“The most wonderful thing about Tiggers is I’m the only one,” boasts AA Milne’s exuberant creation. But Hundred Acre Wood is the only habitat where devastatingly low population figures are a subject of joy and mirth. For species like the northern white rhino, with only five individuals left in the world, the struggle for survival is no laughing matter.
Smaller populations are more vulnerable to environmental catastrophes, the negative consequences of inbreeding and even sheer bad luck, such as randomly skewed sex ratios. “If there are very few of you and you all have offspring, there is a good chance that maybe all those offspring will be one sex,” explains Dr Philip Stephens from Durham University. Lifespan, rate of reproduction, area of occupation and genetic history also contribute to the outlook, as does the presence of existing threats and, crucially, the rate of decline. And while the factors that push species to the brink are myriad, our fingerprints are everywhere.
“Human impacts in the last century or so have intensified and pushed natural systems to isolated small areas and otherwise affected natural systems and their functioning,” says professor of ecology H Resit Akçakaya at Stony Brook University, New York.
But just how low can numbers go before a population is doomed? One approach is to evaluate the minimum viable population (MVP), a parameter often described as the smallest number of individuals needed for a population to have a particular probability of being alive and kicking in a chosen time frame (say 99% and 1,000 years). “It is certainly useful in theory and in guiding our theoretical understanding of population risk,” says Stephens.
But MVP and the figure varies between species and populations and can be estimated in several ways. “Two populations of the same size, one growing or stable and one declining rapidly, clearly are not in the same boat at all,” he adds. “So we need to be wary about judging things exclusively by the size of population.”
Indeed the role of MVPs in shaping practical conservation efforts is much debated while Akçakaya says attempts to make broad-brush estimates of MVPs could undermine conservation efforts and lead to species being written off. “If you generalise for lots of species and say such and such is the minimum number for an animal population, you risk pushing [species with very small populations] to extinction because you don’t bother to protect them.” Discerning and removing threats to a population should be the top priority, he says.
Yet, while safeguarding the future of a population becomes increasingly challenging as numbers fall, conservation know-how, political will and good governance can join forces to avert disaster. Siberian tigers numbered around 30 in the 1930s and have grown to number around 450 while the northern elephant seal, nearly driven to extinction in the 19th century as a result of hunting, now number hundreds of thousands of individuals. Indeed, as Dr Samuel Turvey, senior research fellow at the Zoological Society of London (ZSL), points out, some species – such as the black robin of the Chatham Islands, off the coast of New Zealand – have even bounced back from just one breeding pair.
“One of the factors that really influences if that kind of conservation is effective or not isn’t any kind of scientific theory,” he says. “It is basically if you have got a really bloody-minded individual determined to save it, who cuts through the politics and red tape.”
Turvey remains optimistic that the Hainan gibbon – down to 25 individuals largely due to habitat loss and hunting – will be saved by both reducing threats and encouraging numbers to rise. Plans drawn up by an international team and led by ZSL will employ both forest corridors and artificial canopy bridges to connect their fragmented habitat in the rainforests of Hainan Island, south China. “They can disperse out across a wider area and then there will presumably be more room for more social groups to form and the population to grow,” says Turvey. But while the Hainan gibbon may yet bounce back, Tigger is undeniably doomed. As Turvey says: “If you have one individual, then it’s too late.”