Inheritors of the Earth

by Chris D. Thomas

  These figures already incorporate the successes of modern conservation efforts, and particularly the protection of national parks and other preserves and actions to save individual endangered species. If these efforts were abandoned, the extinction rate would escalate. A major task of conservation is to keep the losses towards the lower end of the likely range–as well as to encourage biological gains. Although I have been advocating a more flexible approach to the environment, and specifically to conservation, nothing I have said should be used to undermine attempts to save existing species or maintain protected areas.

  Yet in this book I have chosen to focus on the gains, which are so often overlooked. Virtually all countries and islands in the world have experienced substantial increases in the numbers of species that can be found in and on them, particularly when it comes to plants. Some of these additional species have colonized human-created habitats under their own volition, and others have been accidentally or deliberately introduced by humans. Most places for which good data are available now have between 20 per cent (for state or country-sized areas within continents) and 100 per cent (especially on islands) more plant species growing in the wild than they did before humans arrived. These percentages may be higher than the global norm, since good data mainly come from countries for which there is a relatively long history of trade, and hence higher rates of importation. However, all parts of the world are now involved in international transport, and these more recently trading countries can be expected to catch up pretty soon. If the recent pattern of more arrivals than disappearances keeps up for another thousand years, most regions will experience at least a doubling of the number of plant species present, and some will see five-fold increases.

  Once plant diversity is in place, the diversity of animals that feed on plants, and then of other animals that consume these herbivores, will eventually catch up. Insects are establishing on introduced plants throughout the world, and regular outbreaks of pathogens imply that the same must be happening for fungal and microbial diversity (most such invasions will not be noticed unless they cause a major epidemic). The number of vertebrates in each region is increasing too. Despite the extinction of birds and reptiles on oceanic islands, most of these islands now support more vertebrate species than they used to, if you add up all the introduced mammals, birds, reptiles, freshwater fish and amphibians. The number of species living in virtually every country or island has already increased during the period of human influence, and numbers continue to increase.

  These diversity increases go beyond a simple count of the number of species. Each island now supports land mammals as well as birds–the addition of a major new group of animals. Each continental region now contains representatives of plants and animals that evolved for tens of millions of years on separate continents. Adding Australian gum trees to the flora of California increases the variety of major plant groups (Eucalyptus is a new genus for California) more than would be achieved by importing an additional pine tree (Monterey pine and other species in the genus Pinus already grow there). With this increased diversity at the base of the food chain, there are increased opportunities for insects and fungi and bacteria that did not initially accompany the plants to spread around the world. Almost all countries, states and islands are now more biologically diverse than they used to be.

  These ecological changes have added diversity to each region,4 but they have not added species to the world list–that requires evolution. Evolutionary additions are likely to take place by three main processes. The first is hybridization, as I discussed in Chapter 9. Human-moved species from different parts of the world meet up, reproduce and generate hybrid offspring. In some cases, as with ragworts and monkey-flowers in Britain, the hybrid offspring are not compatible with their parents and new species arise almost as soon as the pollen tube from one species meets the ovule of another. More information is still needed, but the hybrid generation of new plant species in Europe and North America already seems to be as fast as the process of extinction, and possibly even faster. The New Pangea in which distant species are meeting up and generating hybrids is so unusual in the history of life on Earth that the current generation of new plant species, in particular, could be higher than it has been at any time in the 700 million years since there have been land plants. In other cases, the new hybrids (such as between rhododendrons, or between red and sika deer in Scotland) do not form new species immediately, but the new mixed-gene animals or plants will slowly head off in a new evolutionary direction–perhaps generating new species in hundreds of thousands of years, rather than overnight.

  The second process involves animals, plants and microbes that take advantage of new ecological situations, as I considered in Chapters 7 and 8. The apple fly and the parasitic insects that attack apple flies became adapted to a new ecological opportunity (introduced apple trees) on a timescale of decades to a century and a half, to the point that they are already on the verge of forming new species. This will be a major process, generating new species not only for decades and centuries but for hundreds of thousands of years to come. A million years hence, we can expect all the plants that establish persistent populations in new parts of the world during the human epoch to have acquired substantial numbers of specialized insects and diseases. The rate of evolution is plenty fast enough to achieve this. Gum tree forests in California are likely to have as many associated insects and diseases in the ten thousandth century as gum trees in Australia have now.

