3
THE PLANET IN A GARDEN

The Welsh village of Llanbadoc, in Monmouthshire, today has some 800 inhabitants, a sawmill, a college, a church, and a prison. Two centuries ago, one would guess that it probably had rather fewer in the way of both people and amenities. A place far too insignificant, one might surmise, to illuminate the grand designs of life on this planet. Nevertheless, in 1823 it was birthplace to Alfred Russel Wallace, who shed a good deal of light on those designs.

His beginnings were unpromising. Alfred was born to English parents, who, during his schooldays, fell on hard times. The 13-year-old was forced to leave grammar school and learn a trade. After briefly living in London, he joined his oldest brother William to work as a landscape surveyor in the Welsh Borders. In 1843, at the age of 20, he moved to the Collegiate School in Leicester to teach map-making and surveying. There, his fortunes changed. He met Leicester-born Henry Bates, and an adventure began that would take both men across the world and spark important insights into the fundamental patterns of life on Earth.

Wallace struggled with finances throughout his life and so did not fit the mould of the typical Victorian gentlemen scientist, making even more extraordinary the endeavours that took him into the jungles of the Amazon and South East Asia. This is in sharp contrast to his more famous contemporary, Charles Darwin, who was financially secure through the industrial legacy of his grandfather, Josiah Wedgwood. Nevertheless, like Darwin, Wallace assembled his knowledge of biology through extensive travelling and by making prodigious collections of specimens. This interest seems to have been sparked by Henry Bates, who at the age of 19 had published a short article in the journal The Zoologist on beetles from damp places in Leicester and the surrounding hills.¹ Grubbing around in ditches, Bates already displayed the keen eye of a naturalist, observing seasonal changes to insect faunas. This interest soon infected Wallace too, who for the rest of his life would be an enthusiastic collector of beetles.

Bates and Wallace avidly read many of the influential natural history texts of the first half of the nineteenth century, and after Wallace left Leicester in 1845 to look after the business of a brother who had died, they kept up an active dialogue in letters. One of the books they read was A Voyage up the River Amazon, by William Henry Edwards, an American businessman and natural history enthusiast, who went on to write a classic monograph of North American butterflies. Enticingly written and conjuring images of exotic and beautiful landscapes, plants and animals, A Voyage up the River Amazon was a key inspiration.² The two men determined to follow in Edwards’ footsteps.

After hatching their plans for an Amazonian expedition in 1847, Wallace and Bates left the port of Liverpool in April 1848, travelling on a tall ship destined for South America. They arrived at the Brazilian coastal city of Belem in late May. After a year acclimatizing and collecting biological specimens around the city, the two men separated. Bates was to spend the next 11 years in the Amazon basin, amassing an extensive collection of insects, and only returning to England in 1859. He was to become a famous naturalist in his own right through his work on mimicry in butterflies (now called ‘Batesian mimicry’ in his honour), a widespread phenomenon in which a harmless species copies the colour patterns of a toxic species as a deterrent to predators.

Wallace meanwhile made for the Black River (Rio Negro), a tributary of the Amazon, so called because its waters are tea-coloured from masses of decaying plant matter. And, though being only a tributary of the mighty Amazon, it is itself one of the largest rivers in the world, equivalent in scale to the Mekong River of East and South East Asia. Wallace’s expedition along the Black River was to end, though, in disaster. The ship on which he was travelling to England in 1852 caught fire on 6 August, taking all his carefully collected specimens and most of his notes to the bottom of the sea. Wallace himself had a narrow escape, spending 10 days aboard a small rowing boat drifting in the Atlantic before being rescued by a passing Norwegian ship en route to London. Despite this loss, Wallace managed to publish a number of articles on his discoveries in Amazonia. And, unperturbed by the misadventure, in 1854, he set off on more travels, this time to South East Asia. This expedition was to last until 1862.

Wallace’s travels in South East Asia were prodigious. With little by way of formal funding, and often living rough in a physically challenging climate, he covered over 22,000 kilometres from the Malay Peninsula through the Indonesian archipelago to Papua New Guinea. Wallace collected over 100,000 specimens of his beloved insects, as well as many thousands of bird specimens.³ His collections would help to build the foundations of the science of biogeography, the study of the patterns of distribution of plants, animals, fungi, and microbes across the world. They would also lay the foundation for his own insights into biological evolution.

The revelation that organisms evolve by adapting to their environment appears to have come to Wallace during a bout of fever in early 1858.⁴ His ideas on natural selection and the origin of species were developed quite independently from Darwin. Nevertheless, he was aware of Darwin’s interest in the theme, and wrote to the older man outlining his ideas. It was one of those key moments in science, making Darwin realize with a shock that his long delay and many hesitations about publishing his voluminous evidence that species were not forever fixed but ‘mutable’ (‘it is like confessing a murder’, he wrote) meant that he, and the central part of his life’s work, were in danger of being scooped.

