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 Paleontologist Bruce Archibald compared fossilized insect levels in B.C.’s McAbee fossil beds with those of today’s tropical and temperate forests to determine that the tropics’ greater biodiversity is due not to more heat or light but to lower seasonal temperature variations.

Stable temperature promotes biodiversity

July 22, 2010

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Scientists have generated more than 100 hypotheses over the last few centuries to explain why tropical rainforests have so many more plant and animal species than temperate rainforests, but they’ve all been difficult to test—until now. 

SFU paleontologist Bruce Archibald, who spent the last decade investigating the question, says he and his colleagues may have finally found the answer: it’s the difference in temperature variation between the two climatic zones.

The climate is obviously warmer in the tropics, but it’s not that simple, says Archibald. There’s not only more heat and light in the tropics, but also much lower seasonal variation in temperature, or seasonality, where the average temperature of the hottest and coolest month may vary by only a few degrees.

“These factors tend to change together as you travel away from the equator toward the poles,” he explains, “leaving it difficult to separate their individual effects on diversity.” 

But Archibald and his colleagues found a way. They began by trapping insects in both Costa Rica’s hot, low-seasonality jungles and in Harvard University’s cool, high-seasonality research forest in Massachusetts.

Then, to isolate the possible effects of heat and light from seasonality, they compared the contemporary insect samples with fossilized insects from the 53-million-year-old (Eocene age) McAbee fossil beds near Cache Creek, B.C. The area back then had an average yearly temperature similar to that of southern B.C. today, but with low seasonality like the current tropics, says Archibald. 

The scientists’ eureka moment came when they found that the McAbee fossil record indicates an insect diversity at least as high as today’s Costa Rican tropical forests.  
“This indicates that it’s low seasonality, not greater heat and light, that promotes the modern tropics’ high diversity,” explains Archibald. 

Their findings also imply that the world may be less diverse now than it was 50 million years ago, when low seasonality extended globally into high latitudes. 
Archibald’s team’s research was published this month in the journal Paleobiology.

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