Pushing the Boundaries of Life: Tropics

Contents

For many years, it was thought that tropical rain forests were essentially unaffected by climate change. Now studies are showing that not only were they changed during past events like ice ages, but some areas are being affected right now by warming. At Monteverde Cloud Forest Reserve, Costa Rica, clouds are forming higher, drying out some of the habitat and causing changes in flora and fauna.

The most celebrated peer reviewed case is the disappearance of the golden toads, Bufo periglenes. Each year Dr. Alan Pounds and others search for the distinctive orange amphibian (see photos) in its restricted habitat along a narrow, fog-bound ridge. About 1500 toads were sighted in 1987. But now the breeding pools remain empty -- the toad has not been seen since 1991 and is feared extinct.

The golden toad and other amphibians and lizards studied by Dr. Pounds are in decline apparently due to regional temperature increases lifting the level of clouds, effectively drying out the cloud forest moisture on which they depend. This species, Antelopus varius, once common throughout Costa Rica, was not found at all in a recent survey, according to Dr. Pounds. Some of this change may be due to deforestation in lowland Costa Rica, and a group of scientists at Monteverde is planning more study. At the same time, research by Dr. Nalini Nadkarni shows drying will drastically change the composition of the diverse epiphyte community that inhabits the cloud forest canopy.

Dryer conditions in the cloud forest concern Dr. Karen Masters, who studies tiny Pleurothallic canopy orchids. Lengthening dry periods could drive some into extinction. "We are now seeing 2, 3 even 5 days in a row without moisture." she reports. "This is very challenging to these orchids." Also, recent repeat surveys of bats by Dr. Richard LaVal, and of birds by Debra DeRosier (repeating a 1979 survey by Dr. George Powell) shows lowland, dry habitat species are already moving higher into former cloud forest areas.

Other big changes are being monitored in the tropics, too.  Sixteen years of data on tree growth, tropical air temperatures and CO2 readings indicate that a warming climate may cause the tropical forests to give off more carbon dioxide than they take up.  This would upset the common belief that tropical forests are always a sink for carbon, taking huge amounts out of the atmosphere.  

The study, by Deborah and David Clark of the La Selva biological station in Costa Rica, and Charles Keeling and Stephen Piper of the Scripps Institution, reports that rainforest trees grow much more slowly in warmer nighttime temperatures, which is a hallmark of climate change in the tropics.

In other parts of the tropics, even in places that have been undisturbed for more than 4500 years, the rise in atmospheric CO2 appears to be changing the composition of the forest.  In a paper in the 11 March 2004 issue of Nature, William Laurance and colleagues document that many tree genera in Amazonia are growing faster than  they were in the 1980s. 

Other tree types are declining in vitality.  The study of 13,700 trees in 18 very isolated plots in Brazil concluded that increased carbon dioxide is the most plausible explanation for the abrupt shifts in species growth.  This "could also have serious ecological repercussions for the diverse Amazonian biota" wrote the scientists.