Why ecosystems rely on dung beetles
One animal’s waste is another’s opportunity. Richard Jones answers a call of nature to celebrate the world’s unsung yet industrious dung beetles.
Ever since my dad showed me a massive blueblack dumbledor, probably Geotrupes spiniger, I’ve loved dung beetles. It clawed its way out through my clenched eight-year-old fingers and flew off across the South Downs, and my world was never the same again. It’ll be no surprise, therefore, to learn that my favourite exhibit at the British Museum is a giant stone sculpture of a 1.5m-long dung beetle. The sacred scarab was carved from green diorite sometime between 399 and 300 BCE, during the Ptolemaic era – already late in an enduring Egyptian fascination with dung beetles.
The first beetlemania had reached its zenith 2,000 years earlier, when there was a Mediterranean-wide industry in carving scarab pendants, from Sardinia to the Levant. Dung beetles were revered by the ancient Egyptians. Whether this was a circle-of-life recycling awareness, or simply a marvel at their bizarre dung-ball-rolling behaviour, is still open to polite scholarly debate. Nevertheless the ancient Egyptians created a deity in the beetles’ image – Khepri, the god of rebirth and sunrise.
How things have changed. Today, dung beetles are declining. Twelve per cent of the world’s 10,000 species are thought to be endangered or threatened. In Britain, worries over our own dung-beetle fauna led to the launch of the dung beetle UK mapping project (DUMP) last year.
For all their – to some – unsavoury habits, dung beetles are relatively well known in the UK, helped by good identification guides. Nevertheless, things are looking grim for the English scarab, Copris lunaris (not seen since 1960, in Sussex), Aphodius subterraneus (last glimpsed at Scarborough in 1954) and Onthophagus taurus (the last one was possibly observed in the New Forest in 1967).
ANCIENT ADMIRERS
Since dung is still being dropped by animals across the e globe, there must be some ecological shift to account for the scarabs’ now-altered fortunes. There are major ecological upheavals going on in the world of dung. The first is that dung beetles go unappreciated. How different it was in ancient Egypt! Scarabaeus sacer is usually reckoned to be the iconic dung scarab, but ancient Egyptian amulets portray no fewer than five or six genera, distinguished by the finely carved details of head fluting, presence or absence of ornate horns, and the delicate shape of thorax and wing-cases. There is no suggestion that the ancient carvers had much knowledge of scarab classification, yet they were obviously intimately familiar, on a personal and day-to-day basis, with these insects.
Sadly that fascination has been lost: part of the widespread disconnection between nature and an increasingly urban human society. Dung beetles – and dung flies – are now unsung heroes. Since most of us no longer have to deal with our own ordure (thank the flush toilet for that), it’s easy to forget what a debt is owed to these tiny creatures. Without them we’d be up to our necks in our own and our farm animals’ output by now.
In fact, this nearly happened in Australia, where 200 years after European colonisation dung from the nonnati ive farm animals was overwhelming the meadowlands. Th he problem is that not all dung is the same. Sheep, horseh and cow droppings are approximately 65 per cent, 75 per cent and 85 per cent water, respectively. But because the native Australian dung beetles were adapted to much drier marsupial dung, which is about 40 per cent water, they could not cope.
So the stock animal droppings lay where they dr ropped or splashed – unburied, uneaten, unrecycled.
WITHOUT DUNG BEETLES WE’D BE UP TO OUR NECKS IN OUR OWN AND OUR FARM ANIMALS’ OUTPUT BY NOW.
The usually quoted figure is that just 10 cows would smother a hectare of land in dung in a year. Something had to be done. Between 1968 and 1984, 1.73 million beetles of 43 Eurasian and African species more used to moister dung, were released into thousands of Australian meadows. Twenty-three imported species survived, and some of them did very well; the African Onthophagus gazella was off like a gazelle, extending its range by 50km a year, making a leap of 800km one year, and easily crossing 30km of open sea to colonise offshore islands.
There is still much to be done on what is really a terraforming project to alter the ecology of an entire continent. New releases continue, and all Australian dung beetlers have their own species import wishlists. Yet while the mechanics of dung removal can be applauded – measured in financial terms according to meadow clearance, grass growth and meat and milk output – a cloud of doom has descended.
DEADLY DUNG
Throughout the developed world, intensive animal production is now the farming norm, but to counter an increased disease and parasite load from large, close-packed herds, systemic insecticides known as ivermectins are routinely given in food, drenched onto the skin or injected into farm animals. These synthetic, ‘broadspectrum’ pesticides are highly effective at killing off flukes, intestinal worms, skin warbles and botflies, but can pass almost unaltered in the dung. The result is an insecticidal pat, which kills the very organisms that would normally recycle the stuff.
In one study, cows treated with a 0.2 mg per kg, by subcutaneous injection, were found to produce dung toxic to that saviour of the western Australian meadowlands, Onthophagus gazella, for seven days after administration. Excretion of just one microgram (that’s one-millionth of a gram) per kg of dung was toxic to the larvae of the yellow dung fly, Scathophaga stercoraria, once found everywhere.
There is much talk by pro-ivermectin agri-businesses of the ‘sublethal’ effects on dung beetles – a euphemism, if ever I heard one, for ‘not dead yet’. Similar arguments are used to justify the non-lethal effects of neonicotinoid pesticides on bees, butterflies and other pollinators, but that doesn’t necessarily make these chemicals safe.
