Inheritors of the Earth Page 13
Some will hate this, saying that it is like a zoo, with predators controlled, foreign plants to eat and the ‘wrong species’ of takahe introduced to a new island home. But, in a world where everything is somewhat changed, it is reality. We have no option but to get used to it.
The challenge is that some of the world’s species are rising to the top, whereas others are losing out. The takahe is one of the losers, while rats, stoats and their ilk are thriving. This is a difficult problem. The history of life on Earth is one long story of successful animals and plants replacing those that proved to be less successful. Should we just let these winners replace unsuccessful species, or intervene so that the losers might live a little longer?
Nowhere is this quandary greater than in New Zealand, where the human inhabitants have decided to take action on behalf of the losers. They are aiming to slow the rate at which continental species arrive through customs, to wage war on the ‘foreign species’ that have already become established in New Zealand and to nurture every precious individual of their rarest and most endangered species. This is understandable. New Zealand is full of unusual island forms that do not live anywhere else, and many of them are a source of national and cultural pride. They are also of considerable scientific interest. Because New Zealand is a large area of land that has remained above water for a long period of time (it is a fragment of continental land), it represents the last stand of many ancient groups, including parrots (a group that contains the alpine kea, forest kaka and the nocturnal flightless kakapo) and tuatara reptiles, which used to be more widespread in the world. It is the only place where they have not yet been replaced, in the same way that lemurs survive only in Madagascar. Faced with so many endangered and unusual species, the human inhabitants of this island nation today dream of removing the mammalian predators their ancestors imported so that the ‘rightful’ inhabitants of the land can be saved. And they have started.
The idea is to eliminate rats, mice, feral dogs, possums, cats, stoats, weasels and the odd Mrs Tiggiwinkle so as to re-create a mammal-free world on large numbers of New Zealand’s offshore islands, of which Tiritiri Matangi is one. In so doing, New Zealand conservationists have become world leaders in extermination. Setting off in helicopters and planes, they have air-dropped all-purpose mammal poison across entire islands, killing off any pests that take the bait. The aerial bombardment is followed by on-the-ground maintenance of poison stations and traps to ensure that the islands become, and then stay, completely predator free.
Not content with killing mammals on offshore islands, New Zealanders have started to build predator-proof fences around some areas on the mainland, and then they kill off the rodents, mustelids (ferrets, stoats and weasels) and possums that were unfortunate enough to be living inside the cages. The mammal-killing public have even assembled themselves into volunteer groups to poison and trap uncaged areas. As these areas lack fences, this requires continuous effort–every weekend spent killing mammals in the bush–to maintain a cordon sanitaire around locations where endangered New Zealand animals and plants might again thrive. The aspiration is that New Zealand’s unreasonably large snails, giant insects, ancient tuatara reptiles and native plants will all benefit from this war on mammals, but it is the native birds that have really captured the public imagination. As soon as an island, mainland cage or uncaged death zone is declared predator free (or the number of predators sufficiently reduced), the local citizenry want to reintroduce as many of New Zealand’s endangered birds as possible.
Much as I admire the endeavour, and appreciated the opportunity to see the galumphing birds, the whole approach suffers from three major drawbacks. The first is that many of the birds that used to live in New Zealand before humans arrived are extinct already, so it will never be possible to restore the original biological communities. The second is that it will not be feasible to rid Tiritiri Matangi Island, or anywhere else in New Zealand, of every insect, plant or fungus that originated in one of the world’s other continents. In terms of the whole fauna and flora, the removal of terrestrial mammals only scratches the surface. The third, and most problematic, challenge is that all the effort merely staves off failure. A few of the world’s most successful animals can be kept at bay for a while, but not for ever. We can kill boat-hopping rats and swimming stoats the moment they dare to place their paws on predator-free islands, but if humans were ever to depart New Zealand or get fed up with trying to save takahes, those islands would be reinvaded within a few decades or centuries. They are close enough to the mainland for stoats to swim back across to them, and all the ‘protected species’ would die out again. The challenge of saving takahe, kakapo, tuatara, mouse-sized crickets and saucer-like snails has been postponed, not solved. This is the fundamental difficulty for all conservation programmes when some species (the threat) are more successful than others (the threatened). They are saved only for as long as active intervention continues.
Conservationists recognize that reinvasion by predators is a problem, and they also appreciate that each offshore island (and onshore cage) is too small to support a viable takahe population. They are committed not only to guarding against reinvasion but also to moving animals back and forth between islands for ever more, so as to turn many tiny enclaves into one reasonable-sized population. Both of these problems would go away, however, if there were no mammals on the mainland either. There would be no rats left to reinvade the offshore islands and, better still, it would be possible to release walking birds and dopey reptiles on the mainland once more, where they would be free to wander off and reclaim their native lands. And so it has come to be that the new national strategy is to make the whole of New Zealand ‘predator free’. This cunning plan has a slight flaw. No one knows how to eliminate the offending mammals from the entirety of New Zealand, or how to keep New Zealand predator free once they have.2 It might be more realistic to release takahes on more modestly sized and remote islands elsewhere in the Pacific, in areas which have lost their own flightless birds and where the risk of reinvasion by predators is lower, or on some of the colder islands of the southern oceans, where rats have already been exterminated.
