Furthermore, heavily pregnant flying mammals are not airworthy, so bats produce only one offspring each time they breed. For the greater short-tailed bat, this amounted to one pup a year, in contrast to the brown rat, which can become sexually mature when it is about five weeks old, has a gestation of three weeks, and litters that contain seven and sometimes even more kittens. This allows the rat population to at least triple in two months when food supplies are plentiful, whereas it would take the bats three years or more to achieve the same level of population growth. Thus rats have a much greater capacity to take advantage of temporary gluts and to bounce back from times when their numbers have been reduced by predation. Rats, stoats and cats were a fatal combination for New Zealand’s greater short-tailed bat, which was last seen in 1967. RIP. Conservation efforts are now under way to save the lesser short-tailed bat, which typically spends rather less of its time hunting on the ground and is therefore somewhat less susceptible to ground-dwelling predators. It would certainly be a shame to lose such an evolutionary oddity.
Sad as this may be, it is hardly surprising. And the future evolution of the world’s mammals is not threatened by the loss of ground-dwelling bats. The walking bat was an eccentricity that was unlikely to catch on, even if the vampire bat can do it, too, hopping around the legs of animals whose blood it wishes to drink. In contrast, bats undeniably outperform rats in the sky, and their ability to echolocate–to bounce high-pitched sounds off moths and other nocturnal insects and listen for the echo–makes them better than most birds at hunting in the dark. The global supremacy of bats as airborne night-time hunters is not under threat.
At least the bats still had some capacity to fly, whereas many of the island birds faced an even greater evolutionary challenge. The ancestors of all birds sacrificed their front legs over 70 million years ago,9 when their forelimbs evolved into wings. Wings have great advantages. They enable birds to obtain food that cannot be reached from the ground, to find new places to forage and to escape from ground-dwelling and tree-climbing predators by taking to the air. Wings make sense. However, birds use up enormous amounts of energy to grow, maintain and use their flight muscles, so flight can be a disadvantage when there are no predators to escape.
If you live on a remote island where the only predators are crabs that can be evaded by walking away, any ground-feeding bird (as opposed to those that feed in trees or catch insects on the wing) that puts its energies into flight is likely to leave fewer offspring than its waddling relatives. So, time after time, flying birds colonized isolated oceanic islands and then started to lose their power of flight in worlds of mammal-free bliss.10 They evolved into stocky beasts, with big legs for walking rather than wing muscles to take off. This is what happened to the takahe. Its ancestors were flying swamphens of the genus Porphyrio which settled in New Zealand about two and a half million years ago and proceeded to evolve into the heavy, flightless birds that we see today.11 Island birds also lost their fear–seals and sea-lions were not going to chase them. The story is similar with disease. Why expend metabolic energy preparing to fight off bird malaria and other non-existent pathogens?12 Any birds that redirected this energetic expenditure towards other bodily functions would be at an advantage. In the end, the world’s archipelagos were filled with oversized, tame, disease-prone, slow-breeding, flightless birds.
Such was the situation when humans turned up in New Zealand about seven hundred years ago, accompanied by our personal menagerie of continental carnivores. Rats and pigs would have depleted the birds’ food and plundered their nests, while humans and our canine friends could easily polish off the adults. It was far more probable that heavyweight, tame, walking birds would become extinct than that they would remaster the air. By sacrificing flight, they had ended up with only two functional limbs, and they were no match for agile four-limbed mammals. Takahes were not capable of defending themselves, even though their ancestors, other Porphyrio species that lived in Australia and elsewhere, did have this capability. Rats, dogs, pigs, stoats and humans won.
Flight-worthy island birds have also declined in more recent times, mainly after the arrival of European sailors and settlers who brought cats, mice, new species of rats and sundry other animals to torment the natives. New Zealand’s grey-and-rusty-red kaka parrot nests in tree holes that are accessible from the ground, making them vulnerable to clambering rodents, Eurasian stoats and Australian possums. Chestnut-winged New Zealand saddlebacks roost at night on branches near the trunks of trees, which was a perfectly sensible strategy until nocturnal tree-climbing carnivores arrived–why would they have sat on inconveniently small twigs if nothing was coming to get them? Then we inadvertently released a variety of pathogens and mosquitos that initiated epidemics on many islands. Introduced continental birds acted as carriers, which prevented the native birds recovering once the first outbreak had taken place–the demise of two-thirds of the flight-capable Hawaiian honeycreepers can be credited to their inability to fight off avian malaria, in addition to their susceptibility to predation by mammals. A capacity to fly was not sufficient protection. When we look across all the heavyweight, predator-naive and disease-prone birds that used to exist, there is no single cause of death. It was the combination of the characteristics of the island birds that did for them.13
The pukeko colonized New Zealand only five hundred to a thousand years ago. It can still fly, and continues to thrive, despite the presence of predatory animals.
In contrast, its relative the takahe (seen here on the island of Tiritiri Matangi) arrived about 2.5 million years ago and evolved into a heavy and flightless bird that is incapable of surviving predation by ground-living mammals.
