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Fish poop boosts distant forests

Researchers put a radio tag on a characid fish in the Amazon. The fish disperse seeds up to 5 kilometers from where they eat fruit dropped by trees and other plants.
Researchers put a radio tag on a characid fish in the Amazon. The fish disperse seeds up to 5 kilometers from where they eat fruit dropped by trees and other plants.Jill Anderson

The Amazon's big fish poop seeds far from where they eat fruit, helping to maintain the genetic diversity of the tropical forest, according to new research that shines light on a little-studied mechanism of seed dispersal.

The seed excretions occur during the six- to eight-month-long flood season, when the characid fish Colossoma macropomum swim from lakes and rivers into vast floodplains where they gobble up fruit dropped by trees and shrubs.

They poop out the seeds up to 3 miles (5 kilometers) away in the floodplain where they are likely to germinate, according to Jill Anderson, an evolutionary ecologist at Duke University who led the research.

"That's an important point, because if these fish go into floodplain forests for short feeding bouts and then return to the main river channel they could be depositing seeds in rivers or lakes, permanent bodies of water, and then those seeds would die," she told me today.

Study mechanics

To make the finding, Anderson and colleagues radio tracked 24 fish for three seasons at Peru's Pacaya-Samiria National Reserve. The results showed that the fish roam as much as 3.7 miles (5.9 kilometers), spending up to 90 percent of their time in the flooded forests.

The team combined this data with information on how long captive fish retained seeds in their guts before they were excreted.

"Every hour we would collect the seeds they had defecated and so we would know what proportion of seeds they defecate at certain time intervals," Anderson explained to me today. "When you combine that data with data on how the fish are moving, you can model how the seed is moving around the landscape."

They modeled a mean dispersal distance of 1,100 to 1,800 feet (337 to 552 meters), and a maximum of 3.4 miles (5.5 kilometers), though Anderson thinks this is conservative.

Colossoma macropomum can reach a weight of 66 pounds (30 kilograms) but the fish in her study were in the range of a couple of kilograms. "So I'm pretty sure our model is an underestimate of how really big fish would be moving seeds around," she said.

Fish in trouble

The findings may add pressure on efforts to conserve these fish, which have been swimming in the Amazon for 15 million years.

"Because they are so large and they are also tasty, they are really important in human consumption throughout tropical South America," Anderson noted.

In some parts of the fish's range, the population size has declined by up to 90 percent, she added. What's more, the fish that remain are smaller since the big fish fetch a higher price at market.

"We have data showing that these smaller fish are not as good as the big fish at dispersal," Anderson said. "The smaller fish eat fewer seeds, they disperse fewer seeds and they don't bring seeds as far. Basically, overfishing is removing the best seed dispersers from the system."

What this means for the tropical forest is unknown. The researchers have yet to determine how important fish seed dispersal is compared to other dispersers such as monkeys and birds. "That's a question I would like to pursue," Anderson said.

The current findings were published Wednesday in the Proceedings of the Royal Society B.

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John Roach is a contributing writer for Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following's science editor, Alan Boyle, on Twitter (@b0yle).