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For Seeds, Success Means Striking Out on Their Own

John Roach
for National Geographic News
July 16, 2004
 
If seeds could talk, they might sound like recent high school graduates making a beeline for jobs and colleges as far away from their parents as possible: "See ya 'rents, we're outta here."

Several studies have shown that seeds that stick too close to home have to put up with their parent's diseases and fight with the whole family for access to life's essentials: light, water, and nutrients.

Homebody seeds are also easier targets for predators. And some plants—like those that specialize in colonizing forest gaps or regenerating fire-ravaged landscapes—can only find fertile ground away from home.



Life, at least for a seed, is better out in the world.

"In general, seed dispersal away from the parent plant increases the chances that a seed will reach maturity," said Chris Birkinshaw, a biologist with Missouri Botanical Garden in St. Louis.

Seeds Get Around

Seeds know how to get around. Some fly with the wind, others go with the flow. Many hitch a ride with unsuspecting critters. But they all have the same goal: to take root and give rise to the next generation.

Scientists can understand what type of dispersal strategy a plant employs by looking at its environment. For example, dispersal by sea currents is important for plants that grow on seashores, wind is important in open grasslands.

And for each type of dispersal, there is a type of design. Wind-dispersed seeds are generally lightweight and have adaptations such as wings and parachutes so they can catch a ride on the breeze.

Water-dispersed seeds, such as coconuts, are buoyant. Seeds dispersed by animals usually offer a nutritional reward so that they are eaten, or they are sticky or barbed so they can latch on to passing bodies.

"Among species with seeds dispersed internally by animals, the size of the seed or fruit, its color, and the presence of protective adaptations—for example, the husk—will reflect the swallowing, visual, and processing abilities of the seed disperser," Birkinshaw said.

For example, seeds spread by small birds will be small in size, covered with plant flesh (to give the birds a reward for eating it), huskless (since most birds are ill equipped to remove such an outer shell), and brightly colored (since birds have good color vision).

Lemurs, a group of primates on the African island of Madagascar, go after a wide range of seeds, including big fleshy seeds encased in a husk. But owing to the primates' poor color vision, the fruits are dull in color, Birkinshaw said.

Nondispersal

Some seeds have no specific dispersal strategy, like the coco-de-mer, a palm tree that only grows in the Seychelles, an island chain in the Indian Ocean. These palms have the largest seeds of any plant and lack any seed dispersal method other than gravity, Birkinshaw said.

In other cases, as with the rare Madagascan palms Satranala decussilvae and Voanioala gerardii, the seeds collect in piles beneath their parent trees. Researchers believe that perhaps their animal dispersers are long extinct.

According to John Dransfield, an expert on Madagascan palms with the United Kingdom's Royal Botanic Garden, Kew, both of these palms have very large seeds that animals alive today are incapable of moving. "You start dreaming up stories that it could have been distributed by a now extinct animal," he said.

Possible extinct dispersers of the palms are large lemurs that once roamed Madagascar or flightless elephant birds, which were the largest birds known to have lived and were endemic to Madagascar.

There are only a few of these Madagascan palms left standing. If researchers can confirm that indeed the animal disperser of the palms' seeds are extinct, then the only way to prevent the trees themselves from becoming extinct may be to reintroduce seedlings into the forest with a controlled program of replanting, Dransfield said.

Donald Drake is a botanist with the University of Hawaii in Honolulu who studies how plant and animal interaction affects reproduction of native plants and food for native animals in the Pacific Ocean islands. He said loss of animals to disperse seeds certainly impacts a plant's viability, but "hard, conclusive data are difficult to come by."

He and colleague Kim McConkey are currently engaged in research that suggests animals may stop performing ecological functions such as seed dispersal long before they go extinct.

"We found this to be the case with flying foxes," Drake said. "Flying foxes are among the few remaining large animals that disperse seeds on islands in the Pacific and Indian Oceans."

Drake and McConkey found that the flying foxes cease being effective seed dispersers when their population densities fall below a point that induces them to compete over food resources—the foxes stop bothering to scatter and hide their food stores.

In order to maintain effective seed-dispersing populations, the researchers say it is important to take conservation actions before seed-dispersing animal species drop below this threshold.

"Many flying foxes are either rare or extinct," Drake said. "If they cease to be effective dispersers long before reaching that stage, there is a possibility that the results we found are of wide applicability."

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