Inheritors of the Earth

by Chris D. Thomas

  This does not in itself tell us whether we should think of the Italian sparrow as a variety of Spanish sparrow (as my mother and I had supposed), as a kind of house sparrow (which our bird book had suggested), or as a separate species in its own right. In any event, more work was required. Glenn-Peter and his band of merry geneticists found that the Italian sparrows hardly ever still interbreed with the Spanish sparrows, and they remain quite distinct from house sparrows, despite the fact that some hanky-panky does continue to take place in the foothills of the Alps. The Italian sparrow is a new species–separate and genetically self-perpetuating.

  Biologists usually think of new species taking hundreds of thousands, or millions, of years to evolve. Yet the Italian sparrow must have come into existence within the last eight thousand, after agriculture first began to be practised in the Italian Peninsula12 and the previously Asian house sparrow established itself in the region. This is astounding. The expansions of town pigeons, eastern bluebirds, house sparrows and tree sparrows, which are the urban sparrows in parts of eastern and southern Asia, are impressive enough–many previously existing species are benefitting from human-caused changes to the world. But the Italian sparrow is something different. It is an entirely new species that has originated in the human era, and for which we are directly responsible. This species exists only because humans created the towns, villages and farmland that allowed the Asian house sparrows to spread, permitting them to meet up with Spanish sparrows and produce a new kind of hybrid. Moreover, humans have created all the urban and agricultural habitats where Italian sparrows now live. Because hybridization between the house and Spanish sparrows must first have happened in a single breeding season, the birth of Italian sparrows would have been extremely fast. Separation is likely to have been achieved in a few hundred generations, and it may only have taken a few decades, which is at least a thousand times faster than would be expected by our ‘conventional’ understanding of evolution. It is the virtually instantaneous biological genesis of a new species.

  Humans have not only changed the world’s ecology, enabling house sparrows to spread around the world, we have also altered the trajectory of evolution.

  Amid all this change, our attitudes are failing to keep up with the reality of the modern world. The news is full of stories that we are causing the loss of species and that we are transforming the Earth. As I write, in December 2016, the recent news contains dozens of stories explaining how we are harming the world’s biological diversity, pointing out that climate change is causing wildlife populations to die out; that 7 per cent of all mammal species, including giraffes, are endangered by poaching; that thirteen recently discovered bird species are already listed as extinct; that Mexico’s vaquita porpoise is close to extinction because they drown in fishing nets; that half of the shark and ray species in the Mediterranean are endangered; and that plastics threaten marine life throughout the world.13 Stanford University professor Paul Ehrlich sums it all up: ‘We are basically annihilating the life on our planet.’14 Bombarded by thousands of bad-news stories every year, we are motivated to try and fight back. It is right and proper for us to attempt to stop climate change, to prevent unsustainable hunting and fishing and to avoid polluting our environment.

  On the other hand, we are often ambivalent or even hostile to the biological successes of the human epoch. When successful species turn up in new locations, we resist their arrival. The same trawl of current news revealed over a hundred reports highlighting the ‘negative impacts’ caused by mammal, insect, snail, mussel, worm and plant species that are today living in a continent, country or habitat they did not previously occupy. We have declared them to be in the ‘wrong’ place, but nearly all these examples could be rewritten as biological success stories. Each of these species is now more numerous and widespread than it used to be. We take a particularly dim view when successful ‘foreign’ species have the temerity to interbreed with ‘native’ residents. For example, the European Commission, the UK government and the Royal Society for the Protection of Birds, among others, have been behind recent attempts to exterminate the ‘foreign’ American ruddy duck, which escaped from British wildfowl collections in the twentieth century and then started to spread into mainland Europe. Ruddy ducks can hybridize with the rarer ‘native’ Eurasian white-headed duck, so the consensus in the conservation world has been that all the ruddy ducks and hybrids in Europe must be killed.15 In this quarter, hybrids are deemed to be bad.

  Similarly, at the opposite end of the world, pied stilts are elegant, pointy-billed, black-and-white birds that wade through water on long, pink legs in search of tasty insects and worms. Pied stilts colonized New Zealand by flying across the Tasman Sea from Australia in the early nineteenth century. However, they represent a genetic threat to New Zealand’s native black stilts, according to the New Zealand Department of Conservation and the International Union for the Conservation of Nature. Hybrid stilts are ‘controlled’, even though the primary cause for the decline of black stilts has been predation by stoats and other introduced mammals rather than hybridization.16 Given that introduced carnivores are the greatest threat to black stilts, the long-term effect of hybridization could potentially be beneficial if it acts as a source of genes that enable the hybrids to evade their predators a little better, which the pied stilts evidently can. But the black-and-white hybrids are so obviously ‘impure’ that they must be removed–it would be a national disaster if the nation’s ‘all black’ stilt were to become extinct.17

  These human responses to recent biological invasions and to the hybridization that followed suggest that, if the Italian sparrow had arisen within the last few years and had started to spread at the expense of Spanish sparrows, there is a strong likelihood that we would be doing our ‘best’ to kill off the wicked Asian house sparrow and our guns would be turned on the new hybrid Italian form. It is hard to see the logic of liking Italian sparrows, which are extremely recent hybrids in the history of life on Earth, but being quite so antagonistic towards other hybrids that have even more recent origins. Seen through the telescopic lens of life on our planet, all these events have taken place at more or less the same time. And they all have the same underlying cause: humans.