  The greatest contribution to new species, however, is likely to be the third process–through geographic separation in the human, and perhaps then post-human, world. This is when two or more populations of the same species, which we have transported around the planet, live in widely separated geographic regions. Eventually, they become distinct species in different places and, just as hybrid speciation is the short-term signature of the Anthropocene, this will be the long-term consequence of the human-dominated Pangean Archipelago. This process of evolutionary divergence in (partial) isolation is how mockingbirds came to be different species on different islands in the Galapagos. It is why, at a larger scale, the South American southern lapwings (the bird that was expressing its vocal irritation at the crowds during the Olympic golf tournaments in Rio de Janeiro) evolved into a species that is different from the related northern lapwing of Europe and Asia, and from other lapwings in Africa, Asia and Australasia. Star-thistles that were introduced from Europe to California have started off down this road. The Californian plants are already partially infertile when crossed with the pollen of their European ancestors, less than a century after the plants were introduced. In a million years’ time, a substantial proportion of all the species that have been transported to new continents and islands will be different enough from their ancestors that they will have become new separate species. By 2 million years AD, almost all of them will be different. Californian and Australian blue gums will no longer be the same species.

  To put this in historical context, white-eyes represent a group of tropical birds that spread out across the Old World islands and continents in the last 2 million years.5 Apart from the mode of transport (flying rather than human-assisted colonization), this is exactly the situation facing every species that has become globally distributed during the Anthropocene. Human-introduced species now live in many different parts of the world, and there are only modest rates of genetic contact today (via continued human movement) between their original populations and the new places where they now live. This is similar to the white-eyes, which needed the capacity to move long distances in the first place but then became adapted to local conditions and became separate species. White-eyes have already diverged into about eighty species.

  This is not a one-off. Over 375 species of New World rats and mice evolved after they colonized South America, and rodents have evolved into over 130 species in Australia and New Guinea just a few million years after their first ancestors (presumably) rafted over on floating vegetation from Indonesi
a. And about eighty species of lupin plants evolved in the Andes in the last one and a half million years, after they invaded from North America. It would take only a modest 5 per cent of the world’s species to repeat the white-eye, rodent and lupin’s feats (say, generating twenty new species in different geographic locations in a million years) to double the total number of species on our planet. Each of these new species is then likely to acquire unique parasites (including associated insects, in the case of plants) and diseases, further bolstering numbers.

  Diversity increases on islands are likely to exceed this. Because introduced mammals can’t fly, the populations on each island will become genetically isolated; or at least the rate of continued arrival (gene flow) during the human epoch is unlikely to be sufficient to prevent them from becoming adapted to local conditions. The world’s islands will end up with considerably more species in total: tree rats, ground rats, fast rats (where there are predators, they need to escape), slow rats (where there are not), and vegetarian and predatory rats will come into existence in different locations. Give it a few million years after that and many of them will seem truly bizarre. New lineages of reef-diving mammal might be born. Can we put numbers on this? If a thousand of the Pacific Ocean’s twenty thousand to thirty thousand islands are large enough to support populations of two kinds of rodent (starting off with one rat and one mouse species) for an extended period of time, we will end up with an enormous number of new species added to the world list. This alone would add, approximately, an extra 40 per cent to the world mammal list. Many of these islands also have cats, dogs, goats, pigs and mongooses, among others, which would also diversify. The authorities in New Zealand have decided that it would be a good idea to wipe out as many introduced mammals as possible but, assuming they fail, there will be dozens of new mammal species that live only there. It seems likely that a third of all mammal diversity might be species that live only on islands, in the year one million AD.

  Introduced lizards and birds also abound–there are at least as many introduced bird populations in these scattered locations as there are bird species that have died out. These, too, would turn into separate species, but this time they would be species that are capable of surviving in the presence of mammals. The world’s islands will be full of unique vertebrate species again, and probably at least triple the number there were in total before humans arrived (even though there are far fewer unique island species today than there were ten thousand years ago).

  Back on the world’s continents, we will start to see the largest surviving mammals, including descendants of domestic livestock, reclaiming their place. We can expect the feral horses that already live in North America, South America, Europe and Australia to become separate species in a million years or so; and there might be more than one species on each continent. Descendants of tapirs may be on their way to becoming the next multi-tonne beasts. Giant jaguars will have returned to prey on them. However, these new biological communities will probably be dominated by even-toed ungulates. Relatives of cattle, goats and fleet-of-foot deer and antelope have survived the human epoch pretty well. These are the mammals that will be available to start evolving into the next generation of megabeasts. However, there may be unexpected additions. The fastest kangaroos, if they escape from captivity during the human epoch, could by then have become major herbivores in all the world’s grasslands and savannas.

