February 18, 2010

Big Plant Seeds Don’t Always Beat Out Small Seeds

College of Biological Sciences researcher Helene Muller-Landau has developed a new theory explaining why some plant species produce a small number of large seeds while others produce a large number of small seeds.

Using mathematical modeling, Muller-Landau demonstrated that plants having different size seeds can coexist when regeneration sites vary in stressfulness.  Species that produce large seeds (e.g., coconuts) have the advantage under stressful conditions -- such as drought or shade -- while plants that produce large numbers of small seeds (e.g., fig species) have the advantage in areas with adequate water and light.

The research was published in the Early Online edition of Proceedings of the National Academy of Sciences during this week of Feb. 15.

"The standard explanation has been that big seeds beat out small seeds under all conditions, but that's not necessarily true," Muller-Landau says.  "Big seeds have the advantage in stressful conditions and small seeds have the advantage when sun and water are abundant. It's a trade-off between tolerance and fecundity."

Muller-Landau's "tolerance-fecundity model" explains why different plant species have different size seeds and may also provide insight into the variation of the number and size of offspring among animal species, she says. It also helps to explain why there's so much diversity among species, a key finding that advances understanding of evolutionary biology.

As a staff scientist at the Smithsonian Tropical Research Institute and head of an international effort to quantify carbon in forests worldwide, Muller-Landau has visited forests in China, Malaysia, Ecuador and Panama, among other exotic destinations. Her experience has enabled her to observe a broad spectrum of plant species and the conditions under which they grow. This led her to question the prevailing theory about seed size.

Research in tropical biology has long focused on natural history and basic biology, as the bewildering diversity and complexity of these ecosystems has made them seem beyond the reach of quantitative ecological theory. In recent years, however, as larger datasets have accumulated, and some general patterns have begun to emerge, mathematical models have been increasingly been applied -- and have provided important insights.

"This simple, elegant theory, so well grounded in sound natural history, is a considerable advance in our understanding of plant species and how they coexist," said Egbert Leigh, of the Smithsonian Tropical Research Institute.

Financial support was provided by the HSBC Climate Partnership, a Packard Fellowship in Science and Engineering, the University of Minnesota and the U.S. National Science Foundation.


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