Inheritors of the Earth

by Chris D. Thomas

  It is not just the forest. The lake itself is full of foreign species, most of which have been introduced in the last century. The pike-like zander fish is an import, although only from elsewhere in Europe. An oversized specimen of this denizen of the deep had to be tracked down by police harpoonists after a troublesome individual bit chunks out of six bathers–in compensation, its walleye-flavoured meat was served to the tourists. One hopes that the fish had had time to digest its last meal before their feast. Most of the additions, however, have originated from North America. Largemouth bass, which is so feisty and prized a catch that it is an official state fish in Mississippi, Alabama, Georgia, Tennessee and Florida, was introduced as early as 1930. The enthusiasm of anglers has provided this fish with a ticket to the world. The pumpkinseed sunfish is also American. It wears impressive black-and-orange ‘false eye’ markings on its sides to give large predatory fish and diving birds the impression that their quarry is far more dangerous than it is. The American bullhead catfish has a different strategy: it keeps out of sight, hoovering up anything from plant debris to insects and rotting animal matter in the murkier parts of the lake, escaping notice by feeding at night. Altogether, over a third of the thirty-two types of fish in Lake Maggiore are introduced species without, as far as is known, any ‘native’ species becoming extinct as a consequence.5 So the lake contains many more kinds of fish than before humans appeared on the scene, just as the forest now supports newly arrived populations of exotic trees and shrubs.

  The palm fruits are consumed by European blackbirds.

  The blackbirds deposit many of the seeds in the nearby forest, forming a dense palm under-storey, through which it is possible to see the bare stems of the original forest trees.

  Palm crowns, Nepalese camphor trees and other Asian and North American evergreens now join the deciduous trees to form a forest unlike any other that exists.

  The lower slopes surrounding Lake Maggiore in Switzerland are home to exotic gardens, where windmill palms are grown.

  And so Maggiore continues its journey into the Anthropocene epoch–an ecological and evolutionary melting pot of the world’s species. European and Asian birds and mammals mix with African-origin humans. Together, these animals are moving the seeds of North American, Asian, South American, European and Australasian plants into a forest of what used to be European species. It is an international blend, rich in diversity. It is a novel ecosystem or, in the parlance of some ecologists, an Anthrome.6 It is a human-altered land.

  The success of Asian and North American trees is partly down to the absence of similar European evergreens, which died out during the ice ages. Geologists and botanists have spent the last century staring down their microscopes at the rot-resistant walls of pollen grains, preserved leaves, and at the internal structure of fossilized tree trunks, which they have unearthed from the bottoms of lakes and the alluvial outwash of rivers that flowed three million years ago. In so doing, they have discovered the remains of large numbers of extinct trees that used to live in Europe, including magnolias, which still survive in eastern Asia and in North America. Jens-Christian Svenning, a tall, crazy-golf-playing scientist from Aarhus University in Denmark, worked out that as many as thirty-one genera7 of trees that were native to Europe between 5.3 and 2.6 million years ago have since become extinct, whereas thirty-five have survived in the region.8 If you had taken a grand tour of Europe 3 million years ago, you would have encountered double the diversity of native trees.

  This rich European forest became impoverished by a succession of ice ages, which eliminated nearly all the cold-sensitive trees (those unable to persist in places where the average annual temperature is below 0°C), including most of the broad-leaved evergreens. In contrast, cold-sensitive trees and shrubs did manage to survive the ice ages in more amenable climates along the Caribbean fringe of North America and in south-eastern China. Now, globe-trotting humans are bringing them back. Relatives of the original European trees are coming ‘home’ during a period when the human-warmed European climate increasingly resembles the conditions that existed 3 million years ago. Although the returnees are not quite the same as the original species, they can nonetheless be thought of as nearly native trees that are flourishing in Europe once more. The rediversification of European forests is under way.