  We have to accept that a world without change is not an available option. Because humans are involved, we feel uncomfortable, giving us a sense of negative responsibility for all the transformations–including new biological successes–we observe. We are confused because a particular animal or plant may not necessarily be where we expect, and confused again because species are neither all good nor all bad from a human perspective. House sparrows are certainly a mixed blessing. They do nest in our roofs, block gutters, eat crops, leave their droppings in stored grain and on windowsills, carry avian diseases and oust other birds from their nesting holes, and they do sometimes peck their competitors to death. But they eat some insect pests and consume weed seeds as well. And, on occasion, we have eaten them as food. They also amuse us. In other words, they are much like any other bird, apart from their particularly close association with humans. They do all these things wherever they live in the world, regardless of how long they have been there. The more recently they arrived, the further they are from their origins and the greater the level of human intervention in getting them to far-flung parts of the world, the more we point to the downside of their existence. But this ignores the reality that house sparrows behave pretty similarly and have comparable effects on humans and other species wherever they live in the world. These different opinions seem to have at least as much to do with human prejudice as they do with any biological reality.

  Whatever our individual opinions, these sparrows have been remarkably successful in the modern world. This leads us to an important question. Are they part of a broader movement in which life on Earth has set off on a new course, coping with and adapting to the impacts of humanity? As the remainder of this book will reveal, the answer is ‘yes’. Huge numbers of species are thriving in t
he human-altered world, and we may even be stimulating a mass diversification of new species. The present day is indeed the regrettable end of the evolutionary story for many animals and plants, but it also represents a new opportunity for many others.

  PART II

  New Pangea

  Prelude

  The house sparrow reminds us that species are taking advantage of new opportunities that have been provided by humans. Sparrows are ecologically successful in that they are more numerous and inhabit a greater part of the world than they did before humans appeared. They have also evolved–the hybrid Italian sparrow has come into existence, with the consequence that the world’s biological diversity has increased by one species. Ecological and evolutionary changes are both of great importance. Ecological success will determine the species that will live among us in the short term, and evolutionary success will alter the future direction of life on Earth. I will consider ecological success in Part II and evolutionary responses to the human-altered world in Part III.

  In order to investigate whether opportunists like the house sparrow are rare or common, I have selected four human-caused changes that are critically important to the world’s terrestrial biodiversity, and I will explore each in turn. These are the killing of animals for food and other products (Chapter 2), habitat destruction to make space for agriculture and cities (Chapter 3), climate change (Chapter 4), and the biological invasions that take place when species are transported to new parts of the world (Chapter 5). These are the four greatest known threats to the biological diversity of the land but, as we shall see, they have brought unexpected opportunities as well.

  There are far too many successful species to chart every one individually, so I have intentionally selected examples from various parts of the world to demonstrate that biological gains are genuinely global. However, we need to complement these illustrative case studies with a broader assessment of how the diversity of life is changing. Is biological diversity going up or down? If we count the number of species on Earth, the answer is undoubtedly down. The ‘extinction crisis’ is real, as I mentioned in the prologue, and I will return to it later. Nothing that I write in this book contradicts the evidence that we are in the process of losing many species that existed before humans arrived on the scene.

  However, it is not so obvious that diversity is declining in any particular district, county, province or nation. Animals and plants are still being extirpated from them, for sure, but fresh immigration provides a counterbalance, introducing new species to each region. We need to enumerate these additions before we can conclude whether biological diversity is increasing or decreasing in the landscapes that surround us. So let us contemplate how we are changing our planet, and tally up the gains as well as the losses.

  2

  Fall and rise

  Ten minutes down a leech-infested forest trail came the first crash. An orangutan, perhaps? More crashes, followed by a hint of musty air. Probably bearded pigs, I thought. Then, rounding the massive buttress of a forest tree were three exceedingly large grey shapes guzzling wild ginger plants and grabbing fruits with their trunks: pygmy elephants. Despite the name, the female was about two metres high and weighed several tonnes, heavy enough to leave plate-sized ponds wherever she trod on boggy ground. Her elder daughter was already a metre and a half at the shoulder. Slightly smaller than their Indian relatives, the pygmy elephants I’d encountered in the Danum Valley Conservation Area of northern Borneo are still among the heaviest animals on Earth. And there were more than three. Surprisingly well hidden by the undergrowth, they were revealed by periodic crashes as they attempted to squeeze through smaller than elephant-sized gaps in the vegetation.

