Brooks and Balmford focused their research on the forest birds rather than the monkeys. They estimated the amount of forest that had been lost in each part of the region and then used the species–area graphs to work out how many of them might be doomed.5 Their answer was that 51 (or 41 per cent) of the original 124 forest-dependent birds that they looked at could be expected to disappear. Of these 51, some 45 have already been designated as globally threatened with extinction.6 Brooks and Balmford concluded that, without remedial conservation actions, all of the threatened species might be expected to die out over the coming decades and centuries.
One already has. The Alagoas curassow was previously found in the north-easternmost part of the Atlantic forest region. This fruit-feeding, glossy-black creature is reminiscent of a slimline turkey but with a prominent and curiously flattened red beak that gives it the appearance of having had its snout trapped in a door. Its misfortune was to have lived in Alagoas, the most densely populated area of Brazil, aside from the metropolises of Rio, São Paulo and Brasilia. Consequently, all but the very steepest 2 per cent of the former forested land in this region has been converted into sugar-cane fields, tropical farmsteads and urban developments. This misfortune was compounded by the fact that this particular bird makes a delicious meal. Luckily, around 130 individuals still survive in two small captive populations, albeit with a hint of razor-billed curassow genes in the mix–an attempt to restore genetic variability to the inbred remnant population. Nonetheless, it is extinct as a truly wild species.
The Alagoas curassow was not the only one, or so it was thought. The other candidate to have joined the dodo on its path to extinction was the small green-and-yellow kinglet calyptura. According to English ornithologist William Swainson’s charming nineteenth-century painting, this bird was characterized by a dashing topknot of rusty-red feathers. Unseen for a hundred years, it seemed reasonable to assume that this species was also extinct when Brooks and Balmford penned their article in 1996. The birds themselves were entirely oblivious to deliberations about their fate, however. As if both to annoy (by making their article immediately slightly out of date, though not in any important way) and to delight (because it was still alive) the authors, the calyptura revealed its presence once more in the very same year that Brooks and Balmford published their work. Nonetheless, the curassow and the calyptura (which has not been seen since) are under threat, as are many additional species in the region.7
The situation is equally worrisome for the other inhabitants. Surveys of forest mammals in one part of the region estimated that 78 per cent of their populations (but not whole species) have already become extinct.8 A frog seen just once, ninety-eight years ago,9 is unlikely to enjoy a calyptura-like resurrection, given that it was only ever spotted in the outskirts of urban São Paulo. And at least five plant species are presumed extinct in the region. The threat that Brooks and Balmford sought to quantify across the Atlantic region of Brazil is certainly real.
On the other hand, we need to consider both sides of the equation. When people have counted changes to the number of species of animals or plants in any given location over time, they usually find that local diversity has stayed about the same–a conclusion that is based on the analysis of large volumes of data that have been collected over many decades.10 If anything, the average number of species is increasing slightly. That must mean that new species are arriving at least as fast as any previous occupants are disappearing. And crops, pastures and urban environments still average about 60 per cent as many species as can be found in equivalent areas of the habitats that preceded them.11 The original habitat is not so much destroyed as replaced by a new environment that still contains quite a lot of species. Once one appreciates that there may be several different human-created habitats in any given region, containing somewhat different species (i.e., the species found in crops, pastures and urban areas are not all the same), then the total number of species found within a region may be just as high, or even higher, than it was before.
In the Atlantic region of Brazil, only one of the forest birds has died out and over half of the remainder are not threatened. These survivors are successful species of the Anthropocene epoch by virtue of their capacity to tolerate living in small areas of forest, or to adjust to some level of disturbance. Then there is an influx of new species, which are genuine beneficiaries of humanity. Rusty-white cattle egrets strut in great numbers, hunting for insects and frogs that are disturbed by the hooves of domestic cattle, using skills that their ancestors honed following herds of buffalo and zebra. Originating in Africa, these enterprising herons managed to fly right across the Atlantic Ocean and colonize South America in the 1870s, eventually arriving in São Paulo and Rio in the 1970s. Now, they stalk cattle in Atlantic pastures, joined by yellow-bellied cattle tyrants, ground-dwelling birds that are South American natives but which were formerly excluded by the forest. An ecological auditor might ask: how many species have been lost and how many gained? Alagoas curassow: debit. Cattle egret: credit. Cattle-tyrant: credit. One loss, but two gains–and the list of arrivals goes on. Burrowing owls have invaded farmland that was previously covered by forest, these long-legged hunters liking to chase their prey across open ground rather than to swoop through the trees that used to cover the land. There seem to be more globe-trotting inheritors of this human-transformed part of the world than there are casualties. This suggests that the present-day mixture of small areas of relatively undisturbed habitats and all sorts of human-created environments might in fact contain more species than we started off with.