The catalyst acted quickly. Friends of Darwin assuaged his despair by suggesting that the two men collaborate, which Wallace agreed to do. They published their revolutionary idea together on 1 July 1858 at the Linnaean Society in London, as a presentation ‘On the Tendency of Species to form Varieties’. Neither man was present. Darwin was ill, and also grieving over the death of a son, while Wallace was in the midst of his travels, on the Moluccan Islands in Indonesia. The presentation of their joint work was marked by silence, with no discussion: the audience perhaps sensed that the subject was too new—and too ominous. If so, this awareness was not shared by the then President of the Society, Thomas Bell. Reviewing the year, he said, ‘The year which has passed … has not, indeed, been marked by any of those striking discoveries which at once revolutionise, so to speak, the department of science on which they bear.’ One wonders if he reconsidered his view, when Darwin’s famous book On the Origin of Species was published the following year, and controversy over the theory of evolution was definitively ignited.

Being part of such a major revolution in scientific thought should have been enough for any one person, it might have been thought. But Wallace was puzzling over other big ideas. Crossing from Java and Bali into the small East Indonesian island of Lombok he wrote in his journal, ‘Plenty of new birds … Australian forms appear. These do not pass further West to Baly & Java & many Javaneese birds are found in Baly but do not reach here.’ Wallace may have relied on some local knowledge to make this judgement, having at that time not travelled farther east, and having never visited Australia. But he also carried with him a compendium of birds by the French naturalist and nephew of the Emperor Napoleon, Charles Lucien Bonaparte, and his keen eye would have spotted the absence of those birds characteristic of the islands of Java and Bali to the west.⁵ Wallace had thus observed and recorded the ‘Wallace Line’, though it only became known by that name some years later.

The Wallace Line is one of the most fundamental divisions between the animals and plants on Earth. It demarcates the faunas of South East Asia with their monkeys and apes, from those of Australasia with their kangaroos and wombats. These strikingly different faunas were separated by an impassable deep-sea barrier—the Lombok Strait—that cuts between the small islands of Bali and Lombok and runs on northwards between the larger islands of Borneo and Sulawesi. This line is a physically small but important part of the mosaic of animal, plant, fungal, and microbial biogeographical patterns identifiable across the Earth, which are controlled by climate, geography, and oceanography.

While Wallace was scratching around in the jungles of South East Asia—and Bates was still amassing evidence for insect mimicry in Amazonia—another English biologist, Philip Lutley Sclater, was establishing the global distribution of the world’s birds from the relative safety of published accounts. As a young boy, Sclater had already developed a strong interest in ornithology, and this became his life’s work. He published many important works on natural history, amassing over 1,400 in all, many with lavish illustrations of both birds and mammals, but his most famous work laid the foundation for the modern science of biogeography.

In a study published in 1858, and compiled before evolution had yet entered the arena of discussion, Sclater identified six areas of ‘creation’, neatly outlining the distinction between the birds of North and South America, and drawing the line cleanly through the ‘Table-land’ of Mexico. Sclater’s work was remarkably prescient and, almost willing someone to find the ‘Wallace Line’, he wrote that it is ‘not yet possible to decide where the line runs which divided Indian zoology from the Australian’.

The geographical patterns of animals identified by Sclater, when assembled on a map, form a jigsaw puzzle of interlocking land regions. Eight are now recognized, of which the Australasian region, east of the Wallace Line, is immediately obvious from its geographical isolation by seaways. The biggest of these regions, called the Palearctic, is more enigmatic, as it runs through landmasses from Iceland, through Europe and North Africa, on into Central Asia and East Asia, to finish in Japan. Its demarcation is a combination of marine barriers on its eastern and western margins, and topographical and climatic barriers to the south, though exactly where the southern boundary is drawn in the deserts of North Africa and Saudi Arabia remains a matter of scientific debate. Sclater had already identified the Palearctic when he wrote, ‘Europe and Northern Asia are in fact quite inseparable. So far as we are acquainted with the ornithology of Japan—the eastern extremity of the temperate portion of the great continent, we find no striking differences from the European Avi-fauna.’ Wallace later recorded these major patterns in his book The Geographical Distribution of Animals, but it was Sclater who first discerned them.

Within this jigsaw pattern of the major regions of life on Earth, animal and plant communities also respond to the patterns of rainfall and temperature. And so, the Palearctic includes, from north to south, large areas of tundra, conifer and deciduous forest, grasslands, and the deserts of North Africa, the Arabian Peninsula, and Central Asia. Southwards into the Neotropic, Afrotropic, and Indo-Malay regions, extensive areas of rainforest grow. Here and there mountain ranges burst through these patterns, and grow temperate forests at tropical latitudes, as Humboldt and Bonpland had observed on their journeys through the forests of equatorial America. These climatically controlled patterns are called ecoregions, and any one biogeographical realm, like the Nearctic of North America, will contain many of them.

