The European bison, the largest herbivore in Europe, went extinct in the wild in the early 20th century due to habitat degradation and fragmentation, forest logging and unlimited hunting and poaching. The bison that survive today are all descendants of around 50 bison from Bialowieza in Poland that were held in zoos. 

Bison have now been reintroduced into conservation areas in France, Denmark, Lithuania, the Czech Republic, Germany, Spain and the Netherlands. They are broadly considered to be a keystone species with huge potential benefits for conservation projects. In the Kransvlaak, in the Netherlands, for example, in an area of only 

There is an ongoing debate about whether bison were ever present in Britain after the last ice age. No bison bones have yet been found in the UK. But fossil evidence is notoriously difficult to come by. No fossil bones of the wolf, for example, have ever been found in the Netherlands though it was widespread there until only a few centuries ago, and the last one was shot in 1845. Indeed, fossil evidence is so rare that when it comes to light it often explodes all previous theories. A single accidental find of mammoth bones in 2009 in Condover in Shropshire moved the presence of mammoths in Britain closer to the present day by 7,000 years, to only 14,000 years ago. Absence of evidence, as the saying goes, is not evidence of absence.

Moreover, bison bones have recently been discovered in Doggerland under the North Sea dating to the beginning of the Holocene (our current post-Ice Age epoch which began around 11,700 years ago), along with remains of other Holocene fauna such as the aurochs, wild boar, elk, beaver, roe and otter. Doggerland was the land bridge that connected Britain to Europe until rising seas separated us 8,200 years ago. It is inconceivable that, when we were still physically part of the continent, animals tamely stopped at Calais. 

As the Kransvlaak reserve demonstrates, bison are not a significant threat to humans. But they don't like dogs, and the main objection to using them in conservation projects in the UK is likely to come from dog-walkers.