of them—it was also the room where we taught guests about bats. I remembered circling sleeping ones with my flashlight or laughing as they swooped low overhead on their way to darker realms.
Somehow, in my brief moment of distraction, mood shifted. When I turned around, two members of the team stood fixed in place, examining a corner of the ceiling near the hallway entrance. Another joined them, shining a black-light over the rock. I crept forward, unwilling to disturb the abrupt quiet, and peered between shoulders and heads.
Speckles of glaring white stood out on the little brown bats’ wings and noses, stark in the purply-blue blaze of the black-light. I hung back, but I caught snippets of words.
“Is that it?”
“Yeah. They have it.”
The team took out the sampling kit, gathering cotton-bud swabs and tubes. One of them wiped the swab gently on a bat, snapped the swab’s tip off into a tube, and moved on to another bat while the tube was labeled. The samples of powdery white fungus would need to be tested to confirm what they suspected, but they were almost certain: WNS was here. The team shifted between groups of bats in the room, and I approached the original cluster for a moment of privacy. The white spots weren’t as easy to see without the black-light, but I could still make them out.
My bats… I had thought there was more time. It was too soon.
In February 2006, Howes Cave, a hibernaculum in New York State, was stricken by a brand new, deadly disease. Dozens of bat bodies, mostly little browns, were found dead in the snow with a strange white fungus growing on their skin.
Since then, the fungus—the cause of the disease—has spread from population to population, hopping to new states and new hibernacula. By 2008, it had spread to four states and there was a documented two-year population decline of over 75%.1 By the next year, a team of Wisconsin researchers classified and described the new species of fungus.2 First designated as Geomyces destructans but reclassified by USDA scientists to the genus Psuedogymnoascus,3 the fungus has, according to data from WhiteNoseSyndrome.org and as of the time of this writing, spread to 33 states and 5 Canadian provinces.4 It’s just begun spreading west of the Mississippi River and to the Southeast United States, and it’s even popped up in a county in Washington State.
The prognosis for infected bats is grim: according to the Wisconsin DNR, the disease, which has killed over 7 million total bats as of May 2017, is reducing populations in infected sites in Wisconsin by anywhere from 30-100% by the second and third year of infection.5 This was my cave’s first instance of infection, so there were no deaths, but by the next year we’d probably see an impact in line with the DNR’s report.
A third to all of them: gone. I’d seen them almost every day I worked there; even in the summer when they were supposed to be outside I’d often see one or two dark fuzzy lumps on the support beams above the cave entrance or catch the flittering shadows as they darted, restless, between rooms. What would happen to them? How would they die?
The Wisconsin researchers who first described the white-nose fungus discovered a crucial fact in its particular virulence in bats: it is a cold-loving fungus, unable to grow at temperatures above 24 degrees Celsius (or about 75 degrees Fahrenheit).2 Bat hibernacula typically remain at constant, relatively low temperatures year-round (this one maintained a chilly 50 degrees Fahrenheit). The bats, adapted to long periods of energy-conserving winter torpor, lower their body temperatures to more closely match their environment. When spores of the fungus get into the environment, they are able to take hold and grow on the hibernating bats. The fungus “invades living tissue,” growing inside hair follicles and in the skin.2
Another major, early discovery: dead, infected bats have depleted fat reserves.1 Researchers hypothesized that the skin infection causes enough irritation to awaken bats too early from hibernation, and thus with heightened metabolisms, they use up their body fat. But it’s not quite that simple. New research has shed light on the horrific, slow-killing physiological effects of the white-nose fungus.
Infected bats are likely to face catastrophic disruptions to the functioning of normal bodily systems.6 Much of the white-nose fungus infection is centered on the large regions of exposed skin on the bats’ wings. A bat’s wings are involved in crucial aspects of homeostasis (the maintenance of normal body cycles). They regulate water levels and blood flow in the body, and the lesions of dead and invaded tissue disrupt these processes.
Ultimately, death results from a cascading string of effects stemming from the infection in the wings.7 Like burn victims, the bats lose fluids and electrolytes through their lesions. This leads to dehydration and blood loss, which awakens the bats to drink. Waking periodically to drink or lick moisture is normal for bats, but doing it too much leads to a depletion of fat, a reduction of electrolytes, and thus to hypotonic dehydration and acidifying of the blood. Any of these things could be the cause of death, but for the bat the result is the same: a slow, painful spiral. This was to be the fate of my bats.