Earth’s terrestrial biogeographic patterns have been a key feature of life on Earth for hundreds of millions of years, changing in their shape and size as continents drift apart and then collide. But changes are now afoot that, uniquely in Earth history, are affecting every part of the globe more or less simultaneously. To examine them, we will travel across some far-flung regions in these pages. But it is easiest just to start in the garden.

The planet in your garden

The horticulturalist Robert Hart was a pioneer, but one who acknowledged he was standing on the shoulders of a body of human knowledge stretching deep into prehistory. His temperate forest garden, built on the Silurian rocks of Wenlock Edge in the Shropshire hills of England, was constructed as a source of food, a natural and self-sustaining ecosystem, and a place where he and his brother could find well-being and connect with nature. Hart organized his garden from observing the structures of naturally growing forests, using six levels from the tree canopy at the top, to the soil biota of fungi, roots, and rhizomes at the bottom, with herbaceous and shrub layers in between, and connected by a seventh component of vertically creeping plants. In the foreword to Patrick Whitefield’s book⁶ How to Make a Forest Garden, Hart wrote passionately about our deep connection with woodlands, which in prehistoric times provided our ancestors with food, shelter, and clothing.

What kinds of plants, then, might have surrounded a prehistoric community in Shropshire—one that might have existed 10 thousand years ago say, not long after the end of the last Ice Age? There would have been such trees as oak, hornbeam, aspen, beech, yew, hazel, lime, willow, and blackthorn in the forests that clothed the ground after the ice retreated. Clearings may have been lined with juniper, buckthorn, dog rose, and sweet briar. Our ancestors may, perhaps, have delighted in flowers such as primrose, wood anemone, lily of the valley, Pasque flower, and hellebore, cursed the abundant flying, walking, crawling, and stinging insects, hunted the wild boar and deer, feared the wolf and auroch, and kept a safe distance from the lynx. If they ventured out at night, they might have happened upon the eerie lights of the will-o’-the-wisp dancing above the sodden ground, that led them to weave legends of lost souls and ghosts that tempt unwary travellers to their doom.

Fast forward 10 millennia, and the gardens of most homes consist of straight lines, shaven lawns, concrete patios, and neatly trimmed borders, microcosms of how we try to control nature in our cities. The ramshackle corners of our gardens hide behind the bins, or at the rear of the garage, or behind the compost heap. In such places a remnant of the natural ecology might try to get a temporary toehold. A bluebell or buttercup might be tolerated. Occasionally a weed, like a dandelion, will poke its head through the grass, only to be clipped to its roots by the mower, or shrivelled by the spray of the weed killer. The plants in such a modern garden would be mostly bewilderingly unfamiliar to our ancestors. The Japanese Sakura, say, growing in one corner, with its beautiful pink blossom in March. A Chinese rhododendron, dwarf or full size, in another corner, and perhaps a Dicksonia tree fern from the temperate rainforests of New Zealand.

The biodiversity of these gardens is high, if measured by individual species alone. And, the effect is felt well beyond the gardens. In the UK alone the Royal Horticultural Society suggests there are about 1,400 species of introduced plants⁷—roughly equivalent to the number of native plant species—that have escaped the confines of gardens to live freely, part of the new biological landscape. In ecological parlance, they have become ‘naturalized’.

Of these introduced plant species, a little more than a hundred have become invasive. Either because they have arrived without the natural enemies of their native land, or because they spread very quickly, or simply because they are superbly efficient at taking over local resources, they have pushed native species aside and come to dominate local ecosystems. Some of these newcomers are already well known to gardeners and homeowners, like the Japanese knotweed—unproblematic in its native Japan but a highly expensive problem in the UK, the giant hogweed, originally from the Caucasus, and the rhododendron spreading relentlessly across hillsides. Five introduced water plant species have had such a severe environmental impact that they have been banned from sale, including the Australian swamp stonecrop, which has a devastating effect on the ponds it invades, strangling the life out of them by forming a dense green mat of vegetation at the surface.

Some of the longer-established species already seem to be traditional parts of gardens and the countryside. In Britain, the horse chestnut tree, the source of conkers for generations of schoolchildren in times before electronic games, was introduced from its native Greece in 1616, while the sycamore, with its characteristic winged seeds, arrived only a little later from central Europe and Asia. In the branches of these trees one might see collared doves from Asia and grey squirrels from North America. Hopping around on the ground there may well be rabbits that were brought from their native France and Spain to Britain by the Romans, but only really became established in the wild in the twelfth century. They have famously spread more widely, being introduced to Australia in the late eighteenth and nineteenth centuries for food and sport, before escaping and growing to populations of hundreds of millions that, despite the desperate efforts by humans to shoot, trap, and poison them, have nibbled their way across the continent.

