The sundews of Kent and Sussex are far from safe and many of the insectivores sent to Down House from across the globe are in deeper trouble. The wide fields of Venus flytraps and of pitcher plants that once covered parts of North and South Carolina have been destroyed. Agriculture and drainage tear up their homes and the gardeners who dig them up do not help. A more subtle threat comes from fire control, because such beings thrive best in places often burned - which in today’s carefully managed countryside happens less than once it did.
Darwin’s other subjects, the orchids, face the same problems. Their enemies are those of the insectivores: aggressive farmers,
fragile habitats and greedy collectors. A third of the fifty British species are under threat, and several have populations of fewer than a hundred individuals - and one, the Lady’s Slipper Orchid, was for a time reduced to a single plant in the Yorkshire Wolds (thousands of greenhouse specimens have now been sown in the hope that the species can be rescued). The Victorians suffered from ‘Orchidelirium’, and paid large sums for rare specimens. Traders destroyed whole beds to ensure that their own stock kept their price and Darwin’s colleague the botanist Joseph Hooker noted how the area around Rio de Janeiro, visited by the young naturalist thirty years earlier, had even by then been pillaged of its orchids, which never reappeared. Unlike the Dutch tulip fever of the seventeenth century, which faded away, the orchid mania is still upon us, with a global trade worth ten billion dollars a year. Expensive specimens sell for thousands. Some of the business is legitimate and the plants are cultivated or cloned in huge numbers from cells taken from one or a few individuals. Plenty more is not, and many wild species from Thailand, China, Brazil, Guatemala and elsewhere are at risk. One orchid species in ten is threatened and the continued loss of tropical forests means that many more will disappear before they are known to science. Even those on ‘Orchis Bank’, near Down House, survive only through the vigilance of local naturalists.
Britain itself, like many other places, faces a mirror image of the loss of its insect-eaters, orchids and more: a revenge of the immigrants, a wave of creatures that have appeared from almost nowhere and have attacked its natives. A New Zealand flatworm introduced to Belfast in the 1960s has run wild and an Australian cousin has also begun to move. It kills earthworms as it wraps itself around them and digests them alive. The pest has spread through Scotland, northern England and Ireland and in some places worm populations have collapsed.
The losers in the post-Victorian battle have been replaced by others that have thrived in the new global economy. Many have migrated to new places. There, they cause havoc. Weedy plants are a nuisance but weedy animals are even worse.
Modern society has always depended on aliens, creatures moved from their native lands, be they maize, chickens or cattle. They evolved in the Middle East, in Asia or in the New World, but have been transported to all parts of the globe. Many have become pests in their new home and many more have hitched a ride with those who cultivate them. Darwin himself noticed the invasion of British plants into the United States and asked his American colleague Asa Gray, ‘Does it not hurt your Yankee pride that we thrash you so confoundedly?’ The New World soon got its own back on the Old, with grey squirrels that eat woodland birds’ eggs and Canadian pondweed that blocks streams. The third millennium is the era of the weeds and the weediest species of all -
Homo sapiens -
is to blame.
Plenty of weeds stay at home. They live in disturbed ground, flourish for a short time and move on to a new patch when conditions change. They do little damage except to the good temper of gardeners. When they escape, they are the botanical equivalents of pigs: they move in, exploit what is available and destroy the locals. Many imports from the Old World have thrived in the Americas. A common European roadside species, the knapweed, a small thistle with a pink or yellow flower, has covered tens of millions of hectares. It secretes a poison that kills native plants, which - unlike those at home - have not evolved resistance. As a sinister side-effect it also kills horses. The knapweed is now out of control. Others, such as the Brazilian water hyacinth, which has become a pest on Caribbean islands, find themselves in a place without their native pollinators and take up self-fertilisation. Some even hybridise with a relative, hijack its genes and gain renewed virulence as a result. The bright yellow and poisonous Oxford Ragwort common in disturbed ground in England is a hybrid between two Sicilian species brought to the Oxford Botanic Garden in the seventeenth century, which escaped and is still spreading.