While it is a great pity that a thousand species of island birds have disappeared, this event may not have much, if any, effect on the long-term evolutionary future of birds. Nearly all these island forms were ultimately descended from flight-capable, predator-savvy and disease-resistant birds that started life on the world’s continents.14 Once stranded, they became suited to mammal- and disease-free worlds, making them unable to survive on continents ever again, or to survive on islands once continental species arrived. Eventually, the island-adapted birds would have disappeared, either as the islands ducked back under the waves, or as fresh continental species arrived and displaced them.15 By mixing up the world’s species, humans have accelerated their demise rather than altered their eventual fate.
The flip side is the increased success of those species that originated on the world’s continents. Flitting white-eyes, for example, still peek out of shrubberies in many of the Pacific and Indian Ocean islands. Having colonized the world’s islands ‘only’ in the last million or so years, most of the island white-eyes could still outwit feline killers and deal with continental lurgies, when they arrived. The New Zealand pukeko has also survived perfectly well. Just like the original takahe, it is a Porphyrio swamphen, but it arrived from Australia only about five hundred to a thousand years ago, and so it can still deal with ‘outsiders’. Pukekos can fly, usually have four to six eggs in a clutch instead of the takahe’s two, and they continue to thrive in the presence of predatory mammals. The takahe represents New Zealand’s past, and the pukeko its future. Loquacious myna birds, red-whiskered bulbuls and a host of other birds that originated in the world’s continents now greet visitors to numerous oceanic islands. As a consequence of these introductions, the diversity of birds on most of the world’s remote islands has increased over the course of the last century or two. Birds, as a whole, are doing well, even if a substantial minority of the species that used to exist in the pre-human era have disappeared.
The consistent failure of island forms provides insight into what types of animals are most successful. It is no great scientific revelation. Successful species need to be resistant to disease and have the ability to avoid predators, and they must have the capacity to reproduce fast enough to replace individuals that die–abilities which apply to nearly all continental species. There ar
e no great surprises, either, when we contemplate the mechanical design of land-dwelling vertebrate animals: four-legged mammals are effective at living on the ground and clambering up trees, birds move through the daytime air and bats hunt in the night-time air. Of course, this is a simplification (owls hunt at night, for example, and ostriches walk), but this is a true reflection of the design features that have worked best for the last 50 million years. It is no different now. In the absence of four-legged land mammals and the insect vectors of disease, the rules used to be different on oceanic islands. Now they are not. Continental life has been spreading throughout our island realms, with the consequence that already successful animals and plants just became a little bit more so.
Now that we have investigated the state of island diversity, the next challenge is to understand what happens when species move from one continent to another. There is no better place to contemplate this than in a tropical forest, the ultimate representation of successful life on Earth. I first visited the tropics in 1979, although a day later than originally planned, having accidentally flown to Panama City in Florida en route to my intended destination of Panama City in the Republic of Panama. Not put off, I was soon sweating my way up the endless flight of steps that led away from the small dock on Barro Colorado Island. Before I knew it, a chattering white-faced capuchin monkey had appeared. Then a brown-throated three-toed sloth was spotted, hanging like a slow-moving sack in a tree. Now surrounded by water, Barro Colorado Island is a former hilltop that became marooned when Lake Gatun rose around it in 1913, the lake having been created as part of the Panama Canal that connects the Atlantic and Pacific Oceans. Barro Colorado Island sits bang in the middle of the Isthmus of Panama, which became a crossroads of biological life when the enormous, continent-sized island of South America ‘bumped into’ North America several million years ago. The connection of the Americas permitted bears, cats, gomphothere elephants, peccaries, tapirs and the ancestors of llamas to walk southwards into South America for the first time, and monkeys and sloths to clamber through the trees in the opposite direction. Armadillos, opossums and porcupines also journeyed northwards towards their Central and North American dream–as did ground sloths and armoured glyptodonts, which were later exterminated by humans. Pausing for breath on the steps of Barro Colorado Island, I stood on a piece of land that had allowed tapirs, peccaries, capuchin monkeys and arboreal sloths to move through just a few million years ago, and they are still there, living together.
The exact sequence of events remains somewhat disputed, mainly because there was no single day on which the Isthmus of Panama miraculously emerged from the waves. The gap between north and south became an archipelago of emerging islands as the Cocos geological plate in the Pacific ground its way beneath the Caribbean plate (then mainly land with decreasing gaps), until a continuous strip of solid ground was achieved perhaps 3 million years ago.16 This lack of a single contact date makes interpretation of the fossil record rather tricky. It has been further complicated by the continuing trickle of animals from Asia into North America over the last 2.6 million years, when sea levels dropped sufficiently during successive ice ages for Siberia and Alaska to be periodically joined together into the Beringian Plain. It has been a time of increasing connection between the biological worlds of South and North America, just as we are now seeing human-mediated increases in the connections between continents, and between continents and islands.
