Rainforests Are Regrowing: Will Displaced Species Return?

Scientists have predicted that up to half of all species may be lost in our lifetimes. But some researchers contend that rainforest regrowth has not been adequately factored into estimates of future species loss and that the biodiversity crisis has been overstated.

Others maintain that only 50 to 80 percent of plant species may return to logged or altered forests, and many animal species will not survive the transition.

Pasture with remnant trees and small forest patches in northeast Costa Rica (Photo courtesy Robin Chazdon)

Still others warn that the continuing rapid expansion of logging and mining roads makes forest access easier for commercial poachers and hungry people. Animals are being hunted for exotic food, trophies, medicine and pets on levels that threaten the continued existence of many species, they argue.

This increasing harvest of animals, combined with the emergence of devastating wildlife diseases, habitat loss due to industrial scale development, climate change and other factors, is a recipe for catastrophic biodiversity collapse, despite encouraging evidence of rainforest regrowth, says this group of scientists.

The need to explore these issues has prompted the Smithsonian to invite experts to present their ideas at today's symposium on the tropical extinction crisis.

Cristian Samper, director of the National Museum of Natural History, said, "By bringing together the world's foremost authorities on different aspects of rainforest science, we hope to achieve new insights into a situation with potentially profound implications for all species, ours included."

Symposium presenter Greg Asner of the Carnegie Institution says the tropics originally had almost 20 million square kilometers of rainforests. Today's best available but rough estimates, based on a combination of satellite data and field research, show:

• 10 million square kilometers have been cleared of at least half their forest cover for human use


• 5 million square kilometers have been selectively logged, often with high-impact methods that leave forests degraded


• Of the intact forest remaining, about 275,000 square kilometers were clear-cut in the five years from 2000 to 2005


• 350,000 square kilometers, or 1.7 percent of the original forested area, are in some stage of regrowth today, most in south Asia and Latin America

Often the inhabitants of upland areas that offered marginal farming opportunities leave to pursue better income opportunities in lowlands and cities.

Once the land is abandoned, the regrowth is relatively quick, scientists have found. The forest canopy closes after 15 years. After 20 years, about half of the original biomass weight has grown back.

The symposium is co-chaired by William Laurance and S. Joseph Wright of the Smithsonian Tropical Research Institute based in Panama. The two scientists authored differing papers in academic journals that sparked much of this scientific controversy.

Wright notes that over 20 percent of all land within 10 degrees of the Equator now has protected status, and that the tropics have a percentage of protected land greater than North America, Europe or Japan.

The impact of climate change on tropical biodiversity is his primary concern today, but Wright believes that forested areas will never fall to the lowest levels predicted. He says extinction will threaten a smaller proportion of tropical forest species than predicted.

This position is based in part on United Nations predictions of growing urbanization and slower population growth as mid-century approaches.

Logs cut from Lac Tumba in the Democratic Republic of Congo (Photo by Filip Verbelen)

But Laurance argues that secondary and degraded forests will sustain only a fraction of existing animal species. He notes that birds and mammals are more vulnerable to the altered habitat than insects and other small organisms.

The world now loses the equivalent of 50 football fields of old-growth forest every minute, he says. "Rainforest regrowth is indeed occurring in regions but most old growth is destroyed."

Symposium speaker Elizabeth Bennett, director of hunting and wildlife trade for the New York-based Wildlife Conservation Society, says access for hunters to tens of thousands of square kilometers of virgin rainforests worldwide is being created annually and huge regions are being virtually drained of wildlife.

"Hunting has long been known as a primary cause of wildlife species depletion in tropical forests," she says. "Logging companies frequently regard wild meat as a free subsidy to feed their workers."

The "empty forest syndrome" affecting much of Asia and Africa is spreading rapidly to other parts of the tropical forest world, she says.

Bennett points to the recent seizure of two shipments of scaly anteaters from Sumatra bound for China where they are used to make soup. Fourteen tons seized were in Sumatra, Indonesia and 23 tons seized in Vietnam, more than 7,000 animals in total.

In Vietnam alone, 12 species of large animals have gone extinct, or virtually extinct, in the past 50 years mainly due to hunting, she says.

The chytrid fungus, meanwhile, has wiped out hundreds of amphibian species worldwide.

Chimpanzee displaced by logging in a cage in Cameroon (Photo by Filip Verbelen)

Disease can compound the impact of hunting. The Ebola virus, for example, has reduced gorilla populations in northwest Congo by up to 95 percent, chimpanzee populations by an estimated 83 percent and threatens great ape populations elsewhere in Central Africa.

Says Bennett, "The implications of all this for loss of ecosystem function are still not fully understood, although many studies show that tropical forests depleted of large vertebrates experience reduced seed dispersal, altered patterns of tree recruitment and shifts in the relative abundances of species.

"The loss of top predators and other 'keystone species' has a disproportionate impact on ecosystems and can result in dramatic biodiversity changes."

Thomas Rudel of Rutgers University says rainforest destruction is no longer small-scale but industrial large-scale. He suggests working in a more focused way with managers of large natural resource corporations operating in tropical countries and supports certification programs that identify products produced under sustainable conditions.

Robin Chazdon of the University of Connecticut told the symposium that biological corridors between remnants of old-growth forest and patches of younger forests and agro-forests will enhancing their role as arks to protect species.

"If we can protect, expand and enhance forest cover in these altered landscapes," Chazdon says, "the prognosis for conserving many forms of plant and animal life will improve in many regions."

Copyright Environment News Service (ENS) 2009. All rights reserved.