It's a vertebrate's world out there. Of the almost 1.5 million known animal species on Earth, owners of backbones lead in number and show up in a vast array of sizes and shapes. Fish, amphibians, reptiles, birds and mammals are included within that group, but the number of species in each group can really vary. For instance, lung fish have only six species; and a mere 25 crocodilian species exist. However, 9,700 lizards and snakes fill the world with their scaly selves, and 10,000 birds flutter or hop or fly on its surfaces or in its air.

University of Arizona evolutionary biologist John Wiens recently published research in the journal Biology Letters on why that is. Habitat, he says, is the driving force. For example, groups that live on land really win out over those in water, as the research concluded.

While this hypothesis has existed in other studies, no one had tested the idea with quantitative analysis, according to a release. Wiens looked at the net diversification rates for 12 major vertebrate groups. This is similar to the number of species in a group divided by its age. In this way, he learned how fast species within each clade were proliferating.

Birds, for instance, have many species and are also relatively young in global time: Their clade is hundreds of millions of years old, but younger than that of, say, ray-finned fishes, which is ancient. So birds have a high net diversification rate, says Wiens in the release.

"In contrast, sharks and rays have a smaller number of species and they're a much older group, so they have a lower rate," Wiens said, according to the release.

All that said, ray-finned fishes still have far less species richness than land vertebrates, the release noted.

Wiens is still working on why terrestrial group proliferate faster than water-based ones, but conjectures that it could be related to ocean acidification and other reasons that marine and freshwater creatures experience higher rates of extinction. While land-based animals can sometimes migrate to a different climate in a time of change, marine animals cannot escape the alteration in their environment. Lakes and freshwater, too, can restrict species with their size and geographical isolation, a statement noted.

Up next: Wiens will evaluate invertebrates as well, to see if the trend holds and can explain biodiversity patterns for all species, as the release said.

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