6 
6 Insects as Janitors
I know of few things as beautiful as mighty, ancient oaks. There they stand proudly, a legacy from a bygone age; oaks that sprouted and grew before the days of streetlamps and social media, in a time when trolls still lived among the trees and not in the shimmering websites on your computer screen.
Today’s great oaks have retained their magic. And where Pippi Longstocking once found lemonade, we scientists can go in search of rare insects. Because inside the ancient oak trees, hollows form where the wood slowly rots. Inside, it is dim but not quite dark. There is a scent of fungus and damp earth, like a faint suggestion of autumn. At the same time, a sweetish hint of warm timber is like a promise of spring to come. Here you discover another world, in which the meaning of time and space is altered. Time goes faster because a beetle lives its entire life over a single summer. And a fistful of reddish brown wood mold, with its raw tang of fungus, damp, and life’s decay, is an entire world for a millimeter-long pseudoscorpion.
Inside live brightly colored red velvet mites and pallid beetle babies, enormous scarabs and tiny springtails. Nurseries and pickup joints stand side by side. There are life and death, drama and dreams, all on a millimetric scale.
The quest to find ancient oaks and their inhabitants has led me to many forest areas I would never otherwise have seen and given me many encounters with nature that I wouldn’t have missed for the world: picnic spots on bare, rocky hills with blue mountains in the distance and the warm spring sun upon my face; late spring evenings in Telemark on my way back to the car after my day’s work is done with only the cry of the tawny owl and a sickle moon for company; steep slippery slopes that I barely managed to scale in pouring rain; boulder scree in western Norway, where all the oaks bear traces of pruning from earlier times, when people harvested leaves as winter fodder for their livestock; avenues, pastures, wooded hillocks in cultivated fields, private gardens. Usually solitary—yet never alone. Because more individuals may live in these ancient oak trees than there are human inhabitants in Oslo.
An ancient hollow oak tree is like a fortress—a fortress of biodiversity, no less. The shell of resilient oak wood provides shelter from rain, sun, and hungry birds for the many hundreds of different insect species that live inside. The intricate oak bark, reminiscent of the ornamental carvings of intertwining dragons and serpents on Norway’s stave churches, provides a habitat for minuscule pin lichens. Some fungi live in close cohabitation with the roots of the oak, while others help the insects break down the dead timber.
The factor largely responsible for the presence of all these species is wood mold, a life-giving blend of rotting wood residue, fungal threads, maybe an old bird’s nest, and a smidgen of bat guano. For insects, wood mold is like a gourmet restaurant: even the most exacting bugs will find a menu to suit their tastes. Hundreds of different insects may live in the dim, humid atmosphere inside a hollow oak, contributing to nature’s eternal circle by slowly converting mighty trees into mold and soil where new acorns can sprout.
Herbivores eat just a tenth of all the plants that sprout and grow. All the rest, 90 percent of all plant production, is left lying on the ground. Plants and trees aren’t the only things to die; life also comes to an end for animals of all sizes, from midges to moose. As a result, there are impressive amounts of protein and carbohydrate to be recycled. On top of that we have to consider all the waste these animals produce over their lifetimes; dung, plain and simple, needs to be taken care of, too—a pretty crappy job, you might think, but insects are on hand to help us out, as usual.
This is where nature’s caretaking service comes in. Just as in schools, offices, or apartment buildings, it’s often the janitor who must clear up after everybody else. And this is how it works in the forests, the meadows, and our cities, too, where thousands of fungi and insects perform the crucial task of decomposing dead organic matter. Nature’s tiny janitors devour the mess on the spot. It can take time, and it requires a sophisticated collaboration in which different species have different roles to play.
And even though very few of us think about it as we take a Sunday stroll through the park or in the forest, these processes of decomposition are crucial to our life on Earth. Insects’ patient chomping on dried-up trees and rotten remains doesn’t just clear the ground of dung and dead plants and animals; just as important, the insects’ contribution returns the nutrients in the dead organic matter to the soil. If substances such as nitrogen and carbon are not returned to the earth, it will be impossible for new life to grow.
When the insect mom is house hunting in the forest, her priorities are different from ours. Take beetles that live in dead trees, for example: whereas we fear moisture damage and rot, beetles think they’re fantastic because they’re like a fridge full of food for the family’s greedy kids.
So Madame Beetle goes for a viewing. Softly, she sets down all six legs on the dead tree. With antennae and toes, she tastes and smells the spot where she’s landed to see if it will make a good nursery for her beetle babies. If she’s satisfied, she swiftly lays her eggs in a little crack in the bark and moves on in search of more trees in need of caretaking services.