The cave owner returned in time to hear the news.
“I’m sorry. We all thought you had another season or two,” the lead researcher consoled, her shoulders sagged.
“It’s okay. We knew it was coming,” the cave owner replied. I had never heard his voice so sad.
The survey complete, we began the slow ascent, absent our easy smiles. My mind reeled from our discovery, so I tried to focus my senses on my surroundings.
I climbed past layers of stone, forward in time, toward the end of the Ordovician and the secret of this place: it’s now seen two great deaths.
Continental shelves flooded by high, warm seas marked much of the Ordovician. The largest continent at the time, Gondwana, drifted down over the South Pole, allowing an ice sheet to form in the previously-hot world. At some point in the very last two million years of the time period (the geologic blink of an eye), the delicate balance of geochemical and climate cycling slipped, and the world changed.
The Ordovician extinction, one of the five great mass extinctions that wiped out most of the life present on Earth at their respective times, occurred in two distinct pulses.8 The first was associated with a drop in carbon dioxide levels and a subsequent glaciation event that locked up water and drastically lowered sea levels, exposing continental shelves to the air. These formerly rich habitats were eliminated, thus wiping out the reefs of coral and shellfish and their assorted ecosystems. Sea levels eventually rose again, but with rising seas came rising anoxic (low-oxygen) waters, formerly restricted only to deep ocean environments. It is believed that the suffocating, anoxic conditions caused the second, worse extinction pulse. This kind of language should be familiar to anyone keeping up with current issues of climate change: atmospheric carbon dioxide is tightly linked to climate and temperatures. So why did carbon dioxide suddenly drop after a long period of stability?
Mineral weathering may be the cause, and this could have taken effect in a few different ways. Land plants spread and diversified during the Ordovician; their spread would’ve broken up rock and minerals and sent pulses of nutrients into shallow ocean waters. With higher nutrients would come a sudden growth of life, specifically plankton that would draw vast amounts of carbon dioxide out of the atmosphere. Growing too quickly to be consumed, much of the plankton would’ve sunk and kept the carbon dioxide from returning to the air. This was probably a contribution, but not a primary cause of CO2 loss. Weathering of silicate rocks and volcanic deposits on land can also draw down large amounts of CO2 from the atmosphere. Indeed, in 2017 Massachusetts scientist David Jones and colleagues found evidence—higher than normal mercury levels—of a Large Igneous Province (a very large region of widespread volcanic activity) that existed just long enough before the Ordovician glaciation to have caused the documented drop in CO2.9
Whatever the causes of the disrupted cycling, the consequences were dire. In all, the Earth saw a loss of 85% of all species. Members of each of the groups represented by the fossils the naturalists had stored in the cave were wiped out. 70% of all genera (a level above species) of trilobites were eliminated, including all free-swimming forms. 70% of crinoid families were eliminated. Cephalopod predators starved without the shallow sea bed and its prey.
In spite of the widespread damage, life recovered quickly after the Ordovician extinction. Ecosystem functioning was disrupted little compared to other mass extinctions, and life returned to the seas in the following Silurian Period.
As we ascended the cave, moving through the layers time, I actually couldn’t see any evidence of the extinction here. In much of the Midwest there is an unconformity—a missing chunk of time—where the Ordovician-Silurian boundary would be. With lowered sea levels, there was a period of heavy erosion and comparatively little deposition of sediments, and so rock from the end of the Ordovician can only be found sporadically in Wisconsin, particularly in the Neda formation in the southeast.10 The Neda is made up of iron-rich siltstone and oolites (spherical grains of sediment formed in layers around a “seed” particle). The presence of oolites means the Neda must have formed in a high-energy, shallow-water environment such as an intertidal zone—that is, the edges of a dying sea.11 Other locations, where the boundary is preserved, display glacial deposits and isotopes of oxygen and carbon consistent with glaciation and lower sea levels.8
This cave is a place where past and present coexist: an improbable link forged across deep time. Its story is not just of the Ordovician sea life and the bats—it is about their deaths, too. And the similarities don’t end there: the Ordovician extinction event has a parallel today.
