zaterdag 13 december 2014
Posted: 11 Dec 2014 11:31 PM PST
The interactions between wildlife and the physical planet are more complex and fascinating than we could ever have imagined.
By George Monbiot, published on the Guardian’s website, 12 December 2014.
I can hear you muttering already: he’s completely lost it this time. He’s written a 2,000-word article on whale poo. I admit that at first it might be hard to see the relevance to your life. But I hope that by the time you have finished this article you will have become as obsessed with marine faecal plumes as I am. What greater incentive could there be to read on?
In truth it’s not just about whale poo, though that’s an important component. It’s about the remarkable connectivity, on this small and spherical planet, of living processes. Nothing human beings do, and nothing that takes place in the natural world, occurs in isolation.
When I was a student, back in the days when mammoths roamed the earth, ecologists tended to believe that the character of living systems was largely determined by abiotic factors. This means influences such as local climate, geology or the availability of nutrients. But it now seems that this belief arose from the study of depleted ecosystems. The rules they derived now appear to have described not the world in its natural state, but the world of our creation. We now know that living systems which retain their large carnivores and large herbivores often behave in radically different ways from those which have lost them.
Large carnivores can transform both the populations and the behaviour of large herbivores. In turn this can change the nature and structure of the plant community, which in turn affects processes such as soil erosion, river movements and carbon storage. The availability of nutrients, the physical geography of the land, even the composition of the atmosphere: all now turn out to be affected by animals. Living systems exert far more powerful impacts on the planet and its processes than we suspected.
I’m talking about trophic cascades: ecological processes that tumble from the top of an ecosystem to the bottom. (Trophic means relating to food and feeding.) It turns out that many living processes work from the top down, rather than the bottom up.
Trophic cascades have often been detected in places in which large carnivores still exist or have been reintroduced. But what has been discovered so far is likely to underestimate their natural prevalence. For what we now describe as “top predators” are often – from the perspective of palaeoecology – nothing of the kind.
Species such as wolves and lynx, for example, would be more accurately described as mesopredators: belonging to the second rank. They would once have had to contend with lions, hyaenas, scimitar cats, sabretooths, bear dogs and other such monsters, throughout their ranges. Even the giant lions and giant sabretooths that lived in North America until the first humans arrived could not unequivocally be considered the kings of the jungle. The short-faced bear, which stood 13 feet in its hind socks, appears to have been a specialist scavenger: specialising in driving giant lions and giant sabretooths off their prey.
One hypothesis which might help to explain the sudden disappearance from many parts of the world of the megafauna, following the first arrival of human beings, is that we triggered trophic cascades of destruction.
For example, before humans reached Australia, the continent teemed with great beasts. There was a spiny anteater the size of a pig; a giant herbivore a bit like a wombat, which weighed two tonnes; a marsupial tapir as big as a horse; a ten-foot kangaroo; a marsupial lion with opposable thumbs and a stronger bite than any other known mammal, which I believe was a specialist carnivore of giant kangaroos; a horned tortoise eight feet long; a monitor lizard bigger than the Nile crocodile. Most of them, and many other marvellous creatures, disappeared between 40,000 and 50,000 years ago. At roughly the same time, the dense rainforests which covered much of that continent began to be replaced by the grass and scrubby trees which populate much of the outback today.
One paper suggests that the first humans in Australia hunted some of the large animals to extinction, and that this caused the destruction of the rainforests, which in turn wiped out much of the remaining fauna. How? It postulates that when the giant herbivores disappeared, the leaves and twigs that would otherwise have been browsed began to build up on the forest floor, creating a fuel supply that allowed wildfires to rage unhindered through the rainforests. This catalysed the shift to grass and scrub.
In Europe, ecologists are beginning to wake up to the fact that our ecosystems were and remain shaped by elephants, rhinos, hippos and the other great beasts that lived here during the last interglacial period, when the climate was similar to today’s. You can still see evidence of co-evolution with elephants and rhinos in the way that our deciduous trees respond to attack.
In other words, the natural world is even more fascinating and complex than we had imagined. And we are only just beginning to understand just how rich and strange ecological processes might be.
I promised whale poo, and whale poo you shall have. Studies in the 1970s proposed that the great reduction in the large whales of the southern oceans would lead to an increase in the population of krill, their major prey. It never materialised. Instead there has been a long-term decline. How could that be true? It now turns out that whales maintain the populations of their prey.
They often feed at depth, but they seldom defecate there, because when they dive the stress this exerts on the body requires the shutdown of some of its functions. So they perform their ablutions when they come up to breathe. What they are doing, in other words, is transporting nutrients from the depths, including waters too dark for photosynthesis to occur, into the photic zone, where plants can live.
In the southern oceans, iron is a limiting nutrient, without which the plant plankton at the bottom of the food chain cannot reproduce and grow. By producing their poonamis – sorry, faecal plumes – in the surface waters, the whales fertilise the plant plankton on which the krill and fish depend. This effect, known as the “whale pump” has been hypothesised for several years. But now there is some experimental evidence to support it. A team of scientists at the University of Tasmania collected some pygmy blue whale poo (who knew that marine biology was so rich with possibility?) and grew plankton in water containing varying concentrations of it. They found that the richer the mix, the greater the productivity. No surprises there.