Each egg hatches a larva that boldly sets about chewing its way through bark and wood—a gargantuan task in which, fortunately, it is not alone. Thousands of beetle larvae may be at work in such dead trees, ably assisted by bacteria and fungi.
New dead wood is fantastic fun: there’s plenty of sugary sap beneath the bark, and when it ferments there’s a real party atmosphere among the visitors. Every type of wood has its beetle specialists, who greedily gorge on this delicacy. Bark beetles are a typical example. But speed is of the essence, because by the end of the first summer the platter is empty: all that lovely sugar is gone.
By contrast, dead, dry timber is a pretty dismal lunch for a beetle. Cellulose and lignin, two of the most important components of timber, are about as juicy and digestible for an insect as a sack of bran is for us. So it’s a good thing that some fungi just adore cellulose and others love lignin. They cast their fungal threads—bits of fungus they carry on their bodies or in their guts—into the wood, making it more appealing to the beetles because the nutritional content increases and becomes more accessible. Bacteria also provide a great garnish. Some beetles even have tiny collaborating partners in their bodies that help them extract nourishment from even the most indigestible parts of the tree. All in all, myriads of organisms are involved in the decomposition of dead wood.
Dead trees, branches, and roots are home to a surprisingly large number of species. As many as 6,000 species live in dead wood in the Nordic countries—a third of all the species found in our forests! Approximately 3,000 of these are insects. By comparison, there are only around 300 bird species and fewer than 100 mammals in the region.
Once fungi and insects, mosses and lichens, and bacteria have moved in, there are more living cells in the dead tree than there were when it was alive. So ironically enough, dead trees are actually among the most living things you can find in the forest. And every species has its particular cleaning job to do, not to mention having its own precise demands when it comes to the kind of dead wood it wants to live in or on.
Why are there so many species in dead wood? Part of the reason is that insects that live on dead timber have differing demands when it comes to the type of dead wood they’re after. For us humans, who don’t find timber toothsome, it is difficult to grasp all the nuances when it comes to types of wood, stages of decomposition, size, and surroundings.
But to insects, a dead spruce tree is quite different from a dead birch. And an aspen that has just died is quite unlike an aspen that has lain dead in the forest for several years. As I mentioned earlier (see page 94) plants and trees have an active, species-specific defense against grazing animals and insects. This persists after the tree’s death, especially early on, meaning that the first insect arrivals at the newly dead tree must be specially adapted to cope with this.
Size is also important: a dead oak branch offers a totally different habitat than the rotted innards of a giant oak. And a dead resinous pine tree on a hill in the baking sunlight is home to totally different species with different dietary habits than a dead pine in a dark, dense forest.
In other words, a stick isn’t just a stick; dead timber has more nuances than a fine wine, and many insects are exacting connoisseurs. Since insects have such differing demands, there needs to be enough of all these various types of dead wood in the forest to provide enough housing to enable every insect to find its own little hovel and get its job done.
But there’s one more vital point when the beetle mom is hunting for a suitable dead tree trunk in which to house her children. The window of opportunity is short in the insect housing market, and it’s a question of getting to the suitable tree in time. If the distance between rare and specific types of logs (such as large-diameter oak trunks) is too large, as is often the case in modern managed forests, the beetles dependent on such logs might not be able to get there at all.
That is why natural forest, forest that isn’t affected by modern logging, is so important. It contains much more dead timber than managed forest, and there is a great deal more variety in the dead wood, which means there are more beetle homes on the market. They are so close together that the beetle mom can pop into several of them in a single evening of viewings, laying a few eggs here and there. That’s the way to create beetle diversity.
The goings-on in dead trees are one of my favorite topics and a subject we do a lot of research into in the research group I belong to. It may not all be “rocket science,” but we’ve certainly been involved in some projects that went off with a bang. Such as fifteen years ago, when we did a really bang-up experiment: we wound yards of detonating cord around trees in the forest, 15 feet above the ground, and lit the fuse. And then we ran . . . With an enormous bang the trunk was blown right off, and the top of the tree crashed to earth!
The point of the exercise was to create standing dead trees. We created sixty such trees, and in each of the years that followed we checked which beetles were visiting them. This taught us a great deal about different insects’ dietary preferences. We also saw that the forestry sector’s environmental measure of tree retention—leaving trees in the clear-cuts that eventually become high stumps—actually works.
