A gripping tale of exploration, the pursuit of ice-age records, scientific invention and controversy, and revelations about the great Amazon forest In this vivid memoir of a life in science, ecologist Paul Colinvaux takes his readers from the Alaskan tundra to steamy Amazon jungles, from the Galapagos Islands (before tourists had arrived) to the high Andes and the Darien Gap in Panama. He recounts an adventurous tale of exploration in the days before GPS and satellite mapping, and a tale no less exhilarating of his battle to disprove a hypothesis endorsed by most of the scientific community.
Colinvaux’s grand endeavor, begun in the 1960s, was to find fossil evidence of the ice-age climate and vegetation of the entire American equator, from Pacific to Atlantic. The accomplishment of the task by the author and his colleagues involved finding unknown ancient lakes, lugging drilling equipment through uncharted Amazon jungle, operating hand drills from rubber boats in water 40 meters deep, and inventing a pollen analysis for a land with 80,000 species of plants. Colinvaux’s years of arduous travel and research ultimately disproved a hotly defended hypothesis explaining bird distribution peculiarities in the Amazon forest. The story of how he arrived at a new understanding of the Amazon is at once an adventurous saga, an account of science as it is conducted in the field, and a cautionary tale about the temptation to treat a favored hypothesis with a reverence that subverts unbiased research.
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|Publisher:||Yale University Press|
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About the Author
Paul Colinvaux is senior research scientist, Marine Biological Laboratory, Woods Hole, and professor emeritus, The Ohio State University. He has published extensively on ecological topics and hosted the twenty-part PBS series, What Ecology Really Says. He lives in Woods Hole, MA.
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My Quest for the Ice-Age Equator
By Paul Colinvaux
Yale University Press
Copyright © 2007 Paul Colinvaux
All right reserved.
The Reason Why
The reason why was diversity. Why are there so many species in the tropics? This has long been one of the knottiest problems of ecological theory. Life gets richer as you go from temperate regions toward the equator. In warmer climes there are more kinds of living things than in the colder north; many, many more kinds. But why should this be?
The temptation is to say, "Obvious! It is nicer in the tropics; more productive; wet and warm; no winter; living is good and lots of species take advantage of it. Next question please." But that answer is no answer. Lots of living things in Europe and North America, thousands of kinds of animals and plants. The problem is that the wet tropics have more kinds still, many more.
In the tropics, the Amazon being the most spectacular example, there are perhaps eighty times as many species of plant as in a northern region. I get this absurd statistic by comparing the Amazon with Alaska, big places both. Accepting the commonly touted (especially by me) figure of 80,000 plant species known from the Amazon basin and comparing it with the known species list of Alaskan plants which is something over 1,000, I get that ratio of 80 to 1. As a statistical exercise this calculation is absurd, but like the generality of lying statistics, it adds drama to a mundane observation. The plant species list of the Amazon basin is hugely diverse.
More honest statistics can make the same point. The island of Britain is credited with about a dozen kinds of native tree (taking "tree" to mean anything that grows at least 30 feet tall): oak, beech, ash, elm, willow, alder, birch, sycamore, pine, larch, and yew; allow two species each of oak and birch and that is a generous dozen. A good splitter, perhaps armed with DNA technology, can break out a few more species but not many. The island of Britain, for all that it is rich in agriculture and well worth fighting for, is a poor place for botany, though good for gardening.
A portion of the great Amazon forest the same size as the British island would have possibly a thousand distinct kinds of tree. This seems grossly unfair. Why should a British naturalist get away with learning just a dozen or so tree names while to be truly competent a Brazilian should learn a thousand?
Even this statistical comparison, however, has its components of falsehood. The most immediate untruth is hidden in comparing an island with a continental expanse because it is as hard for trees to invade islands as it is for armies. Islands always have fewer species than continental bits of the same size. But however dud the statistics, the wet and warm places are always more richly diverse than the lovely temperate lands of the "West." Why should an eternity of natural selection have left more species in wet, tropical places like the Amazon than anywhere else?
The sublime explorer, Alfred Russel Wallace, found this problem awaiting him when he thrust himself into the Amazon jungle in 1848. Wallace was to find fame as the co-discoverer with Darwin of evolution by natural selection. But he started out as a restless young man in England, with little formal education, in straightened circumstances but with a thirst for reading, and, by his own admission, a passion for collecting beetles. He worked, saved, and went with his friend Henry Bates to the Amazon by the cheapest route.