Many exotic species lie hidden. In the soil, earthworms are becoming less common in growing numbers of places, preyed upon by invasive flatworms from Australia and New Zealand which have few of the soil-conditioning properties of their victims. And these are only the larger, more obvious, well-studied species of the extraordinarily complex soil ecosystem. Changes to the smaller, cryptic yet (originally) highly diverse meso- and microfauna are poorly known, let alone those of fungal and microbial soil communities. Among the casualties, though, are the ghostly lights of the will-o’-the-wisp.⁸ These eerie flames are now hardly ever sighted, probably because those wide, swampy wildernesses have now been drained, and converted into tidy plots of agricultural lands and gardens. As the microbial communities were transformed, the will-o’-the-wisp flickered out.

Since his death, Robert Hart’s garden at Highwood Hill on Wenlock Edge has lain fallow for two decades.⁹ It is slowly returning to a more natural state, the elevating tree canopy now blocking out much of the light to the forest floor, though here and there a few shrubs near ground level still grow. This is a fitting end to a forest garden, that now becomes a natural forest in its own right. Elsewhere, we humans have profoundly altered the ecosystems around us by introducing new animals and plants. But animal and plant populations have mixed, too, before humans appeared on Earth.

The Great American Interchange

South America and Australia are currently separated by an expanse of about 7,000 kilometres of Pacific Ocean, which has acted as a highly effective barrier to species migration. The native faunas of these two continents clearly show their separateness, exemplified by the llamas and capybaras of the former and the kangaroos and duck-billed platypuses of the latter.

And yet surprisingly, some of the plants and animals that survive in the forests of South America and Australia—biogeographical regions that are now far apart—retain a distant relationship with each other, through common ancestors that lived more than 100 million years ago, when these two continents were joined together in one gigantic landmass called Gondwana. The different pieces of that leviathan were broken up by the actions of continental drift, driven by the extraordinary planetary mechanism that is plate tectonics.¹⁰ It was Philip Lutley Sclater—the man who discerned the geographical patterns of birds—who was also the first to observe these remnant relationships when, in 1864, he published The Mammals of Madagascar, noting that lemurs occurred in India and Madagascar, but not in Africa, and suggesting that long ago Madagascar and India must have been joined in a single continent which he called Lemuria. Although he was working long before the theory of plate tectonics was developed, Sclater had observed a phenomenon that is now well known to geologists: that, as previously conjoined continents drift apart, their faunas and floras retain some similarities, even though the individual plants and animals gradually change, as evolution in areas that have become geographically and climatically isolated takes separate paths. And, conversely, when continents are brought together, whole ecologies may change, as invaders arrive across land bridges to conquer new lands.

As they drifted apart for millions of years, the faunas and floras of South America and Australia retained a memory of each other, in monkey puzzle trees, lungfish, and marsupials. Antarctica—another part of Gondwana—drifted too, remaining an island continent, but moving southwards to keep its lonely sojourn over the South Pole, whilst its land-based flora and fauna simply froze to death. Other parts of Gondwana—Africa, Saudi Arabia, and India—broke away to drift northwards, eventually colliding with Eurasia. For more than 100 million years South America drifted in near isolation, evolving many remarkable creatures like anteaters and armadillos. In its past there were also elephant-sized ground sloths, gigantic crocodiles, and giant flightless birds—the ‘terror birds’—that were ferocious meat-eaters. As Africa and South America remained close for a time, before the South Atlantic fully opened, some animals managed to island-hop to become the ancestors of the capybara and South America’s monkeys. Some may have made this journey on driftwood, and these natural invaders diversified in their new landscape of South America. But for all this time there was no connection with North America.

The two continents, North and South America, finally began to approach each other about 10 million years ago, and the appearance of the first American invaders, north and south, begins shortly after that. Before the two continents were bridged by an isthmus, rodents and skunks had already travelled south, and some sloths had made the passage north. Thereafter, from a little less than 3 million years ago, the Great American Interchange took place. Horses, deer, bears, sabre-toothed cats, and cougars headed south. Sloths, armadillos, and even one species of terror bird headed north. The invasion of South America by North American cats led to the demise of many of its indigenous large predators, but some groups, like ground sloths, fared better. Although South American faunas witnessed widespread extinction, none such occurred in the north. Nevertheless, many migrants from the south fared well in the north, and some extended their ranges as far as Canada.