As humans, we have long known of our outsize influence on nature. We have been enacting effects ranging from land transformation, losses and additions to biota, influence on global biogeochemical cycling, loss of biological diversity, and instigating climactic change.12 In 2001, Harvard biologist Stephen Palumbi posited that humans are the “world’s greatest evolutionary force.”13 There has been a massive shift in biomass on Earth since the end of the last Ice Age: the population crashes of megafauna like mammoths and woolly rhinos and the subsequent explosion in biomass related to humans (ourselves and our cultivated species).14 We have replaced other life on Earth, dominated it, and continue to influence its change. But what is the true magnitude of this shift? Is the loss of diversity and life on par with mass extinctions like the end-Ordovician?
A loss of seven million bats in less than ten years, a loss for which my own bats might now represent a drop in the bucket, suggests an affirmative answer to me, but scientists have already examined this question from a global scale.
In 2011, a group of researchers led by Anthony Barnosky calculated the current rate of extinction of species as well as the expected “background extinction rate.”15 Species originate and end naturally all the time—it’s a part of life on Earth. But usually there is a balance to this process: a normal, expected rate of extinctions over time. Barnosky’s group found that the rate of current extinctions far exceeds the background rate—in fact, it’s the highest extinction rate since the mass extinction that killed non-avian dinosaurs at the end of the Cretaceous Period. A later report in 2015, using conservative estimates of extinction rates, corroborated this finding.16 The two reports proposed that we are entering another period of mass extinction: another event like the end-Ordovician and the sixth such in the Earth’s history of complex life.
This sixth mass extinction hypothesis is supported by more insidious and silent mechanisms than merely extinction itself. Life on Earth is experiencing “trophic downgrading:” the widespread loss of large animals from worldwide ecosystems—a loss which will have cascading effects down the food web.17 Remove something important—population controlling predators like wolves or pest-controllers like bats, and the chain of life from top to bottom shifts and suffers.
This is part of a larger trend now termed “defaunation.” Defaunation isn’t just extinction: it’s a loss in population as well. When I venture into the forest or prairie near the cave, I am likely to encounter fewer animals than someone would have just a few decades ago in the same spot: this is true of both land and ocean habitats. Of non-extinct terrestrial vertebrates, there has been a 25% average decline in population abundance in the last half century.18 Defaunation has affected even remote marine ecosystems, and will get worse with continued habitat degradation.19 Defaunation is coupled with “biological annihilation:” 32% of all vertebrate species are decreasing in population size and habitat range.20 Other life is dwindling on a global scale, and the bats, once incredibly populous and familiar, are now part of it.
Another parallel to the Ordovician: carbon dioxide levels and climate are shifting out of balance. Human dredging and burning of prehistoric carbon deposits—fossil fuels—has pumped carbon dioxide into the atmosphere at an astounding rate and to a dangerous level. As in other periods in Earth’s history, this will likely spell disaster for many species. In 2004 a joint effort of scientists from many institutions calculated extinction risk due to climate change: 15-37% of species in their sampling region would be “committed to extinction” by 2050, based on the severity of climate warming.21 Shifts in climate alter everything in every ecosystem—no being nor place is spared—and many creatures will simply be unable to adapt or escape in time.
All of these measures taken together tell a story that is both new and old. They speak of a series of endings for life on Earth and a reduction, a monumental shift, in what we see around us. Even white-nose syndrome isn’t a special case; the past two decades have seen a rise in new fungal diseases, including one which has been devastating to amphibians.22 Earth is undergoing a complete and total overhaul.
I examined the rocky strata as we left the cave, wondering at my impact—at the impact of my species upon this world. I had already transported myself mentally back in time: what if I performed the trick again? What would I see if I moved forward in time?
As in the Ordovician, human impacts will be writ in layers of stone for eons to come. Units of geological time are divided into segments based on their physical and chemical make-up, with different stratigraphic layers representing different times. The Ordovician was one of the major “periods” of Earth’s history, lasting from 488 million years ago to 444 million years ago (so about 40 million years of time). Our current time period, the Quaternary, began about 2.58 million years ago and is divided into epochs. Although the current epoch is recognized as the Holocene (beginning about 11,700 years ago), geologists now debate whether to recognize another, newer epoch: the Anthropocene, from the root “anthro” or human.
