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.
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 resourcesthe 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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