The antics of natural historians give us a clue that this could be so. In the movie The Big Year, Jack Black, Owen Wilson and Steve Martin caricature a real-life story of an obsessive competition to spot more species of bird in one year in North America than anyone else ever has. We birders are a strange lot. We have life lists, country lists, year lists, garden lists and holiday lists (but ‘holiday’ sounds far too frivolous, so we call them ‘trip lists’). When a birder goes on holiday, they aim to visit as many different habitats as possible, including those that have been created by humans. Eschewing another day on the beach, where they face the horrendous prospect of getting sand in the workings of their binoculars, the dedicated birder sets off to sample the delights of the local sewage works. They are off to spot a few waders that are probing the soft ground for invertebrates, or wagtails sallying after juicy flies attracted by the human effluent. The goal is always to maximize the number of different species that can be spotted.
Similar strategies are adopted by plant and butterfly hunters, though only rarely with quite the same level of obsession, and with a welcome reduction in the emphasis on sewage. The naturalist’s desire to visit different types of habitat implies that the total number of species in a mixture of ecosystems is likely to be greater than if an entire area was covered by one type of vegetation. But are these naturalists correct? If so, the current mixtures of habitats that exist all across the world, which have been modified by humans in a variety of different ways, are likely to contain more species in total than existed in the original vegetation.
African-origin cattle egrets can be seen coming in to roost at night after they have spent the day hunting for insects on human-created pastures in the Atlantic forest region of Brazil. Despite so many species being endangered by deforestation, the total number of species of bird has increased in this area. More species have arrived to exploit human-created habitats than have been lost to deforestation.
If commandeering land for human purposes is going to reduce the diversity of species anywhere, then we should head for a tropical forest. This is where there is most biological diversity to lose. It is where the colonization of human-created habitats by additional species is least likely to compensate for the losses.
After six hours of driving through alternating clouds of red dust and tyre-sucking, muddy pools, arriving at the entrance to Korup National Park in western Cameroon is about as spectacular as you can ima
gine. A sweeping suspension bridge ushers intrepid travellers towards the forest, capably assisted by the loquacious Chief Adolf of Mundemba’s machete-wielding guides. One of the most diverse rainforests in Africa, Korup teems with life: over 600 species of trees and shrubs, 500 herbs and climbers, 1,000 kinds of butterfly, 130 or more types of fish, and some 200 reptiles, frogs and salamanders can be seen. It is home to chimpanzees and to the endangered drill, a relative of the baboon, whose black-faced males sport incongruous white pencil moustaches above their pouting pink lower lips. Both these primates are threatened by the dual pressures of habitat loss and the hunt for bushmeat. The park is also a long-standing home to the Korup people, now relocated into a forest support zone, a buffer on the fringes of the park.
The villagers of Basu, Bajo, Abat and Mgbegati in this buffer zone grow annual crops and maintain agroforests, where scattered trees shade groves of coffee, cacao and plantains. Regenerating ‘secondary’ forests grow on steep hillsides that have previously been logged, along with ‘near-primary’ forests, that is, those which are just about as undisturbed as you can get in this peopled forest. Has all this disturbance added or subtracted species? German scientists from the University of Göttingen, working with Serge Bobo of the University of Dschang in Cameroon and Moses Sainge, the park’s field manager, set about discovering the effects of these human interventions on the diversity of birds.12 Rising before dawn, they traipsed out to their field sites to count the number of species, the early rise enabling them to see the birds at their most active, when they are easiest to locate and to identify by their calls. Small parties of the rather dull-green Fraser’s sunbird were noticed moving through the branches of the near-primary forest, whereas the shimmering-green and scarlet, olive-bellied sunbird and the iridescent, blue-green superb sunbird staked out flowering trees and shrubs in the more open habitats. What might have seemed like the ornithological short straw of being assigned to count birds in patches of annual crops near to the villages turned out to be a boon. The field workers saw and heard around one and a half times as many individual birds per visit to the annual crops as they did in the forest. And, overall, the numbers of species they observed were pretty much the same in all four types of habitat. The more human-modified habitats contained lots of species–not exactly the same species as in the forest, or in the same abundances, but as many different species. Birds that feed on insects and follow the massed ranks of driver ants (hoping to catch insect prey that the ants scare away) are most abundant in the near-primary and secondary forests where the ants themselves are most commonly found, but the reverse is true for those that eat seeds and for flower-visiting sunbirds. Because some species are found only in forests and others are confined to agroforests and annual crops, the human-created mixture of forest and agricultural habitats thus supports more species than could be found in a forest-only landscape.
Meanwhile, the researchers discovered that butterfly diversity was in fact higher in the agroforests and secondary forests than in the primary vegetation,13 and more species of under-storey plants were found among the annual crops than elsewhere. Across all groups of animals and plants that they considered, the mosaic of habitats around these villages results in an astonishing diversity. This is not an argument to stop protecting near-primary forest, which contains the greatest variety of trees and many rare animals that are confined to West Africa (whereas the agroforestry and annual crops contain higher proportions of more widespread and successful species).14 Protecting the largest, least disturbed and least poached forests in this region is the only effective way to ensure that all the species are safe. But if we try to answer the scientific question ‘What happens to the total number of species present when uninterrupted forest is converted into a mixed landscape that contains a patchwork of forest remnants and various human-created habitats?’, the answer is that the number of species increases. This is because new species move into human-created habitats faster than the previous residents of the region die out.
