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Amazon Expeditions: My Quest for the Ice-Age Equator Christopher D.Muir Paul Colinvaux
New Haven, Yale University Press, 2008
$32.50/Hardcover

What makes a scientific hypothesis “beautiful”? In my experience, there are three crucial qualities. Beautiful hypotheses are simple; they explain a lot; and they are sufficiently counter-intuitive to buck the tide of conventional wisdom. Most accepted scientific theories are imbued with these qualities (think of Darwin’s proposal that all life descended from a single common ancestor). The graveyard of disproven scientific hypotheses is also littered countless beautiful ideas that turned out not to fit the data. But beautifully wrong ideas, like a beautifully wrong lover, take much time and effort to depose. Data are fit to the hypothesis rather than the other way around. Worse yet, the most elegant hypotheses never truly die, but rather persist like Zombies, always returning to life after every empirical refutation. Paleoecologist and palynologist (one who studies pollen) Paul Colinvaux has worked in opposition to one such beautifully wrong hypothesis, which he elevates to the level of Kuhnian paradigm, for most of his career in an effort to better understand the climate and vegetation of the ice-age Amazon.

The Paradigm

The Amazon is an embarrassment of richness for biologists, mostly trained in universities situated in relatively depauperate North America and Europe. It is estimated, roughly, that nearly half of all species are packed into a belt around the equator that covers less than 5% of the land on Earth. Known in the field as the Latitudinal Diversity Gradient, the pattern has defied convincing explanation, though over 100 hypotheses have been proffered. One early idea came in 1969 from German ornithologist Jürgen Haffer. He proposed that ice-age climate change, which extended the reach of glaciers in temperate areas, had fragmented that giant Amazonian forest through increased aridity. In these fragmented forest refugia, tropical flora and fauna persisted and diverged, unhampered by gene flow, into new species. Haffer’s hypothesis—that the Amazon was a climatically static museum of ancient diversity—flew in the face of the prevailing wisdom. The refuge hypothesis, as it has come to be known, is simple, explanatory, and certainly counter-intuitive.

Paradigm Lost

Paul Colinvaux, an Englishman who spent his academic life in the US, is very direct in both prose and research. His scientific hammer is the sediment core, which is quite literally hammered into lake bottoms and taken up. Pollen in the lake mud are identified, counted, and matched to time through radiocarbon dating of organic material. Colinvaux’s very specialized hammer is matched by a single nail – reconstructing an ice-age climate for the Amazon. He arrived at this goal well before Haffer’s paper through similar work first in the Arctic, then the Galapagos. While these episodes are given a brief treatment in Expeditions, the stories, especially those in the Galapagos with their inextricable historical connection to Darwin, are compelling. Especially remarkable is Colinvaux’s discovery and treacherous descent into an unknown crater lake for coring. The difficulty of finding and arriving at a lake suitable for pollen coring, even in an area as small as the Galapagos Islands, is a common theme in Expeditions. Despite its penchant for rain, the Amazon and nearby areas contain surprisingly few lakes that date back to the ice-age, most of them having formed recently due to shifts in rivers’ courses. To find ancient lakes, one must search “above the rivers” on mountainsides and geological oddities called Inselbergs. To make many long stories short, when Colinvaux and colleagues analyzed pollen from these lakes, they found forest all the way down. The only difference they found during ice-ages was the presence of pollen from present-day high-altitude species, indicating that the climate in the Amazon, like that away from the equator, cooled dramatically during the ice-ages.1 Refugialists strike back at every turn with increasingly contrived reinterpretations of the data. Each new pollen core failing to yield an arid flora is simply another forest refuge! Meanwhile, no incontrovertible positive evidence has come forth in favor ice-age aridity.

The Paradigsmal Science

Colinvaux’s case against the refuge hypothesis is convincing, if at times ridiculing. The problem is that he never lays out what is really at stake in the debate. Presumably, biologists are not especially interested (or maybe some are?) in whether the Amazon was arid during the Ice-Age, but rather in the ecological and evolutionary processes involved in creating and maintaining the Latitudinal Diversity Gradient. It follows from the refuge model that high biodiversity in the tropics was generated through rapid speciation over the past two million years, resulting from pronounced periods of geographic isolation. Besides the absence of evidence for Ice-Age aridity, it is also now clear that the refuge hypothesis, even if true, has lost much of its explanatory power. The Latitudinal Diversity Gradient extends to all taxa, even marine ones that could not possibly have been subject to forest fragmentation. DNA evidence also indicates that speciation in the tropics has not been rapid or recent across the board (though there are exceptions). In fact, a recent study indicates that speciation rates may actually be higher in temperate areas, but that this is countervailed by increased extinction. The dismal truth is that there is no broad consensus explanation for even the coarsest grained patterns of biodiversity. More dismal yet is the realization that, at its current pace, anthropogenic extinction may soon obscure the causes of biodiversity before we truly understand them. It is little wonder that a beautiful hypothesis, even a wrong one, could capture the imagination of many biologists by instilling a (false) sense of epistemological security.

Expeditions is not intended as a book of science, but rather a memoir of a scientist’s career. Its weakness is that there is too much jargon and assumed knowledge for the casual reader, but not enough data to be useful in the field. Historians and sociologists of science may find it a useful resource, as it records the myriad false starts, dead ends, and unintended successes that rarely make the pages of polished scientific papers. That said, the book’s many anecdotes are fascinating, even to the non-specialist, and Colinvaux’s impressive breadth of ecological, geological, anthropological, and historical knowledge shines through. Ultimately, Expeditions is much more than a memoir; it is one scientist’s personal account of a Popperian falsification. Falsification is a good thing, but what the science of biodiversity needs next is a better, positive, and correct theory of how diverse ecological communities are formed and maintained. If Colinvaux’s career, having dispelled one popular hypothesis, is any indication, then we have a long time to go.