Habitat Range Has Bearing on Survival or Extinction of Animals

The new study of marine fossils provides clue regarding why certain ocean animals are more prone to extinction than others.

Paul Harnik of the National Evolutionary Synthesis Center and colleagues Jonathan Payne of Stanford University and Carl Simpson of the Museum für Naturkunde in Berlin wanted to know which aspects of rarity best predict why some species survive and others die out. 

In order to proceed with the finding the team looked at a fossil data base for marine invertebrates that inhibited the world's oceans from 500 million years ago to present. That is because the oceans represent more than 70 percent of Earth's surface. The dataset included 6500 genera of sea urchins, sand dollars, corals, snails, clams, oysters, scallops, brachiopods and other animals.

On carefully hunting for links between the extinction rate and measures of rarity, the researchers were surprised to know that the key predictor of extinction risk for ocean animals was small geographic range size.

Researchers have long assumed that rare animals are more likely to die out. But "rare" could mean multiple things. The word "rare" could be applied to species that have restricted geographic ranges, or small populations, or that tolerate a narrow range of habitats, or any combination thereof, the authors say.

The researchers stated the example of False killer whales who are rare because they occur in small numbers, even though they're found in oceans throughout the world. Similarly, Erect-crested penguins are considered rare because they're geographically restricted to remote islands off the coast of New Zealand even though they're fairly abundant where they occur.

They noticed that habitat played a secondary role and population size had a little effect. The concluded that, Ocean animals that had both, small geographic ranges and tolerated a narrow suite of habitats were six times more likely to go extinct than common animals were.

"Environmental changes are unlikely to affect all areas equally, or all individuals at the same time in the same way. If something terrible happens to some part of a species' range, then at least some populations will still survive," Harnik explained.

"The findings don't mean that when populations dwindle we shouldn't worry about them," Harnik said.

"But the take home message is that reductions in range size -- such as when a species' habitat is destroyed or degraded -- could mean a big increase in long-term extinction risk, even if population sizes in the remaining portions of the species' range are still relatively large."

The results were published in the journal Proceedings of the Royal Society B.