What’s even more fun is that now, fifteen years later, we can hear a kind of echo from those earlier beetle visits. It turns out that there are different fungi on the trees these days depending on which insects paid a visit all those years ago!
That made us wonder whether fungi and beetles have become a bit like bees and flowers—are they mutually useful? Perhaps certain wood fungi simply hitch a ride with certain beetles and get dropped off at the restaurant door? We know of some bark beetles in which the collaboration is so close that both parties depend upon it. But could this collaboration be much more common than that in a looser form—without codependency but with advantages for both parties?
To check this out, one of our PhD students has been putting trees into cages—or rather, parts of trees. She cut down living trees, made logs of identical sizes from them, and randomly chose which logs to cage. Logs that were caged did not receive insect visits, as the bugs couldn’t get through the netting of the cage. For the purposes of comparison, the other logs were placed outside the cages, so that insects could land as usual.
It turned out that the fungal society was totally different in the logs that insects couldn’t access. We think this is because many insects carry spores or fungal threads, in their body or their guts. When the insect lands on a newly dead log to lay eggs, this fungus is sprinkled or excreted along with the insect dung and thereby finds a new home.
In addition—and this is the really exciting bit—our study showed that the caged logs decomposed more slowly. The cleaning job takes longer when the insects don’t get to help.
I love running, especially on soft forest paths. A half-hour run from my home takes me to a forest reserve strewn with dead trees, like a game of pickup sticks. I can look around and try to count species, of which there are around 20,000 in Norwegian forests. Of course, not all of them live in “my” forest, but still—how many can I see? I can count several trees, a dozen plants, lichen, fungi, perhaps an elk or a large bird if I move quietly. If it’s summer, the insects will do wonders for my list of species, but even so, I spot barely more than a hundred. Even here in the reserve. So where are all the other tens of thousands of species?
A lot of the other species are tiny insects and related creatures that live out their lives in hiding. As I mentioned, a third of forest species live in and on dead trees. The other important habitat is the soil; there is no other place where species are packed together so densely. The tiny patch of earth stuck to the sole of my running shoes after a trip to the forest may be home to more bacteria than there are human beings in the United States, not to mention thousands of thin fungal threads. Here in the soil are also found a myriad of important critters and small insects. A whole zoo of little creatures lives down there in the darkness: earthworms and mites, roundworms, pot worms, springtails, and wood lice. All these species, which we don’t give a hoot about on a day-to-day basis, have important jobs in the recycling sector. They chew and dig and aerate and mix. In the blink of an eye, rubbish is reconverted into soil, ready to sprout new life. It’s pretty miraculous, really.
Soil is important, but masses of it vanish every year—not because runners dash off with chunks of it stuck to their shoes but because of erosion by wind and water. Some of this is natural, but in many places the soil loss is high because we humans have removed natural vegetation. Consequently, there’s nothing left to retain the soil, which is blown away or runs off into the sea and elsewhere. This is losing us billions of tons of topsoil every year—and along with the soil, we are losing the diversity of decomposers, which are our guarantee that the recycling of nutrients will continue.
The thin layer of earth is the planet’s skin, a thin living layer over the magma and the rocky crust. Perhaps we ought to pay a little more attention to the earth’s skin care? Like a teenage girl anxiously checking her complexion in the mirror, we, too, should be aware of the well-being of the topsoil and forest soil, along with all their inhabitants. Because we need them and—to stick to the language of the cosmetics industry—because they’re worth it.
Finger food at festivals, picnics in the park. The summer propels us and our meals out into the city. But what about the scraps of food—the bits of hamburger we drop on the pavement or the hot dog bun left lying on the grass? This is where ants come into the picture.
Many people think of ants as a nuisance, even yucky. But it’s actually good to have them around, in the urban environment, too. A group of insect scientists studying ants in Manhattan did a back-of-the-envelope calculation and estimated that there are two thousand ants for every person in the city. And what do ants do there? They live out their tiny ant lives, which consist mostly of food gathering and reproduction. They are undemanding when it comes to diet and have a healthy appetite. Another of the scientists’ back-of-the-envelope calculations estimates that the scraps of junk food put away by the ants of Manhattan add up to the equivalent of sixty thousand hot dogs a year! It’s a damn good thing we have them.
In an experiment, scientists compared how much food waste found its way into ant bellies in different parts of Manhattan. Precisely weighed quantities of food were set out in tiny “food scrap cafés” in parks and on pedestrian traffic islands. The scientists offered the ants a comprehensive fast-food package, New York style: hot dogs, and a cookie for dessert. At the same time, they measured the species richness of ants and other urban bugs in the same places, establishing that there were more species of ants (and other bugs) in the parks than on a busy street.
