Inheritors of the Earth

by Chris D. Thomas

  The rest of the Australian birds continue to fly. If you go camping in the Cape York Peninsula of northern Queensland, keep your eye out for brush turkeys that come looking for scraps. Brush turkeys have vulture-like necks of vermilion-red skin, a vivid yellow collar and a black body that ends with a tail that has the weird appearance of having been ironed into a vertical position. Approach them, and they scuttle off into the undergrowth. But, if pushed, they explode into the air in a somewhat ungainly manner, flying up to land on a branch that is out of reach of the potential predator. They and many other Australian birds seek their food on the ground, but they have retained their ability to escape from ground-living predators when required.

  Whether Australian mammals could invade the world’s larger continents is largely untested. Brushtail possums are thriving in New Zealand despite the authorities’ attempts to exterminate them, and there seems to be no particular reason why they would not be successful elsewhere. Small populations of several kinds of wallaby are living in New Zealand, on Oahu in the Hawaiian Islands, in the British Isles, and in various other locations around the world. There is even a wild-living wallaby population to the south-west of Paris, in France. However, few introductions have been made of Australian mammals to continental areas where they really might be able to thrive. The larger and faster kangaroos are perhaps the most interesting because the kangaroo design has no equivalent among large mammals in the rest of the world. Kangaroos lived with marsupial cats for millions of years, out-hopped humans for fifty thousand years, and then managed to flee from dingoes for thousands of years. They might well be able to outwit predators on all the world’s larger continents, given the chance. Their mode of locomotion, combined with an impressive ability to survive and reproduce in regions with sporadic rainfall, is yet to be tested in the grasslands and savannas of South and North America, in the Mediterranean region or on the Indian subcontinent. I put my money on the kangaroos.

  While species that originate from larger, more diverse and better connected locations tend to win the evolutionary struggle, there are also plenty of successful species that originate in ‘smaller lands’, although they are, in most cases, species that come from large and biologically diverse21 habitats within these more island-like places. Species from the vast rainforests of South America were successful further north, and it might be presumed that Australia’s dryland plants and animals, including kangaroos, could be effective elsewhere–Australian wattle trees are already spreading across parts of southern Africa. New Zealand is renowned for its extensive cool rainforests, which support some two hundred species of earthworm that live nowhere else in the world. It is perhaps not surprising, then, that a flatworm predator of New Zealand’s earthworms is now happily consuming European species, following its introduction to the cool and damp British Isles. New Zealand is, perhaps, like a mini-continent for the enemies of earthworms.

  It should also be remembered that most species are not steam-rollered out of existence when biological worlds collide. The majority of all species in every region remain quite successful, presumably because their prior experience of competition, predation and disease, and their adaptations to local conditions, enable them to ward off most incomers. New Zealand still has nearly all its original plants, most of its invertebrates and about half its birds. Australia is still mainly populated by species with a long history of living there–and the highly successful passerine birds seemingly originated there before colonizing and diversifying in the rest of the world. Likewise, South America is still full of species whose ancestors evolved in South America millions of years before it became connected to North America. Subsequent evolutionary diversification of rodents within Australasia and South America has more than made up–if we simply count up the number of species–any deficit associated with the initial extinctions.

  Great replacements have frequently been at the heart of large-scale and long-term evolutionary change, most of which took place so long in the past that we now think of the consequences as simply the way the world is. Standing in the humid tropical luxuriance of Barro Colorado Island, I was in a place that bore witness to such events. I was at the crossroads between the southern and northern continents, where one of these great transitions unfolded. It was also the place where humans passed through as they moved into and once more transformed the biology of South America. In more recent times, the Panama Canal has increased the flow of marine life between the Atlantic and Pacific for the first time in millions of years. Wherever one looks around the world, humans are accelerating the transport of species from one location to another.

  Mix the species up and see who wins. Based on the lesson of the coming-together of South and North America, and of Eurasian animals arriving in Australia and New Zealand, it will be species from the most biologically rich parts of the world (for each particular type of environment) that are most likely to thrive. But we can also see, both today and in the geological past, that the overall consequence of these biological exchanges is to increase the diversity of life within each region.22

  In the middle of a period of change, as we are today, it is perhaps natural that the emphasis is often on the losers. If we were to look back on today from many millions of years in the future, we would see that humans caused an increase in extinction. The Anthropocene would represent an epoch that saw the final extinction of ancient groups which had hung on in remote locations where they had not yet been displaced. But we would also see the success of plants and animals that will form the basis of life in those distant post-Anthropocene times.

  A geologist 10 million years hence will notice how the world’s geography changed in a remarkable manner during the Anthropocene epoch. Individual species that were previously restricted to single continents became global in their range and, by this point, they will have diversified in their new homelands. A future geologist would remark on some great replacements, and on others that received the final nails in their coffins. Unless we prevent it deliberately, the Anthropocene will be the last stand of the tuatara, a primitive group of reptiles that arose about 220 million years ago but proved incapable of surviving in the presence of a full set of modern vertebrates.

