Currents, geologic upheaval, global warming affect local diversity
October 22, 2004
Jon Witman, associate professor of biology at Brown, said this finding was a surprise. At the start of the project, Witman expected to find that forces specific to a small area of ocean -- predation, species competition and disturbances such as hurricanes or landslides -- would play a central role in limiting the number of species found there. But Witman and his team found that species diversity in local areas, no bigger than a half-mile square, was directly proportional to species diversity in that region, which can span thousands of square miles. Researchers came to this conclusion after examining 1,500 photographic samples taken of invertebrates clinging to rock walls in every corner of the world.
For example, Witman and his team sampled five sites from Maine to Massachusetts and found anywhere from 26 to 51 species. This reflects the comparatively low marine diversity in the Gulf of Maine. In contrast, in the warm, teeming waters off of the Palau Islands near the Philippines, divers counted as many as 300 species living in an area smaller than a basketball court. This reflects the high level of diversity found in Micronesia. With a few exceptions, these patterns held true around the globe.
Witman believes that local interactions, such as storms and predators, still exert a strong influence on biodiversity, but the associate professor in the Department of Ecology and Evolutionary Biology now sees that regional forces are critical to maintaining species variation. These large-scale influences include currents that disperse larvae across hundreds of miles or the creation of new species caused by geological upheaval and biotic isolation millions of years ago. Global warming and pollution are other regional forces that can impact local diversity. "The work is a wake-up call," Witman said. "We need to think about regional processes if we want to preserve biodiversity."
Witman said results from the project, published in the current early online edition of the Proceedings of the National Academy of Sciences, have implications for conservation efforts.
Governments or non-profits interested in maintaining biodiversity in the ocean -- or on land -- shouldn't simply create single preserves or parks. Instead, Witman said, they should create as many as possible across a broad area. Of particular importance, he said, is safeguarding "source areas" for high biodiversity that act as wellsprings of eggs, seeds or vital nutrients or that provide important habitat for critical species. While scientists know that tropical coral reefs and the Amazon rainforest act as source areas, Witman said more areas must be identified.
"This is particularly true in the marine environment," he said. "We don't know much about source pools. We need a lot more research in this area."
The project focused on invertebrate species found in shallow water, such as sponges, corals, mollusks, worms, barnacles, anemones, urchins and sea fans. These animals were studied in one habitat: flat, vertical rock walls, such as ones found along reefs, in fjords, or in other parts of coastline. Witman said the choice was practical: These invertebrates can't move, so they could be counted. And rock walls can be found from the poles to coral reefs and leave few places for creatures to hide, so estimates would be comparable and highly accurate.
To get a true snapshot of the diversity of these species around the world, Witman and his team chose 12 distinct biogeographic regions and randomly sampled at a total of 49 sites within these regions, which included the Gulf of Maine, Iceland, the Northeast Pacific, the Galapagos Islands, Chilean Patagonia, the Antarctic Peninsula, the Eastern Caribbean, Southwest Africa, Southwest New Zealand, the Seychelles Islands, the Norfolk Islands and the Palau Islands.
At each site, scientists dove down 30 to 50 feet below the surface. Then they took standard-sized (18 by 24 inches) photographs of a rock wall area. They took anywhere from 18 to 200 of these photo samples at each site. Back in the lab, they examined a total of 1,500 slides and counted the species found in each frame.
The project took more than 14 years to complete. It led to another key finding: Latitude also plays a big role in local species richness. At the poles, partly due to the harsh environment and glacial scouring, there are fewer species. But moving closer to the equator, the number of species increases. While this is a long-held and widely accepted phenomenon on land, it has been brought into question in the past decade as scientists have found a surprisingly varied array of ocean animals in Antarctica. This is the first global study to show that latitude affects species richness in shallow-water ocean animals.
The research team also included
Ron Etter, a professor of biology at the University of Massachusetts--Boston
and Franz Smith, a former research associate of Witman's who
is currently a marine scientist in New Zealand The National Science
Foundation primarily funded the work. The National Undersea Research
Program, the Andrew Mellon Foundation, and the Helen and Merrill
Bank Foundation also provided support.
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