The Anthropocene is, in fact, geologically distinct from the Holocene.23 In my forward-in-time moment, I would examine layers that were deposited during the Anthropocene. I might find “technofossils:” synthetic materials like plastics, concrete, and aluminum. I might find a thin layer of ash and carbon from fossil-fuel burning. If I analyzed samples geochemically, I might find signatures like hydrocarbons, pesticide residues, lead isotopes from leaded gasoline, nitrogen and phosphorus from fertilizers, and even radionuclides from the detonation of atomic bombs. If I studied the isotopes of the carbon and oxygen in the layers, I would find evidence of climate change.
And if I searched long and hard enough, I might realize that the fossil remains of many species started to disappear around this time.
It’s an overwhelming amount of evidence for our unwitting dominion. A timestamp in stone: we were here, and we burned brightly.
The Anthropocene Epoch has its own unique mineralogy.24 Human activities have affected the minerals of our planet’s upper layers in three major ways: the introduction of synthetic mineral-like compounds, removal of large amounts of rocks and sediment, and the redistribution of some minerals such as gemstones. These three activities are likely to be “preserved as distinctive stratigraphic markers far into the future.” In other words, our behavior will be laid down as part of Earth’s story, just as millions of years of sea life was required to form the layers of dolostone in the cave. Our impact is geological in scale.
In this context, white-nose syndrome is a symptom of something bigger: humans as a dominant force of nature. Lisa Warnecke of the University of Winnipeg, along with other biologists in 2012, inoculated North American bats with strains of the white-nose fungus from both North America and Europe (where it is also found, without resulting in disease).25 If the fungus has always been in both regions but only just became deadly in North America (perhaps due to mutation), then European strains shouldn’t have harmed the subjects. However, the bats sickened with both strains, meaning that the disease is not naturally present in both locations. It is an invader from Europe, where the bats are adapted to it. Thus, we know that humans are the original cause of WNS as well: we’ve accidentally introduced it from Europe to North America, where it is spreading rapidly and with deadly consequences.
Our actions will, as always, affect us in turn. Bats control insect pests—insects that act as vectors for diseases like malaria and that destroy food crops. The Wisconsin DNR reported in 2017 that bats save Wisconsin farmers alone anywhere from $600 million to $1.5 billion in pest control expenses.5 Systems of biological control and cycling are breaking down around us.
This hasn’t all been to say that the situation is hopeless—beyond our control. In fact, technically speaking, it’s been in our control the whole time. There are countless efforts to stop the extinction crisis: restoration of habitats, protection of endangered species, reduction of pollutants, and development of renewable (non-carbon-emitting) energy sources.
There have even been attempts to stop WNS: research into direct fixes ranging from vaccines,26 to antifungal treatments, to naturally-occurring, antifungal bat skin bacteria,27 and even to thoughts of genetic engineering. But nothing is viable for large-scale use, yet.
Nothing has come in time to save my bats.
Months later, I’ve had time to process the prognosis. The test for WNS came back positive, as expected. This was my last season as a naturalist at the cave, and it may have been the last for the bats as well. Every interaction carried with it the gravity of finality: the angry chittering as my flashlight beam accidentally finds them in the dark; the rush of wings past my face as I stand breathless near a hallway entrance; the tiny sleeping forms with their paper-thin, cupped ears and delicate, bony toes gripping the rock; and the sheer excitement as visitors spot them, something vibrant and breathing and still-wild in arm’s reach. I’ve left in the last season of plentiful, healthy bats—an ecosystem functioning as it’s supposed to, a memory intact—but it won’t stay this way. How can I ever return and face the diminishment of this place?
As the team concluded the bat survey, emerging from darkness into piercingly bright sunlight, I was afforded a moment of peace. Winter was ending, and the remaining patches of snow glistened wetly under the sun, much like the flowstone had under my headlamp. Green spots poked through the mud, and birds moved amidst budding trees. The bats slumbered beneath us, in the world of calcite and old stories.
This place was still healthy—one last season of full life and beauty. I vowed to relish every sign of life—of pups in June, of insects feasted upon, of nights lit with sound, of beating wings. I would finish the season with pride and love and sadness, all fused together in the heart of a naturalist at the end of nature.
I would finish the season with a sense that this event, monumental though it might be to me, is only a small part of something vaster that’s happening to the world: something geologic.
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