  Yet the geologist would also see that, apart from several groups of exceedingly large animals, which humans hunted to extinction, nearly all the major types of animals and plants that were widespread before the Anthropocene epoch would have survived. The most successful will have been completely ordinary, everyday species: ground-dwelling mammals, flying birds, bats whizzing past in the night air. The heirs to the world are not bizarre, weird things and ancient evolutionary relics–those are the species that are dying out. The success stories are already all around us. Look out of your window and the chances are you will be staring at the future.

  7

  Evolution never gives up

  The Generals Highway snakes its way through a forested land interspersed with boggy meadows and rocky domes in California’s Sierra Nevada range. Black bears root their way through delicate, pink shooting-stars in the meadows, and lightning-struck pines stand as sentinels on high ridges. The highway ushers travellers between the Grant and Sherman groves of giant sequoias, where the visitors stop and gaze upwards at the reddened, fissured trunks of the most massive plants in the world. They enjoy the splendour of grand landscapes and experience a wilder, unchanging world. But this is no pristine landscape. Loggers moved through the forest in the late 1960s, leaving behind clearings of powdery granitic soil where the trees failed to regenerate. It was on one of these dusty slopes that I found myself sitting in 1984–contemplating why it was absolutely teeming with Californian butterflies, while the same species was far rarer in the less disturbed parts of the forest. We do not normally expect species to be commoner in places that have been so seriously ‘damaged’ by human actions.

  As I sat there, Edith’s checkerspot butterflies fluttered by on patterned wings, a mosaic of rusty-orange, black and creamy-white flecks. Every so often, a female l
anded inexpertly and laid a batch of eggs on the delicate blue-eyed Mary plants; when the eggs hatched, the caterpillars would consume the foliage. Despite their enthusiasm for depositing eggs on blue-eyed Mary plants that were growing in the dust, elsewhere the checkerspots laid their eggs on louseworts, which are distinctly less attractive-looking plants with grubby-yellow flowers that grow in the forest’s relatively undisturbed dappled shade. In choosing blue-eyed Mary plants, the butterflies in the human-altered habitat seemed to be behaving differently from those in the surrounding landscape.

  To find out if this was true, my PhD supervisor Mike Singer and I decided to ask the butterflies themselves. How this worked was that Mike wandered around wielding his butterfly net, and from time to time brought me female checkerspot butterflies that he had caught either on the dusty slope or in the butterfly’s original habitat. Meanwhile, I sat cross-legged, cramped, back aching, notebook open, a pyramid-shaped cage placed on the ground in front of me. To either side were spherical butterfly-containing cages, known as Singer balls, so named because of my supervisor’s own bodily deficiency and because Mike had designed them. I took each female butterfly out of one Singer ball, and then placed her on blue-eyed Mary plants in the pyramidal cage. If she liked what she tasted (through chemical receptors on her front legs), she would curl her body into a semicircle and settle down to lay a batch of eggs. Then I quickly removed her before she had had a chance to deposit the first egg, so that she would still be ready to lay if offered another plant. Once all the females had been tested, I gathered up my cages, stretched my legs and walked over to the nearest lousewort. There I sat down again and repeated the whole exercise, this time placing each female butterfly on the lousewort. After that, I went back to the blue-eyed Marys and did it all again. Then back to the lousewort, and so on.

  Because each butterfly was numbered with a Sharpie pen, I could track exactly how each female had behaved over a number of days. Lots of them would curl their body around the lousewort leaves very readily, and they would do so every time I placed them on this plant. But if I placed the same butterflies on a blue-eyed Mary in between times, they would just sit there and sun themselves. This would go on for hours, and sometimes for days. Clearly, they liked the lousewort better. This was not particularly surprising because lousewort was the plant that had traditionally supported the development of checkerspot caterpillars along the General’s Highway. However, some of the females seemed to like the blue-eyed Marys just as much, and a few of them liked them better than the lousewort. The individual female butterflies had their own personal opinions of how much they liked to lay their eggs on the two plants.

  Edith’s checkerspot. Female is laying on an introduced white man's footprints plant, Plantago lancelota.

  Female number 6 is laying a batch of yellow eggs at the base of a lousewort.

  Eggs can be found on this plant.

  The caterpillars eat the plant.

  The adult female shows the endangered subspecies, Taylor’s checkerspot, which survives mainly because it has switched its diet to feed on the introduced plants.

  Mike Singer and his students were witness to an amazing event. The butterflies were in the process of evolving a love of blue-eyed Marys in front of our eyes.1 Those that Mike had caught in the undisturbed dappled shade mainly liked to lay their eggs on louseworts, especially where the plants grew below scattered pine trees that surrounded natural rocky openings in the forest. But those from the disturbed dusty slope were perfectly happy to lay their eggs on blue-eyed Marys. It transpired that their caterpillars also survived better when they were eating this plant, so females that were prepared to lay their eggs on it, and carried genes to do so, left more offspring–they had evolved a new egg-laying behaviour that increased the consumption of the blue-eyed Mary plants.2 In less than twenty years, foresters had cut the trees down, butterflies had moved in, the butterflies had evolved to use the plants that were now growing in the new forest clearings and, by the middle of the 1980s, the checkerspot population had built up to record numbers. They had become adapted to a human-created habitat.

