Some animals have a knack for dispersing seeds and spreading plant species.
It comes naturally to them: They eat in one spot, travel here and there, and pass the seeds along their path.
Birds, elephants, and monkeys are among the most recognized long-distance seed carriers. But when it comes to spreading seeds of fruit-bearing trees and woody vines in the Amazon region, fish may be the unsung heroes—until recently.
Tim Nuttle was part of a research team that spent six years studying seed dispersal by fish species Colossoma macropomum in the Amazon’s wetland forests. The
team’s findings were published in U.K. biological research journal Proceedings of the Royal Society B in March 2011.
Colossoma, commonly known as tambaqui or pacu, are known to enter the Amazon’s floodplains during the annual flood season, eating large quantities of fallen fruit and defecating viable seeds.
The researchers set out to determine how far the fish carry the seeds and whether they tend to disperse them in areas where they’re likely to grow, such as floodplains, or in settings where they’re not, such as rivers, lakes, and other permanent bodies of water.
Data collection for the study was twofold. Radio transmitters were placed on twenty-four wild tambaqui in northeastern Peru over three flood seasons, from 2004 to 2006. This allowed researchers to track the tambaqui’s movement and habitat use.
How long a seed spends in the digestive tract was measured by feeding fruit pulp with a known quantity of seeds to captive tambaqui and recording on an hourly basis the number of seeds defecated.
Nuttle, who joined the study in 2007, developed the simulation model used to integrate data on travel distances, habitat affinities, and gut-retention time and determine how effective tambaqui are at dispersing seeds.
Aided by their long gut-retention time (averaging seventy-four hours from the time of eating the seeds to passing them), the tambaqui carried seeds a mean distance of 337 to 552 meters and a maximum distance of 5,495 meters, according to the model. In addition, 5 percent of seeds were predicted to have been dispersed a distance of at least 1,707 to 2,114 meters.
Those statistics put the tambaqui on par with the longest-distance seed dispersers known, the African hornbill and the Asian elephant, and ahead of the white-tailed deer. Long-distance seed dispersal is defined as 100 meters or more.
Furthering the tambaqui’s effectiveness at dispersing seeds is their apparent preference for floodplain forests and savannahs over permanent bodies of water. More than 90 percent of seeds were released in areas that promote growth of the plants, the model showed.
According to the researchers, the study’s estimated seed-dispersal distances were conservative, based on a few factors: The dense floodplains decreased the range of the radio transmitters, underestimating the amount of time the fish spent there; some fish in the study were lost; and none of the tagged fish were close to the maximum size reported (tambaqui can reach 30 kilograms in weight and more than a meter in length), probably because of overfishing in the area. The study found that larger fish had significantly greater mean distances for seed dispersal.
Findings from the study support previous research suggesting that tambaqui have dispersed seeds for millions of years and have probably contributed greatly to the evolution of Amazonian wetland plants.
“These trees are clearly adapted for seed dispersal by fish,” Nuttle said, noting the trees fruit specifically during flood season, from January to June.
While the study measured the tambaqui’s seed-dispersal abilities, it also explored a larger issue: the fish’s role in promoting plant diversity and, thus, healthier wetland forests.
Long-distance seed dispersal results in greater genetic variation among plants, both in the species present in a given area and in the genetic makeup of plants of the same species, Nuttle said.
Those variations make plant populations more resilient to environmental changes, such as climate changes. “Diversity helps them to persist,” he said.
Healthy forests, Nuttle explained, are crucial to the ecosystem, aiding with the cycling of water, removing pollutants such as carbon monoxide from the atmosphere, and supporting plant and animal life.
While plants in wetland forests are an important source of new medicines and chemicals, they also have an intrinsic value: “They’re part of our biological heritage,” Nuttle said. “They enrich our experience as human beings.”
Nuttle hopes to take part in a follow-up study that will delve deeper into the effects of tambaqui seed dispersal on plant diversity in the Amazon region. In that study, the diversity of tree populations in areas where there are many tambaqui would be compared to diversity in areas where there are few. Researchers would expect to find greater genetic variation among plants in areas where tambaqui and long-distance seed dispersal are present.
According to Nuttle, this research would also help to raise awareness of and build a case against the overfishing of tambaqui, a dietary staple in the Amazon region. The fishing industry tends to target the largest tambaqui—the longest-distance seed dispersers.
“Overexploitation probably disrupts an ancient coevolutionary relationship between Colossoma and Amazonian plants,” the researchers wrote in Proceedings of the Royal Society B.
The project was funded by the Wildlife Conservation Society, the National Geographic Society, and the Center for the Environment at Cornell University.
Nuttle specializes in forest ecology, restoration ecology, and ecological modeling.
His research interests focus on how forests develop into and function as diverse communities, including how plants and animals interact to shape the community structure.
In another recent research project,
Nuttle explored how the overabundance of white-tailed deer in forests of Pennsylvania reduces tree-species diversity, foliage density, and the presence of insects and birds.
Classes Nuttle teaches at IUP include Botany, Plant Ecology, and Environmental Science. He is the coordinator for the Biology program’s Ecology, Conservation, and Environmental Biology Track.
Nuttle received his bachelor’s degree in wildlife ecology and management from Michigan State University, his master’s degree in wildlife ecology from Mississippi State University, and his Ph.D. in ecology from Utah State University.
This graph, created by Tim Nuttle, IUP Biology professor, shows cumulative seed dispersal curves generated by Colossoma macropomum and eleven other seed dispersers. The curve of the Colossoma macropomum, represented by the solid black line, is comparable to those of the African hornbill (green dotted line) and the Asian elephant (blue dotted line).
The photo at the top shows Colossoma macropomum swimming in the Pittsburgh Zoo aquarium. It was taken by Jill T. Anderson, the team’s lead researcher.