Some of the most aggressive aliens are among the climbers. They are global pests. Even the hop has become a nuisance, with a Japanese variety that has spread across the United States. Kudzu, a climbing pea, is also native to Japan. In a gesture of amity, it was transplanted into that nation’s ornamental garden at the 1876 Philadelphia Exposition. Gardeners liked the flowers and it was dispersed across the country. At first sight, the immigrant seemed helpful. It lays down roots two metres long and in the South was used to reduce soil loss after the forests had been cut down. The railroads gave free kudzu to farmers in the hope that they would cultivate it for fodder that their trucks could then transport. That was a considerable mistake. The weed grows so fast that the locals recommend, with an attempt at wit, that windows be closed at night to keep it out. In some places, it extends by thirty centimetres a day - twenty metres a season - and can soon smother a huge tree. Kudzu is out of control over an area that straddles Alabama, Georgia and Mississippi, and has spread as far north as Massachusetts and as far west as Texas. Attempts to subdue it cost half a billion dollars a year.
Other climbers are just as busy. Florida has ‘air potatoes’, yams from West Africa that sprawl over trees and block the light. It also suffers infestations of climbing ferns from Asia. English ivy has shaded out tracts of maple forest around Seattle. In Australia, the humble blackberry is a nuisance, as is the mile-a-minute vine, a morning glory introduced from the Old World tropics. Most are harmless at home, but a lifestyle that depends on a burst of growth when a sudden open space appears in the forest is lethal when exported to a place not adapted to their wiles.
Some of the climbers’ success emerges from another by-product of human activity. The effects of the carbon crisis on climate are familiar enough - but it has unexpected side-effects, for climbers thrive in the new and enriched atmosphere. Over the past two decades, the proportion of the Amazon jungle taken up by lianas has gone up and up; in part because the forest has been opened up by loggers, but also because of the increased carbon dioxide, which they can soak up and lay down as wood. As a result they flourish at the expense of trees. Ivy, too, now grows at an exceptional rate as it gains extra carbon from the air.
Plenty more subjects of the Down House experiments have moved. Even barnacles are a menace. Some species, helped by the spread of shipping and by water (larvae included) dumped from ballast tanks, have begun to gallop across the globe. A barnacle the size of a tennis ball, once restricted to the Pacific coast of South America, has, within the past five years, made its way to Florida, Georgia and South Carolina. It infuriates boat owners because it acts as an unwelcome brake on their slippery bottoms. A species common on the coasts of California and Oregon has, in return, spread in huge numbers to Argentina. It was first recorded in the 1970s and has taken over the shoreline for two thousand kilometres. The animal has driven many local sea snails, seaweeds and more to extinction. It has found its way to Japan and at the present rate will soon reach Chile and wipe out the creature that drew the young naturalist’s attention to the joys of the barnacle in the first place.
On land, too, a subterranean revolution is under way. Since Darwin’s day the worm has turned. The animals he studied in his kitchen garden have crossed the world. The northernmost part of the Americas is almost devoid of native earthworms for they were wiped out by the last Ice Age, which left just a few remnants in the Pacific Northwest (one of which is still abundant on glaciers) and in a few patches elsewhere. Native Americans may deplore what the white man brought, but they did at least import earthworms, carried in pots or on the mud of immigrants’ boots. Now fifty and more exotics have arrived. They can advance at several metres a year as they burrow and even faster when they hitch a ride. Escapes from anglers’ bait tins mean that the creatures move at speed into remote forests as fishermen search for a new lake. Some are parthenogens and a single clone can take over huge tracts of land.
Unlike most invaders, they have been of some use. The West was won with the help of worms. Before the white man arrived, the northern prairies fed no more than herds of buffalo, but as the aliens spread soil fertility soared and maize and cattle moved in. Millions of hectares of soil were churned up and a dense and surly coat of acidic humus that sat on top of a sterile mineral layer was transformed into a well-mixed light soil with plenty of nutriments, just right for farmers.
