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Volume 11, No. 2, Fall 01 | Return to Table of Contents
Mud Studies in the Amazon Help MBL Scientist Understand Species Diversity
Paul Colinvaux samples Brazilian waters and uncovers information to challenge a popular theory
Many pretty theories have been devised on the streets and beaches (and in the taverns) of Woods Hole. And many have met their ends here, as well. Study science for any length of time and it becomes clear that some theories die harder than others. As a young man, Paul Colinvaux took on one of the prettiest (and toughest) ecological theories around.
He didn’t mean to discredit it. He meant to confirm it.
He had completed his Ph.D. work in the arctic: reconstructing a history of the Bering Land Bridge from pollen records. Moving to warmer climes, he had worked in the Galapagos Islands and in the high Andes. He was casting about for fresh work to tackle, when a new theory caught his eye. The refugia hypothesis, posed by Jurgen Haffer, suggested an answer to the long-standing question of why are there so many species in the Amazon. It was a very attractive hypothesis, and Colinvaux saw straightforward ways to test it.
New species arise when existing groups of the same species get separated. Water will do the trick. Or mountains. Or large spans of urban development. Or a sea of grass, isolating islands of rain forest. That was Haffer’s hypothesis. During glacial periods, the otherwise wet and hot rainforest grew cooler and much drier. Grasses took over the lowlands and patches of rainforest withdrew to the highlands. The rainforest species specialized within these patches until the climate warmed again and the forest melded back together. In the intervening centuries, though, they had grown different from each other. New species were born.
If that’s the way it went, reasoned Colinvaux, he should be able to see more grass pollen in the lake cores during glacial periods. The test may be straightforward, but it’s not easy. Finding the right lakes to core was only the first difficulty. The lakes had to lie in the lowlands or he couldn’t expect to see the grass pollen signal. The sediments had to be undisturbed by river inputs or he wouldn’t get a clear chronology. In thirty years of searching, only a few lakes have met those criteria.
Then came the problem of recognizing what he was seeing. In the first pollen diagrams he published, half the taxa were listed as unknown. Colinvaux and Mark Bush from the Florida Institute of Technology, began trapping modern pollen along transects and comparing it to local species lists. In about a month, their list of unknowns went from 50 percent to five percent.
That work led to the publication of the first pollen identification manual for the tropics in 1999. It also led to Colinvaux’s conviction that Haffer’s refugia hypothesis was simply wrong. His cores showed no increase in grass pollen through the last ice age. The Amazon had almost certainly cooled (he did observe more high elevation species), but it hadn’t dried out, according to his data.
By 1996, when Colinvaux published the record of cores from Lake Pata, the theory had collected many followers. “Paul, I hear you’ve been disputing the refugia hypothesis,” Colinvaux recalls Ernst Mayr saying to him, years ago, “even if you get the cores and the data are there, no one will believe you because the hypothesis is so beautiful.” Mayr’s statement was prophetic. Adherents of the hypothesis simply decided that Colinvaux had happened on a refuge.
In the last few years, Colinvaux has collected a 30,000-year sequence from the remote lake Maicuru, 1000 miles away from Pata. Colleagues with the Deep Ocean Drilling Project have also cored the fan at the mouth of the Amazon, which should collect pollen from throughout the Basin. Any substantial shift in species would be observable there, but neither of those records gives any indication that grasslands took over major portions of the Amazon Basin.
The refugia hypothesis won’t truly die unless a better idea arises, but several theories exist. Using genomic data, in addition to traditional paleontology, researchers are testing whether shifting rivers or geologic changes might have split once-common species. Like oceanographers a century ago, evolutionary ecologists are gradually revealing the full diversity of the Amazon, and finding that the variety of species found there is supported by a complex mosaic of soils and climate.
Colinvaux has turned his attention to other projects. He is hoping to core Lake Nicaragua, the only great lake in the neotropics, which he thinks could yield a sea level record for the entire Pleistocene period. Returning to the Galapagos ranks high on his list, as does a planned excursion to Siberia.
Paul Colinvaux joined the Marine Biological Laboratory as an adjunct scientist in 1999, after seven years at the Smithsonian Tropical Research Institute. He and Llewellya Hillis have nearly completed renovations of the charming cottage at the corner of Clapp and Brooks Roads in Woods Hole, which now houses the libraries and laboratories of two long and active careers. |
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