In Australia and New Zealand, where a land bridge to another continent was never formed, their plants and animals, especially their marsupial mammals, continued to thrive in splendid isolation. So too did the inhabitants of oceanic islands far removed from continents, such as Mauritius and Hawaii. But even here all was about to change, though not at the hands of continental drift. The jigsaw puzzle of life, the patterns of biogeography controlled by the position of the continents and oceans that had existed for countless millennia, would begin to break down as the alien invaders arrived.

Alien invaders

Humans have been refashioning the ages-old geographical ranges of animals and plants since the beginning of domestication, some 14,000 years ago or more. This likely began with ‘man’s best friend’, the dog, and evidence is found of such early canine domestication in many places from Kamchatka to Europe. Still earlier, some 23,000 years ago, hunter-gatherer peoples living in a tiny village between the Mediterranean Sea and the Sea of Galilee in Israel were already consuming the wild forerunners of barley, wheat, lentils, peas, and oats, and they may have been practising a rudimentary type of farming.¹¹ Here people were living in small huts made from willow, oak, and tamarisk, and artefacts within the huts include wooden objects, beads, flint and stone tools, and animal bones and shells. Alongside them in this more settled life were those ubiquitous cohabiters of humans, rats.

Beyond the shores of the Sea of Galilee, other animals and plants would become domesticated and subsequently spread across the world. Crops like maize, that were first cultivated in Mesoamerica 9,000 years ago, have become global in their distribution, as have animals like turkeys and chickens, which are eaten in huge numbers each year. Wherever geologists look, these changes to plants and animals have left a fossil signature, identifiable—to take just one example—in the sediments of cores sunk into the bottom of a Kenyan rift valley lake, that record many centuries of history. These cores show maize pollen appearing about a metre below the lakebed surface, marking its arrival in this area during the seventeenth century. And above the maize pollen, the pollen of pine trees appear in the cores, with their distinctive shapes that mimic the head and ears of the Disney character Mickey Mouse, and indicate the introduction of pine trees to this region by European colonists in the early twentieth century.¹²

There are thousands of alien species globally. The rate of their introduction has been increasing over the past few centuries, and further accelerated since the mid-twentieth century.¹³ These species range from bees, frogs, rabbits, and snails, to oil palm and acacia, and all continents are affected, with the exception of the Antarctic landmass. Some of these aliens have crept in by stealth, and others by deliberate introduction. They have proliferated in landscapes to the extent of entirely changing ecosystems. Many are also remarkably good at proliferating in environments that have suffered environmental damage at the hands of humans.

The introduction of new species also threatens to upset the delicate balance of the oceans. Here species are sometimes moved deliberately for aquaculture, like oysters, but many have been moved around the planet invisibly and unwittingly, in the ballast tanks of ocean-going ships. Some of these marine invaders have had a serious impact on local ecologies, like the Asian lionfish, which has taken up residence along the south-east coast of the USA.¹⁴ Its name is something of a misnomer, given that its orange-red and cream stripes are more reminiscent of a tiger than a lion, and it is in reality two distinct species of closely related fish. These venomous invaders have no natural predators outside of their native range of the Indo-Pacific, and their numbers have proliferated. In the Gulf of Mexico and Caribbean, the lionfish consume many small fish relied upon as food for local predatory fish like groupers and snappers. Lionfish also feed on fish that are herbivores, those which clean the coral of its surface algae and thus help maintain the health of the reef. Some marine environments have become pervasively infested by the invaders, local animals and plants being pushed to the edge of existence.

The infestation of San Francisco Bay

James Wilson Marshall was unlucky in both love and money. He left his native New Jersey after twice failing as a suitor. As a farmer in Missouri in 1844 he became ill and was advised by his doctor to seek out a better climate. His second farming venture in California also ended in disaster, when having returned from the army during service in the Mexican–American war he found all of his cattle gone. He then entered into a partnership to build a sawmill, constructing one near the village of Collumah on the south fork of the American River, about 180 kilometres to the north-east of San Francisco. On 24 January 1848, he was examining the tailrace from the mill when he noticed some shiny metallic specks, which on further inspection turned out to be gold. News of the ‘discovery’¹⁵ spread very quickly—Marshall had told his workers at the sawmill of the find—and, unsurprisingly, the lumber venture was soon doomed, as everyone turned their attention to hunting for the Californian El Dorado. Marshall never benefitted from his discovery and was soon forced off his land by the influx of prospectors. His further business ventures, a vineyard and a gold mine, also ended in failure. Dying almost penniless as an eccentric recluse in 1885, Marshall fared better in death, when 9,000 dollars was raised to erect a monument to commemorate his historic find. But even then, he was to be dispossessed in a final ignominious insult. The original piece of gold that he found in the tailrace now resides in the collections of the Bancroft Library at the University of California, Berkeley, where it is named the ‘Wimmer Nugget’, after Marshall’s assistant Peter L. Wimmer.¹⁶

