Hunting Leads to Rapid Change in Tropical Trees

  • Credit: Christian Ziegler/Smithsonian Institution

    Green treasure. More than 1100 tree species are found in Lambir Hills National Park in Malaysia.

  • Credit: Khoo Min Sheng

    Painstaking. Every 5 years, researchers measure the size of 370,000 individually tagged trees in a 52-hectare plot. Here, they work in neighboring Brunei.

  • Credit: Christian Ziegler/Smithsonian Institution

    Vanished. Most of the seed-dispersing animals, such as this hornbill, are gone because of hunting.

  • Joshua Tewksbury/University of Washington and the Luc Hoffmann Institute/WWF International

    Credit: Christian Ziegler/Smithsonian Institution

    Plummeting. Seeds that were once spread widely by birds or gibbons now fall straight down to the ground.

  • Credit: Christian Ziegler/Smithsonian Institution

    Too close. The seedlings of these trees are now crowded together, raising the risk of disease.

  • Credit: Christian Ziegler/Smithsonian Institution

    Denser. Deer and pigs have been hunted, so fewer animals eat the young saplings, leading to thicker understory.

In the early 1990s, Lambir Hills National Park was a paradise for tropical ecologists. They were attracted to this dark forest in Sarawak, Malaysia, by its extraordinary diversity of trees—more than 1100 species—and its rich array of birds, gibbons, flying foxes, and other animals. Then, the market for bushmeat exploded, and hunters arrived in droves. Within the decade, almost all the larger animals had been shot. "It's a total tragedy," says Stuart Davies, an ecologist with the Smithsonian Tropical Research Institute (STRI), who is based in Washington, D.C.

From that tragedy, Davies and his colleagues are learning about how the forests are affected by the loss of animals that disperse seeds and feed on young plants. In the most thorough assessment to date, the team has found that saplings are overcrowded, putting them at risk of disease. And in some places, tree diversity has started to decline over just the past 15 years. "The rapidity of change is astounding," says Joseph Wright, a tropical ecologist at STRI who was not involved in the study. He and others think the outlook for the forest is only going to get worse.

The broad effects of hunting are fairly straightforward to predict. Killing herbivores such as deer should lead to an increase in vegetation. The loss of animals that spread seeds by eating fruit, such as birds or monkeys, ought to crimp the distribution of these plants. These trends have been seen in previous studies that compared forests with more or less hunting and in experiments that excluded animals with fences. But no one had looked at a single forest through time as hunting emptied it of animals.

Lambir Hills is a scientific gold mine because researchers have been surveying its plants in great detail since the early 1990s. As part of a network of research sites in tropical forests, researchers identify, map, and measure about 370,000 trees in a 52-hectare plot every 5 years. It takes a team of 20 people about 9 months to complete the survey.

Tropical ecologist Rhett Harrison of the World Agroforestry Centre in Kunming, China, has been studying Lambir Hills since the early 1990s and witnessed the animals disappearing firsthand. He decided to look at the consequences for seed dispersal. First, he had to comb through books and museum records to figure out how the seeds of each of the 1100 tree species are spread. Working with Davies and others, he examined what had happened to the trees over the past 15 years.

The first thing the team found was that regardless of how seeds are dispersed, the density of saplings in Lambir Hills has increased by 25%, probably because no deer or pigs are nibbling them. "Overall, there was a great flush of seedlings," Davies says. As for the distribution of tree species, there was no change among species whose seeds are wafted by the wind, pop apart, or just drop to the ground. That contrasts with species that had depended on hunted animals to disperse their seeds. Their saplings were much more tightly clustered around the adult trees. That is a problem because it makes the plants more vulnerable to outbreaks of disease. "Hunting is the silent killer of forests," Davies says.

When the team analyzed trees within 20-by-20-meter plots, they discovered that on average, sapling diversity was 1.9% less than in the early 1990s. "That's pretty massive given that it was just the animals being extirpated," comments seed-dispersal expert Patrick Jansen of STRI, who is based at Wageningen University in the Netherlands and was not involved in the study. Although no species have been lost from Lambir Hills as a whole, the changes in the diversity of saplings will probably alter the future makeup of the forest , the team reports online this week in Ecology Letters. Wright adds that diversity could decline even more in the future as adult trees die. "This is the tip of the iceberg," Jansen warns.

Jansen says it's likely that Lambir and other "empty forests" will be less diverse in the future. Because every tree hosts other kinds of life, such as insects and flowering plants called bromeliads, the disappearance of tree species may have ramifications that cascade through the ecosystem. The loss of animals, he says, is a "time bomb." Moreover, the impact of hunting could be even greater in other parts of the tropics. In South America, for example, up to 90% of tree species depend on animals to disperse their seeds.