Photo credit: adKinn/Creative Commons
Understanding this discrepancy, between estimated and officially documented extinctions, is critical to contextualizing threats to endangered species—as well as seeing the direction in which a changing planet is headed.
What Does it Mean to be Extinct?
Extinction, in simple terms, occurs when the last existing member of a species dies. Determining that this has happened, however, is not a simple task.
First, the taxonomy of the species must be very carefully and clearly defined. The species in question must be uniquely identifiable from any ancestor or daughter species. Studies of the fossil record have suggested that 99 percent of all species that have ever existed have, in fact, evolved into another distinct species.
Once this definition has been developed, population surveys must be conducted to determine the number of remaining individuals. For many years, the World Conservation Union, a group that works with scientists to develop extinction declarations, used a "50-year" rule. This rule held that if a species wasn't seen in 50 years, it could be declared extinct.
In the 1990s, this rule was revised. Today, the WCU states that an extinction label will only be applied if "there is no reasonable doubt that the last individual has died." This means that scientists must perform regular surveys of a species' habitat—each showing no evidence of the species' existence—in order to obtain the label.
Estimating extinction rates, however, is less precise. To get a sense of this number, researchers use the list of 1.5 million known species and calculate factors such as the number official extinction declarations, rates of habitat loss, and the number of endangered and critically endangered species. This results in an estimated 2.7 and 270 species reaching extinction every day.
The number is, admittedly, only a guess, but most researchers believe it to be conservative, if anything.
Extinction by any Other Name
Complete extinction—being "as dead as a dodo"—is not the only label conservationists have to describe a dramatic decrease of a species' range.
Indeed, when researchers talk about extinction, they are usually referring to local extinction or extirpation. This means that one population of a species has disappeared. The extinction of wolves in the Rocky Mountains of the United States during the early 20th century is an example of this.
Other species are defined as extinct in the wild, meaning that the only known individuals survive in captivity or as a naturalized population well outside their original range. The most famous example of a species that is extinct in the wild is Lonesome George, the last living Pinta Island tortoise.
Any of these labels fall under the category of functionally extinct, which generally includes historical extinction, populations that have been reduced to the point at which they no longer play a significant role in their ecosystems, and populations that are no longer viable.
When Extinction is Not Forever
These various labels highlight an important fact: Determining an active estimate of a rare and elusive species is difficult, if not impossible. Historical extinctions, which rely on the fossil record or historical accounts, are comparatively easy—when a species no longer appears, scientists can safely assume it is extinct.
Modern species, however, have a tenancy to disappear and then show up again years—and sometimes decades—later. Known as "Lazarus species," the misplaced extinction label is often found to be a result of imprecise sampling.
Even the reappearance of species formerly thought to be extinct is problematic. For those that have been missing for an extensive period of time, the reemergence is often attributed to "Elvis species," or lookalike.
On the Brink of Mass Extinction
As difficult and problematic as the process of determining extinction may be, it is absolutely critical for understanding the health of the planet.
Using the fossil record, researchers can establish a "background rate" of extinction—the number of natural extinctions that occur outside of major events. This allows them to assess current and historic events. Based on this research, the rate for mammals is thought to be two extinctions every million years.
Taking this as the baseline, the number of extinctions that have occurred since 1500, is significant. Stripped of its context, the extinction of 80 mammal species in the last 500 years seems smalls, but compared to the background rate it is a signal of a troubling trend—one that has a cascading effect across the world.
Anthony Barnosky, a professor of integrative biology at UC Berkeley, believes it's a trend that indicates the approach of a major extinction event. "If currently threatened species—those officially classed as critically endangered, endangered and vulnerable—actually went extinct," he explains "and that rate of extinction continued, the sixth mass extinction could arrive within as little as 3 to 22 centuries."
When it comes to extinctions, he says, the rate is much more important than the magnitude and, even with the uncertainties extinction research presents, the rate is alarming.
Read more about extinction:
The Earth's Sixth Mass Extinction May Be Underway
Invasive Species May Trigger Next Mass Extinction
20% of World's Plant Species Threatened With Extinction - Yes, Human Activity is Main Cause