Inheritors of the Earth Page 8
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Steaming ahead
Three groups of primates were perched above the dramatic gorge of the Jemma River, a tributary of the Blue Nile. Agricultural terraces lined the gentler slopes, interspersed by rugged crags where rusty-breasted, moustachioed vultures soared on the up-draughts. Each group had its own purpose. Droves of smartly dressed, brown-skinned Homo sapiens made their way along the paved road, heading on their pilgrimages to the nearby Debre Libanos monastery, seeking absolution from the humdrum drudgery of mortal life. The second group, to which my wife Helen and I belonged, was considerably scruffier; sun-hatted, pink-skinned Homo sapiens resplendent in dangling binoculars and telephoto lenses. We scurried in a more chaotic manner towards the cliff edge, engaged in a wildlife pilgrimage. One of the group, a septuagenarian greybeard, was particularly dogged, seeing how close he could get in his enthusiasm to capture a fine portrait of his quarry–the third primate. This turned out to be a few metres closer than anyone else thought was wise. The third group of primates was not afraid of pale, pink humans.
The baboon-like geladas sought their own solace in a bachelor troop, sitting comfortably in the dappled shade of imported Australian Eucalyptus trees, sniffing the air for the scent of fertile females. They were relaxing, avoiding the heat of the day, while the somewhat less hairy females and their infants were happily munching their way through a field full of grass; their penchant for eating crops explains why the locals quite reasonably throw stones to keep the greedy animals away. Little did the geladas of Debre Libanos know, but greybeard Roger and his companions had just flown thousands of kilometres in their quest. In so doing, they had just contributed more than their fair share of greenhouse gases to the atmosphere. Our contribution to climate change was potentially more dangerous to the geladas than a few stone-wielding locals.
The mountain geladas are covered in a thick pelt of silvery-brown hair, punctuated by a bizarre, hourglass-shaped pink expanse of chest skin. Their dense fur enables them to cope with cold mountain conditions, but they would overheat–and their diet of grass would be of poor quality during the dry season–at the sweltering lower elevations where olive baboons can be found. Confined to altitudes above 1,800 metres, the only places in the world where geladas can be found are in Ethiopia’s towering Simien Mountains and in gorges like that of the Jemma River.1 Temperatures in mountain ranges decrease by between about 0.5°C and 1°C for every hundred-metre increase in elevation,2 so we can work out how far they might be forced to retreat as the climate warms. This part of the world may heat up by as much as 5°C over the coming century;3 if this were to happen, geladas would need to move upwards by 500 to 1,000 metres–potentially to elevations above 2,800 metres. Fortunately, these hairy primates can already be found higher than this in the Simian Mountains, so they should survive global warming, even if populations below this disappear, like those in the Jemma Gorge.
Not so the Ethiopian wolf, squeezed between humanity and the clouds. While the gelada population numbers around two hundred thousand, only five hundred orange and pale buff Ethiopian wolves remain, of which barely two hundred are mature, reproducing adults.4 They are restricted to much higher altitudes than the geladas. About 113 of these adults, over half of the world population of the species, are confined to the Bale Mountains, and this is where they are easy to see. Or so we were told. Alas, our early-morning departure to watch wolves foraging at dawn was thwarted because a section of the precipitous road to the high plateau had been washed away by a storm in the night.
Abiy, our Ethiopian guide, and an assorted collection of British holiday-makers leapt from the vehicle to gather stones, filling the gullied track with whatever came to hand. An eternity later, or so it seemed, the brave troop of hominids had filled in the deepest of the ruts and our driver could attempt the climb. But alas, we were too late: the wolves had gone back to bed for the day. Refusing to give up, we spent the following six hours with numbed fingers, dizzily rushing back and forth in the thin 3,500 metre-plus mountain air–but the closest we got to a wolf was to spy a distant orange spot on the horizon. Reluctantly, frozen to the core, we set off home across the undulating plateau when, suddenly, over the brow of a ridge, there it was: a full-sized wolf standing next to the road.