  Having turned the whole world into a global archipelago, we have set in train processes that will increase rather than decrease the long-term diversity of the Earth. If I were to hazard a wild guess, and humans were to disappear today, I imagine that there will be approximately double (between one and a half and five times) the number of species alive on the Earth in a million years. If the current rates at which species are accumulating in new parts of the world were to continue for another millennium, and then humans were to disappear quietly (without a major calamity), then the multiplication of diversity over the course of the next million years could be far higher than this. Five million years later, once these new species have evolved into increasingly distinct forms, the advent of humans could be seen to have substantially increased the biological diversity of the Earth. A sixth genesis.

  Acknowledgements

  The bulk of the credit for everything in this book goes to those scientists, conservationists and many others I have met throughout my life, and who have influenced my thoughts. I would also like to thank dozens of others who accompanied me, hosted me, argued with me and made various trips around the world the enjoyable times they were. I am not going to try to thank everyone individually–but thank you. I would like to thank all the authors of the scientific articles and books I have read on the influences of humans on the biological world, and those who have studied how species and biological communities have responded to climatic and other environmental changes in the geological past–this historical context is essential if we are to understand and then respond to present-day changes. I cite some of these works directly, especially where I dwell on particular examples. However, thousands of articles have been consulted (and tens of thousands over the years), hence it is impossible to list them all without making the Notes longer than the whole of the rest of the book. My particular thanks to all those whose work I should have cited but have not.

  I have done my best to represent all factual information accurately, even when my own interpretation of the evidence is not always the same as that of the original authors. All the opinions represented in this book remain my own, although I have of course borrowed liberally from others. I am as grateful to those who have argued against my ideas as to those who have agreed–honest discussion is how science advances. I apologize to those who feel offended by my conclusions. I hope that we can shake hands and productively discuss the best way forward.

  In a couple of places, I have included a new analysis in this book (such as the effects of sparrows on bluebirds). In these circumstances, I have provided a very brief summary in the text or in endnotes to the relevant chapters.

  All the events described in this book are accurate to the best of my recollection and knowledge; for the purposes of the narrative, some things that happened or were observed up to two years apart are presented as though they took place as a single event. I have personally visited most of the locations described in this book, but on a few occasions I have used accounts by others (including information in the methods sections of scientific articles) and online photographs in an attempt to bring places and species ‘to life’. Where I have done so, I have taken in good faith the information that is publicly available. The Leverhulme Trust and the Georgetown Environment Initiative supported the Chernobyl trip; thanks to Victoria Beale and David Moon for organizing it.

  A number of people have kindly read draft chapters and sections, or otherwise provided information, in an attempt to spot any factual errors or misinterpretations I may have inadvertently made. They are: Richard Bailey, Colin Beale, Jon Bridle, Roger Butlin, Matthew Collins, Erle Ellis, Jane Hill, Daniel Montesinos, Dov Sax and Stuart Weiss. Very many thanks. Any errors remain my own.

  I thank my agent, Peter Tallack, for persuading me to write the book in the first place, and Peter and Tisse Takagi for their wise advice throughout the process. Many thanks to my book editors Laura Stickney (Penguin Random House) and Ben Adams (Public Affairs Books), and Sarah Day, for their suggestions and valiant attempts to convert my prose into a readable volume. I hope it was not too painful. I also thank Shoaib Rokadiya, Pen Vogler (publicity) and Richard Duguid (editorial) at Penguin, and Melissa Raymond (production), Jaime Leifer (publicity) and Lindsay Fradkoff (marketing) at Public Affairs Books, and their colleagues.

  Thanks to my wife, Helen, daughters Rose, Alice and Lucy, and son, Jack, and for everyone at work for tolerating my lack of attention to my ‘real job’ while I was writing this book. My sisters, Phil, Julie and Anthy, and brother, Jeremy, helped recall anecdotes from my youth and found old family photos; they and my brothers-in
-law, Steve and Erik, and my sister-in-law, Sarah, provided information and photos relating to one or more of the trips described in this book.

  Then to the disappointments. I really did not need to be asked quite so often: ‘How is your book going?’ You know who you are. Now you can judge for yourselves.

  Picture Acknowledgements

  All photographs by the author, unless specified–many thanks to the other photographers. Copyright of photographic material remains with the original photographer.

  CHAPTER 1

  Photo: House sparrow copyright © Kevin Phillips.

  CHAPTER 2

  Maps of megafauna diversity: Thanks to Søren Faurby and Jens-Christian Svenning for providing the data for present and potential diversity, and to Phil Platts, who kindly produced the maps.