Monkeys and apes play a crucial role in dispersing the seeds of fruit trees in tropical forests. But across the tropics, habitat loss and hunting are decimating local primate populations, and may be putting many tree species at risk. Véronique LaCapra reports.
How a plant spreads its seeds – and how far – are key to its survival. Some species rely on wind or water to disperse their seeds, but for trees in the tropics, animals often play a critical role. Primates in particular are one of the most important seed dispersers in tropical forests.
Kathryn Stoner is a research ecologist at the National University of Mexico. She has been studying how certain monkeys contribute to seed dispersal in a tropical forest in the southern Mexican state of Chiapas.
"They'll eat a fruit," says Stoner, "then they move. They can move up to kilometers sometimes, seven or eight hours later before they actually defecate the seed." Wherever the seed falls, it will germinate and grow to an adult tree.
Stoner notes that some species of primates may actually help degraded areas of forest to re-grow, by dispersing seeds as they move between forest fragments.
The role of primates as seed dispersers has been observed throughout the tropics: in Latin America, in Asia and in Africa.
For the past 17 years, anthropologist Joanna Lambert has been studying seed dispersal in Uganda's Kibale National Park, which is known for its unusual abundance of primates. In a presentation sponsored by the National Science Foundation, she described the results of her research:
"What I've found is that depending on the species of tree and the fruit that's being consumed," Lambert said, "primates are responsible for 78-92% of the removal of the seeds, relative to […] the major fruit-eating birds." Lambert adds that on average, the monkeys and chimpanzees of Kibale move 34,000 seeds in a square kilometer in a day.
And many of those seeds grow into trees used by local people for fruit or for medicinal purposes. According to Lambert, 42% of the fruit species dispersed by primates in Kibale have resources that are used by people.
But according to the International Union for Conservation of Nature, close to one third of the world's primate species are in danger of extinction.
In Asia and Latin America, tropical forest clearing for human settlement, agriculture, and commercial logging are destroying primate habitat.
Hunting is also taking a huge toll on primate populations, as logging operations open up new roads through previously inaccessible areas of forest, and rural subsistence hunters exchange their traditional weapons – bows and arrows, wooden traps, and blowpipes – for guns.
In Western and Central Africa, hunting is largely being driven by demand from urban markets. Known as "the bushmeat trade," the commercial hunting of primates and other wild game species has eliminated all the large mammals from some areas of forest.
Joanna Lambert says that the direct consequences are obvious: "The direct consequences of a bushmeat trade or a bushmeat crisis are the loss of species, the loss of actual animals in a forest."
But, Lambert explains, for people with no other way to earn a living, bushmeat hunting may seem like their only option.
Both she and Kathryn Stoner emphasize that hunting and habitat loss will affect more than just primate populations. "We're actually in the very beginning stages of what we may see as a very large change in composition and structure of tropical forests," Stoner warns, "as a consequence of elimination or reduction of primate populations."
The process may be already underway. As primate populations decline, preliminary studies have found effects in tree seedlings, including a different mix of species and less genetic diversity.