Measures to enhance survival of calves, and particularly females, are key to saving the Asian elephant.
Judged by their numbers in the wild, endangered species like Asian elephants are doing poorly. Populations of the pachyderms have plummeted in recent decades in countries like Laos, Cambodia and Myanmar where they were once nearly ubiquitous.
Yet the longer-term prognosis for the survival of these large mammals in their shrinking habitats may well be even worse for a simple reason: they are slow-breeding animals. This means that even relatively minor changes in mating opportunities and offspring’s survival could deal a severe blow to entire herds.
That is why a combination of so-called key vital rates that influence population growth in large mammals is a better indicator of a species’ long-term viability in the wild than short-term trends in population size and distribution, say the authors of a study published in Frontiers in Ecology and Evolution.
“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,” stresses the paper’s lead author Dr. Shermin de Silva, president and founder of Trunks & Leaves, a nonprofit seeking to preserve Asian elephants in the wild.
“History bears this out,” de Silva adds, pointing to the extinction of the last remaining wholly mammoths of Eurasia as a result of human interference some 4,000 years ago. Mammoths on the isolated island survived several thousand years past the time their relatives on the mainland had died out. Yet they, too, eventually succumbed, probably because of intense inbreeding that weakened the genetic health of the local population, causing harmful mutations.
“Genomic studies of the last mammoths isolated on Wrangel Island – between Russia and Alaska – have shown that although they were able to persist for thousands of years beyond the extinction of mainland populations with just ~300 individuals, they had accumulated numerous genetic mutations that may have eventually contributed to their extinction,” de Silva notes.
The same could happen to Asian elephants, only around 40,000 to 50,000 of which persist in diminishing habitats in the wild. They have been largely fragmented into smaller herds whose members could end up losing their biological viability decades before they actually go extinct if the pachyderms are pushed beyond their “demographic safe space.”
The pachyderms are like humans in that routinely they bear only a single offspring, which then needs years of devoted care by its mother to reach a level of maturity when it can survive on its own. Most Asian elephant mothers produce one calf every six years or so.
De Silva and his colleagues found that it will take near-optimal reproduction rates as well as high rates of calf survival for the species to maintain its current population number even if only a relatively low number of adult females, which can live up to 60 years, were to die .
“Measures to enhance survival of calves, and particularly females, are key to saving the Asian elephant,” de Silva says. “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 [the] illegal trade in live animals and parts,” he adds.
“Habitat loss can create something known as ‘extinction debt’ by slowing down birth rates and increasing mortality rates,” he explains. “For slow breeding long-lived species, even incremental changes make a big difference, but their longevity can obscure the risk of extinction.”