Inheritors of the Earth

by Chris D. Thomas

  This is a story of gain as well as loss. Around two-thirds of the species that researchers have studied in recent decades have shifted their distributions in response to climate change, becoming commoner in those places where the climate has ‘improved’ for them.24 Animals are moving towards the poles at around 17 kilometres a decade.25 This is the equivalent of picking up every individual of every animal species and moving it just over four and a half metres every day. This has been going on every day of every year since human-caused climate warming became evident in the mid-1970s. Of course, this is not quite how it happens. In reality, a butterfly occasionally flies an unusually long distance, perhaps five kilometres, and establishes a new colony further north–or south in the southern hemisphere. A few generations later, the new colony has grown large enough that individuals leave it and set off on the next leg of their journey towards the poles. Likewise, a seed may be blown a long distance by a storm, travel inside the stomach of a bird, or be moved in the mud attached to the tyre of a car. The journey is a series of steps, not a smooth progression across the world. But averaged out over a few decades, the overall consequence of all these steps is the progressive movement of vast numbers of species, as they die out in places that are now too hot or dry and colonize new locations that have recently become warm enough for them to breed. Keep this going for a few centuries and we have a new biological world order. If my garden butterflies are anything to go by, we are already a third of the way there.

  The ubiquity of this movement across the surface of the Earth means that something like two-thirds of animal species are already living in at least some ‘new places’ where they could not have survived as recently as fifty years ago, while they have disappeared from other locations. As the present century unfolds, the overlap will progressively decrease between the ‘historical’ distribution of each species and where that species will then be living. As we found when we contemplated the ice ages, the idea that the distribution of a species is fixed is outmoded. It can still be convenient to think of species as being native to South America or to Eurasia, for example, but it is increasingly unrealistic to suppose that there is anything special about the precise locations where humans first documented the presence of a given plant or animal. It was one frame in the passage of time and the movement of species, of no more and no less significance than any other frame.

  As the climate warms, these new opportunities will increase. The average temperature of the world is only about 14°C, whereas most species reach their peak physiological performance well above this. Many tropical and subtropical plants are damaged below 10°C to 15°C, so they have the potential to spread when temperatures increase. Planktonic algae commonly reach peak performance at 15°C to 30°C, so they are likely to spread as the world’s oceans warm.26 The average annual temperature is about 10°C in cool Britannia,27 yet most British insects become fully active only when temperatures are in the upper teens, and they generally do best above 20°C. On the whole, more species like it hot than cold.28 This means that the 1°C global warming that has taken place between the late 1800s and the 2010s has unleashed large numbers of heat-loving species. Tropical species can start to spill out into the subtropics, subtropical species colonize temperate regions, and the inhabitants of the temperate zone can try their luck in the polar world; and this has the potential to increase the diversity of those places where they arrive.

  This is already happening. Many immigrants from the south have arrived in my garden in recent times, but I am not aware of any northerners that have disappeared. If we consider this more systematically, the diversity of butterflies has increased in most parts of Britain as a consequence of southern species expanding their distributions.29 It is similar elsewhere. The number of low-elevation bird species that moved uphill into Costa Rica’s Monteverde cloud forest between 1979 and 1998 greatly exceeded the number of higher-elevation birds that died out:30 golden-crowned warblers, lesser greenlets, flamboyant keel-billed toucans and twelve additional bird species from the lower slopes began to nest high up the mountain, alongside the emerald-green and scarlet resplendent quetzals that characterize Central America’s dank cloud forests. More species arrived than disappeared, increasing the number of bird species in the forest. If we consider the entire world’s land surface, climate change is liable to increase plant diversity across much of it.31 Given how many species live in the hottest parts of the world, it is not surprising that the average biological diversity per square kilometre of the world goes up when the climate warms.

  There are three important caveats. First, we should not forget that the Ethiopian wolf, Kinabalu moths and other species are endangered. This will reduce the total number of species on Earth in coming centuries–although geological history tells us that additional species are likely to evolve if the Earth remains hot for a million or more years.32 The second is that species need to be able to reach the new locations where they will be able to thrive. Plant diversity will not increase unless seeds are able to get to the places where they can grow–most likely, this transport will be aided by us. The third caveat is that diversity goes down where there is not enough water. This is the real concern. Biological diversity thrives where it is hot, but it does so only if there is enough moisture for plants to grow, and for plant-eating insects to digest them. When my former colleagues Rob Wilson, David Gutiérrez and Javier Gutiérrez hiked around the increasingly hot and dry mountains that surround Madrid in central Spain in the 2000s, they discovered that the average number of butterfly species was lower than it had been in the early 1970s, before the climate had warmed.33 This is because the relationship between temperature and diversity is reversed there. Go butterfly hunting in the Sahara Desert, and you will find fewer species than in the cooler Spanish mountains–the Sahara is too dry. If increasingly Sahara-like conditions start to spread, the mountain butterflies will disappear, and there are not so many drought-adapted species available to replace them. Thus, climate change brings reductions in diversity in some places, just as it generates increases in others. Averaged across the whole world, however, the amount of rainfall is increasing. More water evaporates from hotter oceans, and it comes back down again as more rainfall. Places that become warmer and remain wet usually gain biological diversity, as do dry places that experience increases in rainfall.34