One of the most insidious side-effects of ivermectins
WE ARE CREATING AN INSECTICIDAL PAT, WHICH KILLS THE VERY ORGANISMS THAT WOULD NORMALLY RECYCLE THE STUFF.
is that they seem to make the dung more attractive to dung beetles, compared to pats from untreated heifers. There are fears that this will over-expose some keystone beetle species to the deadly dung. And the long half-life of the chemicals in the environment must reawaken worries about accumulation further up the food chain, as happened with DDT in birds of prey in the 1950s and 1960s. In Britain for example, owls and smaller raptors such as the hobby target dusk-flying dung beetles, while in autumn two-thirds of the UK’s cow pats get picked apart by corvids such as choughs, rooks and carrion crows, which feast on the fat dung beetle grubs inside.
Farmers need to be able to raise healthy and profitable livestock, but the unthinking prophylactic administration of powerful pharmaceuticals is fraught with difficulties. It doesn’t take much to disorient a dung beetle so that maybe it forgets how to bury dung. The slowing of dung removal from ivermectin-treated herds has already started to occur. In an echo of the Australian cow dung debacle, ivermectins ns may eventually prove to be counterproductive.
Meanwhile, out in the wilderness, specialist forest dung beetles are the ones we know least about. They often have small fragmented populations, rely on erratic transient or migratory dung suppliers, and are the ones most likely to suffer from human interference. These make up the majority of the 12 per cent of dung beetle species for which alarm bells are now ringing. You have to admire an entomologist who, when discussing dung beetles, refers to a mammalian anus as a “life-giving portal”. Such, though, is exactly the relationship between many animals and their associated faecal fauna. It’s a real shame that the common name ‘wombat anus beetle’ has not been readily adopted for Onthophagus parvus, which has prehensile claws to hang to the hairs on the wombat’s rear end, and which drops off to lay its eggs whenever its host defaecates. Similar anal-hair gripping has evolved in the tiny Pedaridium beetles on sloths in South American jungles. Finding the dung in which to lay eggs may be easy, but weeks later, when the newly emerging generation of adult beetles is appearing, they now face the daunting task of locating a new host out in the forest. Habitat fragmentation and declining forest mammal
numbers make this ever harder; there may come a tipping point at which time hosts are too scattered and too few for the beetles to find, and these strange but fascinating insects will face oblivion.
Most associations are not quite this species-specific, but there is a host–beetle size ratio that needs to be addressed. Large dung beetles, especially when there are lots of them, need a considerable pile of droppings. There are verified reports of 4,000 tropical dung beetles arriving at half a litre of excrement exposed for 15 minutes – that’s nearly four and a half beetles per second.
There is a mad scramble for possession whenever that ‘portal’ opens and, in the tropics at least, the only way to secure enough of the raw material in which to lay an egg is to roll it away in a bolus the size of a tennis ball, or to dig a tunnel and stuff it out of sight as quickly as possible. These beetles simply could not subsist on rabbit crottels or deer fewmets alone. Remove the large mammalian megafauna, and the dung beetles will go too.
In Argentina, palaeontologists are still unearthing fossilised dung balls as large as (or larger) than those rolled around in Africa today. But there are no dungrolling beetles this big in South America today, nor indeed are there any large mammals to supply the dung. With the end of the last glaciation about 15,000 years ago, the ice sheet retreated, allowing the first human colonisation of North America, across the Bering Strait from Chukotka into Alaska. Within a few millennia humans had spread right down to Tierra de Fuego – and thatt spelled the end of many big mammals.
Glyptodon (a giant armadillo the size of a smart car), mastodon (a large, hairy elephant) and megatherium (a ground sloth as big as a polar bear) were soon gone, either hunted to extinction or because of climate change that was at least exacerbated by the newly arrived humans. The large South American dung-roller beetles were apparently unable to make do with howler monkey droppings or coatimundi scats. They are no more.
VITAL INDICATORS
The megafauna extinction in Australia occurred 30,000–50,000 years ago, and while palaeontologists may mourn the passing of diprotodon (a hippo-sized marsupial), procoptodon (a horse-sized kangaroo) and the marsupial lion thylacoleo, we as yet know nothing of the dung beetles that they once supplied. Today, the so-called Anthropocene era threatens to offer up another mass extinction, as elephants, rhinos, giraffes, gorillas and all the other charismatic species are hunted for meat and trophies, or driven out by climate change, habitat destruction or human encroachment.
These potential extinctions make dire headlines, yet their dung beetles are equally threatened. Smaller, less endearing, less cute, less well known, they are nevertheless important cogs in the intricate ecology of the world. Like so many other insects, dung beetles are hugely diverse, yet amenable to scientific study – if we look closely at them, they can be a barometer of environmental health, an early warning system of impending disaster in the biosphere.
Dung beetles are chunky, lustrous, handsome, aweinspiring. Just ask anyone who, as an eight-year-old, had a large scascarab heave its way out through tightly clam mped fingers, then fly off into the evening
sk ky like a miniature helicopter.
THERE ARE REPORTS OF 4,000 DUNG BEETLES ARRIVING AT HALF A LITRE OF EXCREMENT EXPOSED FOR 15 MINUTES.