Beavering away

The American cousins

There are two species of beaver: the North American Castor canadensis and the European Castor fiber. The North American beaver, although still fairly numerous, is now down to about 10% of the numbers pre-hunting. The species builds much more extensive dams than the European beaver and the largest recorded was picked up in a satellite image – it is 2790 ft (850m) long. There are some anatomical differences between the two species. They are very unlikely to interbreed successfully as Castor fiber has 48 chromosomes but Castor canadensis has only 40.

What went wrong?

Beavers lived in Scotland until the 16th century and had been widely distributed throughout the British Isles in earlier centuries. There is evidence of dam remnants and beaver bones in past wetland areas of the landscape. But why did the beaver disappear?

A number of reasons have been put forward and all involve human activity – the most obvious being hunting for pelts, although habitat destruction is also a major factor. Beavers were also a source of medicine. The willow trees that are their favourite food contain salicylic acid (similar to aspirin) and this reappears in the liquid produced by the castor sacs near the base of the beaver’s tail. The fluid is called castoreum and was used to treat a range of diseases. It has also been sold as an aphrodisiac. Curiously, it was possible to eat beaver flesh on a Friday as, in the 18th century, the Catholic Church had ruled that beavers were fish, but by then beavers had gone from the British Isles, although they were still found in Europe.

The dam-builders

Beavers are a ‘key-stone’ species because they play a major role in supporting the habitat they occupy, through their engineering efforts. The dams that they build produce additional areas of water and wetland, increasing the local biodiversity. There is evidence that the ponds they create are beneficial for fish populations. The trees that beavers fell usually sprout new shoots in the same way as trees do when they are coppiced. However, not all trees can be coppiced successfully. When beavers were introduced to Tierra del Fuego, South America, the tree species there did not grow new shoots and so the beavers became agents of deforestation. This is not a great risk in Europe.

David Robinson

Spotlight on... beavers