Alien plant species are defined as those taken from their native territory and introduced to a different ecosystem that enables them to spread rapidly. Kate Evans/Foto CIFOR.

BOGOR, Indonesia — Little green invaders are poised to cause mayhem in the tropics, new research indicates.

Outside the tropics, the scale of invasion by alien plant species has become unprecedented over the last few decades in terms of areas affected, species involved and the extent of economic, ecological and health impacts.

Without greater attention to prevention, it may just be a matter of time before alien plant species wreak havoc inside the tropics, according to a recent literature review.

The paper, published in Forests, noted that tropical forests cover some 1.7 billion hectares, more than one-third of the tropical land surface. They supply an estimated 9 percent of global demand for timber and wood products, and their production and processing generates significant income in many tropical countries. Little is known, however, about the risk and impact of external invaders inside the tropics, especially in continental forests.

Some reported or suspected impacts of plant invasions in production forests include direct damage to timber trees, reduced recruitment of canopy trees, delays in filling gaps left by fallen trees and promoting fires, said the review. The costs of invasions included reduced timber yield, as well as those spent for control efforts.

“There are no studies on the potential economic impact of invasive alien plant species on tropical production forests,” said co-author Michael Padmanaba, a research officer with the Center for International Forestry Research (CIFOR).

“Once we have a dollar figure, we can use this number to increase awareness of production forest managers about the issue. For better or worse, it’s the bottom line that often provokes people to action.”

THEY GET AROUND

Alien plant species are defined as those taken from their native territory and introduced to a different ecosystem that enables them to spread rapidly. These little green invaders typically use mundane forms of transport to invade new territory.

Clidemia hirta, for example, is native to Central and South America and the Caribbean islands. A woody shrub with dense branches that produces edible berries, it grows in open areas such as pastures, and along rivers and roads. In the late 19th century, a man named Koster accidentally introduced seeds of the plant into a coffee nursery on the island of Fiji.

Now known as “Koster’s curse,” the plant is found throughout tropical Asia where it is more shade-tolerant than in the Americas. Apart from the plant’s presence in logged forests and plantations, Padmanaba observed Clidemia hirta thriving in undisturbed forests where it is often the only alien species able to grow away from trails. It grows quickly, filling in gaps left by fallen trees or fires.

Although recent research suggests Koster was not the true culprit, no one disputes that human actions were behind the introduction of Clidemia hirta to the tropics. Still, as the recent CIFOR review pointed out, the pathways of invasion into tropical production forests are often poorly understood.

“Logging, shifting cultivation, fires or natural catastrophe all make forests susceptible to invasion,” Padmanaba said. “Usually, the invasion is stopped as soon as the canopy is re-established because so many of these alien plants need light.” While dense shade acts as a natural barrier against many invasions, shade-tolerant species such as Clidemia hirta have the potential to invade even undisturbed continental forests.

PLANTATIONS ALSO A RISK

Plants are not the only concern. According to the review, more than 100 alien tree species have been introduced in tropical plantation forests since the middle of the 20th century. Seedlings are raised in nurseries where they acclimatize to local conditions before being planted over large areas at multiple sites.

“Exotic plantations are emerging as an important source of invasion, partly because they are surrounded by road networks that help spread plant invasions,” Padmanaba said. “ ‘Plantation escapes’ are particularly serious invaders because they are trees. They are more likely to compete with timber trees and are more expensive to remove than shrubs and herbs.”

Species grown for fuelwood, such as Acacia nilotica, appear to be particularly invasive. Given the recent expansion of pulpwood plantations in the tropics, and the planned expansion of biofuel crops, some researchers predict a large “invasion debt” will have to be paid over the next few decades.

Prevention is better than control, Padmanaba said. Best practices include continued surveillance for invasive species; minimizing canopy opening during harvesting and other silvicultural operations; closing canopy rapidly in plantations; minimizing the width of access roads; and ensuring that vehicles and other equipment are not transporting seeds of invasive species. Foresters need to be aware of potential sources of invaders, as well as natural habitats vulnerable to threats from plantation species.

“So far, tropical continental forests have natural barriers against most invasive species,” said Padmanaba, “but given what we know about the speed of invasion elsewhere, there is no excuse for complacency.”

For more information about the issues in this article, contact Michael Padmanaba, m.padmanaba@cgiar.org

This study was supported in part by the Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden/Chinese Academy of Sciences. This research forms part of the CGIAR Research Program on Forests, Trees and Agroforestry.