Inheritors of the Earth

by Chris D. Thomas

  Summer melt-water fed into Lake Humber. As the ice melted, stones, sand and other coarse materials embedded in the ice were dropped as long ridges at the toe of the glacier, forming the Escrick and York moraines; today, low, gravelly hills that I drive up every morning on my way to work. But the fine minerals–the clays–were free to float on into the middle of the lake. To the south and south-west were the river systems of eastern England, bringing more suspended clays, which also floated out into the lake before settling at its base. There was no escape for these products of erosion, blocked by ice to the north, east and west, and by low hills to the south. The impounded waters spread out into a massive lake. Gradually, Lake Humber filled with clay, silt and sand.

  If I take out my microscope, I can find occasional head capsules of water-inhabiting midges in the grey gunge, and signs of long-gone Arctic diatoms, the suspended algae that once formed the base of the food chain and which would have supported the Arctic char and dagger-billed great-northern divers that were hunting fish.13 Arctic fox and snowy owls would have inhabited Lake Humber’s southern shores. None of these species lives in this region today, though divers continue to fly past the Yorkshire coast in winter. Still, it is relatively easy to imagine the world that existed between twenty thousand and fifteen thousand years ago.

  So it was until the day of the great Yorkshire flood. As the climate warmed approximately fifteen thousand years ago, the ice dam broke and Lake Humber drained away in a flash into what is now the North Sea,14 revealing a plain of clays, silts and sand. As frigid and desiccating conditions returned a few millennia later (11,500 to 12,800 years ago),15 the meagre vegetation cover that could gain a footing across this muddy plain was insufficient to prevent the wind from rearranging the former lake’s deposits. Wind-blown sand formed dunes across parts of the lake bed–now the Vale of York and Humberhead levels–and my house and land are perched on top of one of them. It was wind that dumped the enormous pile of sand on the clay-covered plain. The marshes of the Vale of York and these ancient dunes, which would have been covered by steppe-like vegetation, were home to furry lemmings, honking flocks of wild geese, antlered red deer, bone-crunching spotted hyenas, ancestral bison, woolly rhinoceros with their sweeping horns and ivory-bearing mammoths: a perfect hunting ground for summering ice-age humans.16 Biological community number four was a mammoth steppe. Most of the animals and plants that lived on that ancient steppe are today citizens of the high Arctic, although spotted hyenas, like geladas, went in the opposite direction. They survive only in Africa. The rhinoceros and mammoth were consumed by our ancestors.

  Coope was absolutely right. These ancient communities of animals and plants reveal that the biological world has been turned on its head repeatedly since the end of the last ice age. Lake species were replaced by dune, steppe and wetland species. These were supplanted by forest species that were in turn usurped, as humans converted the land, by species of open farmland and field margins. Then I made the land into hay pasture. And today, new arrivals are turning up as the climate warms once more, changing the biological community again.

  The moment we focus on timescales that exceed ten thousand years, we come to realize that species move around the surface of the planet when the environment changes. The set of species present on my own bit of land has been almost completely replaced at least four times in the last fifteen thousand years, and at least forty times in the last million years of repeated climatic changes. Biological communities are transient. Rearrangement followed by rearrangement is the norm. That is how species survive climate change. They move around. This is true even of species that are today confined to shrinking distributions in isolated mountains. They have survived because they have been able to move, even if that has only been up and down the same mountain. The greatest risk they face in the future is the possibility that they may not be able to move again.

  Standing in the squidgy hole I have just dug in my garden, I begin to appreciate something quite fundamental about life on our planet. Not only have the inhabitants of my small spot on Earth always been interlopers, but this is also true of every spot on Earth. Species come and go with the vagaries of the Earth’s climate, and with any other great environmental change. In due course, today’s interlopers will be replaced by the next set of temporary residents. Any attempt by humans to keep things just as they are is utterly pointless.