Does this conclusion hold elsewhere, in other kinds of habitats? At the opposite end of the ecological spectrum from the rainforests of Cameroon is Flanders, the flat, Dutch-speaking part of Belgium, which was once described by butterfly ecologists Dirk Maes and Hans Van Dyck as Europe’s biological ‘worst-case scenario’.15 It is a region of intensive agriculture and densely populated towns and suburbs. It is certainly a good candidate for a reduction in habitat diversity and for more extinctions than gains. And, truth be told, Flanders is not top of my list for a wildlife tour of the world. But if we consider the whole of Belgium, the animal and plant lists for the country include species found in remnants of the original woodlands, bogs, rivers and sand dunes, plus those associated with the human-created rough grazing land, heathland, hay and silage fields, managed woodlands, plantations, horticulture, different types of cereal and root crops, orchards, ditches, road verges, suburban gardens and parks, farm ponds, canals, reservoirs, barns, glasshouses, the outsides and insides of a variety of buildings, brownfield sites and even parking lots. Just as in England, house sparrows, yellowhammers and meadow butterflies can be found in Belgium because rather than in spite of humanity. Add up the species in all these habitats, and the region has a greater diversity than we started with, despite the fact that many of the original species have been lost.16
Losses of the original species can be minimized if 10 per cent or more of the land surface remains in a relatively undisturbed condition–ideally, 30 per cent should be protected in some landscapes within each biologically distinct region so as to maintain animals and plants that are particularly sensitive to habitat change.17 This level of protection is not achieved everywhere, but, even so, most landscapes that today contain a mixture of human-created habitats and at least some remnants of undisturbed vegetation hold more different species than they did before humans arrived on the scene. Perhaps biodiversity has never had it so good. But are we sure about this? A provisional answer was provided back in 1995, when Michael Rosenzweig, a cheery scientist from the University of Arizona, deduced that places with a greater diversity of habitats do indeed contain more species.18 When investigating the species–area relationship of the type that Brooks and Balmford used to estimate the risk of extinction among Brazilian Atlantic forest birds, Rosenzweig discovered that habitat heterogeneity (the mixture of habitats) was an even better predictor of how many species would be found in a given location. Other researchers have come to similar conclusions, including Jonathan Hiley, who leads a quadruple life as a school teacher, Mexican cricket international, ornithologist and PhD student based at the University of York. He discovered that a combination of human-modified and relatively undisturbed habitats in the Sierra Gorda Biosphere Reserve in central Mexico has increased the number of species in the region.19 Brown-headed cowbirds and white-winged doves, for example, have moved into the farmland and villages, adding to the area’s diversity, while other species continue to survive within protected habitats. Comparable conclusions have been reached for butterflies in Borneo, insects in Hungary and plants in Germany. All these individual studies have culminated in an overarching analysis by Anke Stein and Holger Kreft from the University of Göttingen, together with Katharina Gerstner from the Helmholtz Centre for Environmental Research in Germany. They rifled through the scientific literature from across the world and unearthed over a thousand relationships between the number of species seen and various measures of habitat diversity. The answer was conclusive: the number of species almost always increases with the diversity of habitats.20
This is all very well, provided that the diversity of habitats is in fact increasing. There is a common perception that humanity is ‘simplifying’ nature and reducing ‘habitat heterogeneity’. Researchers and conservationists often suggest that we are reducing the variety of habitats in a particular region. But this perception is rarely valid unless we omit human-generated habitats from the equation–which these studies usually do. Of course, there are places where ferrous herds of p
loughs, motorized seed drills, fertilizer and herbicide booms and combine harvesters sweep the landscape on such a massive scale that it is hard to find any trace of the former vegetation, as in parts of the Midwest of the USA, and possibly even in bits of Flanders. We are definitely capable of reducing the variety of habitats that can be found, and wildlife diversity will have declined in some of these regions. But we have in fact increased the number of habitat types in most Belgium-, Maine-, Panama- or Lesotho-sized regions of the world. Stein and her colleagues’ research shows that this will normally enhance the biological diversity of those regions, not erode it. Now we know this, we can actively increase the variety of habitats and thereby the numbers of species in any given region–if we wish to.
This all means that there are myriad successful species growing, crawling and flying across human-modified landscapes. Agriculture and other human-made changes to the world’s land surface are not inexorably reducing all forms of diversity. Most of the species that I have at home in England are there only because they live in habitats created by generations of Britons; large parts of Belgium are just the same; numerous species in the Korup region of Cameroon are found only in places where humans have replaced or disturbed the forest; species are thriving in former farmland at Sirena in Corcovado National Park in Costa Rica; and newcomers to the Atlantic forest region of Brazil have exceeded the number of species that have been lost. It is probably the same where you live too. The number of species in most regions is increasing, not decreasing.
Inheritors of the Earth Page 7