Fruit-eating fish does far-flung forestry

Species’ seed-dispersal skills go the distance

In the Amazon, Johnny Appleseed may be a fish.

TREE PLANTER A tambaqui, a kind of fruit-eating fish, turns out to carry seeds a respectable distance from their mother plants as the fish feasts in flooded Amazonian forests. Tino Strauss/Wikimedia Commons

When rivers in the Amazon Basin flood into surrounding forests and savannas, a fruit-eating fish called a tambaqui proves itself a champion at excreting seeds in distant new homes, says Jill T. Anderson of Duke University in Durham, N.C. In extreme cases, seeds hitchhiking with the fish can land almost 5.5 kilometers from the mother tree.

Those distances put the tambaqui (Colossoma macropomum) into the ranks of elephants and big birds for long-distance planting, Anderson and her colleagues report in an upcoming Proceedings of the Royal Society B.

In tree reproduction, distance matters, especially as loggers, farmers and builders clear more and more patches of forest. Fruit-eaters are “the mobile links that keep forest fragments connected,” says ecologist Pedro Jordano of the Do±ana Biological Station in Seville, Spain. Otherwise patches could become so isolated they lose healthful genetic diversity.

Biologists have begun to gather evidence on how much impact a seed-courier fish might have on a terrestrial forest. The tambaqui, also called a gamitana, is one of roughly 200 known fruit-eating fish species worldwide and gets its chance to forage in the floodplains where rivers swell over their banks and cover some 250,000 square kilometers for months each year. “In some places, you can canoe in the tree canopy,” Anderson says.

Plenty of plants fruit during the flood season. “A lot of these fruits are not especially sweet or tasty to humans,” Anderson says. Yet she and her colleagues found that the tambaqui eats the seeds of 21 plant species.

To see how well the tambaqui might serve as a disperser, Anderson and her colleagues radio-tagged 24 fish and followed the signals to see how far the animals moved. The fish didn’t seem to have territories but rather moved from food patch to food patch. To estimate how long a fruit would stay in the fish’s gut, the researchers fed captive fish some fruit pulp with known numbers of seeds and kept hourly records of fish excretions.

Putting this information together, the researchers calculated that fish were likely to excrete seeds into another, plant-friendly part of the floodplain instead of into a dead-end location like the river bottom.

Fish seed-delivery distances averaged around one-third to half a kilometer, Anderson and her colleagues calculated. And at least 5 percent of seeds end up around two kilometers from the mother tree.

Larger fish appear to carry seeds the greatest distance, a conclusion that worries Anderson. Fishing targets big individuals, and the practice has been intense enough to cut tambaqui numbers by 90 percent during recent decades in certain places, Anderson says. “Overfishing may be removing the best dispersers,” she says.

The effects of overfishing on a forest may be harder to spot than the outcomes of overhunting on land, warns conservation biologist Francis Putz of the University of Florida in Gainesville. The new fish paper, he says, “provides field researchers with a strong impetus to look for these impacts.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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