Because it has been shown that food gathering in many other natural systems is more efficient in a species-rich society, the scientists expected the park ants to eat more of the food scraps than the traffic-island ants did. But the result they got in Manhattan was the opposite: the ants on pedestrian islands carried off more than twice as many of the scraps. There may be several reasons for this. First off, it is warmer on the pedestrian islands. And since ants are cold-blooded animals, everything goes faster when the temperature is high.
Second, it seems that a European immigrant, the pavement ant, really has a taste for American junk food. This species was much more common on pedestrian islands than in the parks, and wherever it was present, up to three times as many fast-food scraps vanished as when it wasn’t. In other words, environmental conditions and individual species turned out to be more important than species diversity when it came to cleaning up Manhattan’s food scraps.
Pavement ants are territorial, and, like other urban gangs, they fiercely defend their little patch of the city against interlopers. But the ant gang isn’t alone on the streets of Manhattan. There are regular episodes of gang violence between rats, which are less common but bigger. They want their share of the junk-food plunder. These clashes between gangs ought to be of some interest to us humans, who are even bigger. Because although mice and rats make a positive contribution in the sense that they eat our food scraps, they are also notorious transmitters of disease. The same can hardly be said of ants. This means that ants are much better suited to the role of cleanup patrol in the city’s outdoor spaces.
It is time to acknowledge that even our cities are ecological systems, in which crawling critters are an essential ingredient. On a pedestrian island on Broadway alone there are thirteen different ant species. In all, forty ant species have been found in New York; that’s almost two-thirds of the ant species in the whole United Kingdom. And since more than half of the world’s human population now lives in cities, we ought to spend more time finding out how urban ecosystems operate.
The thing is, urban nature also performs significant services for the ecosystem. Trees provide shade, muffle noise, and clean the air. Green areas absorb water after heavy rainfall and reduce flooding. Open water cools the air, and the species in ponds and streams filter the water, making it cleaner. The tiniest patch of earth can provide a habitat for masses of useful bugs that pollinate plants, spread seeds, or clean up the streets—such as ants.
Economists in Oslo have studied that city’s ecosystem services and their worth. An attempt to measure the value of the green structures in and around the capital to the health and well-being of the inhabitants, for example by calculating their use value, among other measures, added up to millions of dollars. And that was without including the value of the ants’ contribution.
A greater knowledge of urban ecology will enable us to plan and maintain our cities better. Even something as simple as raking pedestrian islands less frequently has proven important, as it ensures more hiding places and a happier life—if you happen to be an adventurous Manhattan ant.
Hot dogs on the streets of big cities are one thing. But there are also other types of dead meat that need to be cleared away out in the natural world. Think of all the animals, big and small, that die and are left lying where they dropped. It would be quite unpleasant if they didn’t get recycled pretty quickly.
From the insects’ point of view, carcasses are a handy source of food—they can’t run away, and they can’t defend themselves. But the insects have to be quick, because carcasses are rich in nutrients and therefore much-sought-after food; what’s more, the competition includes a large range of species of varying sizes. Here, insects are literally in the flyweight class, whereas their opponents are heavyweights such as foxes and ravens, vultures and hyenas. One trick is to lay not eggs but ready-hatched larvae in the carcass, as some flesh flies from the Sarcophagidae family do. Another is to eat quickly, grow even more quickly, and generally be flexible when it comes to how big you need to be before pupating.
Another cunning solution is to hide the carcass by burying it. The beautiful red-and-black burying beetles of the Nicrophorus genus are masters of such a vanishing act. They work in pairs, digging out soil from underneath the carcass and placing earth on top, and in this way they can bury a dead mouse in a single day. Beneath the earth, they wrap up the carcass in a ball and lay their eggs on it. And despite the slightly jaw-dropping choice of nursery, they are attentive parents: they chew off tiny scraps of the carcass and regurgitate them into the mouths of their larvae, which are incapable of digesting the food themselves. This is one of the few examples of parental care in the insect world other than among the social insects (see page 44).
Burying beetles also have some good friends that are not insects. When newly hatched sexton beetles leave their childhood home, masses of tiny mites climb onto them and hitch a lift to the next carcass. This species of mite lives only with burying beetles; it cannot fly and is reliant on transport to find its way to a new, fresh carcass. In return for the lift, the mites eat up the eggs and larvae of other competing fly species in the carcass.