Wallace existed on practically nothing at all for four years along the rivers and in the forest; collecting, always collecting. His younger brother came out to help him but died of yellow fever within a year. Wallace collected with gun, net, or forceps, whatever it took; every living thing was his target. In four years he had amassed thousands of specimens. He lost the lot when the old tub of a ship taking him home to England burned and sank off Bermuda. But the memory of what he had seen and learned did not burn. He had lived four years in a world so rich in life and species that England was a barren rock by comparison.
Two years in England were enough for young explorer Wallace before he was off again to another spell of penury and illness in the jungle, this time to the Malay Archipelago. This second self-imposed sentence was for eight years. All the way down the archipelago, through the islands that are now Indonesia, and on to New Guinea was that same incredible richness of life he had seen in the Amazon, so different from England. Again he collected with dedicated vigor. But the catch was not the catch made familiar in the Amazon. Animals might look the same, come in as many kinds, and do the same sorts of things, but the species were different. Wallace had shot and trapped them on two continents, and he knew the differences when he saw them. From this observation came the two great contributions to learning for which Wallace is known: biogeography and the discovery of natural selection. But he also wondered why England was different.
Wallace's self-education by reading had included Charles Lyell's Principles of Geology, the stem treatise of modern geology. Wallace knew about the ice age. He knew the havoc the ice must have wrought in Europe. No sign of ice works in the Amazon or the Malay Archipelago, of course. Here was a prime hypothesis to explain the relatively barren life of England: the glaciers had killed off so much of what ought to have been there. In the tropics, there were no glaciers, thus no extinction.
A reasonable idea, this; and one that has partly survived the test of time, though only partly. As explained in the preface it accounts for the poverty of trees in Britain compared with continental Europe, and the poverty of both compared with the American Midwest.
On continental Europe the southern reach of the ice sheets was made more effective as tree destroyers when they stretched southwards to meet other glaciers coming north or west from the Alps or Carpathians. Thus the vegetation of central Europe was caught in an icy pincer movement: ice sheets to the north, glaciers to the south, bitter cold between. In the western bits of Europe retreat southwards from glacial cold was impeded by the Mediterranean Sea. The forest diversity, not just of Britain but of the whole of Europe, was savagely reduced by extinction as the forests were caught in this glacial trap. These events were very recent, ending just 12,000 to 10,000 years ago. People saw them happen. Pity they had not yet invented writing, their memoirs would make good reading.
In European strata older than the ice ages we find fossils of trees that still grow elsewhere in the world, China and North America mostly. That these were killed off in Europe by the ice and have been unable to get back since seems certain. These trees, not seen in Europe since the Miocene, include the long list of trees familiar to American naturalists but known only as ancient fossils to European botany; trees like sweet gum, black gum, tulip poplar, and sassafras which so delight European naturalists when first they see them. Russian visitors too. Back in the days when the USSR exerted its power to restrict travel, a distinguished Russian paleobotanist, expert in the fossil history of Russian plants, walked through a forest in the American Midwest for the first time during an international congress. In emotional awe he turned to his hosts to say, "I am walking through the Miocene of the Russian plains."
The extinction of this Miocene flora in Europe derives from the peculiar properties of European geography that set up the land for catastrophe by ice. The principal mountains run west to east, making possible that deadly pincer movement of mountain glaciers running northwards to meet the ice sheets heading south.
But in North America the mountains run north and south. When the ice sheets reached down from the Arctic, snuffing out forests in Canada and the northern United States, escape was always possible to the south. Forests might not escape intact; we are now sure that they didn't. But most of the species did escape, to become southern colonists living in forests quite different from modern forests.
Pollen analysis was a European invention, devised essentially to plot the return of forests after the glacial retreat. Those forests were simple, with few species, from which two great ecological fallacies followed: that plants lived in obligate associations (after all, you always found the same ones together) and that you could use the most prominent or persistent species as "indicators" for the whole group. The inventors of pollen analysis in Europe and their well-trained successors convinced themselves that they could identify whole associations by finding "indicator" pollen. Thus were pollen analytical interpretations made simple.
