If the forests of South America are the world’s lungs, their rivers and wetlands must be its veins and arteries. This is the most diverse continent when it comes to plant life, and it’s thanks in part to the fish swimming through its waterways.

Freshwater fishes consume about 600 neotropical plant species. The largest of fruit-eating fish in Brazil's Pantanal—a 70,000 square-mile floodplain the size of Washington state with as much as 55 inches of annual rainfall—are disproportionately responsible for dispersing seeds and growing habitat. During bountiful summers, trees adjacent to wetlands often flood, dropping fruit that fish happily swallow, then pass through their excrement. The biggest fish have the biggest stomachs, and the largest potential for dispersal. As much as 95 percent of woody plant species in tropical forests are spread this way.

A recent study in Biotropica shows that when these large fruit-eating fish are eliminated from an ecosystem, anglers “fish down” to capture smaller fish, which can have drastic results on seed dispersal and germination. Essentially, large fish excrete whole seeds while smaller ones poop out seeds destroyed by chewing. Sixty-three percent of seeds counted in the stomachs of sardines, for instance, were demolished by mastication.

“Seventy million years ago there were already forests in the Amazon and the Pantanal,” says the study’s co-author, Sandra Bibiana Correa, an assistant professor at Mississippi State University’s Department of Wildlife, Fisheries and Aquaculture. “That means that for 70 million years, those fish have interacted with fruit and helped plants to diversify.”

Correa’s previous work in the Pantanal and Amazon established that overfishing—up to 90 percent for some fruit-eating fish species—hinders plant growth and regeneration.

View Images The Pantanal, the world's largest wetland, straddles Brazil, Bolivia, and Paraguay. Its forests often flood, providing food for fish and other aquatic animals. Photograph by Carl De Souza, AFP/ Getty

Critical Roles

Fruit-eating species disperse as much as 95 percent of tropical plants and large fish consume a high diversity of seeds, which enhances germination. When fruit is pulpy, or soaked in water, the chances are that the seed isn’t damaged when eaten. Bigger mouths help, too. “Once they break the seed, the probability of germination drops.”

“Imagine you eat a tomato,” she explains. “It’s highly unlikely you chew the seeds. Seed size determines whether it will be broken down. Big fish are able to swallow whole seeds, but even little fish regularly gobble intact seeds.”

At least 150 species of fruit-eating fish inhabit South American wetlands—even one subspecies of Colombian piranha eat fruit, Correa says. The fruit bloom coincides with the annual flooding, when fish spend as much as 87 percent of their time in the floodplain. When waters recede, those fertilized seeds easily germinate. Long digestive tracts retain seeds for long periods, enabling dispersal across wide distances.

“Over five kilometers,” marvels Raul Costa Pereira, a study co-author and Sao Paulo State University doctoral biologist. Pereira says casting this wide net is a key to this little-known method of spreading forests, a phenomenon that isn’t confined to the Americas.

“Fruit consumption by fish has been documented all over the world,” he adds.

In South America, where wetlands extend over 15 percent of the entire land mass, this fish-forest relationship was first documented in the late 1970s.

One main farmer is the pacu, a ray-finned fish that can live to 60 and grow 40 pounds. With its wide mouth (and human-like teeth), the pacu disperses seeds of 27 percent more plant types than smaller fish. It’s also a delicacy. And when large fish like pacu are overfished, the quantity and diversity of plant life drastically drops.

“The loss of dispersers can have greater consequences than just declines in a few tree species,” confirms Daniel Wenny, a senior biologist at the San Francisco Bay Bird Observatory who previously studied seed dispersal.

The biggest consequence is no new forest growth.

Threats to Forests

Flooding here lasts four to seven months a year, with waters rising as high as 30 feet. Fish have historically dispersed up to 90 percent of the flooded forest, a co-dependence Correa worries about. With dams, cattle ranches and development, “they now only disperse 30 to 40 percent of the floodplain.”

Correa and her peers are pushing for regulations for fishing small and large species. That means establishing a body of evidence that shows this interplay. With 99 percent of the Pantanal privately owned by ranches, free-ranging cattle are gobbling up all the natural grass. Correa believes smaller fish here can travel into shallow areas and colonize new territory. But with locals eating these shrimps with impunity, “we really don’t know the population size and structure of the small fish.”

Regulations are hard to enforce in the Amazon, a region bordering eight nations, and in the Pantanal, both places feeling the effects of overfishing. When the fish no longer grow large, the forest doesn’t grow either. And when ranchers expand and dams end cyclical flooding, fish are shut out and trees begin dying with no natural way to regenerate.