Conservation decisions based on population counts may fail to protect large, slow-breeding animals from irrevocable decline
To better protect larger, slow-breeding species, conservationists, biologists and other decision makers rethink the "endangered species" definition, the authors of a new study suggest.
Researchers warn that slow-breeding giants, like elephants and rhinos, might not reveal themselves as "endangered" until it is too late. A slow decline among a population of slow-breeders can, in some cases, be more worrisome than a more precipitous decline among fast-breeders.
To account for this, scientists suggest conservationists pay less attention to the size and distribution of a population, or the speed of its decline, and focus instead on the relationship between mortality and fertility rates.
"Critical thresholds in so-called vital rates -- such as mortality and fertility rates among males and females of various ages -- can signal an approaching population collapse long before numbers drop below a point of no return," Shermin de Silva, founder of the Asian elephant conservation charity Trunks and Leaves, said in a news release.
"We propose that conservation efforts for Asian elephants and other slow-breeding megafauna be aimed at maintaining their 'demographic safe space': that is, the combination of key vital rates that supports a non-negative growth rate."
The Woolly Mammoth
Previous studies suggest small populations of woolly mammoth, which disappeared from continental mainlands some 10,000 years ago, persisted for a few more thousand years on islands in between Russia and Alaska.
One of those populations persisted on Wrangel Island in the Arctic Ocean until about 3,700 years ago. According to genomic analysis, these megafauna holdouts suffered from a mutational meltdown.
The fate of the inbreeding mammoth, according to de Silva and his colleagues, serves a reminder that large, slow-breeding species can become doomed long before they actually disappear.
In other words populations of megafauna can become biologically inviable long before they disappear, if pushed beyond their ‘demographic safe space.’
The new research -- published this week in the journal Frontiers in Ecology and Evolution -- suggests a greater emphasis on vital rates could boost the effectiveness of conservation efforts.
The group applied the ‘demographic safe space’ concept to the case of the Asian elephant.
“Asian elephants are classified as ‘Endangered’ under the IUCN Red List because populations are thought to have declined by at least 50% in less than a century,” explains de Silva. “There are fewer than 50,000 wild Asian elephants living today.”
De Silva and his colleagues used models to identify which conservation strategies would be most beneficial to slow-breeding Asian elephants. Their simulations showed Asian elephants would benefit most from a boost their reproductive rates. Without more calves, reductions in mortality rates will be for not.
"Measures to enhance survival of calves, and particularly females, are key to saving the Asian elephant," said de Silva.
"But while the attention of the world has been focused on the ivory trade, for critically endangered Asian elephant populations the greatest threat is habitat loss -- followed by illegal trade in live animals and parts."
"Habitat loss can create something known as 'extinction debt' by slowing down birth rates and increasing mortality rates," de Silva said. "For slow breeding long-lived species, even incremental changes make a big difference, but their longevity can obscure the risk of extinction."
Asian elephants aren't the only slow-breeders in need of more strategic protections. Giraffes, rhinos, Bactrian camels and eastern gorillas would all benefit from an emphasis on vital rates.
"Rather than rely on simple population counts or estimates of near-term extinction probability, we urge that conservation resources for slow-breeding megafauna also be invested in identifying demographic tipping points and how to maintain populations within their safe spaces," de Silva said.
"Populations of slow-breeding taxa need proactive management well before numbers become critically low, when returns on investment are potentially greater and populations less likely committed to extinction."