This belief in obligate associations of plants took root in North America too, particularly in the first half of the 20th century. An eloquent teacher, Frederic Clements, persuaded three generations of plant ecologists that not only was the taxonomic unit of vegetation the association but also that this "super organism," as some called it, could grow and colonize fresh ground by plant succession. In Clements' own words, "As an organism the formation arises, grows, matures, and dies.... The life-history of a formation is a complex but definite process, comparable in its chief features with the life-history of an individual plant."
This was the European idea of plants living in obligate associations pushed to the limit of credulity. For a botanist steeped in this viewpoint, the glacial history of eastern North America was both obvious and simple. The associations migrated south as neighborly groups, some jostling, perhaps, but the whole thing rather orderly until all the old associations were hunkered down in warmer latitudes. When the glaciers melted, they all moved north again, in sequence. Simple.
Too simple, America was soon well supplied with skeptics of the whole permanent association idea that made this glacial history seem highly improbable. Trees were individuals of individual species, provided with independent traits; they did not, and could not, belong to organizations of different species. Careful plotting of long transects in mature America forests, or up mountainsides, demonstrated continual change and overlapping populations of species. These careful examinations were enough in themselves to show that real associations did not exist except as abstractions, they were merely collections of individuals with common interests in a local habitat.
The definitive demolition of this idea of forests or associations migrating across landscapes was done by two pollen analysts, Margaret Davis and Tom Webb working in the eastern United States, in what I think of as the finest achievement of pollen analysis in my professional lifetime. They severally collected together all the pollen diagrams from lake sediments known from eastern North America, filling in gaps with new lake cores of their own, then mapped the relative importance of tree pollen types individually each millennium for ten thousand years. At the end of glacial time none of the associations familiar to modern botanists could be found, although all the species could, scattered well to the south of the ice front, even down to the Mexican border, but in eclectic array. Then the successive maps showed the northward migrations, species by species, at different rates and by different routes, until that final fateful meeting with the New Englanders and their axes. A by-product extremely useful for squashing error in Amazon interpretations was that since associations had no fixed existence, there could not be indicator species for those associations. Ignore claims for indicator species of Amazon forests or, more likely, Amazon savannas!
The Davis and Webb studies were completed only in the 1980s, when they gave immense confidence to the attitudes to Amazon pollen analysis we had already developed. This immense forest (and all other vegetation there) should not be regarded as static, but in a constant state of reshuffle. There should be no "indicator species" of anything at all beyond simple physical needs like "needs a swamp" or "needs frost to break seed dormancy" (not likely in the Amazon, but we did encounter one such species in southeastern Brazil).
This elegant demonstration of the independence of plant species was to strengthen our hand in the 1980s and 1990s when old-fashioned beliefs in indicator species and the integrity of associations incredibly were still guiding pollen analysts trained in the old European traditions. They tended still to look for indicator species. They, and their Brazilian colleagues, were also avid supporters of the dry Amazon hypothesis so that finds of pollen types dubbed as indicators of open space, or genera known to be found in savannas, were taken to allow reconstructions of Amazon vegetation that conformed to the expectations of the refuge hypothesis. The hypothesis, in its more powerful guise as a paradigm, was to come at the last to call on outmoded and falsified science to its defense.
The Davis and Webb work in America gave final understanding of why Wallace's hypothesis of extinction by northern glaciers could not account for the ubiquitous phenomenon of richer diversity as you move from a pole towards the equator. It works fairly well for his native England, for Europe, and for the Russian plains, but perhaps only for trees, the least mobile of things. That seems to be the limit. The trees of the Miocene, more than five million years ago, survived the coming of the glaciers in America and China. I wish I could claim that hearing that Russian paleobotanist talking in the 1960s of "walking through the Miocene of the Russian plains" had set me to thinking that the Amazon forests might be Miocene also, but I cannot. Forty years later, however, that is where we were to end up.
Back in 1876, when Wallace published his great treatise on zoogeography there seemed to be no alternative to the hypothesis of glacial extinction to explain low diversity in the north. The corollary was high diversity in the Amazon because no glaciers and thus no extinction. In the absence of an alternative hypothesis, this was to become the textbook explanation for the best part of a century.
Excerpted from Amazon Expeditions by Paul Colinvaux Copyright © 2007 by Paul Colinvaux. Excerpted by permission.
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