Arlington, Va. -- By analyzing data on tree pollen extracted from ancient lake sediments, ecologists have sharpened the understanding of how forests can maintain a diversity of species. Their findings indicate that stabilizing processes have been more important than previously thought, and that the human-caused loss of species could upset that stability in ways that remain poorly understood. "Quantifying the link between stability and diversity, and identifying the factors that promote species diversity, have challenged ecologists for decades," said Saran Twombly, program director in the National Science Foundation (NSF)'s division of environmental biology, which funded the research. "The contribution of this study is unique, as the scientists used a clever blend of long-term data and statistical modeling to test the opposing hypotheses of neutrality and stability as key factors promoting community assembly and diversity."
Scientist James Clark and graduate student Jason McLachlan of Duke University, published their findings in this week's issue of the journal Nature.
According to Clark, the purpose of their study was to address a central scientific problem in explaining the diversity of tree species in a forest.
"In the mathematical models ecologists use to describe how different species compete for resources such as light, moisture and nutrients, it can be difficult to get species to coexist," he said. "In models, slight advantages allow one species to 'outcompete' the other, leading to extinction, that is, loss of biodiversity. And so, ecologists have put a lot of effort into trying to understand the differences among species that would allow one species to coexist with another species."
Explaining such coexistence is critical, if ecologists are to truly understand forest biodiversity and the forces that sustain or reduce it, said Clark. According to Clark, two basic hypotheses have arisen to explain forest biodiversity. One theory holds that stabilizing forces are required for many species to coexist.
"For example, one might imagine that, if one species is limited by light, and another by moisture, they could coexist because they're not really competing that much," said Clark.
"An alternative 'neutral model' hypothesizes that species are so similar it just takes a long time for winners and losers to be sorted out by competition," said Clark. "But eventually the better competitor would drive the other to extinction."
Direct observations to distinguish which model is correct would take centuries, said Clark, "So in this study we came up with a way to test the neutral model based on long-term changes in species abundance that are evident in the pollen record."
So, Clark and McLachlan examined existing data on pollen from red maple, birch, beech, ash, oak, hemlock and elm trees isolated from cores of lake sediments in southern Ontario.
"This record covers about 10,000 years, so if we look at the relative abundance of different species over that time--which encompasses perhaps several hundred generations of trees--we can estimate long-term growth rates," said Clark.
According to Clark, the neutral model would predict that the variation among the sites would increase over time, as random chance caused different species to go extinct in some areas but not others.
Some sites, just by chance, should come to be dominated by one species, while others would come to be dominated by another species.
However, the researchers found that variance among the sites did not increase over the millennia, leading them to conclude that stabilizing forces were maintaining forest diversity.
"Our findings indicate there are factors that regulate populations at relative abundances that are consistent from one place to another," said Clark." The variation from place to place is not 'neutral.' Ecologists have long known that, within a region, some species tend to be more abundant and some species less abundant. Our study doesn't identify what those stabilizing forces are, but it clearly shows they do not arise from neutral dynamics." Indeed, said Clark, a major challenge for ecologists is to attempt to understand what the stabilizing forces are in forest biodiversity.
"Ecologists have devoted a lot of research to that understanding, but substantial challenges remain," said Clark.
"A long-held view that there are tradeoffs among species, in which each has a unique set of competitive advantages and disadvantages that determines its abundance, is often not supported by data."
"What we are seeing is huge variability within populations," said Clark. "And this variability means they overlap in ways that determines who's going to win and who's going to lose. And that variability might itself represent a stabilizing mechanism."
Clark emphasized, however, that even though the role of stabilizing mechanisms remains unknown, the results from his and McLachlan's studies offer cautionary lessons.
"Our findings suggest that forest biodiversity has probably been stabilized in some important ways, so extinction of species should cause us greater concern than if we believed that biodiversity was maintained in the past by continual replenishment of random extinction by generation of new species," he said.
"So, there's a lot for ecologists to do to really understand the extent to which differences among species are responsible for diversity."
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