The decomposing crew that turns up to break down the carcass belongs to a segment of the insect world that is rarely mentioned or rewarded. There are no fan groups for burying beetles as there are for bumblebees. Yet they are tremendously important animals.
In South Asia, people have learned to their cost what can happen when carrion eaters vanish. It’s true that the animal in question was the vulture, which might be said to be the blowfly’s massively big brother and enjoys a similarly bad reputation among most people. Nonetheless, the point is the same. Around the turn of the millennium, the veterinary medication diclofenac was introduced into India as a treatment for sick cows. Just fifteen years later, the medicine had dispatched an insane 99 percent of all the country’s vultures because residual amounts of the substance were left in dead cows and were passed on to the vultures that ate them. The vultures suffered kidney failure and died. Although scavenging insects almost certainly worked at top gear, they were unable to deal with such large amounts of carrion alone. As a result, dead cows were left lying on the ground. Once the vultures had vanished, other large scavengers appeared on the scene: feral dogs, whose numbers exploded. Since many of them are rabies carriers, the population boom in dogs that resulted from the disappearance of the natural carrion eater has been blamed for an additional 48,000 rabies deaths in the Indian population.
In fact, carrion eaters can also help the police in criminal investigations. There is a pattern to when which species come to a corpse, and this can be used to help connect the dots in criminal investigations and ultimately solve crimes. The first time insects helped identify a murderer is supposed to have been in a Chinese village in 1235. A man was brutally murdered with a sickle, and the local peasants were called into a meeting. They were instructed to bring their sickles with them. The investigator made them wait, and, since it was a hot, sunny day, it wasn’t long before flies appeared. When all the flies landed on the same sickle, the owner was so shocked that he confessed on the spot. With their peerless sense of smell, the flies were drawn to the traces of blood even though the sickle had been cleaned.
Today the methods are more advanced, but the basic principles remain the same. Insect species appear in a dead body in a given order and following a particular logic. This fact can be used to calculate the time of death and may, in some cases, also tell you something about the cause of death. Drugs and toxins accumulate in the insects present, and thus they can be more easily detected. Such chemical substances also affect the growth rate of feeding maggots and is therefore important information when forensic entomologists are estimating the time passed since death occurred.
Moreover, species are distributed over defined geographies. Knowledge of this fact can be used to determine whether a body has been moved if the species that are present are normally widespread in other areas or in other parts of the country. One example of this was a case where a body was found in a sugarcane field in Hawaii. The oldest larvae that were found in the corpse belonged to a fly that lives primarily in urban areas. And indeed it turned out that the corpse had been kept in an apartment in Honolulu for a couple of days before being dumped in the field.
Insects can also make a more indirect contribution to crime solving. Insects mashed into a car’s radiator grille were used to trap a killer in the United States. He claimed to have been on the East Coast when his family was murdered in California, but the species found on his rental car could be found only on the West Coast.
All animals eliminate waste in the form of dung. Dung from large animals, like mammals, represents a significant biomass. Dung may contain useful nutrients, but it also contains large quantities of bacteria, disease-causing parasites, and other things the body has expelled. Not all animals are up to eating this waste, but insects stand ready. Beetles and flies are particularly likely to have muck on their menu. In this working group a special expertise is required: a good sense of smell and rapid reflexes. When the battle for cowpats is under way, you have to be quick if you want to secure your slice of the pie.
Some participants, such as the horn fly, are renowned for starting to lay their eggs before the cowpat is even finished. This is dangerous, but some parents will go to any lengths to ensure the best conditions for their kids to grow up in—because fresh dung goes quickly, especially if it’s warm; you might say it sells like hot cakes. For example, one study showed that as many as four thousand dung beetles fell upon a 17-fluid-ounce dollop of elephant dung doled out by researchers in a mere fifteen minutes. Other studies found that it took a couple of hours for 3 pounds of elephant dung to vanish once 16,000 dung beetles went in and did their job.
Dung beetles have three main strategies: they can be dwellers, tunnelers, or rollers.
The dweller likes to live right in the middle of the meal. It crawls down into the dung, content to eat and lay its eggs there. Many Norwegian dung beetles (members of the Aphodiinae subfamily) belong to this category. The dwellers’ strategy is risky. They never know how many others are laying their eggs in the same dung, and in the worst case the larvae may eat one another out of house and home and all end up starving to death.
