The calamity of the Pacific, the loss of hundreds, if not thousands, of fish that once lived in land-locked African lakes, which have been disrupted by imported predatory fish, and the demise of chytrid-infected harlequin frogs in the New World, help explain why foreign predators and diseases have gained such a terrible reputation. Invasive species (if we include ourselves as hunters) have probably been the largest single cause of vertebrate extinctions since humans spilled out of Africa into an unsuspecting world. Given a choice, I would happily repatriate some of the most inconvenient of these super-successful species and resuscitate those that have become extinct. But this is not the world that we inhabit. Furthermore, invasive species are no longer near the top of the list of threats,17 perhaps because the isolated ecosystems and species that were particularly susceptible to interlopers from distant lands have, in the main, already been invaded.
More remarkable than the losses, however, is quite how few foreign species have caused any others to go extinct at all. Most of the Pacific extinctions were accomplished by the aforementioned trio of big (human), medium-sized (dog) and small (rat) predators, and the rest by the arrival of a mere handful of additional carnivores and diseases, such as avian malaria and the mosquitos that transmit it. Only a few dozen out of thousands of foreign species that humans have transported across the constellation of Oceania’s islands have driven any native species extinct. Even fewer have caused other species to become extinct on the world’s largest continents.18 With notable exceptions, including the chytrid diseases of amphibians and chestnut blight in North America,19 foreign species hardly ever cause native species to become extinct from entire continents.
This is not surprising. Researchers who study the interactions between different organisms long ago demonstrated that a small number of species have a major effect on other species, but that most do not. This was first appreciated in 1969 by North American ecologist Robert Paine, who spent his days scrabbling around on rocky shores near the University of Washington, where he worked. He discovered that one species, which was a relatively rare starfish, had a huge effect on almost all the other species on the shore.20 He christened it a ‘keystone species’. When he experimentally removed this mussel-guzzling starfish, beds of mussels grew in great abundance, crowding out other rock-encrusting animals and plants that used to grow there. The entire biological community changed. However, although Paine is deservedly remembered for the keystone concept, it is worth recognizing that most of the other species he experimentally manipulated had very little effect–otherwise, the starfish would not have seemed remarkable.
The ever ebullient Dave Raffaelli, enthused by this research, set out to test Paine’s ideas in the estuary muds of the Ythan River, north of Aberdeen in Scotland–a landscape so bleak and windswept that it perhaps accounts for his decision to grow a wind-cheating protection of dense facial hair. Disappointed not to find a keystone species in his initial work, he hit on a plan: to add and remove as many species as he could, in turn. He reckoned that persistence would pay; eventually, he would find the keystone species. After a decade or more of experimentally adding and subtracting species in search of the elusive keystone species, he gave up. There wasn’t one. Lots of species had relatively small effects, a conclusion that is critical to our understanding of ecosystems. Sometimes there are one or two species in a particular place that have large and disproportionate impacts on all the others, like Paine’s starfish, and sometimes there are not. Most species that are added to or subtracted from an ecosystem have little impact on the others.
This is equally true of foreign species, caricatured by York biologist Mark Williamson’s ‘tens rule’,21 whereby only about one in ten species that arrive in a new part of the world escape from captivity or gardens, only about one in ten of these then become fully established in the wild, and only about one in ten of the established species go on to be regarded as pests or weeds. A tenth of a tenth of a tenth. This ratio varies quite a bit from place to place, but roughly one in a thousand species that arrives causes a real issue for the native animals and plants, consistent with Paine’s keystone ideas in community ecology and Raffaelli’s failure to find any such species. And when people say that these species become pests or weeds, this usually just means that they become common, without actually endangering other species with extinction. Foreign species are acting like any other species: a few have major impacts, but most don’t.22 Because a large majority of them have such limited impacts, the importation of lots of new species almost always increases the numbers of species in any given location, just as we saw in the forests and waters of Lake Maggiore. When lots of new arrivals establish breeding populations, hardly any ‘natives’ die out as a consequence.