  The changing soils of the last twenty thousand years, in the Vale of York in England. At least five completely different sets of animals and plants have lived here during this extremely short space of time (‘Arctic’ lake, mammoth steppe, forest, cropland, pasture), demonstrating the dynamism of the biological world. Human-caused changes to the climate are now allowing a sixth set of species to become established.

  Wherever you live, it is worth taking out your spade. Not everyone is lucky enough to be living on top of geological deposits that were laid down during the last ice age and, even if you do, town planners may take a dim view when you start digging up the local park. But at least take out a virtual spade and imagine the history of your home.

  Twenty thousand years ago, Canada and large parts of northern Europe were under ice, so all the species that live in these areas at present have arrived since that time. Ice sheets locked up so much of the world’s water that sea levels were about 130 metres lower,17 hence the coral reefs that form the atolls of the Pacific and Indian Oceans have grown since the ice melted. All the species that live at the intertidal boundary between the land and the sea have moved to today’s coastlines since the climate warmed and the ice melted. At the height of the last ice age, many North American broad-leaved trees were living on now-submerged continental shelves under the waves of today’s Caribbean Sea–the animals and plants that live in North America’s great eastern forests are nearly all recent arrivals. The Amazonian forest contained mixtures of Andean, Amazonian and dry forest trees not seen today,18 the trees having since gone their separate ways. There were enormous lakes in the now-arid Great Basin of western North America, and the Sahara Desert was covered in trees and lakes as recently as 10,500 to 5,500 years ago.19

  The species that are found wherever you happen to live were different–often radically different–from what you see today. When the climate warmed after the last ice age, the distribution of species started to shift, dramatically rearranging themselves over the ensuing thousands of years. Boreal willow grouse and Arctic ptarmigan, which turn white in winter as camouflage against the snow, were hunted by our ice-age ancestors across the lowlands of central Europe, in locations which these birds no longer inhabit; they subsequently spread into the high mountains and to the far north of the continent, where they live today.20 Greenland collared lemmings did not live in Greenland but were found in a band across the United States, far to the south of their present range. This is perfectly normal. The redistribution of species in the last twenty thousand years has simply been the last act of the Pleistocene epoch, which has experienced a roller-coaster of alternating extreme cold (ice age) and warm (similar to now) interludes for the last million years. The ancestries of all species stretch across this period of instability. Individual species have survived by moving, and every location has seen a kaleidoscope of changing biological communities as new species arrive and others depart.

  It is hard for durations of longer than ten thousand years to grab our attention. Even so, it is essential that we learn lessons from the past. The magnitude of human-caused change to the climate between the middle of the twentieth century and the end of the twenty-first century will be on a par with changes that normally take tens of thousands of years. Moreover, the new world climate is likely to become warmer than it has been for three million years. If we wish to understand how the biological world responds to and survives climatic shifts, it is these longer timescales to which we must pay attention. Today, we regard these past transformations as natural, and we accept without question that species are found where they should be. We think of the new vegetation
and reefs that developed at the end of the last ice age, and of Greenland lemmings living in Greenland, as the way that the Earth is ‘meant to be’. However, while there are plenty of reasons why we would wish to reduce the rate at which humans alter our planet’s climate, there is no logic in defining this past change as good and natural and at the same time describing more recent and future change to the distributions of species as regrettable and unnatural. This is to impose an inappropriate human sense of territory and duration on the unique histories of every population of every species. Looking back from the twenty-first century to the last ice age, we appreciate that our ice-age selves would have been wholly mistaken to have resisted changes to the distributions of species that took place as a consequence of the climatic warming at that time.

  Whatever period we are considering, species have moved to take advantage of new opportunities that have arisen from time to time, just as they died out in places where conditions became unsuitable. It is the way our biological planet works. It is the same again today.