Gelada families.
Graze by picking grass blades with their rather stubby fingers, in a pasture perched above the Jemma Gorge in Ethiopia, where they retreat to steep crags to avoid disturbance (notice a few geladas on the rocks in the foreground).
Groups of males sit out the midday heat in the dappled shade of Eucalyptus trees.
The wolves had come back out again for their evening snack of giant mole-rats, an endangered rodent that is completely confined to the Bale Mountains. It was hard not to feel sorry for them. Like enormous hamsters or lemmings, they excavate underground passages, only coming up to graze on mountain vegetation that is close enough to their burrows that they can retreat if danger threatens in the form of a wolf or an eagle. The landscape is full of meal-sized rodents, and this allows the wolves to live at higher densities in this mountain range than they could anywhere else.5 There are about two and a half thousand giant mole-rats per square kilometre of suitable habitat, which explains why over half of the world population of Ethiopian wolves lives in the Bale Mountains, and why the wolves have muddy paws from digging them out. Although mole-rats can be found from 3,000 metres to nearly 4,200 metres altitude, most of them are restricted to a much narrower band, primarily in the short Afro-alpine vegetation at 3,500 metres and above. This means that they, along with the wolves, are truly endangered by climate change.
The Ethiopian wolf is confined to the mountains of Ethiopia, with more than half of the world population living in the Bale Mountains.
The wolves inhabit the freezing-cold Afro-alpine zone, where giant lobelia plants pepper an otherwise tundra-like mountain vegetation. There, the muddy-pawed wolves dig…
… for burrowing giant mole-rats.
The wolves’ world distribution is restricted to the Bale Mountains. Both species are endangered by human-caused climate change.
The wolf itself would probably be perfectly happy living in the warmer lowlands, but this would bring it into conflict with people or, rather, with their domestic dogs. Dog-borne rabies and distemper have extinguished the susceptible wolves from lower altitudes (there are no recent wolf sightings below 3,000 metres) and periodic epidemics threaten even those on the high plateau. Warmer temperatures may bring people and their dogs to higher elevations and confine the wolves yet more.6 Perhaps the greater risk from climate change, however, is the disappearance of their food. Move down just a couple of hundred metres from the frigid Afro-alpine zone and one arrives in a shrubbier vegetation where mole-rat and wolf densities are barely a tenth of those on the high plateau. If human-caused climate warming causes this somewhat lower-elevation vegetation to start growing 500 metres to 1,000 metres higher than at present, virtually the whole of the area would turn into a rat- and wolf-poor shrubbery. The maths is grim. A nine-tenths reduction in density applied to the 113 mature breeding wolves that currently live in the Bale Mountains would be 11, and there would probably only be 20 to 30 animals in total if non-breeding adults and juveniles are included. This would not be enough for a viable population to survive several centuries of a warmer climate. Unlike the geladas, there is no opportunity for the mole rats and wolves to survive anywhere higher.7 The world’s climate is getting hotter, and they have nowhere to go.
Future threats of the kind faced by the Ethiopian wolf seem clear enough, but we should remember that global temperatures have been rising since the 1970s. Climate change is not only about the future. This gives us the opportunity to find out whether these dire predictions are likely to be borne out. Have mountain species begun their retreats to higher elevations?