This is true even in New Zealand, one of the world’s hotspots of extinction. When my wife, Helen, and I accompanied invasion biologist Jacqueline Beggs for a jet-lagged view over New Zealand’s largest city, Auckland, we could almost have been at home. Species-rich meadows now cover the tip of the old volcanic cone of Mount Eden–Maungawhau, to give it its Maori name–that protrudes above the city, forming a vegetation comprised largely of European plants. Meadow plantains, wild cranesbills, sorrels and European grasses generated the scents of our own hay meadow. But walk into the remaining areas of forest near Auckland, and hardly a foreign plant species can be seen. A few introduced plants do live in the forest: Kahili ginger from the Himalayas, check; wandering Willie, or Tradescantia, from South America, check; Plectranthus blue spur flowers from South Africa, check. But even though these plants grow in the forest, the forest is still dominated by native New Zealand trees, shrubs and ferns, and there is no indication that they will cause native species to disappear.23
This story is told and retold. On average, for every new species that arrives, less than one of the species that was originally there dies out.24 The arrival of foreign plants has not only approximately doubled the diversity of New Zealand’s flora,25 it has also done so in the Hawaiian islands and elsewhere in the Pacific. The botanical diversity of these islands seems to just go up and up and up as new species arrive, with no obvious limit in sight.26 This is also true of vertebrates. Despite the extinction of many native birds from the Hawaiian archipelago, the islands are now home to many different introduced mammals, lizards, frogs and freshwater-fish species, as well as to imported birds.27
This all leads to an increase in the diversity of each region. Take Britain, which now has six species of wild deer, rather than the original two. Some 1,875 foreign species of plants and animals have established wild populations in Britain in the last two thousand years, and mostly in the last two hundred; yet, as far as we know, no native species has died out as a consequence (although some have died out for other reasons).28 This pattern repeats itself across the world’s continents.29 American states have experienced approximately 20 per cent increases in plant diversity through imports, and a similar level of increase has taken place for fish in American river catchments.
It is worth reflecting on the British ratio of 1,875 arrivals to zero extinctions caused by invasive species. With odds that low, I might cease to worry so much about legislating against new arrivals. Of course, some of the new arrivals do engender ecological changes–the replacement of one introduced crayfish by another, for example–that fall short of the extinction of native species, but change is how the biological world works.30
Many of these successful invaders have simply filled ecological voids and seized new opportunities that have been created by humans. We have changed the world’s habitats: creating forests where there were grasslands, meadows where there was once forest, and cities along the coast. The burgeoning human-mediated traffic of microbes, fungi, plants and animals is accelerating the rates at which these human-made habitats are filled with species. The New Zealand forest is still largely populated by plants that have lived there for millions of years, whereas Auckland’s human-created meadows and suburbs are dominated by species from Europe, Australia and Asi
a. Woodlands in England and in New England are often composed of native trees, shrubs and under-storey herbs, whereas human-derived habitats are dominated by plants that originated from further afield, be they from sand dunes along our coastlines, mountain crags or imports from other continents. In Missoula in the American state of Montana, the university campus teems with imported European birds, while the forest behind is inhabited by natives. Other opportunities arise because we have altered the Earth’s climate, and incomers that we have transported from warmer regions are able to thrive under the new conditions. The biological world is on the move, taking advantage of new human-created opportunities.
However, ecological and evolutionary voids also exist for other reasons. For example, many oceanic islands were too remote to be colonized by land mammals, and their arrival has filled a void. And the dearth of broad-leaved evergreen trees in Europe was due to the history of the ice ages; introduced species have now plugged this gap. The myriad species that are now filling these historical voids and taking advantage of the new opportunities we have created are the initial inheritors of a human-altered planet.