Not all the news was good. Before the immigrants put in their appearance, many of the Northern states and large areas of Canada were covered by vast, fern-filled forests that sprang from deep mounds of leaf litter, or ‘duff’, that mouldered over years rather than being dragged into the ground by worms, as is the habit on the other side of the Atlantic. The duff was the home of beetles, salamanders, mice and more. The immigrants ate it, to leave a naked and unprotected surface. Most of the natives evolved to deal with undisturbed ground and suffer as a result. Thick undergrowth gives way to horsetails and pitcher plants. The local plants also depend on a relationship with the fungi that cluster around their roots - and they too have been lost under the assault. Aspen and birch trees die, native forests of sugar maple are parched as the water runs through the newly permeable soil, and prairie herbs disappear and are replaced by their European equivalents. In northern Minnesota, great tracts of hardwood have been destroyed in the past forty years and the problem is spreading. The once ponderous economy of those ancient forests has speeded up and vast quantities of carbon and nitrogen have been washed away. The danger is not limited to North America, for exotic worms have invaded tropical jungles. What they will do, we do not yet know.
Insects, too, followed the farmers and some, like worms, are a help. Most North American bees came from Europe. They arrived within two years of the Pilgrim Fathers for they were brought in by sweet-toothed pioneers anxious for honey. At once the immigrants set up wild colonies and thrived. So much were they an indication of European settlement that the Indians called them the ‘white man’s fly’. A wave of bees moved up the valley of the Missouri at fifty kilometres a year and, it was said, as the bee advanced, so the buffalo retreated. So impressed were the Mormons of Utah by the animal’s hard work that they chose it as their state symbol.
European bees still live in their billions in North America, their numbers boosted by commercial hives. Like the worms, they have driven out local species, and - like them - they also play an important part in the agricultural economy. Many crops - fruit trees above all - need pollinators if they are to thrive. Eight out of ten of the top hundred or so food plants, responsible for two-thirds of global production, depend on them. California has so many almond, cherry and apple orchards that natural pollinators are unable to keep up and a healthy business in rented honey bees has grown up. It has an annual turnover of a hundred and fifty million dollars. A wild guess as to the overall value of bees to American farmers comes up with a figure a hundred times greater. As the season moves on, the hives follow the flowers. Without the added European bees, the yield of some fruit, seeds and nuts in the United States would drop by nine-tenths. The aliens may have damaged their local relatives but they have done a lot to help the humans who brought them.
The pollinators are in crisis and on both sides of the Atlantic the bees are in decline. The number of wild colonies of European bees in parts of North America is a tenth, and of those in hives a third, of what it was fifty years ago. Insecticides, parasites, viral disease and competition from introduced African bees have caused a crisis, as has the loss of hedgerows and other scrubby places that were a home for useful insects. In spite of attempts to keep the bees happy with wild flowers planted around orchids and by controlling insecticide use, the decline goes on. In an airborne twist to the tale, pollution kills the scent of many flowers and further reduces the chance that a pollinator will find its target. That is bad news for honey-lovers, worse for farmers and may be catastrophic for many native plants.
The troubles of the bees, the spread of the weeds and the destruction of the soil laid down by worms are just part of a new global crisis of agriculture. The population boom and the increased price of oil and fertilisers are also to blame. After a long period of stability, or even decline, the price of wheat, rice, chickpeas and other staples has gone up by several times in the years from 2006. The era of plenty may be near its end. India, for many years an exporter of food, has begun to ship in rice. The population explosion in Africa has led to shortage and while Chinese numbers are under better control, the nation’s new affluence means a shift from cheap grains to expensive beef. World production of meat has gone up by four times since the 1960s. The habit is expensive in many ways. It takes fifty times more energy to make a kilogram of beef than a kilogram of maize or soy. Already there have been food riots in Haiti, Mexico and Egypt. The world’s fisheries have been depleted and - in a fatuous gesture of ecological concern - some of its finest land used to grow biofuels. Soils are degraded and, as the climate changes, productive regions have lost their worth. Gloom is an occupational hazard for ecologists, but it is getting harder to be cheerful.