  The next crack was much closer. An even larger female was heading straight towards me, oblivious to my presence. At this point, I realized that I had left home without completing a health-and-safety training course on ‘what to do if you walk into a herd of elephants in a rainforest’. I moved closer to several large trees, figuring that I might be more nimble than a three-tonne Proboscidean and hoping that the trees would hold their weight (without being at all sure that either was true). At fifteen metres, I reckoned a cough was in order so that she knew I was there. It could have been dangerous if she noticed me only when we were standing next to each other. She registered my cough by raising her head and flexing her ears, but carried on, perhaps imagining that I was one of the herd, or that I was too insignificant to be a cause for concern. At ten metres, I coughed again. Adrenaline coursed through the veins of elephant and human alike. She started, almost jumped, and off she ran: to the extent that a hefty not-quite-leaving-the-ground elephant can run through dense undergrowth.

  I realized it was time to head back to the Danum field station for a refreshing drink and to allow my emotions to subside. Safe once more, I was distracted by the Eurasian tree sparrows that had taken up home in this remote outpost, a human-created sparrow oasis surrounded by what for them was inhospitable jungle. They were popping into the canteen through slatted windows and picking scraps of food off plates that had been left stacked up after breakfast. A small bird that had originated in the cool temperate zone of Asia was, thanks to humans, living a mere ten-minute walk from where I had just seen a bunch of forest elephants. This is not how nature used to be.

  My only previous encounter with wild elephants was in Pilanesberg National Park in South Africa, where a family party was standing in the middle of the road, barring progress and threatening to leave us trapped in a small car for the night. We would be locked out unless we were able to reach our camp before sundown. The leviathans had seen it all before; tourists were an everyday occurrence for them. Not bothered, they soon lumbered off into the acacia-strewn landscape, and we could speed on towards our lion-free, fortified lodgings. That was more like the wildlife documentary and holiday brochure version of an elephant–African elephants flapping their ears amid savanna landscapes of browned grasslands and scattered trees. David Attenborough might have popped out of the undergrowth at any moment.

  The two encounters provided a stark contrast. One group of elephants was travelling in a herd through the basement of a towering, verdant-green tropical forest, the other wandering across sunbaked African grasslands dotted with stunted acacias. Indeed, elephants can be found in near-desert-like conditions as well as in the wet rainforests of the Congo Basin. It raises a question. Why, if elephants can live in such different environments, do they not live all over the world?

  In fact, they used to. All manner of elephant-like animals lived across most of our planet’s land surface until very recently (in ecological and evolutionary terms), when ancient clans of human primates hunted them to extinction. Elephants could provide food for the entire group, useful skin, fur in the case of mammoths, and a source of bones and tusks to construct small buildings and to make tools. People still kill them today because they are dangerous animals. Exact figures are hard to come by, but the National Geographic Channel documentary Elephant Rage reported that they are responsible for the deaths of around five hundred people a year. Testosterone-crazed bull elephants that are in a state of ‘musth’ during the breeding season are seriously incompatible with everyday life, so Pilanesberg is surrounded by towering electrified fences for a very good reason. But the fences are mainly to protect the elephants, which wander around sporting extremely valuable overgrown teeth. The ivory in these tusks, which is essentially the same as the dentine in our own teeth, has been used for jewellery and ornaments for at least thirty thousand years. Poachers kill the elephants and hack out the tusks, and then criminal gangs ship the ivory to wherever there is a demand, mostly to eastern Asia.1 It is a deadly battle between humans and elephants, but the odds are stacked: at least fifty times as many elephants have been killed by humans in recent years2 as the other way around. Worse still, humans can kill elephants faster than elephants can breed. No wonder they have disappeared from most of the world.

  The remarkable thing is that any elephants ha
ve survived at all–although there is no scientific consensus on why it is the African and Indian elephants that still exist, when the others have disappeared. African elephants had an advantage because they lived alongside pre-human apes from the start and may have gradually evolved an increased ability to escape or fight, allowing them to cope with hunting parties that were armed with little more than sticks and stones. This may also help explain the survival of Indian or Asian elephants, which lived alongside hominids for a million years or more, prior to the arrival of modern humans; although it remains unexplained why it was Indian elephants that survived, when their relatives elsewhere in Europe and Asia did not.

  As modern human civilizations developed over the last few thousand years, the continued survival of elephants seems to be thanks to two further historical accidents. In Africa, they were partially protected by a virtual fence that had been erected by the tsetse fly, a horsefly-like insect that feeds on blood. The painful, saliva-filled bites of tsetse flies transmit trypanosomes, delivering sleeping sickness to human victims, and nagana and surra to our livestock–diseases that incapacitate cattle, camels and horses and afflict pigs, sheep and goats. An estimated 48,000 people south of the Sahara die of sleeping sickness each year, and the toll on our livestock is far greater. This dual assault checked the progress of pastoralists and reduced the encroachment of mixed-agriculture farms into areas where elephants and other large mammals survived.3 Elephants, in contrast, had a sufficiently tough hide to deter most tsetse flies and enough resistance to trypanosomes to survive in the tsetse zone.