Wildlife conservation: Tigger is doomed, but can other species such as the northern white rhino escape extinction?

Conservation know-how and political will can help animals battling the threat of dwindling populations to survive

Nicola Davis

Saturday 13 June 2015 07.59 BSTLast modified on Sunday 14 June 201500.10 BST

“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.

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