The discovery of gold in California was to have much wider implications, heralding the arrival of some 300,000 prospectors— half arriving by sea—and producing a rapid decline in the indigenous population from a mixture of disease and cruel competition for land. San Francisco now became a boomtown, growing from a village to having over 30,000 inhabitants by the early 1850s. As the city grew apace, so too did its appetite, and oysters were introduced to the bay from the east coast. These shellfish fisheries eventually failed, but other marine creatures, from the Atlantic and from across the Pacific, found opportunities to colonize the waters of the bay. Slowly but surely, their invasion began to gather force.

One of the invaders to San Francisco Bay is a chimaera. Its elongate body and glistening flesh make it look like a worm, but the naval shipworm—indigenous to the Atlantic Ocean—is anything but. At one end of its body there are a pair of shells, which it uses to form a tight grip on its burrow. These shells are a highly modified form of the typical seashells you can find on a beach, for the shipworm is a mollusc in which the body has become exaggeratedly long relative to the shell. For centuries the naval shipworm had been boring its way into the hulls of wooden ships. It became such a nuisance to the British Royal Navy that, when faced with the prospect of simultaneous war with the American colonies, France, Spain, and the Netherlands at the end of the eighteenth century, the navy ordered all of its ships to be ‘copper bottomed’—expensively lined with this metal—to keep the ships afloat and the worms out. Over a century later, when naval shipworms invaded San Francisco Bay in 1913, it was the American Navy that was to be severely tested. The navy had originally chosen the north end of the bay to avoid attack from shipworms. In that part of the bay the water is brackish, being less influenced by the seawater from the south. For about half a century this strategy kept the Pacific shipworm—the native species of that ocean—at arm’s length. But not the naval shipworm. When it entered San Francisco Bay it rapidly spread to the north, being a species tolerant of waters of different saltiness. By 1919 this prolific invader was gnawing through the wooden wharves, demolishing a major structure fortnightly, and continuing this level of destruction for two years.¹⁷

The naval shipworm is not the only foreigner in San Francisco Bay. The bay has become one of the most heavily invaded aquatic ecosystems in the world, and one of the best studied ones. In some parts as much as 97 per cent of the species are invaders, and they may form as much as 99 per cent of the living mass.¹⁸ They include the Amur River clam, whose home is far away on the other side of the Pacific, in the river that marks the boundary between Asiatic Russia and China. Amur River clams were introduced to San Francisco Bay in 1986 and in some places have proliferated to numbers of 10,000 individuals for every square metre, invading the living space of home-grown organisms. These clams are much more than just space invaders. They are greedy feeders, sucking tiny phytoplankton from the water with almost miraculous efficiency, so that other animals have little to feed on. In San Francisco Bay you might also encounter Chinese crabs, Atlantic periwinkles, Manila clams, and Pacific oysters, all brought in through the direct or indirect action of humans. These are the visible signs of an altered ecosystem. But even at the microscopic level, things have changed.

Like a tiny version of the human-eating protagonist in the 1958 sci-fi movie The Blob, foraminifera are amoeba-like organisms that feed using a gliding motion that allows them to form small grasping structures called pseudopodia—false limbs—that engulf their prey. They live in lakes and seas, and like amoebae their bodies are composed of just a single cell (though sometimes this can grow to a couple of centimetres across). Foraminifera have gone one better than their amoeba cousins, and rather than exposing their body to attack from predators, they build a tiny shell for protection. Lurking in large numbers at the bottom of San Francisco Bay is a foraminifer that is native to the coastal waters of Japan. Like many invaders it probably made its way across the Pacific in the ballast tanks of ships, to be unceremoniously dumped into its new environment when the ship arrived in port. This minuscule invader from the Japanese seas (too obscure to have a common name) builds its shell from the available sediment at the bottom of the bay, sticking together individual grains of sand with its own mineral glue. In those parts of the Bay where human influence on the bottom sediments has been very strong, it can account for nine out of every 10 of the foraminifer individuals counted. It is a revolution in this microscopic world.

Jekyll and Hyde species

While rivers and bays, and estuaries and seas are invaded, as in San Francisco Bay, so too is the land. In this modern ‘Great Global Interchange’, almost all areas of the Earth are fair game for invaders. Many introduced species do not cause harm in their new habitats and may co-exist with the natives benignly or even with benefit. Other animals and plants, though, take on a Jekyll and Hyde existence, useful and productive in their home territories, but bringing death and destruction to the places they invade, just as their human carriers did when they arrived in new lands.