Maggiore is emblematic of this new world. The Swiss Riviera may not be typical of the whole world–no place is–but there are many common themes. The consequences of hunting, agriculture, forestry, industry, gardening, fishing and tourism, not to mention climate change, are all evident in this one landscape. The numbers of species in the landscape have increased with the new diversity of habitats: the Italian sparrow, whose existence is a consequence of human arrival, is nesting in the eaves of the lakeside villas; yellowhammer buntings, clovers and comma butterflies are found in the farmland; green woodpeckers, European pond tortoises and cranesbills inhabit the golf course; yellow whirls of Cornelian cherry flowers, chirping tree sparrows and sulphur-powdered brimstone butterflies adorn hedgerows, while hawfinches, blackish-red squirrels and lime trees are found in the forest. The numbers of species have also increased with the importation of thousands of garden plants, and the lake is full of fish that have only just arrived. In addition, the warming of the climate has allowed the broad-leaved evergreens to make their home in the surrounding forests. The world has become a melting pot–New Pangea–transforming our planet in a few short centuries. And one of the consequences is that European forests of the future will contain more species, not fewer.
That’s not so terrible. We have to be realistic and accept the world’s biological systems for what they are. In the long run, it is the species that keep moving and successfully exploit new environments that will survive and prosper and thus ensure the survival of their kin on planet Earth. Successful species will continue to inherit the human-altered Earth.
PART III
Genesis Six
Prelude
In Part II, I concentrated on ecological change. The exploitation of animals and plants by humans, conversion of habitats, climate change and the transport of species have increased the abundances and geographic distributions of many species, as well as triggering the decline and extinction of others. They have generated declines in diversity in some locations but increases in many others. These ecological changes are at the forefront of environmental thinking, and rightly so. However, humans have initiated a period of evolutionary change that is just as fundamental as the ecological transformation of the world–in the long run, more so. When the environment changes, life ultimately responds by evolving. Evolution is how life on Earth comes back from disasters.
The ‘Big Five’ mass extinctions to befall Earthlings–those occasions in the last half billion years when three-quarters or more of all the species that previously existed became extinct–led to periods of biological recovery and diversification over the ensuing millions of years. Each mass extinction gave rise to its own genesis of new lifeforms that would go on to dominate successive spans of our planet’s history. Amid a new human-created mass extinction–some say mass extinction number six–we should consider whether we are on the brink of a sixth major genesis of new life. This is the topic of Part III.
The initial phase of recovery from the Big Five mass extinctions involved some creatures becoming successful while others dwindled. We might just think of this as ecological change, but the characteristics of species that enable them to be successful under new conditions are the products of past evolution. Birds and mammals were successful, whereas the much larger dinosaurs died out at the end of the Cretaceous period. This represents evolutionary change on the grandest of scales. So, in Chapter 6, I contemplate the characteristics of some of the winners and losers of the human epoch. This evolutionary replacement of some kinds of species by others is already an indelible signature of the Anthropocene epoch.
Those species that do survive have begun to live under new physical and biological conditions. Every population of every species has experienced changes, be that to the climate, the acidity of the ocean, the levels of carbon dioxide in the atmosphere, increased levels of nitrogen in the soil and water, the habitats they live in, or the arrival of new invading species from across the seas. Given that genetic variation exists for nearly everything, evolutionary change inevitably follows, as populations and species respond to these new challenges. These are the events that I contemplate in Chapters 7 and 8. In Chapter 7, I evaluate how some individuals and populations are surviving better than others when conditions change in places where they already live. In Chapter 8, I examine how evolution is accelerating in the Pangean archipelago as species arrive in a new part of the world and meet other species for the first time.
Chapters 7 and 8 also reveal something more surprising. New species seem to be coming into existence with immodest haste, adapting to new conditions. Increasing numbers of species that are global adventurers are coming into contact with distant relatives, as we will see in Chapter 9, and hybridizing with them. These events are generating novel evolutionary forms and, on occasion, new species. The Italian sparrow is one such. Remarkable as it might seem, new plant species may be coming into existence faster today than at any time in the history of our planet. A new era has arrived in which we see an acceleration of evolutionary change and the genesis of new lifeforms. Given that many of them would not exist but for humans, they challenge us to contemplate the relationship between humanity and nature.
