Does tropical biodiversity increase during global warming?
Rhett A. Butler, mongabay.com
March 30, 2006
Generally yes, but not necessarily if habitat area is reduced finds study
Forest fragmentation may cause biodiversity loss lasting millions of years according to a new study published in the March 31, 2006 issue of the journal Science.
Using cores drilled through 5 kilometers of rock in eastern Colombia and western Venezuela, Carlos Jaramillo of the Smithsonian Tropical Research Institute (STRI) in Panama and a team of researchers derived a fossil pollen record for a 72 million-year period with samples ranging from 10 to 82 million years ago. They then correlated pollen diversity with global temperature estimates for the middle part of that sequence (20-65 million years ago) and found that plant diversity seems to increase when tropical forests cover large areas.
“We found that pollen diversity tracks global temperature through time over millions of years,” said Jaramillo, lead author of the study. “Diversity increases as the planet warms and decreases as it cools. The mystery is that even when global temperatures vary enormously, average temperatures in the tropics don’t change much, so why do we see global temperature patterns reflected in tropical plant diversity?”
While many theories have been put forward to explain the origins of biodiversity, Jaramillo proposes that changes in area drive speciation and extinction in the tropics.
“There is good correlation between area and number of species: more area implies more species. During global warming, tropical areas expand and diversity goes up, the opposite happens during global cooling. If this is the case, fragmentation of modern tropical forest could be equated to a global cooling period, because forested areas are shrinking dramatically, resulting in plummeting diversity in the forests that remain.”
Fragmentation in the Amazon rainforest
Jaramillo’s work may have important implications for tropical biodiversity conservation. Despite warming trends that could potentially expand the area of tropical rainforests and theoretically boost levels of biodiversity, ongoing degradation and fragmentation of habitat could have an negating effect, similar to what might occur during a global cooling event.
“Jaramillo’s intriguing findings provide an evolutionary perspective on a modern crisis,” said William F. Laurance of STRI and the Biological Dynamics of Forest Fragments project in Brazil. “They suggest that the rapid contemporary loss and fragmentation of rainforests will lead to striking, long-term biodiversity declines.”
Fragmentation and habitat reduction is already having a significant impact on the world’s forests. Last week environmental group Greenpeace launched a set of satellite maps showing the world’s remaining extent of “intact forests” today cover less than 10% of the earth’s land area. Intact forests—defined by the environmental organization as blocks of forest landscapes larger than 500 square kilometers that have not fragmented by infrastructure, such as roads, human settlements, waterways, pipelines, and power lines—are the most biodiverse form of forest.
Meanwhile, the U.N.’s Millennium Ecosystem Assessment reports that the current rate of extinction is running 100 to 1,000 times the normal background rate, suggesting that we are currently in the midst of a sixth great extinction. 800 species are known to have become extinct since 1500.
While conservation efforts have focused on preserving tracts of forests in protected reserves, this may not be enough to stave off species extinction, especially if reserve size in insufficient. “If the size of forested areas does indeed control levels of local species diversity, conserving isolated pockets of tropical rainforest may not be sufficient to prevent high rates of extinction in the long run,” write the authors.
This article used press materials from the Smithsonian Tropical Research Institute