  This is reasonably good news, but it is not an argument advocating that we stop worrying about climate change. A three-quarters-full cup is still a quarter empty. Coral reefs are threatened by hotter and more acidic water and, if we want to keep Ethiopian wolves alive in the mountains of Ethiopia, we must minimize the greenhouse gases that are emitted into the atmosphere. Nonetheless, the climate is changing, and the basic expectation of a warmer and slightly wetter world is that the diversity of many–and perhaps most–regions in the world will increase. This is what we are starting to see, accomplished by the movement of species across the surface of the Earth. We need to accept and even encourage this movement because botanical and zoological world travellers will form the basis of the world’s new ecosystems, just as they have when the climate has changed in the past. These travellers are the future of life on a warmer Earth.

  5

  Pangea reunited

  The exuberant green growth of palm seedlings spilled out of the forest and on to the edge of the winding road. The deeply dissected leaves of Chusan windmill palms flickered in my headlights. This palm grows further north than any other in the world and, at last, I was seeing it for myself, in the wild. I had tried to grow it back home in Yorkshire, but failed. The seeds I planted germinated well enough, but the seedlings had perished, each meagre bout of summer growth cut back by the following winter’s cold. In the slightly milder climate I was now visiting there was hardly enough space to accommodate the chest-high, trembling fronds that formed an evergreen thicket. I swerved closer for a better look. Too close. The high stone kerb penetrated the side of the tyre and immobilized the hire-car that I had picked up earlier that same d
ay. And there I was, stranded in a foreign country, surrounded by a living herbarium of exotic greenery, unable to speak the local language and in a vehicle without sufficient tools to replace the wheel. I was left attempting to communicate in sign language with some locals whose gas station doubled up as the village bar, rather than as a workshop.

  From the surrounding vegetation full of Chinese palms and Nepalese camphor trees, I could have imagined that I would need to speak Mandarin or Nepali, hence my inability to communicate might have seemed understandable. But I was stuck by the shore of Lake Maggiore in Switzerland, and only Italian would suffice. Our educations had failed us all. We gesticulated with enthusiasm and attempted to communicate in pidgin French, German and Spanish. We phoned Giovanni’s wife, who did speak a few words of English. Several hours later, I continued on my way, the kind clientele of the bar-cum-gas-station having persuaded an Italian vehicle rescue service to cross the border into Switzerland and make running repairs to my car.

  The following day revealed a flickering, Hockneyesque palm-fringed hotel swimming pool with a backdrop of snow-capped peaks and forested mountainsides, accompanied by a soundtrack of cheeping Italian sparrows. One could, for a moment, have believed man and nature to be in harmony, yet Lake Maggiore is a postcard from the Anthropocene. The shores of this enormous alpine lake have been altered by humans ever since the ice-gouged trench of the retreating glacier filled with water. Ancient grasslands and forests waxed and waned with the vagaries of the climate at the end of the last ice age,1 yet all these changes already bore the signature of humanity. Our ancestors had killed off Europe’s hippopotamus, the forest elephants and rhinoceros, as well as the lions and Etruscan bears. As a consequence, this new megafauna-free forest was much darker than it would otherwise have been.2 The Stone Age around Maggiore gave way to the Copper Age, about seven thousand years ago, then the Bronze. The forest was cut and cut again: for fuel to cook food and smelt metal, and to build houses and livestock pens. Parts of the forest were cleared to cultivate crop plants that had originated in the Middle East, and domestic animals arrived. The period of humans deliberately importing useful new species had begun. The Romans introduced sweet-chestnut trees for their seeds. They carefully peeled back the prickly rind of their fruits to reveal the glistening nuts, which they ground into flour. By AD450 the chestnut had become established as a major part of the forest in the hills above Ancona, near to the head of the lake–a cultivated species gone wild. The landscape never stood still. Nor did the species that lived in it.

  The speed of transformation accelerated with the initiation of the Industrial Revolution. In 1808, the small lakeside town of Intra became home to Italy’s first steam loom, a picturesque ‘Manchester of Italy’. Industrial wealth meant rich people, and rich people in the nineteenth century craved country villas and gardens. Their enthusiasm for exotic plants to embellish the land became a passion and, to this day, the shores of Lake Maggiore are dotted with plant nurseries and garden centres. A new era had dawned. From a modest start, in which humans had deliberately transported a few different kinds of crops, livestock and medicinal plants over thousands of years, the importation of new species had gone into overdrive. The age of European palm forests was about to commence.

  As I set off the morning after my nocturnal adventure, the early-spring mountainside above Locarno looked surprisingly green. On closer inspection, deciduous trees that had not yet spread their bright summer livery could be seen standing as twiggy sentinels above the billowing sombre green crowns of the shorter evergreens. As I entered the forest, the insanity of it struck me, and my meagre botanical skills were found wanting.

