New York, Aug 8 (IANS): Animals living on land proliferate more rapidly than their aquatic counterparts, says a new study.

For example, among vertebrates, there are only six species of lungfish and only 25 crocodilians -- but roughly 10,000 birds and 9,700 lizards and snakes.

This study that looked into the diversification of vertebrates suggest that as far as variation in species numbers is concerned, habitat is a more important variable than climate or metabolic rate.

"I found that most variation in species numbers has a simple explanation: Groups living on land proliferate more rapidly than those in water," said John Wiens, professor at University of Arizona in the US.

Previous studies have hypothesized a connection between terrestrial habitats and higher rates of diversification, but none had performed a quantitative analysis to test the idea.

To address this question, Wiens calculated the net diversification rates for 12 major vertebrate groups.

The net diversification rate is similar to the number of species in a group divided by its age.

Using this approach, Wiens was able to compare how fast species within each clade (a group with a common ancestor) were proliferating.

"An example of a clade with a high net diversification rate is birds, which, on a timescale of hundreds of millions of years, are a relatively young group. Since they are both young and have many species, their rate is relatively high," Wiens explained.

"In contrast, sharks and rays have a smaller number of species and they are a much older group, so they have a lower rate," Wiens noted.

The study was published online in the journal Biology Letters.