The most impressive of these projects, so far, was inspired by the success of New Zealand conservationists, but it was not carried out in New Zealand. Between 2011 and 2015 Tony Martin, a rugged animal conservationist from the University of Dundee’s Centre for Remote Environments in Scotland, spearheaded a project that involved massive air-drops of poison to remove rats and mice from the 3,700-square-kilometre island of South Georgia.3 This appears to have been spectacularly successful, although follow-up surveys are still required to be certain that no pockets of population have survived. Not only is this the largest area cleared to date, but South Georgia’s minimal human population and remote position in the southern Atlantic Ocean make biological defence of the island far more feasible. Yet, even in this brief five-year period, one subsequent ship-borne rat incursion has taken place and had to be wiped out. This reminds us that we have taken on a for-ever fight. Whatever the scale of a particular project, the problem remains. Any project that re-establishes an unstable ecological system will have to be defended indefinitely against reinvasion by continental beasts.
In the unlikely event that every rat and mouse can be removed from New Zealand, the challenge of stopping rodents from ever reappearing will be humongous, unless all transport of goods and people to New Zealand is outlawed. New Zealand was first invaded by the kiore, or Pacific rat, which was whisked from its Southeast Asian homeland to islands throughout the Pacific Ocean, until the kiore was replaced by the smart black rat, which purloined food stores from our houses and barns in tropical Asia before it was transported across the planet. The black rat was in turn replaced by the scabby-tailed brown rat from northern China. Having unleashed the rodent hordes, people have subsequently done their best to stem the tide, to little avail. History tells us that there is really only one effective way to keep any particular species of rat at bay ov
er a large geographic area–and that is to wait for an even more pugnacious species of rat to arrive and wipe out the previous pest. Kiore, black rat, brown rat: which one next? There are over six hundred mouse- and rat-like rodent species in the world (as well as many more other rodents). There is a long waiting list, and we have no particular reason to suppose that the global succession of rodent life has finished just yet.
Rather than attempt to assuage our ancestral guilt and defend an unending siege, it might be better to go with the flow. If we step away from the presumption that the old species are better than the new and must be saved, it is evident that–if we add up the mammals, birds, reptiles and amphibians–there are more different kinds of vertebrates living in New Zealand today than there were before humans arrived, and there are twice as many plants as before, as well as hosts of imported insects. Most of them are not going to go away, even if a few of the introduced species can be extirpated completely and others removed from specific locations. If we want to have oceanic islands that contain flightless birds, either out of a sense of nostalgia or because we think that they will help maintain the vegetation (which evolved with flightless birds for many millions of years), then they need to be animals that can survive in the presence of rats, cats and other successful continental species. It does not make much sense to try to restore ecological communities with species that pass away at the merest whiff of a stoat.
We should look to the continents where there are plenty of walking birds that still have the wherewithal to deal with mammalian predators. We could introduce cassowaries, rheas and emus to New Zealand to compensate for the missing giant moas.4 We could also encourage the growth of populations of turkeys, chickens, quail, pheasants and other game birds, which are already living in the wild there. These are feral birds that walk while they are foraging but evolved on continents that are full of predatory mammals, so they retained the capacity to take off when attacked. Bustards, tinamous and flight-capable geese are also realistic options. Ideally, those that are released would be species that are themselves threatened, like Brazil’s endangered Alagoas curassow, protecting the introduced species at the same time as restoring walking birds to New Zealand ecosystems.
Another strategy would be to introduce diseases, predators or competitors that would reduce the densities of the introduced mammals to the point that they would no longer kill off the native species. Strictly vegetarian rodents, for example, might be able to displace the more omnivorous rats and mice from most rural and forested habitats. New Zealanders are understandably nervous about this, given that some of the predators they dislike so much were originally introduced in previous failed episodes of biological control. Otherwise, attempts could be made to modify the native species so that they will be able to survive in the presence of introduced mammals, potentially using new genetic technologies, engineering them so that they would breed more rapidly, nest in safer places, defend their nests aggressively, run faster or be more resistant to new diseases. For example, rather than continuing the existing policy of killing the naturally occurring hybrids between Australian pied stilts and New Zealand black stilts, it might be preferable to use these hybrids to breed a new race with the physical appearance of the black stilt but with the ability of their Australian cousin to avoid being killed by predatory mammals. In other words, turn losers into winners.