  Conservationists and many ecologists are not happy about this. A hatred of foreign species is regarded as perfectly acceptable, and to think otherwise is tantamount to heresy. For instance, New Zealand scientist James Russell and British academic Tim Blackburn liken people who do not condemn introduced species in sufficiently strident terms to those who undermine the scientific consensus on ‘the risks of tobacco smoking or immunisation, the causes of AIDS or climate change, [and] evidence for evolution’.9 And this neophobia has been translated into practical action. At a political level, our governments invest in keeping foreign species out through customs controls, and nearly all countries are signatories to the international Convention on Biological Diversity. As such, taxpayers are committed to the costly control or eradication of priority (aka successful) alien species. On the ground, conservation volunteers are poised to kill invaders when they arrive, and take action to reduce their numbers. For example, Locarno locals are chopping down small patches of evergreen forest on the slopes above Lake Maggiore and planting deciduous linden trees in their stead, in a vain attempt to rewind an unrewindable history. However, we need to ask whether we are just responding to a nostalgic impulse that the world should be as it once was. Today’s Insubria forest certainly differs from all previous forests that have existed, but it is still doing all the things that forests do. It is growing, providing wood, nectar, pollen, fruits and seeds that animals eat and transport. It gives shelter, and the roots of the forest trees and lianas stabilize the ground. The forest delivers benefits that humans prize.

  How long will it be before the environmental police force of ecologists and conservationists is prepared to step back and decriminalize introduced species that have had the temerity to be successful? Back home in Britain, members of the eco-constabulary are unanimous in accepting as ‘native’ any species whose ancestors arrived between about five thousand to fifteen thousand years ago. Hence, they are deemed innocent of ecological harm, however common they might be and however large their impacts on other species. Environmentalists are equally of one mind in their condemnation of the gloriously purple rhododendron bush from southern Europe, which was first planted in Britain in 1763. They also love to hate a kind of annual balsam, policeman’s helmet, first grown in 1839 and today popping its explosive seed-pods into my roadside ditch. Much loved by bees, the policeman’s helmet balsam has such startling pink flowers that it has acquired a remarkable diversity of popular names–ornamental jewelweed, to reflect its beauty; Himalayan balsam and kiss-me-on-the-mountain, after its geographic origin; and gnome’s hat-stand, Bobby tops, and Copper tops–as well as policeman’s helmet–in recognition of its domed helmet-like flowers. Hardly a historical drama or documentary is broadcast without it putting in an appearance, as the camera crew line up the most attractive flowers they can find to complement their favourite country house. Ecologists of Russell and Blackburn’s ilk apparently know better. Himalayan balsam is an invasive foreign species. A couple of centuries is too short a time for an introduced species to be accepted.

  The running hares that nibble my meadow and the aphid-sticky sycamore tree in the hedgerow are, however, more of a challenge for the ecological jury; it is unable to decide whether these species belong to a rose-tinted past or are modern interlopers ruining our native ecosystems. The brown hare was most likely introduced by the Romans some two thousand years ago and has been added to the British list of protected species; so it is officially accepted and treated as if it is native. Roman-introduced sweet chestnuts that have been in situ for two thousand years are apparently acceptable around the shores of Maggiore, too, so two thousand years seems to be enough. The sycamore tree was added to the British
flora only about five hundred years ago, in contrast, and many conservationists continue to frown upon it (but bow to the inevitable and rarely dig it up). It seems to take somewhere between five hundred and two thousand years to convert xenophobia into love. Any specific duration is hard to justify.

  The ancestors of all species that are alive today have flowed back and forth across the globe for many millions of years, as we saw in the last chapter. We should never assume that where we see a species today is where that creature’s ancestors originated. Modern humans resided in Africa two hundred thousand years ago and subsequently spread across the world, yet most of us alive today think of ourselves as natives of the regions where we were born, rather than of Africa. When we trace our ancestry, we often focus our attention a few generations back, seeking a sense of place and personal identity. But these past places were also transient locations, and each ancestor transient, as our genes have moved around the planet’s surface. It was ever thus. It is completely illogical, then, to hate a fellow human, or another animal or plant, simply because they or their ancestors were somewhere else at a particular time. The location of those genes in one specific timeframe has no special meaning in the history of life.

