Ancient Insect Species Shed Light on Global Biodiversity

A new study by biologists from Simon Fraser University, Canada, has confirmed the theory that change in species diversity across mountains in modern tropics is greater than in temperate latitudes.

About 45 years ago, evolutionary biologist Daniel Janzen at the University of Pennsylvania theorized about global diversity, suggesting that species change greatly across mountain ranges in the tropics.

Janzen reasoned that the difference between summer and winter in temperate latitudes (high seasonality) provides a wide window for the species to migrate across the mountainous ranges. But a small difference in the tropics provides a narrow window every year for species to migrate from one site to the other. As a result, communities across tropical mountains should have a great diversity of species and lesser number of the same species. The theory is supported by many studies on modern communities.

This new study by biologists Bruce Archibald and Rolf Mathewes from Simon Fraser University also confirms the theory about the change in modern species diversity across the mountainous regions. They also found evidence that the ancient mountains were much more diverse, with many species then than now.

The research team studied fossil beds spread across the ancient mountains of British Columbia and Washington. Fifty million years ago, there was low seasonality outside of the tropics, right to the poles, when the fossil beds were laid down. If Janzen's theory is applied in this case, the diversity patterns described in modern tropical mountains should have extended well into the higher latitudes.

"We found that insect species changed greatly across British Columbia's and Washington State's ancient mountain ranges, like in the modern tropics," Archibald says, "exactly as Janzen's seasonality hypothesis predicted."

This suggests it is the particular seasonality found in the modern tropics that is affecting this biodiversity pattern.

The details of the study are published in the scientific journal Palaeogeography, Palaeoclimatology, Palaeoecology.