Marked for Death?

Why do species go extinct? It seems like a simple question. But if a child asked you, what would you say? There are dozens, probably hundreds, of reasons a species might disappear from earth forever—being outcompeted by other species, disease, climatic changes, overhunting, overfishing, deforestation, natural habitat degradation, asteroids…the list is long.

The more important question, from a conservation-policy standpoint, is, What makes a species prone to extinction? A new study published Thursday in the journal Science takes a first pass at answering it, at least from a historical point of view. The authors analyzed 23 million years’ worth of fossil evidence, found in marine sediments that now reside above sea level due to shifts in the planet’s landscape. They looked for patterns: What factors seemed to favor extinction, and what sorts of creatures tended to survive when others failed?

Despite the many, many causes of extinction, the researchers found that two factors seem to best predict which marine creatures are most likely to vanish. First, having a small habitat range is a serious liability. If an entire genus lives on the side of a volcano, for example, one teeny-tiny eruption is enough to wipe it off the face of the planet. Secondly, the data showed that members of certain taxonomic groups have high extinction rates. (If you’ve forgotten your taxonomy, just remember: King Phillip Came Over for Great Soup.)

Which are the unfortunate taxa? Baleen whales, which comprise the parvorder Mysticeti (a parvorder is a subgroup of a taxonomic order), are very extinction prone. Llanocetus, Janjucetus, Willungacetus—I’m sure you can add your own favorite Mysticetes—are all extinct. That’s bad news for the 10 surviving baleens, such as the blue whale and the humpback, many of which are already endangered. If you’re into less charismatic species, then you should worry about brain coral, of the family Faviidae, which populate warm, shallow waters. Fortunately, brain coral have a fairly large habitat range, which counterbalances their taxonomic history of extinction. Smart move, brain coral.

The future is most scary for the narwhal, the study found. The mythical-looking mammal is the lone surviving member of the genus Monodon. Its parvorder, the Odontoceti, or toothed whales, has an exceptionally high extinction rate. Even worse, the narwhal lives in a very limited area, swimming only in the oceans of the northern part of the globe. The International Union for Conservation of Nature Red List classifies the narwhal as near threatened, but this study suggests matters are somewhat worse for the unicorn of the sea.

In addition to informing our conservation-policy choices, this study is just a great piece of science because it raises more questions than it answers.

“One of the things we don’t yet understand is why different taxonomic groups have different characteristic extinction rates,” says the study’s lead author, Seth Finnegan of the University of California, Berkeley. Unraveling what specific factors influence the extinction risk for a group of species is a goal for future research.

It’s also important to note that this is a study about extinction risk factors from the past. Humans have so fundamentally changed the survival game on earth that it’s not entirely clear how these factors will change in the future. Perhaps the greatest risk for extinction in the 21st century is living in a habitat that humans value for their own use, or lacking the ability to migrate when anthropogenic climate change alters the local environment. (Another study published in Science yesterday suggests one-sixth of the world’s species could disappear on account of global warming.)

Why do species go extinct? Today, the best answer to give a child might be: “Because of us.”

This article was originally published on onEarth, which is no longer in publication. onEarth was founded in 1979 as the Amicus Journal, an independent magazine of thought and opinion on the environment. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. This article is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the article was originally published by NRDC.org and link to the original; the article cannot be edited (beyond simple things such grammar); you can’t resell the article in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select articles individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our articles.