  Humans have not invented the transfer of species between distant locations, but we have dramatically increased the rate at which these events take place. The history of land-dwelling species has been strongly influenced by the movement and locations of the world’s continents, which float like oil-on-water islands over the denser planetary mantle and ocean floor. The Earth’s continental landmasses of granites and sedimentary and metamorphic rocks weigh around 2.7 metric tonnes per cubic metre, the basaltic ocean crust 2.9, and the Earth’s denser mantle is a hefty 3.3 metric tonnes per cubic metre. Gravity dictates that the ‘light’ continents float. And they travel at a few centimetres a year, about the rate at which our nails grow, inexorably transporting their cargoes of animals and plants across the planet’s surface.

  Despite the desperately slow progression of plate tectonics, this movement has been sufficient to transport the continents over considerable distances, sometimes bumping together to form one great blob of continental crust adrift on the sea. This was the case a billion years ago, when the land came together into the supercontinent Rodinia, and again between 300 and 175 million years ago, when the continental crust formed Pangea (meaning ‘all land’). By then the land had already been colonized by plants and insects, and Pangea was the supercontinent where reptiles diversified and mammals evolved. Almost all the world’s land biota lived on a single continent at that time, save island species, most of which subsequently disappeared without trace. Then Pangea split, first into two supercontinents, a northern Laurasia and southern Gondwana, and subsequently into the smaller fragments that became increasingly recognizable as today’s continents. But the bits have started to coalesce. The former island that was India rammed into Asia about 50 million years ago, forming the Himalayas in the crumple zone, and South America (previously part of Gondwana) joined forces with North America (part of Laurasia) a mere 5 million years ago.

  ‘Soon’ the Alps will resemble the Himalayas and the Mediterranean is likely to be squidged out of existence as Africa continues to steam into Europe. Meanwhile, Australia and New Guinea can be expected to ram into the corner of Southeast Asia, bearing kangaroo gifts. Projecting forwards, some suggest that our presently separated continents will again reunite in a few hundred million years, forming a new great supercontinent. Gradual reconnection of the world’s biota a hundred million or more years from now might have been the fate of life on Earth, had it not been accelerated by an unusual ape that evolved in Africa. Ocean-going container ships move species 10 billion times faster than migrating continents, aeroplanes 200 billion times faster. We have set about reconnecting the continents on a much faster schedule.

  This is not to suggest that distant locations were totally isolated before humans came on the scene. Many microbes that are critical to the operation of every ecosystem have near-global distributions, blown as dust or attached to the muddy feet of migrating birds. All the ancestors of animals and plants that today live on volcanic islands which emerged from the oceans must have rafted or flown there at some stage. Tortoise ancestors of the Galapagos and Aldabra giants did not originate on those islands. They floated, hitching a lift on ancient tree trunks or mats of vegetation washed up on distant shores. The ancestors of flightless birds that adorned the Pacific isles flew there first. Most of today’s New Zealand wildlife did not survive the journey from Gondwana but arrived more recently and then evolved in subsequent isolation.10 The flow of species around our planet is far quicker than the movement of continents, albeit still orders of magnitude slower than today’s human-assisted torrent.