  The burnt ochre and chocolate-coloured comma butterfly, flexing its ragged-edged wings on the fissured trunk of my apple tree near York, contemplates its first flight of spring. It is a biological beneficiary of humanity, spreading northwards as the climate has warmed. Likewise, the red mason bee sunning itself on my human-built wall has been aided by our existence: it has been spending most of the day visiting spring flowers that I planted, constructing pollen-filled cells for its offspring in the cracks of my crumbling mortar, and enjoying a warmer climate.

  No specific butterfly knowledge is required to work out that the comma butterfly has spread northwards, nor is there any need to spend hour after hour searching through naphthalene-drenched museum drawers of preserved specimens that have seen better days–much as this activity brings joy to entomologists. The UK Biological Records Centre and the charity Butterfly Conservation have already amassed as many museum records as they can find, inspected the old literature and collated the records of amateur butterfly spotters. These they have brought together as one electronic database, which can be examined by anyone.21 The records show that the comma butterfly started to appear in Yorkshire in northern England during the 1970s, which coincided with the first major survey of British butterflies. This survey allowed John Heath, Ernie Pollard and my brother, Jeremy–the little boy in the wedding photograph in Chapter 2–to publish the first national butterfly atlas for Britain in 1984. At that time, Yorkshire was as far north as commas could be found.22 However, the butterfly was soon to continue its path towards the North Pole, reaching Aberdeen in north-eastern Scotland a mere twenty-five years later.

  The comma butterfly has spread approximately 350 kilometres northwards since the 1970s because humans have warmed the climate by around 1°C, enabling it to live in regions that were previously too cold (black squares represent records between 1970 and 1982; grey squares show new locations that were colonized between 1983 and 2015. The side of each square is 10 kilometres).

  Not only have we provided the comma with a suitable climate, we have also provisioned it with food. The comma’s caterpillars feed on wild hops in my hedgerow, a plant that grows there because it escaped from the hop gardens that were planted in this region to supply the brewers of Yorkshire. These included Webster’s Brewery, founded by Samuel Webster in 1838, my own great-great grandfather, whose portrait looks down on me as I write these words. But for the brewers who needed hops, and the farmers who planted the hedgerows as well as the crop, the butterfly’s caterpillars would not have had any hops to eat. Thanks to Victorian naturalists, we know that commas did indeed fly across the Vale of York in this heyday of hop growing, but then they retreated southwards, before recolonizing in the 1970s. Hops are not that common any longer in this part of the world, since Yorkshire brewers source their ingredients from further afield. But stinging nettles abound, beneficiaries of farmers spreading liberal amounts of nitrogen and phosphorus fertilizers across the landscape to enrich their fields. Nutrient-loving nettles are thriving–a success story of their own–providing yet another opportunity for the comma butterfly.

  My friend and colleague Jane Hill, whose own gleaming ginger pelt gives the mason bee a run for its money, wondered why the comma had been quite so spectacularly successful in recent years, so she and postdoctoral researcher Brigitte Braschler decided to investigate. They set off in search of adult butterflies that had just woken up from their long winter snooze. During the dark, cold winter months, these butterflies sit motionless, the blackened, ragged undersides of their wings virtually indistinguishable from dead winter leaves. As spring arrives, they start to appear, warm themselves up, seek nectar to drink, find a mate and, in the case of the females, begin looking for any suitable plants where they will be able to deposit their eggs. Jane and Brigitte caught some female butterflies from near York, and others from further south, and brought them back to their laboratory, where they kept the butterflies in a toasty-warm room to lay their eggs.

  As the eggs hatched, some of the caterpillars were given hops to eat, the plant that British comma caterpillars traditionally ate, whereas others were provided with a delectable diet of stinging nettles. After months of tending their lepidopteran family, Jane and Brigitte found out that the caterpillars from the newly colonized region of York survived better on a diet of nettles than on hops, compared to the caterpillars whose parents had been caught in the south of England. Moreover, this advantage was enhanced at higher temperatures.23 It turned out that the northern commas, which had established a new population in the Vale of York and then continued to spread even further north, have recently evolved a liking for stinging nettles under a human-warmed climate. This, combined with the burgeoning population of fertilizer-dependent nettles, has been propelling the butterflies northwards.