One way of avoiding this is to build an extension for the kids, with its own pantry. This is the technique used by the tunnelers. They dig passages beneath or right beside the dung that can range in length from four inches to a yard long. We often find the longest passageways in species where the dung beetle mother and father work together. They drag small balls or rolls of muck down into their tunnels, which then serve as a kids’ room for the larvae.
The most advanced variant is the rollers, which grab their share of food and make a swift exit. They pack the dung into a ball, which can often weigh fifty times as much as the beetle itself, and trundle it off—always in a straight line, regardless of whether the sun is hidden behind a cloud or whether it’s a dark, starry night. How do they do it?
Creative scientists have really gone to town on their field experiments: Some placed tiny peaked caps on the beetles’ heads to shade them from the sun. Others used large mirrors to manipulate the position of the sun or the moon. The most creative of all was, perhaps, the researcher who moved the entire experiment into the Johannesburg Planetarium and proved that dung beetles can use the Milky Way to orient themselves! The only other animals known to use the stars for orientation are human beings, seals, and some birds. All in all, the research shows that the dung rollers can steer their course using the positions of both sun and moon, as well as polarized light or the Milky Way.
These particular beetles have fascinated humans for thousands of years. The dung-rolling Scarabaeus sacer played a central role in Egyptian mythology. When the Egyptians saw those beetles trundling off with round balls of dung, it reminded them of the sun’s journey across the heavens. The beetle became their “sacred scarab,” symbolizing Khepri, the god of the rising sun. This insect god is sometimes portrayed as a beetle and sometimes as a man with a beetle’s head.
The Egyptians also saw that the scarab beetles were among the first living things to emerge onto the muddy banks of the Nile after the spring floods. Where the old dung beetles had buried their balls of dung, new young beetles clambered up out of the earth a few weeks later. From there, it wasn’t much of a leap to make a link between the sacred scarab and renewal and reincarnation. It became common for scarab amulets to be used by the living and to be bound into the bandages that swathed mummies.
Perhaps the Egyptians even got the idea of mummification from beetles, because what does a beetle pupa look like, if not a mummy? It has even been suggested—somewhat playfully, perhaps—that the pyramids are sacred representations of piles of dung, in which the dead pharaoh lies like a mummified pupa, waiting for the metamorphosis of reincarnation.
Dung can be used for so many things. In a lot of cultures, dried cowpats, for example, are still used as fuel or building material. In the insect world, too, we see examples of creative use of excrement. How about a dung wig, for example? The Hemisphaerota cyanea leaf beetle lives in dwarf palms in Florida and the Southeast states. As a larva chews its way through a palm leaf, beautiful pale yellow curly threads ooze out at the opposite end. The larva arranges these pale threads of dung neatly over its back until it ends up with a whole wig—not unlike Donald Trump’s hairdo. The point of the wig is, of course, self-defense: no matter how hungry you are, you’re unlikely to fancy a mouthful of hair.
Several leaf beetle larvae adopt similar techniques, but instead of using hair, they count on intimidating and scaring off the enemy. The pale green Cassida viridis is common in Europe. Its larvae fashion a kind of roof or parasol out of old larva skin and black lumps of dung that they hold over themselves with the aid of a special “anal fork.” If an enemy comes too close, the larva can brandish its dung parasol, which may also contain poisonous substances that the larva has produced from the leaves it eats in order to keep enemies at bay.
Case-bearing leaf beetles (from the Cryptocephalinae family) are even more advanced. Their children are equipped with something akin to a mobile home made of poo: The mother lays each egg in a beautifully formed container that she molds from her own excrement. When the egg hatches, the larva opens the door and sticks out its head and legs so that it can carry its house wherever it goes. As the larva itself defecates, it adds to its mobile home, thereby ensuring that it is always big enough. When it is time to pupate, the larva climbs inside and shuts the door behind it. There it can lie, nice and safe, until it has become an adult beetle—and the whole sequence starts all over again.
Some people think sloths are cute. You know, sloths—the creatures who were portrayed as the incredibly slow but smiley clerks in Walt Disney’s Zootopia. I actually got up close to a sloth one time in the wild. And I didn’t find it in the slightest bit sweet.
I was sitting on the outskirts of a village in Nicaragua with my back to a fallow area—half-open forest land with bare earth. It was pouring rain. I heard a noise behind me and turned toward the forest. There, just a few yards away—slowly, slowly, its gaze fixed firmly on me—came the most peculiar creature I had ever seen, slinking toward me, sopping wet. This was thirty years ago, but I clearly remember thinking: Good God, it looks like a nuclear mutant!