  New Pangea is an apt metaphor for the accelerated connections of the modern world. By moving species across the surface of the planet, we are bringing about biological collisions every bit as significant–and just as permanent in the history of life on Earth–as when continents have in the past collided, or species have occasionally floated across the oceans. We are reuniting the biological world. Quite how far along this road we have already travelled is open to debate, but 971 bird species have been released in at least one location where they might be deemed to be ‘alien’ introductions between the years 1500 and 2000.11 This amounts to approximately 9 per cent of all birds already, and the rate has been accelerating dramatically through time. A full quarter of all documented introductions were carried out in the last seventeen years of the five-hundred-year study. A project called DAISIE (Delivering Alien Invasive Species Inventories for Europe) has come to comparable conclusions. It established that Europe contains just over twelve thousand ‘alien’ species so far.12 This amounts to nearly 10 per cent of the total current land and freshwater species in Europe, though the figure is much higher for plants (over a third of the species currently growing in the wild are international success stories that started their lives elsewhere) than for animals (about 4 per cent). Even this is bound to be an underestimate because the lists of imported species are nowhere near complete for fungi and for the smallest insects. Given that the flora represents the base of the ecological food chain, we can expect that the diversity of insects, fungi and microbes associated with each plant will catch up over the coming centuries.

  This process is incredibly fast. Contact between Europe and the New World marks the emergence of genuinely global trade, the time when the transfer of species really started to increase. It has taken about five hundred years to get to where we are now, and each century has seen more movement than the one before. At the current rate of transfer, it might take another millennium or so to complete the job. This would be sufficient time for new arrivals to turn up and for recent colonists to spread more widely within the continents where they have become established. Assuming that the transport of goods and people continues, the geography of the world’s animals and plants will have been thoroughly rearranged in less than two thousand years. Geologically speaking, this is virtually instantaneous. We are in the middle of the biggest biological pile-up in world history, an indelible signature of the Anthropocene.

  We are reuniting Pangea in less than two thousand years, approximately 200 million years ahead of schedule.

  Despite my scepticism about their attitudes to the arrival of new species, ecologists and conservationists do have entirely valid concerns. When species become established for the first time in parts of the world where they did not originate, there are bound to be repercussions. There are good reasons why some foreign species have gained a bad reputation. A combination of invasive humans, dogs and Pacific rats set off in boats from Southeast Asia around 3,500 years ago and spread across the Pacific in three waves of invasion. This deadly trio of big, medium-sized and small predators presented the natives of the islands they invaded with insurmountable challenges, generating ecological havoc in far-flung islands from Hawaii in the north to Easter Island in the east, ev
entually settling in New Zealand just seven hundred years ago.13 The local birds, snails, insects and plants had not met intelligent, ground-living mammalian predators since their ancestors had colonized the islands millions of years previously. A mass extinction ensued, extinguishing virtually one in ten of the world’s bird species14–a topic I return to in the next chapter. This was followed in recent centuries by the arrival of Europeans with guns, accompanied by a crew of two even more tenacious species of rat, cats, stoats, mosquitos, bird malaria, snakes, ants and predacious snails, among a menagerie of new invaders, which were fully capable of polishing off yet more of the native islanders.

  Such losses are not entirely confined to remote islands. Worst of the continental invaders has been the ‘chytrid’ fungus Batrachochytrium dendrobatidis, which is now one of the world’s most widespread and successful species, thanks to pregnancy tests. Before the invention of handy pregnancy-test strips, finding out whether you were pregnant involved a trip to the doctor, urine sample in hand, so to speak. The doctor then asked the advice of Xenopus, the South African clawed frog, which in its time had represented a revolution in pregnancy testing.15 The samples were sent off to the labs, where a technician would inject the woman’s urine into the back leg of a female frog. If it contained sufficient human chorionic gonadotropin hormone, the hapless frog would have ovulated by the morning–indicating that the woman was pregnant. Unfortunately, Xenopus turned out to be a carrier for the chytrid skin fungus and the pathogen was washed out in waste water into the world’s ditches, streams and rivers; it may also have hopped off on the backs of a few escapee frogs. The best guess is that Batrachochytrium dendrobatidis originated in Africa but then romped its way through the skin of the world’s frogs and toads, threatening hundreds of species. Some of the frogs and toads that it infected, such as the harlequin frogs of Central and South America, were highly susceptible,16 and disappeared, whereas others had some resistance, and survived. Bad things can occasionally happen on continents, too.