Irrespective of the merits of any of these particular suggestions (which should proceed only following carefully controlled trials), there is a broader logic. Rather than always try to defend the losers, we could seek to build new biological communities composed of compatible species so that future ecosystems are more robust than those that currently exist. In the end, it is ineffective to put all our efforts into schemes that work against the natural grain of the biological world. Backing losers may be honourable in intention, but backing winners will be more effective.5
The most spectacular success story of the last million years has been the evolution of a fast-running, group-hunting, intelligent, tool-wielding ape–nothing like a human had existed before and, consequently, we have changed the world. Apart from species that have been directly killed by humans or eliminated because we removed their habitats, the remaining losers mainly come from places where the biological world was in some senses ‘incomplete’, in that particular types of animals or plants which are found throughout most of the rest of the world were ‘missing’ until humans came along and introduced them, and the introduced species then filled the void. These biological exchanges reveal ‘what works best’. When it comes to vertebrates, it is clear that small and medium-sized land-dwelling mammals are particularly effective. The moment they arrive in places where they were previously absent, they initiate biological revolutions that are difficult or impossible to reverse.
Despite their poor reputation, rodents are intelligent, resourceful animals; cute even, with their large eyes and ears, twitching noses and elegant whiskers. Rodents represent evolutionary designs that have been extremely successful in the past–with 2,250 species in existence,6 they represent half of all mammal species–and their ability to survive and diversify shows no signs of waning. Humans have been able to extinguish most land mammals that weigh more than a tonne, but we have made only modest inroads into those that are lighter than a kilogram. This is good news for rodents, and good news for mammals in general: 70 per cent of all mammal species are rodents, bats and relatives of shrews, hardly any of which weigh more than a kilo. Giant hundred-kilogram flat-tailed beavers and capybaras are among the few rodents that did disappear when humans spread out across the world, but nearly all the smaller ones survived. There is no prospect of humans exterminating every species of rodent.
Equally, sparrows and their relatives are doing disproportionately well–we could never extinguish all the 5,400 species of perching birds that exist. Individual species may vary in their fortunes, but the overall design of passerine birds is not under threat. They are evolutionary success stories, with or without humans. The same is true of lizards, frogs, fish, beetles, butterflies, trees, grasses and lichens. The ordinary evolved designs of animals, plants, fungi, bacteria, archaea and viruses that have been successful for many millions of years, and that are the most usual forms of life throughout the world’s continents, have coped perfectly well with the arrival of humans, just as the survivors of previous mass extinctions were ordinary species that would have been considered completely unremarkable before those calamities. They were just minding their own business–eating, reproducing, fighting off diseases and trying to avoid being eaten–before the world was turned upside down. The heirs to the world already surround us. However, when we move these really successful types of species to locations where they did not previously exist, there are casualties.
Given that most small mammals and birds seem to be coming through the Anthropocene relatively unscathed, it is perhaps easier to identify the characteristics of the losers; the winners are the remainder, by default. Despite their overall success, some small mammals have disappeared already. The big-eared hopping mouse, which used to bounce its way across sand dunes in Western Australia, was last seen in 1842, close to Perth’s Moore River. This may well be sheer bad luck. Any species, whatever its biological characteristics, is prone to extinction if it is confined to a small part of the globe and humans transform all of that area of land in a manner they are unable to adjust to. This is why it is so important to protect examples of all the different kinds of habitats that exist in the world, especially in places where there are concentrations of species that live nowhere else.
However, it seems not to be just about bad luck. It is small mammals that live on islands, rather than in small parts of continents, which are consistently in the greatest trouble. Christmas Island’s corpulent bulldog rat, for example, shuffled off this mortal coil in the early years of the twentieth century (it was last spotted in 1903), perhaps unable to cope with the arrival of more agile ship rats or the diseases they incubated. We have to face t
he fact that some extinctions take place because the species that arrive are simply ‘better’ at carrying out particular ecological roles than the previous residents, or they are superior in their abilities to kill and avoid being killed. This is how evolutionary replacement works. By moving species from one continent to another, and from continents to islands, humans have accelerated the process by which the eventual winners come out on top.
New Zealand bats, having arrived in the land of the long, white cloud, where there were no ground-dwelling mammals to compete with or attack them, undertook a change in lifestyle. They started hunting for insects and other invertebrates on the forest floor. They evolved a shuffling-swimming motion to crawl through the leaf litter; an ingenious way of folding their wings to stop them being an impediment on the ground; extra talons for walking and catching prey on the ground; and a special arrangement of muscles and tendons to help them take off (much like the extinct flying pterodactyls).7 While these are all fascinating adaptations, the basic problem is that, in becoming evolutionary masters of the sky some 50 to 60 million years ago,8 bats long since ‘compromised’ their ability to walk, sacrificing their fingers to support expanses of skin. The result of this evolutionary digit-conversion is that New Zealand’s greater short-tailed bats walked on the wrists of their forearms, and these are just not as good as hands and feet for pedestrian activities. The rats have it.