Separate research, in the Gulf of Maine, estimates that whales and seals, by defecating at the surface and recycling nutrients there, would, before their numbers were reduced by hunting, have been responsible for releasing three times as much nitrogen into those waters as the sea absorbed directly from the atmosphere. The volume of plant plankton has declined across much of the world over the past century, probably as a result of rising global temperatures. But the decline appears to have been been steepest where whales and seals have been most heavily hunted. The fishermen who have insisted that predators such as seals should be killed might have been reducing, not enhancing, their catch.
But it doesn’t end there. Plant plankton, when they die, slowly descend into the abyss, taking with them the carbon they have absorbed from the atmosphere. It is hard to quantify, but when they were at their historical populations, whales are likely to have made a small but significant contribution to the removal of carbon dioxide from the atmosphere. The recovery of the great whales, which werereduced by between two thirds and 90%, but whose numbers are slowly climbing again in some parts of the oceans, could be seen as a benign form of geo-engineering.
This should not be the only, or even the main, reason why we should wish them to return, but the way in which whales change the composition of the atmosphere provides yet another refutation of the idea that we can manipulate the living world with simple, predictable results.
With the Sustainable Human team, I’ve just produced a second trophic cascades video, about the whale pump. The first – about the unexpected impact of wolves in Yellowstone National Park – has been watched 13 million times. The belief that people cannot handle complexity is a myth. There is a tremendous public appetite to understand the world in all its fascinating detail. Which could explain why my articles have become longer and longer.
Talking of which, I haven’t finished with the whales yet. Another paper proposes that as the great whales declined, killer whales, some of which would have specialised in feeding on them, switched their diet to animals such as seals and sea lions. This is likely to have had major effects on fish populations.
And even that is not the end of the story. It now seems that whaling may have been a leading cause of the decline of the Californian condor. Condors appear to have specialised in scavenging the carcasses of stranded whales. As whales were destroyed, the condors were deprived of a major food source, and were forced to feed on dead terrestrial animals. Some of these carcasses are of animals that die after being shot and then lost by human hunters. The ingestion of lead from bullets and shot has been one of the reasons for the fragility of the condors’ grip on existence.
Who would have guessed that the impacts of whaling would cascade through so many living systems?
(Incidentally, until humans arrived in the Americas, the condor was one of the smaller scavenging birds. The North American roc (Aiolornis incredibilis), had a wingspan of sixteen feet and a hooked bill the length of a man’s foot. No skull of another predatory bird, the Argentine roc (Argentavis magnificens) has yet been found, but the available bones suggest that its wings were 26 feet across and that it weighed twelve stone.)
And it’s not just whales. When plant plankton are attacked by the small animals that eat them, some of them release a chemical called dimethyl sulphide. This compound attracts predators, that feed on the animals eating the plants. It appears that the tube-nosed birds, such as albatrosses, fulmars, shearwaters and petrels, which have a highly developed sense of smell, can detect dimethyl sulphide, and use its presence to find their prey. Not only might this help to protect the plant plankton from some of the animals grazing on them, but by defecating in the feeding zone, the birds help to fertilise the plants that brought them there.
There’s one more twist. Dimethyl sulphide seems to have a powerful role in the formation of clouds at sea. Because the sea has a dark surface, and clouds are white, the greater the cloud cover, the more sunlight is reflected back into space. So as plant plankton are attacked, they might help to cool the planet.
There are similar effects on land. Before serious conservation efforts began in the 1960s, wildebeest numbers in the Serengeti fell from about 1.2 million to 300,000. The result was similar to the hypothesised mechanism for the destruction of much of the Australian rainforest. As dry grass and other vegetation that the wildebeest would otherwise have eaten accumulated, wildfires ravaged around 80% of the Serengeti every year.
As wildebeest numbers have recovered, the frequency of fires has fallen and more dung is incorporated into the soil. The Serengeti has been transformed from a net carbon source to a net carbon sink: a shift equivalent to the entire current emissions of carbon dioxide from burning fossil fuels in East Africa.
But it’s important not to generalise from one example. In other parts of the world, grazing animals can increase the production of greenhouse gases. Domestic livestock are a major cause of global warming. So are some wild herbivores. As moose numbers in Canada have risen, partly due to the destruction of their predators by people, through a series of complicated impacts on both vegetation and soil they have sharply reduced the storage of carbon in the boreal forests. One estimate suggests that the difference in carbon storage between high and low moose numbers is the equivalent of between 42 and 95% of the carbon dioxide Canada produces through the burning of fossil fuels. Allowing wolves to return to their historical levels could make a massive difference to Canada’s greenhouse gas emissions.
Nor should we imagine that wolves and whales and wildebeest and plant plankton and sea otters alone can prevent the climate breakdown that the unchecked consumption of fossil fuels will cause. Annual plant growth cannot match the burning of fossil fuels, which mobilises the stored remains of many centuries of accumulated plant carbon every year. But these first inklings of the unexpected impacts of our destruction should provide yet another reason for treating the living planet gently. Everything is connected.
I would hate to see the protection of wildlife reduced to a calculation about greenhouse gases. For me, there are powerful intrinsic reasons for defending the natural world: because it is wonderful; because it enriches and enchants our lives; because to understand how these magnificent and complex systems work is to pass through a portal to an enchanted kingdom.
But the little we now know of trophic cascades and the unexpected complexities they reveal, which doubtless presages a much deeper and richer understanding in the years to come, enhances for me the awe with which I contemplate our world of wonders. It makes me all the more determined to protect it from destruction.
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