6
Heirs to the world
When the famous naturalist, writer and zoo-keeper Gerald Durrell arrived in New Zealand in 1962 in search of the elusive takahe, he and his film crew headed for the one place where a small surviving population hung on, in a valley high in the Murchison Mountains in the remote south-west of the South Island. Drenched in the pouring rain, and frozen to the core, Durrell temporarily forgot his discomfort, so astonished was he to encounter a multicoloured, scarlet-beaked, goose-sized bird popping out from behind a clump of snow grass. And then Takahe Valley, where they were, once more disappeared, as Durrell later recalled, under the ‘muffling grey paw’ of encircling rain clouds, at which point he and his crew retreated to their mountain hut ‘sipping whisky and tea in equal proportions’ to restore their inner warmth.1 He continued: ‘In the many years I have been hunting for animals in various parts of the world, I can never remember being so acutely uncomfortable as I was during our sojourn in Takahe Valley.’ And that is what saved them. The birds, that is–alas, it was the whisky that accounted for Durrell in the end. The environment was so harsh that it was even worse for the rats, cats, stoats and other introduced predators that kill takahes than for the flightless birds themselves. The endangered, plodding birds had just hung on.
The Murchison Mountains was the last place they survived, the final remnant population belonging to one of the two species of takahe that were entirely confined to New Zealand. Predators and then competition from introduced deer brought their numbers down to little more than a hundred individuals by the beginning of the 1980s. They were on the way out, until an intensive conservation programme was launched. Deer were shot from helicopters to reduce competition
for grazing, a captive breeding project using takahe-shaped glove-puppet feeders with fake scarlet beaks (to stop the chicks imprinting on humans) was developed to bolster numbers, and then hand-reared birds were released on to safe offshore islands, where the predators had been trapped and poisoned. Since then, they have been established on seven such islands. Only around two hundred and eighty individuals exist, even now, but at least their numbers are going up and it is possible to see them again.
Fifty years after Durrell’s visit, my wife and I were back in New Zealand. Annabelle, our volunteer guide, beckoned us past the toilet block, behind the café and visitor centre on Tiritiri Matangi Island, a climatically benign sanctuary in the Hauraki Gulf near Auckland. And there they were, half a metre high, gleaming blue-black and greenish feathers, beaks like the ends of a bright red anvil: a pair of takahe. It was a creature I thought I’d never see. But if the somewhat bow-legged, lumbering parents were astonishing enough, the chick was something to behold: fluffy, grey-black and reminiscent of a miniature B-movie tyrannosaurus in its unsteady pursuit of its parents. No match for a rat or a stoat. Turning back the clock to a mammal-free New Zealand seems to be working, in the sense that these heavyweight relatives of moorhens, coots and swamphens can survive and breed again in the lowlands, provided that they are maintained in places where predators are kept away.
One might reasonably ask how come they were on the lawn near the toilet block rather than in the ‘native bush’ that everyone was trying to restore? The answer, it seems, was that the gangling mini-monster and its corpulent parents were fattening themselves on a diet of the nutritious European-origin grassland plants that dominated the lawns, eschewing the tougher native vegetation. The flora that the takahes have been tucking into has more in common with my own meadow 18,000 kilometres away than it does with the remnant native bush a mere ten metres from where the birds were feasting. Not all foreign species are bad for takahes, and the removal of predators is only a façade of re-creating the past. From the takahe’s perspective, successful alien mammals are terrible but successful alien plants are delicious. Another odd feature of this remarkable project is that there is no historical record of a takahe having ever lived on Tiritiri Matangi, and those that live there today are South Island takahe rather than the now-extinct and larger North Island takahe, which used to live on the neighbouring mainland. Both species of takahe did live in the lowlands previously, but not, as far as we know, on Tiritiri Matangi itself.
Inheritors of the Earth Page 12
