It's been known for more than 150 years that the tropics are home to far greater numbers of animal and plant species than the planet's temperate regions. But despite decades of study, the causes of this striking biodiversity pattern remain poorly understood and hotly debated.

One proposed explanation for the so-called latitudinal diversity gradient is that new species form at faster rates in the tropics. University of Michigan evolutionary biologist Daniel Rabosky and two U-M colleagues tested that hypothesis by reconstructing the history of speciation in New World land birds.

They estimated rates of species formation across 2,571 bird species and found no difference between species from tropical and non-tropical regions. Their findings were published online May 27, 2015 in Proceedings of the Royal Society B.

"This result rejects a broad class of evolutionary hypotheses that try to explain the latitudinal diversity gradient through faster rates of speciation in the tropics," said Rabosky, an assistant professor in the Department of Ecology and Evolutionary Biology and a curator at the U-M Museum of Zoology.

"We haven't yet tested some other potential causes of the gradient, but we provide very strong evidence that it doesn't involve 'faster evolution in the tropics,' or anything related to that," said Rabosky, who studies the evolutionary processes of species formation and extinction to understand why biological diversity varies so dramatically over space and time.

The differences in species numbers among tropical and temperate New World land birds is extreme and has generated decades of study by ecologists and evolutionary biologists.

It's noteworthy, for example, that the entire eastern United States has just one species of resident hummingbird—the ruby-throated hummingbird—while Ecuador has nearly 150 of them. Ecuador, which is roughly the same size as Michigan, has at least 1,500 breeding bird species; Michigan has about 250.

Coauthors on the paper are Pascal O. Title, graduate student and Huateng Huang, postdoc. 

Read the full Michigan News press release