  Some, though, were already familiar to me, especially the leafless species that would have lived in the original forest. The deciduous oaks, linden trees, ash, alder and Roman-introduced sweet chestnuts, all of which drop their leaves in the winter, also grow at home in England. The evergreen holly and ivy were familiar too. Prickly holly trees punctuated the under-storey, and native ivy clambered over the trunks of deciduous trees, clothing their brown stems in green and scrambling across the forest floor. There were also species from elsewhere in Europe, the aromas of which were more familiar to me than the plants themselves. Cherry laurels could be identified by the cyanide smell of their crushed leaves. They were native to south-eastern Europe and the Black Sea but now thrive in Maggiore’s lakeside forests. Bay laurel, familiar to cooks for its essential oils, was prized as a symbol of high status in the Greek and Roman worlds, and a laurel wreath was worn by the ancient god Apollo. This tree grew elsewhere in the Mediterranean in the past, but it has been planted more widely and now regenerates with abandon in these new forests.

  Others were less familiar. Nepalese camphor trees were growing high into the forest canopy, equally identifiable by their odour. Transported from the Himalayas, camphor is today a tree of the Alpine foothills, part of the new Insubria forest (Insubria is the region that spans northern Italy and southern Switzerland; Lake Maggiore straddles the two countries). There they grow, alongside all manner of Asian plants: suckering bamboos, silverthorn, broad-leaved privet and box-leaved honeysuckle, willow-leaved cotoneaster from western China, loquat with their bright orange fruits from southern China, Henry’s honeysuckle, the rampant evergreen Japanese honeysuckle whose vanilla-scented yellow and white flowers give rise to small black berries, and sweet-scented Japanese mock-orange and camellia, whose waxy-pink flowers are visited by European bumblebees. Not to be outdone by floral beauties from the Orient, occasional creamy-flowered bull bay magnolias from the US states of Virginia and Florida can be spotted, as well as Oregon-grapes, and silver wattle from Australia. It is a diverse and fascinating forest, however unlikely its origins. Meanwhile, there seems to be no real threat to the European deciduous forest trees. Higher up the mountain, it is still far too cold for the usurpers to survive. The entire forest that encompasses the shores of Lake Maggiore and the chilly upper slopes is far more biologically diverse than it used to be.

  While industrialists were responsible for transporting these plants from far-flung parts of the world, their journey from garden to forest was accomplished by a team of European and Asian seed-movers and seed-buriers. The nutritious windmill palm fruits seem irresistible to European blackbirds, whose glistening black feathers and gaudy plastic-orange beaks adorn grassy lawns throughout Europe. Even they are not quite what they used to be. Originating as shy, forest-dwelling birds that shunned human presence, it is now hard to find a European town or city where they do not hop across lawns and sing from the shrubberies.3 Maggiore’s waterfront towns and villages abound with these newly confident birds, as well as feral rock doves and collared doves, the latter having spread from India and into western Europe in the twentieth century. Yet more success stories. Then there are jays and wood pigeons, with their own urbanization stories, and Italian sparrows, the new species formed by hybridization between Asian house sparrows and Europe’s own Spanish sparrows. Add a host of mice and squirrels, which run off with seeds and bury them in caches to secure themselves a food supply for the winter, and the full company of seed-movers is assembled.

  This team transports seeds in astonishing numbers. Seedlings sprout along the bases of hedgerows, in the back of every garden border, and under any tree where a defecating or regurgitating bird might sit. A profusion of saplings can be found in the adjacent forest. Wherever the wildling palms have grown large enough to produce their own fruits, the under-storey is a veritable jungle of second-generation fronds. A few seeds have been transported half a kilometre, or even further, from the nearest palm-containing garden, which is about as far as most of the birds can be expected to fly in one trip. Within a palm generation or two–a human lifetime–virtually all of the warm, forested slopes that line the banks of the lake will be verdant palmeries.

  Careful examination of garden records from nurseries and from the large estates of the new industrialists indicate that most of these gone-wild plants were cultivated in
the region’s gardens for over a century before they started to grow in the new Insubria forest. The giant forest trees that clothe the lake shores are deciduous trees. Their autumnal golds and yellows signal that they will drop their leaves in preparation for the cold of winter, when blasts of freezing air descend towards the lake from the surrounding High Alps. The undergrowth of hazel, blackthorn, hawthorns and dogwoods follows suit, although European privet and rambling brambles hang on to some of their leaves if the winter is not too harsh. Ivy scrambling across the forest floor and a few holly bushes were once rare spots of greenery in the winter-brown forest. And so it might have stayed, with hundreds, or even thousands, of evergreen plant species carefully nurtured in lakeside gardens, protected from damaging frosts by a cadre of attentive gardeners, while the deciduous trees continued to dominate the mountain forests. However, the industrial moguls who had acted as plant hauliers from the east also brought us global warming, and the severity of winter frosts abated in this region during the second half of the twentieth century.4 With warmer winters, the evergreens gained an advantage. Palms and camphor could now grow more luxuriantly than linden trees. All that was required was for the blackbirds and squirrels to assist their escape from the industrialists’ gardens. The transport of plants across the world, climate warming and semi-tame animals all combined to bring about the transformation.