University of Minnesota
March 21, 2010
U researcher Helene Muller-Landau has found a new explanation for why seeds vary so much in size.
A new explanation for size variation in plant seeds has wide implications
By Deane Morrison
It seems a simple question: Why do some plants make a few big seeds while others make lots of small ones?
The explanation has been a stumbling block for ecologists, but a University of Minnesota researcher may just have found the answer.
In a paper published in Proceedings of the National Academy of Sciences, Helene Muller-Landau makes the case for a variable environment as the key factor. Her work not only addresses a major ecological problem but holds potential for predicting which species will run the highest risk of extinction from future habitat loss or environmental change.
An improbable balance
A current theory holds that big seeds are more likely to successfully establish seedlings and to survive as seedlings, while small seeds counter this advantage by their greater numbers. This leads to a balance that allows many plants to share a habitat. But the theory requires the balance to be perfect—an infinitely improbable situation—or else some species would go extinct. The theory also takes no account of variability in environmental conditions.
In her theory, Muller-Landau posits that big seeds don't always have an advantage over small seeds. Instead, big seeds specifically have the advantage in tolerating a habitat that is too wet, dry, nutrient-poor, or otherwise stressful, as their larger size allows them to persevere despite these hazards. Small seeds, on the other hand, often can't make it in these environments, but tend to win where the environment is not stressful simply by dint of their larger numbers. Thus, variability in the environment gives the advantage first to one seed size then the other, minimizing the head-to-head competition that can drive some species to extinction.
How it works
Consider the coconut, a very large seed. Its tough coat allows it to float hundreds of miles in salt water before being thrown up on a dry sandy beach. Its ample supply of stored food allows its roots to grow deep enough to tap fresh water and allow the seedling to thrive.
If the coconut landed in a cushy habitat with rich soil and surface water, however, it may well find itself losing out to numerous seedlings of small-seeded plants like figs, which need benign conditions to thrive.
A single forest will contain sites that are both more and less stressful; thus, seeds of different sizes will all have a chance to get established. This habitat heterogeneity contributes to the diversity of plant species. And, says Muller-Landau, if human activity or climate change caused either more or less stress, her model provides insight into which species will benefit and which will suffer.
Her work may also aid in understanding similar tradeoffs between the size and number of offspring in animals or even microbes. For example, it may help predict which strains of pathogens can coexist in systems where the host environment is either benign—i.e., immune defenses are weak—or stressful, and where different strains of pathogens may be either "figs" that reproduce fast but have little ability to defeat a host's defenses or "coconuts" that reproduce slowly but can resist those defenses.