The Beaver’s Favourite Tree?
Beavers, as most people realise, are herbivores: they feed on vegetation not flesh. It is surprising how many people think that beavers eat fish: surprising, but perhaps not that surprising, considering how many people were brought up on the Narnia books by CS Lewis. Readers who remember the opening chapters of 'The Lion, the Witch and the Wardrobe' will remember how the children, fleeing from the White Witch, are rescued by the Beavers. Mr Beaver goes to catch some fish with which Mrs Beaver will make a pie for the children's tea. If CS Lewis had got his natural history right the children would have been fed on a pie of iris and bog bean rhizomes.
So far as their nutritional needs go, beavers rely on bark from trees and the rhizomes of water plants for the winter. Their tastes shift to grasses as the growing season gets under way, and then the whole gamut of herbaceous plants, especially the foliage of aquatic plants, later in the summer. Beavers are known to eat around three hundred different species of plant: they are choosy generalists.These are tendencies, for beavers will continue to fell trees and eat their bark to some extent in the summer as well as through the rest of the year. It all depends on abundance and availability.
As to the beaver’s favourite tree, this is not as straightforward as may seem at first because, like humans, beavers develop local preferences and, if moved to a place where the woodland is made up differently, may reject species that are strange to them. They are a little like those people from this country who go abroad and will eat only fish and chips, or burgers.
Taking all that into account, however, there are some broad preferences. Aspen (Populus tremula) is a favourite with beaver. It is said that beavers will walk a couple of hundred metres from the water’s edge if they can scent aspen, but only up to fifty metres for willow, or birch, important staples of their diet. Here, in Perthshire, beavers eat the bark, twigs and foliage of birch, beech, sycamore, rowan, gean, ash, willows, hazel. I have seen the bark of oak consumed, but I suspect that they are put of by that genus’s bitter tannins. They consume the bark of grey alder (Alnus incana), if necessary, but that species is lower in the order of preference in comparison with the others I have mentioned. Black alder (Alnus glutinosa), is probably less popular than grey from the point of view of consumption, but these species are felled for use in the building of dams and lodges. Rhododendron ponticum is cut for repair work, though not consumed: beavers sensibly avoid eating this poisonous species, just as they avoid elder, whose bark contains cyanide. The young shoots of coppice regrowth in aspen, are protected from beavers for their first eight years by their bitter bark. This is the case, too, with willow shoots. Sometimes, however, beavers will cut the shoots of willow and leave them to steep in water before returning to strip the bark. A lot is going on in the never ending war of survival between beavers (and other plant eaters) and the plants on which they prey. The one develops chemical defences to avoid being eaten and the other evolves enzymes that will breakdown these toxins or, as in the case of the beaver, will evolve means of subverting the poisons and putting them to their own use.
Conifers are not popular with beavers in general, but they will consume the foliage of conifers in spring before deciduous trees come into leaf. Beavers will also strip the bark of Norway spruce, Scots pine, Douglas fir, sometimes in the spring for nutritional reasons, but also to kill the trees to remove shade. The removal of shade may be to reduce hiding places for predators, or to promote conditions in which light demanding trees and shrubs can grow. It has been suggested that a third reason for barking conifers may have to do with nutritional cravings in pregnant females.
How far from the water's edge do beavers go in search of their favourite tree? As I have written earlier in this piece, it is said that they will go a couple of hundred metres for aspen and only fifty for willow. Once again it all depends on location. Beavers in the Rhône valley of France spend 65% of their time within five metres of the water's edge according to René Nozerand of the ONCFS ( Office National de la Chasse et de la Faune Sauvage). In Norway, on the other hand, beavers spend 90% of their time within 5 metres of the water's edge according to Dr. Duncan Halley.
There you have it. There is an outright scale of preferences, but location and availability are what call the shots, and nutritional value and the trees’ chemical defences also come into the question of assessing the beaver’s favourite tree.
12 Mar 14
Photograph courtesy of Ray Scott