One biology degree and many years later, I realized this must have been a rare sight. Sloths are one of very few genuinely tree-living mammals and spend the absolute minimum of time on the ground. But once a week the time comes for them to do their business, and, oddly enough, they have to do it on the ground. That’s when they tend to die, because they are so incredibly slow and can barely defend themselves from predators.
The last thing that occurred to me was to count the toes on the forepaws of this slightly terrifying creature with its fixed grin. Now I know that there are two groups of sloths, three toed and two toed, and a few different species within each group. The two- and three-toed variants are very different. It’s the three-toed kind we’ll be dealing with here.
Neither did it occur to me to go over to the animal and search for butterflies in its brownish green pelt. I regret that now because sloths carry an entire ecosystem in their fur—a fact we have only recently grasped.
Why should three-toed sloths risk going to the toilet on the ground instead of just letting rip from the treetops instead? Especially considering that they expend 8 percent of their daily calorie intake on these climbing trips and risk getting eaten to boot. Scientists have long sought an explanation. Could the point be to provide manure for the tree they are living in, or are they communicating with other sloths via their latrines?
That isn’t it. In the pelt of the three-toed sloth lives a creature entertainingly known as a sloth moth. When the sloth goes for a toilet break, the moth climbs out of its pelt and lays some eggs in the poo. The larva lives there happily, and when it grows into an adult moth, all it has to do is wait for the sloth’s next loo stop to move into a safe, warm sloth pelt.
And this is when the fun really starts, because surely the sloth wouldn’t bother to risk its life just to do a moth a favor? Well, it turns out that this business involves some advantages for the sloth, too.
The moths excrete, die, and decompose in the pelt. This increases the nutritional content of the pelt and improves conditions for a type of alga that grows on sloth hair (and nowhere else in the world, just so you know). The sloth eats this green algal growth by licking it off its fur. The alga has a crucial advantage: it contains important nutrition that the sloth cannot get from its monotonous plant diet. It can also serve as camouflage.
So to sum up: the moth is good for the alga, the alga is good for the sloth, the sloth is good for the moth. It’s a whole tiny ecosystem—all in one animal pelt.
Other large animals also play host to dung insects that have found it wisest to stay close to the source rather than spend their entire lives looking for fresh muck. Among kangaroos and our hairy ape brothers, certain beetles set up house in the pelt close to the animal’s rear end. So there are a few benefits to having a hairless rear that probably hadn’t occurred to you.
In 1788, the first cow to set its four feet on Australian soil arrived. Along with it came a rather motley crew of 1,480 men, women, and children—mostly convicts—along with 87 chickens, 35 ducks, 29 sheep, 18 pheasants, and various other things. That marked the end of the Aborigines’ 40,000 years of isolation, not to mention that of the animal and plant life, which had been isolated since the Australian continent split away from Antarctica somewhere between 40 million and 85 million years before. Consequently, the continent was full of species that existed nowhere else on the planet; 84 percent of the mammals and 86 percent of the plants in Australia were unique.
The four cows and two oxen that accompanied the first European fleet had been picked up during the crossing. They came from Cape Town and were zebus, a race of cattle that is accustomed to a hot climate. A convict named Edward Corbett was given the job of herding the animals, with strict instructions not to let the cattle out of his sight. But alas, just a few months after Daisy had ambled down the gangplank, she and the other cattle vanished. They’d taken off while the herder was having supper.
It was a minor catastrophe. The six cattle were supposed to be used for breeding, milk, and food. The settlers could find no familiar edible plants in Australia. Even though they had grain for planting, many of the prisoners had no experience with agriculture and weren’t especially keen to learn. They weren’t even any good at fishing. The provisions disappeared fast despite ultrastrict rationing.
So there was great rejoicing when they found the cattle again a few years later—by which time they had become a whole herd. They were getting by very well in Australia’s pastureland.
After one or two hundred years, joy turned to desperation. Because what do cows do? They feed, chew, belch, and defecate, and they do all of these things on a massive scale. One cow produces as much as nine metric tons of dung per year, and that’s dry weight. The excrement of a single cow covers an area the size of five tennis courts each year. And when cows thrive, there get to be a lot of them—with equivalently large numbers of tennis courts’ worth of dung.
By around 1900, there were more than a million head of cattle in Australia. But who was going to clear up the crap? This brings me to the point of the story: there were no beetles in Australia that could decompose the cow dung. There were some native dung beetles, but they had been reared on dry, hard marsupial dung for millions of years. They had very little taste for foreign cuisine in the form of the zebus’ mushy manure.