  Something even more impressive was taking place on the other side of the mountains, conveniently close to Carson City, to which we could retreat whenever it snowed or we felt the need to avail ourselves of the cheap but excellent buffets that were provided to entice potential gamblers into the casinos. What better reason could there be to visit Carson City? Naturally, we were there to study butterflies that were in the process of evolving to live alongside humans.

  A century earlier, cattle ranching was the prime activity in Nevada, before the state built an entertainment and tourism business out of activities that are commonly known as ‘vice’ in the rest of the country. While loggers converted the forests in the Californian sierras into new, open habitats, ranchers were transforming the meadows of Nevada. A host of European hayfield plants sprouted in the wake of the settlers. Among these was ribwort plantain (alternatively, long-leaf plantain), known as ‘white-man’s footprints’ to the indigenous human population who had previously hunted and burned the land. White-man’s footprints is an appropriate name. These plants initially followed the clearance of forest in Europe and became exceedingly common in meadows across large swathes of the continent. Centuries later, the whitish people from western Europe inadvertently took the plantains with them as they colonized the world, most likely transporting the seeds in hay that had been brought to feed their livestock. White-man’s footprints thus continued its global journey, arriving on the volcanic cones of Auckland in New Zealand as well as in the meadows of Nevada–a botanical sparrow. Plantain plants popped up all over the place, and especially where the meadows were irrigated to generate fresh, lush growth to nourish herds of cattle.

  From the perspective of the checkerspot butterfly, there was an advantage to any female that laid her eggs on this newly arrived plant. The plantain leaves remained green long enough for the caterpillars to survive during dry summers, which seemed to be getting a little drier with the first signs of climate change. In contrast, the native plants they used to eat shrivelled up and most of the caterpillars starved or desiccated. With this difference in survival, the butterflies started to evolve a liking for laying their eggs on plantains: the proportion of female butterflies content to lay their eggs on this plant rose from under a third in 1984 to three-quarters in 1987.3 A few years later, the switch was complete.4 Human-caused evolution was in full flow.

  Years later, I found myself contemplating this experience as I strolled around my thinking meadow in Yorkshire. Back in the 1980s, lying in a tent in the Sierras listening to the howl of coyotes echoing across the mountain slopes, I presumed that such rapid evolutionary events were unusual. But I was beginning to wonder if they were. If rapid evolution is rare, what are the chances that two out of a few tens of populations that Singer and his students studied would be evolving so rapidly? Was this incredible good luck? Or are bursts of rapid evolution far commoner than we realize?5

  As I pondered the fate of checkerspot butterflies, I also reflected on the voluminous updates Mike had emailed me from time to time over the ensuing years. Neither of these particular evolutionary episodes worked out. Freak spring weather and a decline in blue-eyed Marys caused the butterfly population to crash along the General’s Highway, and the remaining butterflies went back to feeding on louseworts. This was not an evolutionary failure because the population still survives, but neither can it be counted as a new success. The Carson City episode came to naught as well–the population died out when management of the meadow changed. We had witnessed rapid evolution, but neither event generated long-term change.

  Freak weather in the Sierras, in California. A younger version of the author in the middle of the checkerspot season, with adult butterflies waiting to be tested for their choice of plant. The butterflies are being kept in ‘Singer ball’ cages, which are hanging in the trees.

  But evolution never gives up. It only needs a very small proportion
of these spurts of evolutionary change to work out and become new biological success stories. When we look across the entire range of Edith’s checkerspot, we can see that it has switched its diet to feed on ribwort plantain elsewhere: in Oregon and Washington states, and in neighbouring British Columbia in Canada. The federally endangered Taylor’s checkerspot (a subspecies of Edith’s checkerspot, whose historical habitats have been lost) is so reliant on it that conservationists are actively planting plantains out into the wild.6 To provide a supply of butterflies, prisoners at the Mission Creek Corrections Center for Women in Washington state breed checkerspots in a greenhouse so that they can be released into these new habitats. Odd as it might seem, actively encouraging an alien plant (increasing gains) is helping to conserve a much-loved native insect (reducing losses).

  Meanwhile, back east, many populations of the related Baltimore checkerspot eat plantains, too.7 Across all the populations throughout the geographic ranges of all of the checkerspot species in North America, it is virtually guaranteed that some of them will eventually flourish on this introduced plant and start to spread–even if many other populations die out because they fail to adapt fast enough to changing conditions. Given that white-man’s footprints now grows right across the world, these plantain-eating checkerspots have enormous potential to prosper in the future.