Counter to expectations, groups of related marine species with large population sizes have just the same risk of extinction as those with small population sizes, according to new research.

The new analysis of marine fossil records across 500 million years shows that ocean invertebrates (animals without backbones) limited to small geographic ranges typically bear the brunt of extinction, regardless of their abundance there. The findings offer a potentially important clue for present-day conservationists.

"There's this generally widely held belief that species that are more rare should be more vulnerable to extinction," lead study author Paul Harnik, a postdoctoral fellow at the National Evolutionary Synthesis Center in Durham, N.C., told LiveScience. "The average population size has no association with extinction risk over the history of marine animals."

Risk of extinction

Harnik and researchers at Stanford University and Humboldt University used the Paleobiology Database, a community-based fossil database, to analyze the global marine fossil record.

The researchers focused on 6,500 groups of related species, called genera, of sea urchins, sand dollars, corals, snails, clams, oysters, scallops, brachiopods and other invertebrates. They examined which organisms disappeared from the fossil record, along with their geographic range, variety of habitats and relative population numbers. [Image Gallery: Quirky Sea Creatures]

In all, they examined records of hundreds of thousands of fossil observations from over 42,000 collections at museums and universities.

"We see a huge variation in population size in the fossil record, and yet it's not associated with extinction," Harnik said.

Compared with genera that lived both far and wide, related groups of species that could occupy only small regions of the globe had a six-fold chance of becoming extinct.

Of the genera limited to small regions, however, the ability to live in a variety in habitats decreased the risk of extinction by 30 percent.

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Implications for conservation

"The findings don't mean that when populations dwindle we shouldn't worry about them," Harnik said. Instead, limiting a species' range through habitat destruction or degradation could raise the risk of extinction, even for a species with a relatively large population.

However, what once drove extinction may no longer be relevant in an era in which vanishing species are more common, one conservation expert noted.

"Finding small geographic range size to be a driver of risk is par for the course ... in the fossil record. But if dinosaurs drove fishing trawlers, the pattern would be different and more like what we see today, with the largest-bodied species suffering most," Nicholas Dulvy, marine biodiversity and conservation professor at Simon Fraser University, wrote in an email.

"While historic drivers of fossil extinctions have been vast — meteorites and volcanism — they have never been as unprecedented as the unique impact of the growing humanity driving the sixth mass extinction — the Anthropocene," added Dulvy, who was not involved in the current study.

Today's extinction rates, he noted, are one to two times the magnitude of those in the fossil record, with future rates expected to climb further.

"There is a real risk that the threatening processes that have gone before may tell us little about what is to come," Dulvy said.

The study is detailed today (Oct. 23) in the journal Proceedings of the Royal Society B.

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