Therefore, the dung was left lying on the ground. There it dried into a crust that could not be penetrated by so much as a blade of grass. At the height of the problem, up to 500,000 acres of grazing land per year were becoming unusable. Around 1960, roughly two hundred years after the arrival of the first cattle, large areas of the country lay fallow because of dung that had not decomposed.
The only things that could be bothered with the dung were the flies, but they didn’t exactly help. In Australia there’s a type of fly that resembles the European housefly, except that it lives anywhere other than in houses—and now it was particularly keen on living wherever dung happened to be lying around unused. This troublesome fly bred massively, as did other flies that plague humans and livestock—an increased nuisance factor on top of the problem of large areas of fallow land no longer suitable for grazing.
New beetles had to be brought into play. A large-scale project was set into motion, sponsored by the government and the meat industry. Over a period of fifteen years, Australian entomologists experimented with numerous species, and after careful testing they set out 1.7 million individuals from forty-three different dung beetle species.
The project was a success. More than half of the species became established. The dung disappeared, and the plague of flies dwindled notably. Before, only a tiny share (15 percent) of the nitrogen in the cowpats was being returned to the soil; the beetles’ caretaking service increased the level to as much as 75 percent. This example shows just how important decomposition is for nature and for us humans.
Despite their importance, things aren’t looking so good for dung beetles as a group. Globally, 15 percent of the species are threatened. In Norway, more than half of the roughly seventy types of beetles that live in dung are listed as threatened with or near extinction, and thirteen species have apparently already vanished from the country. Norwegian dung beetles are having a particularly tough time in the south of the country—which provides a habitat for species that require fresh cow muck, preferably on sand or unfertilized pastureland beneath a warm summer sun. Changes in agriculture are largely to blame for the disappearance of the dung beetles. Uncultivated pastureland becomes overgrown or is not grazed continuously over time.
Another problem is the widely used antiparasite treatment ivermectin, which is given to cattle and other livestock all over the world. The substance has been found to be excreted in the dung in large amounts and to harm the dung beetles that come to clean up. This may have consequences for both species diversity and speed of decomposition. To reduce the negative effects on dung beetles, it has been suggested that the drug be distributed only by injections to reduce the amount excreted in dung, and only to animals where parasitic infections are severe. Such restrictions could also help in delaying the increasing resistance in the parasites to ivermectin.
Life in the hollow oaks is in trouble, too. The work we have done in my research group shows that the specialized insects that live in hollow oaks are struggling. In many cases, we have found individual species in very few places, perhaps just a couple of oak trees. These species need areas with many coarse trees that are exposed to the sun—trees that contain a lot of wood mold. There are few such oaks.
Along with other scientists and assistants, I have researched insect life in hollow oaks for more than ten years. We have identified the species of more than 185,000 different beetle individuals from as many as 1,400 unique species in hollow oaks. Some of these are specialists that live solely in oaks or solely in hollow trees, preferably oaks. Around a hundred beetle species that live in hollow trees are endangered or threatened with extinction in Norway.
Today, hollow oak trees enjoy special legal status in Norway: they are deemed to be a “selected habitat type” precisely because they are associated with such rich diversity. These trees’ status as a selected habitat type means we must treat them with special care and avoid damaging them. I am involved in a national monitoring program for hollow oaks, which aims to tell us about their status and development. The hope is to follow this up with a monitoring of the unique insects that live there, too.
If we are to secure these fortresses of biodiversity, we must protect the great hollow oaks we still have left. Our research seems to suggest that traces of intensive oak logging several hundred years ago are still reflected in the diversity of beetles in today’s hollow oaks. This may be a kind of delayed reaction, known as extinction debt, whereby species cling on for a long time after habitat destruction but are eventually forced to give up the ghost.
We must also make sure that we prevent the areas around oaks that have developed in an open landscape from becoming overgrown. Many of the most specialized insects do best when the sun can shine on the tree, making it snug and warm. And we must take a long-term view, ensuring that we recruit new oaks that can become hollow well before the old ones die out.
It takes no time at all to fell a hollow oak that’s standing in the way of progress—a widened road or a new building. Five minutes with a chain saw, and the giant that put out its first shoots in the days of the Black Death and saw the Renaissance and the Industrial Revolution take shape and pass away, lies splintered on the ground. It takes seven hundred years to replace it with a new oak of the same caliber. And where are the insects to live in the meantime?