Agriculture and hunting appear to be the main culprits in the animals' decline.
Twenty percent of all mammals are moving rapidly toward extinction, with many other species also in decline.
Expanding agriculture and hunting are driving this unprecedented extinction period for mammals.
Twenty-four mammal species are improving due to conservation efforts.
At least twenty percent of all known mammals are nearing extinction, with large species at greatest risk, according to a recent assessment of the conservation status of 5,487 mammals.
Expanding agriculture and hunting are the primary extinction drivers, according to the findings, published in the latest Philosophical Transactions of the Royal Society B. That means humans causing the most severe mammal extinction period in history.
"The example I often tend to bring up is Tasmanian Devil, familiar to many from the Looney Tunes cartoons, because it's an example of how a species that is common, or at least not uncommon, can suddenly, through the emergence of a novel threat, be plunged into a steep decline," lead author Michael Hoffmann told Discovery News, explaining that a relatively new cancer, Devil Facial Tumor Disease, is wiping out this particular mammal.
Hoffmann, a researcher for the International Union for Conservation of Nature, and his team analyzed data gathered for IUCN's Red List Index. It covered the period from 1996, when all species were first completely assessed, until 2008, when they were reassessed.
The researchers documented changes to extinction risk, where species are moving most rapidly toward zero population. Numerous mammals in the wild are in decline, so this study only highlighted the most severe cases. Many are large animals, such as rhinos, tapirs, elephants, dugongs and manatees. Two species of antelopes in desert regions of North Africa are additionally in dire conservation status now: the Dama Gazelle and the Addax. Only a few hundred of each antelope are left, due to non-stop hunting.
"Large size has often been linked to elevated risk of extinction in mammals because, among other reasons, large mammals tend to occur at lower densities, have a slower reproductive rate, and are more likely to be vulnerable to hunting, by-catch and other forms of exploitation," Hoffmann explained.
In addition to size, location is also a predictor of a mammal's likelihood for near-extinction status. Southeast Asia, for example, is mentioned as a place where over-exploitation is rampant.
Population drops are occurring so rapidly that since 2008, certain mammals may have actually gone extinct. One is the Christmas Island Pipistrelle "As recently as January 2009, there were thought to be as few as 20 remaining individuals," Hoffmann said. "In August 2009, authorities returned to the island to capture some of the remaining individuals for captive breeding, but only a single individual was detected and it evaded capture before disappearing entirely."
Bats in general are now in trouble. The deadly disease White-Nose Syndrome, which was only first recorded in 2006, has killed countless hibernating bats in the eastern U.S.
Despite the bad news, there were some glimmers of hope. Over the study period, conservation efforts resulted in population increases for 24 mammal species.
"The two major success stories to my mind are the Black-Footed Ferret (U.S. and Canada) and Przewalski's Horse (Mongolia), which both were considered "extinct in the wild" until just a few decades ago," Hoffmann said. "Now, largely through captive breeding and reintroduction efforts, they have seen dramatic recoveries in their populations."
He adds that yet another "remarkable example" of recovery is the Golden Lion Tamarin from Brazil. Its numbers have doubled, also due to reintroduction efforts.
Brett Scheffers, a researcher in the Department of Biological Sciences at the National University of Singapore, led another study focusing on mammals, as well as birds and amphibians. His team determined that over the past 122 years, at least 351 species thought to have gone extinct have been rediscovered.
There is always joy with each rediscovery, but also a harsh reality.
"Rediscoveries, without aggressive conservation, likely represent the delayed extinction of doomed species and not the return of viable populations," Scheffers said. "In short, there is hope, but we must step up rapid conservation efforts."
Hoffmann agrees: "What we need is to rapidly ramp up efforts, and to work in a more strategic, coordinated and smarter way than we have up until now."