Europe's largest mammal, the European bison, remains extremely vulnerable to extinction, despite long-standing efforts to save it, new research shows.
One of the two remaining wild herds of pure bred European bison is down to an effective population size of just 25.
That is despite the actual number of wild bison in the herd having steadily risen to around 800.The effective population measures the bison's genetic diversity, and can help predict the animal's survival chances.
At 3m long, 2m tall and weighing up to 900kg, the European bison ( Bison bonasus ) is Europe's heaviest surviving land mammal.
It survives in the wild in just two herds, each living on either side of the Bialowieza forest which straddles Belarus and Poland.
While European bison can interbreed with American bison (Bison bison), they are generally considered to be separate species, having considerable genetic and morphological differences.
However, the species has a tortured history.
For hundreds of years, the European bison was protected across large parts of its European range, being considered 'King's game' protected by the monarchy and Russian tsars that conquered Poland.
But early in the 20th Century, its numbers crashed as people left hungry by World War I and a lack of protection saw ruthless poaching of the animals for meat and hide.
By 1919, none were left in the wild.
Back from the dead
"So in the 1920s, biologists decided to reconstruct the population out of the few individuals left in the public and private collections and zoological gardens," says Malgorzata Tokarska of the Mammal Research Institute of the Polish Academy of Sciences, in Bialowieza, Poland.
Of 54 European bison left in the world at the time, just four bulls and three cows went on to found the surviving pure-bred population. Of those alive today, all originate from just from just one bull, with 90% of all their genes coming from two founders.
Today's population stands at around 1400 spread all over the world. The only wild animals live in the Bialowieza forest, with around 400 individuals on either side of the border.
To quantify the impact of the 20th Century bottleneck, Tokarska and colleagues Agata Kawalko, Jan Wojcik and Cino Pertoldi genetically sampled 178 individuals at 12 different points in their genome.
By testing the skulls and frozen soft tissues of long-dead animals, as well as live animals, the team could analyse bison born each decade from 1950 onwards.
Despite the growth in actual bison numbers, the genetic tests have revealed that the herd contains an effective population of just 25 animals, the team report in the Biological Journal of the Linnaean Society.
The effective population of any group of animals will be lower than the number that actually exist, due to factors such as non-breeding individuals or a skewed sex ratio.
But it's clear that the European bison has not recovered from the genetic bottleneck it suffered during the early 20th Century.
Usually, a population of animals must have an effective genetic population of 50 animals to be considered to be safe from an immediate threat of extinction caused by the dangerous effects of inbreeding or having too few genes to adapt to new environments.
"They are highly inbred and closely related and the genetic surveys confirmed that," says Tokarska.
"We could pretend that we have a big plan, but honestly, there's not much we can do. We can not enrich the genetics using breeding methods, since there are no out-bred animals. They all come from the same seven founders."
"Mostly, we can work on maintaining the bison-friendly environment and widen it," she says.
Tokarska is also just beginning to analyse the remaining Belarus bison.
Though they originate from the same animals, the other herd does have some slight genetic differences.
If they are confirmed it may be possible to bring bison together from the two herds to further improve the species' survival chances, she says.