In South America lives the Patagonian bumblebee—Bombus dahlbombii. It is a beautiful, orange-coloured bee, the queens growing to 4 cm long. Once it was so widespread that local people referred to the bees as flying mice.¹⁹ It has plied its trade in this landscape since the time of the Inca, and long before that, occupying these lands as the single South-American representative of the bumblebee diaspora, and pollinating the local plants. Or so it was until late 1982 when the ‘large garden bumblebee’—Bombus ruderatus, originally from Europe—was introduced into Chile from New Zealand and quickly spread. Fifteen years later a second bumblebee, the European ‘buff-tailed bumblebee’, Bombus terrestris, was introduced to pollinate tomatoes grown in greenhouses. A year after that, it was trialled in the open as a pollinator of avocados, for which there was a growing and lucrative market in the kitchens of Europe. The buff-tailed bumblebee seized its chance of freedom. Acting as a villain in its adopted landscape, it spread extremely quickly, reaching as far south as Tierra del Fuego, and extending from the Atlantic to Pacific coast of South America.²⁰ This bee is projected to invade most of South America.

The deliberate introduction of the buff-tailed bumblebee into Chile has been disastrous for the native bees. Wherever it has invaded, the native bumblebee has declined and is now locally extinct. Buff-tailed bumblebees carry diseases which have infected both the native bees and the introduced large garden bumblebees. They also steal the nectar of flowers that are normally pollinated by hummingbirds.

On the other side of the world, another Jekyll and Hyde species wreaks havoc. The golden apple snail, up to about 6 cm in height and with a thick brown shell, is native to Argentina and Uruguay, where it is considered harmless. The eggs of this snail are bright pink, and it lays these in clusters that resemble a bunch of grapes. A single clutch may contain as many as 1,000 eggs, and the snails are very successful at colonizing new lands. Golden apple snails have spread far beyond their homeland, and can be found in Hawaii, North America, and parts of Europe and the Middle East. But in the wetlands of East Asia they have become a major pest.

These snails were first introduced to East Asia through Taiwan in the late 1970s and are widespread from China to the Philippines. They have rampaged through the islands of Indonesia too, crossing the Wallace Line with impunity, and reaching Papua New Guinea. They feed on aquatic plants like rice, and so readily make their home in paddy fields. Many initial introductions of this snail were deliberate, as a potential food source for humans, although this role was only fulfilled in southern China, where the snails are eaten raw as a delicacy.²¹

Golden apple snails are typical of many invasive species in that they are resistant to human pollution, adapt well to modified ecologies—like rice fields—and can even survive in waters that suffer bouts of low oxygen. It is the young seedling rice plants they devour voraciously, as they slither through every square metre of rice field. In their homelands of Uruguay and Argentina these snails are culled by natural predators like kites, which accumulate piles of spent shells beneath their perches. Beyond their native ranges the snails have become food for rodents, birds, crabs, fish, and even leeches. They may be cannibalistic, too, the adult snails eating the juveniles. Even with this range of new predators, attempts to remove golden apple snail infestations have proved ineffective. The impact on people, through loss of rice fields, the parasites they carry (like the rat lungworm which can cause a fatal form of meningitis), and reduced biodiversity, as the snails decimate aquatic vegetation and clog up lakes, has been profound.

Animals are not the only Jekyll and Hyde species. Across the world there are many plants with names like ‘Devil Weed’ and ‘famine weed’ that reflect their impact as invaders. On the Serengeti-Mara of East Africa one such plant is Parthenium hysterophorus, a native of the Americas. This ‘famine weed’ uses our communication systems against us, spreading through the disturbed land beside roads and highways. Famine weed contains a chemical that causes skin diseases in cattle and breathing difficulties in humans. It is, though, only one of more than 200 introduced plant species on the plains of East Africa.²²

Another invader has used human communication systems as never before to spread death, damage, and disruption across the world—specifically to human ecologies—in a matter of months. This is the virus SARS-CoV-2, which causes COVID-19. Where the virus arose remains a matter of conjecture, with bats cited as the possible origin, whilst chickens and pangolins may be the intermediaries, the passage to humans perhaps occurring in the Chinese city of Wuhan, late in 2019.

Viewed through an electron microscope, SARS-CoV-2 looks like a spiky ball. This tiny package of calamity is remarkably tenacious, able to survive outside of a host for several days, and on many different surfaces. Spreading quietly through the human population, and in many cases causing no symptoms at all, within a matter of weeks this virus had spread through the population of Hubei Province in China, and then via aeroplanes had become an alien invader across the world, leaving many human tragedies in its wake.

