Ice Ages on the Amazon

This review of Paul Colinvaux’s Amazon Expeditions appeared in the Times Literary Supplement, 20 August 2008

In late 2005, the Amazon rainforest suffered its worst drought in a hundred years, prompting the Brazilian authorities to declare a state of national emergency. As army helicopters dropped supplies to villages left stranded by the retreating waters, forest fires spread into Amazonia’s remote south-west, consuming some 3,000 square kilometres of the most diverse habitat on the planet. Droughts and fires are, of course, natural features of any forest system, but the 2005 dry season was unusual both for its longevity and for the fact that, unlike most South American droughts, it was not caused by an El Niño event in the Eastern Pacific Ocean, but by an anomalous bout of North Atlantic warming: the same warming that generated Hurricane Katrina. For an ecosystem as vulnerable as the Amazon Basin, already under sustained attack from large-scale deforestation, global ocean warming constitutes a potentially serious threat: if the next Atlantic heatwave happened to coincide with a severe El Niño in the Eastern Pacific, the resulting double drought could prompt forest dieback on a massive scale, transforming one of the world’s principal carbon sinks into one of its principal carbon sources – a classic positive feedback scenario, leading to ever increasing heat, until the climate finally regains control by doing something drastic.

The Earth has seen this all before, from hothouse periods such as the late Permian (c250 million years ago), when atmospheric concentrations of carbon dioxide were ten times higher than they are today, to sudden climatic crashes, as at the end of the Ordovician (c450 million years ago) when global temperatures fell by more than 10°C, precipitating a mass extinction in which two-thirds of life on Earth abruptly disappeared. Glacial eras tend to be just as inimical to life as hothouse ones, due to lack of rain rather than the cold itself, so it might seem reasonable to assume that rainforests such as the Amazon would be the first to suffer during a dry ice age, withering away to savannah as the ice-bound rains continued to fail. Isolated patches of higher forest might survive by drawing moisture from mist and clouds, but the rest would die back into an arid waste. But according to the ecologist Paul Colinvaux, this assumption is entirely false, and forty years of forensic fieldwork have definitively disproved it. By analysing ancient pollens laboriously extracted from lake sediments across the equatorial Amazon, Colinvaux has concluded that the region remained deeply forested “even through the vicissitudes of an ice age. The climate changes that inevitably happen are taken, as it were, in the forest’s stride”. In other words, the Ice Age Amazon was as green as it is today. If Colinvaux is right – and it remains an “if”, in spite of his insistence that his findings “tear the guts out of the arid Amazon thesis” – the mainstream view of the Amazon forest as a climatically vulnerable ecosystem will need to be updated, to reflect instead the “stability and tolerance” that has ensured the region’s survival.

Such insights are loaded with ecological implications, since a clearer understanding of Amazonia’s past is likely to influence our understanding of its future, so it is unfortunate that Colinvaux devotes so much of Amazon Expeditions to rehearsing decades-old arguments with his academic colleagues rather than pursuing the significance of his findings. The principal targets of his long-nurtured feuds are the proponents of the so-called refuge theory, a hypothesis advanced in the late 1960s as a means of accounting for the peculiar distribution of Amazonian wildlife: many of the bird species found in the Amazon confine themselves to particular patches of forest; indeed many of these patches are populated by groups of species unique to those areas, “as if these animals had carved little nation states out of the great forest”, as Colinvaux puts it. Given that the more abundant trees and flora are present throughout the 3,000-kilometre-wide expanse of forest, the oddity of Amazon bird distribution had long invited scholarly explanation, which eventually arrived in the form of a paper published in the journal Science in 1969 by the German geologist Jürgen Haffer. Haffer argued that Amazonian endemism reflected the region’s Ice Age past, when dieback caused by glacial aridity left only a few scattered patches of forest, the inhabitants of which went on to evolve in isolation from their neighbours. When these isolated hillside patches (which Haffer called “refuges”) were reunited by regrowth during warm interglacials, the local species remained where they were, at home in their long-established niches.

Haffer’s Pleistocene-refuge hypothesis supplied an elegant solution to the endemism puzzle, and was quickly taken up by ecologists and biologists in universities all over the world. Colinvaux, however, who had begun his career shortly before the hypothesis came out, had doubts about the whole idea of Amazon aridity, and in spite of the fact that Haffer’s explanation was quickly established as the consensus view, he set off for the Amazon to prove the theory wrong. Colinvaux’s account of his equatorial adventures is by far the most rewarding part of his book, and there is something rather heroic about his single-minded endeavours; or at least there would be if he wasn’t such a crosspatch. Every argument at an academic conference, every rejection by a scholarly journal, every sharp exchange of words is brooded over and analysed, Colinvaux’s capacity to harbour a grievance extending to his still feeling “narked” about not being invited to a biodiversity conference in 1979. No doubt all this sheds fascinating light on the process of scientific canon formation, but it only serves to remind me of why I got out of academia: not so much the egotism and the endless point-scoring – you encounter those in every profession – but the loss of intellectual perspective that comes with the culture of microspecialization. Refuge theory is of central importance to Amazonian palaeoecology, but Colinvaux’s assertion that it constituted a paradigm shift on a par with the acceptance of plate tectonics is wishful thinking, though perhaps understandable on the part of someone who has devoted his career to disproving it.

So, given that he rejects the idea of Ice Age “refugia”, what is Colinvaux’s preferred explanation for Amazonian species distribution? It is a complex question, particularly as many of Haffer’s “refuges” are in fact vast tracts the size of Ireland or Idaho: can an area of land on that kind of scale really be thought of as “isolated”, even within the immensity of the Amazon forest? And is complete geographic separation necessary for speciation anyway? The vastness of the Amazon and its great antiquity – by Colinvaux’s reckoning, it has been permanently forested for something like ten 10 million years – create plenty of environmental niches, separated by climatic or geographical gradients, as well as by the sheer distances involved. “Take all this into account”, he says, “and isolates seem more likely than not.” As plausible as it sounds, however, even with Paul Colinvaux’s hard-won pollen histories submitted in evidence, the everlasting Amazon theory has not yet seen off Haffer’s refugia, the “beautiful hypothesis” that has inspired a generation of ecologists. So it may be some time before a conclusion is reached on the true nature of the rainforest’s Ice Age past – by which time we may already be dealing with the legacy of the warmer future.

Richard Hamblyn

Paul Colinvaux
AMAZON EXPEDITIONS
My quest for the Ice-Age equator
328pp. Yale UNiversity Press. £20 (US $32.50).
978 0 300 11544 4

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