In 2007, my fearless PhD student I-Ching Chen set out to find the answer. The task she set herself was to climb up and down Mount Kinabalu in Borneo, lugging heavy b
atteries and awkward moth traps along with her. Catching moths on a tropical mountain might seem like an odd activity, but moths are lovely, delicately camouflaged animals. Some of them resemble dead leaves, and others the moss- and lichen-encrusted trunks of forest trees, patterns that enable them to remain hidden from the prying eyes of insectivorous birds that seek them out during the day. More to the point, moths form part of the most diverse set of animals on our planet, tropical insects, and hence they represent a crucial element of the world’s biodiversity–eating the vegetation and in turn being consumed by larger animals. The choice of Mount Kinabalu itself was dictated by the fact that it was possible to repeat a survey of moths which had previously been carried out by three undergraduate students during their university vacation in 1965. After she and her intrepid husband had spent weeks with aching backs and limbs trapping the moths, and another year peering at the specimens to identify them, I-Ching was in a position to answer the question. The moths had indeed moved upwards, retreating from the lowlands. Moreover, the species that live in the ever-damp cloud forest have exhibited steep declines, seemingly caught in a climatic pincer movement between temperatures that have become too hot at low elevations and changing cloud cover higher on the mountain.8 The risk of extinction is real.
Half a world away, in the spectacular cordilleras of Central and South America, dozens of species of harlequin frog have already disappeared. They are–or were–among the most beautiful amphibians in the world; their brilliant oranges, blues, blacks, greens, purples and pinks (depending on which species of frog it is) warn would-be predators that their skin contains a deadly neurotoxin. Sadly, these frogs have suffered from a perfect storm of human-caused climate warming, El Niño currents in the Pacific (which accelerate warming and alter rainfall patterns), and epidemics of a nasty fungal skin disease which is thought to have originated in Africa.9 Each successive hottest year brings new fatal epidemics of the invasive fungus, and yet more extinctions.
The prognosis is not good. Climate-change casualties are set to become the next phase of humanity’s mass extinction–at least 10 per cent of all species that live on the land are expected to perish, and possibly double this number.10 In the face of such dire predictions, what can we do? Of course, reducing our greenhouse-gas emissions is the number-one priority, but even the lowest expected levels of future warming are liable to exterminate many of these species. They are trapped. Animals confined to tropical mountains cannot descend to the lowlands and seek out colder mountain ranges thousands of kilometres away. Insects and plants that live only in moist ravines cannot cross deserts to reach new homes. The only realistic conservation option–if we wish to save them–is for us to start transporting them to new locations where they will find the future climate more to their liking.11 Yet how do we know that species are more likely to survive if they move to new places? We need to know how species have survived periods of rapid climate change in the past.
When, in 1954, the densely bearded and exuberantly eyebrowed English scientist Russell Coope visited Upton Warren, a damp hole that had been created by the extraction of gravel in the English county of Worcestershire, he could not have imagined that he was about to change both his own career and science’s understanding of the distributions of species. Not expecting to do anything other than take a cursory look around, he had not brought any collecting gear with him. But nestling among the mammoth and woolly rhinoceros bones–which he already knew could be found there–in deposits dating from the last ice age, he spotted some dark matter that piqued his curiosity. So Russell rapidly consumed his emergency rations and scraped out the dark material into his biscuit tin. Back in the lab, he inspected it under the microscope, and soon realized that the black material was the wing cases and head capsules of ancient dung beetles which had evidently been feasting on faecal gifts that had been deposited by the mammoths and rhinos some twenty thousand years ago.
Off he set, on a tour of the gravel pits of southern England. The beetle kept reappearing, and he even found 150 different individuals of the same species in a pit near the village of Dorchester-on-Thames in Oxfordshire. Russell was a beetle expert, but he couldn’t identify it. The beetle belonged to the genus Aphodius, of that he could be sure, but what was it? Eventually, after exploring the musty drawers of museums and consulting with colleagues, he tracked it down. It went by the name of Aphodius holdereri. No wonder he didn’t know what it was. It is a species known only at altitudes of over 3,000 metres in Tibet.12 Any contemporary ecologist or evolutionary biologist inspecting the present-day distribution of this insect might have assumed that the species had evolved in isolation, high in the mountains there. But no, the Tibetan population is simply the last refuge of what was once the commonest dung beetle in north-western Europe.