The tiny and invisible virus unleashed a rapid change in human behaviour. People stopped travelling to work, meeting in bars, talking in parks, flying in aeroplanes, and watching football matches. For many months, there were no football matches to watch. As the global economy slowed, oil prices plummeted, and less carbon was, briefly, released into the atmosphere. Grass verges began to grow as lawnmowers fell silent. Within these tiny meadows, insects flourished, and birds and bats that fed upon them grew a little fatter. As the world of humans slowed—albeit briefly—nature began to bounce back. Perhaps this is a brief glimpse of the future—although, as we write, with recovery from the pandemic still uncertain, it is too early to assess long-term consequences.

Long reach of the Homogenocene

In 1999, the entomologist Michael Samways introduced the term ‘Homogenocene’.²³ This was a year before Paul Crutzen’s on-the-spot improvisation of the word ‘Anthropocene’, for a geological epoch of the present that is indelibly marked by human actions.²⁴ The Homogenocene was driven by a similar intuition, that very large changes were taking place around us that were not only impacting our planet at present, but would long reverberate, ultimately to have profound long-term consequences. The word-ending ‘-cene’ implies a geological timescale (think of ‘Pleistocene’, for instance), with time units typically measured in millions of years. Samways had this in mind when noting the global, human-driven, merry-go-round of species. Occurring at a speed and scale greater than any other biological mixing event in our planet’s 4.5-billion-year history, this was not only eroding or ‘homogenizing’ the kind of distinctive regional biological communities that Alfred Russel Wallace had described; it would affect biology on Earth into the far future too. Indeed, its effects, in changing the course of biological history, will likely persist until the end of life on Earth, perhaps a billion years hence.

For, the Homogenocene world, uniquely in our planet’s 4.5-billion-year history, has seen a thorough reshuffling of life on Earth, through the transplanting of species between every continent and every ocean. It is now a world in which Wallace Lines, and Philip Lutley Sclater’s ‘areas of creation’, have been blurred, and in many places scrambled. These ecosystems will now need tens to hundreds of millions of years to slowly establish their own characteristic natures and identities (whatever those might turn out to be), just as happened with the floras and faunas of Australia and South America after the Gondwana supercontinent began to break up about 175 million years ago. Just as in those biological assemblages, the fingerprints of the Homogenocene will remain in the anatomical structures of the animals and plants of the far future.

Value of the outcasts

There is a more contemporary significance to the Homogenocene: it might, quite literally become a life insurance policy, in the most profound sense, on a changing Earth. Nowadays, the introduction of new species to the landscape is not treated with the blithe abandon that was current in the days of the great Victorian-era explorer botanists and zoologists, as they brought their living trophies back from the distant regions of the world. Today, the damage done to native ecologies is all too clear, so great efforts are now expended to keep alien species from arriving—and in trying to extirpate invasive forms that have arrived. There have been costly programmes (some eventually successful, at least for a while) to remove rats and goats from ocean islands, while the costly wars against ecological invaders such as the Japanese knotweed, the zebra mussel, and the rabbit continue unabated.

And yet, might this extraordinary effort, and the wish to preserve and nurture what is left of native species, be misguided?

Some conservationists note that many of the imported species increase local biodiversity, and not all of them seriously endanger local species. This in itself, they say, is a good thing. A more diverse biological community, regardless of what the components are and where they came from, is beneficial, as it increases the resilience of ecological communities. And resilience will be needed, because the Earth System that supports all life is now a moving target, as the Earth’s climate begins to heat up beyond the limits that it has known over the past several million years.

In this scenario (again, still not quite inevitable, but now far more likely than not), traditional conservation cannot work, as ‘native’ species find the climate around them changing beyond their tolerance limits. In such circumstances, animal and plant communities migrate to stay within their preferred climatic belts—but this is not always possible, as communities are pushed to the equivalent of a local cliff edge (say, to the edge of a continent or the top of a mountain). In any case, it is made hugely difficult because the remnants of ‘natural’ biological landscapes have been fragmented into pockets within the growing envelope of farmlands and urban areas.

The Australian husband-and-wife team of ecologists Maggie and David Watson²⁵ make the point that it is the introduced species—currently maligned, chased, harried—that have the best track record of surviving new and difficult conditions. In picking a path through shifting climatic belts to come, they may be best placed to form the basis of a functional biology on a world (likely a much-degraded world biologically) after humans have become extinct. Tough, adaptable, and mobile, the likes of the cat, rabbit, and rat, and golden apple snail, prickly pear, and Amur River clam, may be the best bet for the post-human renaissance.

To the Watsons, it is a kind of solace, and one that, in a small and local way, they try to put into practice. They have ‘adopted’ their own local favourites among the outcasts, in studying mistletoes, and gulls, and tern—organisms normally thought of as pests that need to be controlled—and in educating the next generation of conservationists about the dilemmas they will face and the decisions they will have to make. We cannot predict the world to come, but we can try to tip the balance in favour of life.