A correspondence between related plants and animals in East Asia and southeastern North America, including everything from mosses to magnolias and from alligators to beetles, may help scientists understand the origins of biodiversity in these areas. Although the pattern has been observed for more than two centuries, and was made popular by botanist Asa Gray in the mid-1800s, renewed scrutiny contradicts longstanding theories about the biogeography of such “species disjunctions.”
Drs. James Lazell (The Conservation Agency) and Wenhua Lu (Oklahoma State University) reported on “Grayian” distributions, a term they coined, in the recent Proceedings of the XIXth International Congress of Zoology. Based on decades of work in China and the United States, the authors show how Grayian distributions, and related patterns called austro-boreal disjunctions, hold clues to the origins of the local flora and fauna.
“The big deal is that both of these disjunction patterns have been known for a long time but have escaped the notice of most modern biogeographers,” said Lazell, former principal investigator of Earthwatch’s South China Sea Islands and many other projects. “The ‘rediscovery’ of these patterns, and the wealth of newly discovered examples, provides unparalleled opportunities for international collaborations in genetics, molecular biology, paleontology, ecology, and climatology.”
Current biogeographic theory emphasizes plate tectonics and “vicariance,” or the separation of related plants and animals by moving continental plates. Grayian distributions have thus been explained by the break-up of the prehistoric supercontinent Laurasia, leaving related species in North America and Eurasia. But Lazell and Lu point out that these continents separated more than 200 million years ago, long before the evolution of taxa with Grayian distributions. They assert that several more recent dispersal periods across the Bering Land Bridge, in the north Pacific, explain the species disjunctions, but their findings apparently fall on deaf ears.
“Nothing has changed the focus of most biogeographers from vicariance—a passion for land barges, continents and terrains drifting—despite ever-decreasing evidence that many biogeographic patterns were achieved this way,” said Lazell.
Many taxa with Grayian distributions are better represented in East Asia than eastern North America, and more groups seem to have crossed from Asia to North America than the other way around. The authors provide two hypotheses to explain this bias: A diversity of island archipelagos in East Asia provides an engine for speciation not found in eastern North America; and, continental glaciers held back dispersing North American species but not their Asian counterparts.
“In conservation biology there has been great interest in these findings,” said Lazell. “Because the species involved often have small ranges and are by definition disjunct from close relatives, calling attention to them in the context of ‘little-known’ patterns has helped conserve their habitats. Examples include Florida’s Key Largo woodrat and, in China, Nan Ao Island’s short-legged toad. Of course some ‘flagship’ species like the two alligators—Chinese and American—have been important in conservation for some time.”
Earthwatch began supporting Lazell’s biogeographical research in 1974, and sent volunteer teams to help him in areas as disparate as Massachusetts, Madagascar, and Tasmania. Earthwatch teams helped Lazell sample biological specimens in both Florida and South China on several expeditions between 1975 and 1991, providing data that was integral to his findings on Grayian distributions.
Lazell and Lu’s Proceedings article builds on other recent publications, including a chapter on Grayian distributions in Studies on Biodiversity of the Guangdon Nanling National Nature Reserve, and an article on austro-boreal disjunctions in the Journal of Biogeography. Work continues apace at both ends of the Grayian distribution, and Lazell and Lu now hope to test molecular clock hypotheses and fit particular dispersal events to past geography and climate change. Their findings will help pinpoint the origins and timing of these unique species distribution patterns linking China and North America.
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For more information, see “Grayan Trans-Beringian Distributions: New Twists to the Old Tale.” James Lazell and Wenhua Lu. Proceedings of the XIXth International Congress of Zoology. 2004:16-18.
From Earthwatch Institute