Whatever the exact timing of each crossing, far fewer of the southern mammals made it into North America than vice versa, with modern cats, including the present-day jaguar and now-extinct sabre-toothed Smilodon, replacing the marsupial cat-like animals that used to live in the south. Perhaps the self-sharpening (by moulting the old surface as a sheath), fast-action retractable claws gave cats the edge over their marsupial equivalents. Perhaps it was their ability to produce more offspring at once, or their resistance to disease. Whatever it was, the large land animals that originated in North America, some of whose own ancestors had previously arisen in the connected continents of Africa, Europe and Asia, thrived, whereas those that originated in the somewhat more island-like continent of South America fared less well once the Americas were joined together. Giant ground sloths and some of the other South Americans were highly successful in North America, but, nonetheless, the species that originated in the large northern continents were the ones that were more likely to come out on top: at the time when humans first arrived in the Americas, half of the genera of mammals in the south were of northern origin, whereas only 20 per cent of those in the north had originated in the south.17 In other words, the northern mammals were two and a half times more successful as colonists. It seems as though the animals that evolved in places where the fight for survival had previously been keenest–in the larger continents, where there were plenty of open habitats where these types of species thrived–won the evolutionary contest. South America was a bit more island-like than North America.
When humans arrived in the Americas about fifteen thousand years ago, it was again the large animals that had originally evolved in South America that were most susceptible. If we look at today’s paltry remnants of the original fauna, it is no evolutionary surprise that South America’s largest survivors–the tapir and llamas–evolved in predator-rich North America, and that the jaguar, the largest surviving mammalian predator, had ancestors that are thought to have originated in the Old World. The ‘evolutionarily savvy’ species whose distant ancestors emerged in Asia and North America were simply more effective at coping with the challenge of sophisticated predation by armed, ground-hunting humans. The best-equipped species survived.
In stark contrast, the vegetation of South America and tree-living animals were not overwhelmed by invaders from the north. The flow of rainforest species tended to be in the opposite direction. The tree sloths and capuchin monkeys that welcomed me to Barro Colorado Island evolved in the south and spread northwards towards Mexico (which is geologically, and was biologically, part of the north) when the continents collided. This is consistent with the same idea. Most of the tropical forests of the Americas were in the south, and Amazonia remains the largest area of tropical forest in the world to this day. For these species, South America could be thought of as larger than North America, and these were the species that were most successful in the warmer and wetter parts of the northern continent. The consequence was that, despite the success of ground-dwelling mammals from the north, the overall rate of migration from South to North America was approximately 30 per cent higher than the rate of movement in the opposite direction.18
This male jaguar specializes in killing caiman and capybara that haul themselves up on sandbanks along rivers in Brazil’s Pantanal. Jaguars are the largest surviving cats in South America. Marsupial lions disappeared from South America at around the time that modern cats and the now-extinct sabre-toothed cat, Smilodon, arrived.
A similar pattern applies to species that move between continents and to those that move from continents to islands. Species that originate in vast expanses of a certain type of environment seem slightly better equipped–on average–to ‘win’ the ecological and evolutionary contest when they come into contact with species from smaller areas. This makes Australia particularly interesting. The continent of Australia and New Guinea (which is one landmass during ice ages, when sea levels are lower) is not physically connected to the other continents and can be thought of as somewhere in between an island and a continent. Furthermore, Australia is biologically unique. Most of Australia’s ground-living and tree-climbing mammals were marsupials until humans arrived–not forgetting the odd egg-laying platypus and echidna. Giant grazers, top carnivores and medium-sized omnivores, right down to the smallest mammals, were marsupials. But, unlike in New Zealand, rats arrived about a million years ago, and mouse-like rodents before then; and there were bats, of course. Small placentals had already arrived.
When humans colonized Australia approximately fifty thousand years ag
o, we hunted two-and-a-half-metre-high giant kangaroos, two-tonne wombats and the rest of the mega-marsupials to extinction–just as we exterminated mega-placentals elsewhere. Marsupial lions disappeared around the same time. However, we did not bring new species of mammal with us on that occasion. Dingos were not imported until some forty thousand years later, and predatory foxes, cats and ferrets were not released until Europeans arrived, when we also brought over dromedary camels, water buffalo, horses, sheep, cows, rabbits and sundry other placental grazing and browsing mammals. The stripy, dog-like thylacine vanished during the European era, while the stocky, fierce devil is extinct from the Australian mainland but still survives in Tasmania. However, this is not necessarily because they are marsupials: ten of the sixty-four species of native Australian placental rodents became extinct, too, many Australian marsupials still survive, and opossums live in South and North America, where the mammal fauna is dominated by placentals.
Whatever the specific combination of causes, successful mammals of European and Asian origin–the largest landmass–have replaced a number of their Australian counterparts. On the other hand, Australia’s ground-living birds were able to cope quite well. One-and-a-half-metre-high blue-necked cassowaries and grimy-brown emus survived their encounter with Eurasian mammals.19 Australia has long been full of animals that are fast, venomous and dangerous, which has resulted in emus and cassowaries evolving not only the ability to sprint away from predators but also the capacity to deliver unpleasant blows and even to eviscerate unwary mammals that attack them. These defences proved sufficient for them to cope with the arrival of human hunters and the other imported carnivores, in stark contrast to New Zealand’s apparently quite similar-looking moas, which disappeared almost as soon as humans colonized the islands.20
Inheritors of the Earth Page 14