  Of course, the butterfly itself has no other purpose than to mate and lay its eggs. It is itself oblivious to the changing world. It just happens. Each individual lives and dies, and moves a bit, and breeds. And on it goes. Over the generations, climatic conditions became more favourable and small changes have seemingly taken place in the butterfly’s ancestors’ genetic code that help it eat nettles; changes that have been sufficient for the comma butterfly to have moved the 350 kilometres from York to Aberdeen in just twenty-five to thirty years. It will soon reach the Arctic Ocean. It is easy to see that fairly modest changes to the distributions of each species that we can observe on a timescale of a few decades are sufficient, if replicated across millions of species, to rearrange life on Earth over the thousands of years of human influence. All it takes is a little time, just as it did when species were moving around the world as they responded to climatic changes during the Pleistocene epoch.

  The climatic changes that humans have already wrought have brought a cavalcade of animal adventurers to new locations. A few months after spotting the comma and mason bee sunning themselves, I accompanied my domestic wolf for a spin around the garden and decided to count the butterflies. There was a rusty comma, perhaps the offspring of one that I had seen in the spring, this time seeking out the nectar of a bramble flower. Incoming species number one. Female ringlet butterflies, which have perfect white rings marking their deep-brown undersides, were plying up and down, dropping their eggs into tussocky grasses at the edge of the field. Two. Male speckled woods sat perkily in shafts of sunlight between the trees, flying out to intercept females and chase off rival males. Three. Gatekeepers were flitting along the hedgerow and feasting on marjoram flowers on my rockery. Four. A deep orange Essex skipper was darting back and forth between purple knapweed flowers–a butterfly whose name is associated with the more southerly county in England, whence it came. All five were absent from this land a mere fifty years ago, before they expanded their ranges when the climate warmed. Five out of thirteen different species I saw that day–over a third of the species flying in my garden–were there only because of human-made changes to the climate.

  These were not the only five to
benefit. When I moved into my present house in the year 2000, I saw one sulphur-yellow brimstone in the entire year, whereas a decade and a half later I was able to count half a dozen soaring over the buckthorn bushes in just five minutes. Equally, small skippers–which can be distinguished from the Essex skippers only by the colour of their antennae–would have been much rarer fifty years ago. If we count the brimstone and small skipper as well, then half the species flying in my garden on a summer day in 2016 were beneficiaries of climate change. And it is not just the butterflies. The red mason bee has been joined by tree bumblebees, colonizing from the south in the last few years, and I spotted a little egret earlier in the season. The egret is a gleaming-white heron, identified by its brilliant-yellow feet, which trail behind as it flies along drainage ditches searching for frogs and insect larvae. This bird bred on the south coast of England for the first time in 1996, and now I can see it from home–it has colonized more than half the country in twenty years. Humans have changed the climate, and the distributions of species have changed as a result.

  An inexorable march of the world’s wildlife is under way. Moths deep in the forests of Mount Kinabalu in northern Borneo and trees growing on the forested slopes of the Andes have moved to higher altitudes as the climate has warmed. Amphibians and reptiles now live at higher elevations in the mountains of Madagascar. Birds have shifted higher in New Guinea, and they have moved up from the lowlands in Costa Rica. Mammals and birds have moved to higher elevations in the Sierra Nevada in California. Birds have expanded their ranges northwards across the European and North American continents. Plants in European mountain ranges have shifted upwards. Fish, marine plankton and shellfish are steaming northwards in the seas and oceans of the northern hemisphere, and heading towards Antarctica in the south. Warm-water Australian fish have colonized Tasmanian reefs that used to be too chilly. Even those animals that are unaffected by temperature and rainfall directly are living in places where the vegetation they eat and the animals that prey on them have already changed. The footprint of human-caused climate change is ubiquitous.