Russell was on a roll. The dark matter revealed dozens of species of beetle, all of which used to live in Britain but which now live thousands of kilometres away, many in the Arctic, in frigid environments similar to those that were widespread in Europe during the last ice age. And there were other bizarre examples. One of these once-British beetle species is now confined to the Pyrenees. Another today lives only in eastern Asia. In many respects, they are just like geladas. Fossil bones that date back to the Pleistocene epoch (a time of intermittent ice ages from 2.6 million to 11,700 years ago) reveal that relatives of geladas formerly roamed from the southernmost parts of the African continent, up through the Congo, across northern Africa, Spain and elsewhere in southern Europe, and eastwards into the Indian subcontinent. Now they face their last stand in the mountains of Ethiopia. The gelada and ice-age dung beetles have behaved in rather similar ways.
Russell went to war with the conventional view that species evolved where you find them today. Starting with a bit of dirt scraped into a biscuit tin, he demonstrated that many species currently live in places where they happen to survive, rather than where they originally evolved. Some species were much more localized in the past, others more widespread; some were not necessarily any more or less widespread, but they nonetheless lived in different places. This idea soon took root among scientists who study the history of life on Earth but, surprisingly, its importance is still only partially appreciated by many researchers who are primarily interested in the present-day ecology of the world and by those who attempt to protect it. Dynamism is the norm, not the exception. It is how species survive times when the world’s climate changes.
Half a century later, inspired by Coope and by his botanist contemporaries, I grabbed my spade and set off down the garden. As a child, I may have dreamt of digging a hole through the centre of the Earth to reach Australia but, as a fifty-something academic, I took on a more practical challenge: to dig a hole so that I could visit the last ice age.
The top twenty centimetres of my horse pasture proved to be quite ordinary, sandy soil. This soil is a record of recent history, containing most of the roots of the meadow plants and occasional fragments of brick, glass and pottery from the last three centuries. This hay meadow is biological community number one. I dug on through increasingly sandy soil, which was churned from having been ploughed; it would have supported crops for a thousand or more years. Biological community number two is the cereal field. Before that, forest trees would have grown in place of these crops, in a Holocene epoch landscape that existed from five to ten thousand years ago. Biological community number three–deciduous forest. Then I reached a rusty-looking, crunchy layer, a hard pan of iron-rich encrustations separating the browner soil from what is below. Below it was almost pure orange sand, as though I had bought it from a builder’s merchant, although streaked by the odd root and the darker brown burrows of earthworms. Biological community number four–this was presumably different again, but I could not see anything that told me what grew there. I kept digging and found more dry sand, then, close to a metre down, I came across clay, reflecting an earlier history. The clay was so thick and hard that it required a much smaller and sharper spade to penetrate it at all. Th
en, when it rained, it turned into gloopy, adhesive clay and held the water like a pond. Sand was sitting on top of clay, with the sharpest of possible transitions between the two; it was as if someone had dumped an enormous pile of sand on top of a flat plain of potter’s clay. I celebrated, spade in hand. An hour and a half of digging and I had reached the ice age.
Resting in my newly dug hole, I was standing on clay that was laid down over fifteen thousand years ago. At that time, my ‘land’ was at the bottom of a massive body of water called Lake Humber. Biological community number five–glacial lake. I could imagine myself fishing for red-bellied Arctic char, protected from the cold winds by reindeer-hide clothing, my ears warmed by an Arctic fox hat, its white fur the most insulating in the world. Looking northwards to where the City of York has since been built, I imagine the lake’s ice-covered shore, which at that time would have been the bounds of the habitable world. Crumbling ice cliffs melted into Lake Humber’s chilly waters, the edge of a white massif that was one of the world’s great geographical features of its day. A plateau of ice extended from the western limits of Ireland as one uninterrupted ice sheet over northern Britain, across the North Sea, northern Germany and Poland, up through Scandinavia, and on across the Arctic Ocean, north to the islands of Svalbard.