Beavers in Spain – Castores en España 2

December 8th, 2007 | by nick |

Following on from the Beavers in Spain news, European beaver expert Dr Duncan Halley of the Norsk Institutt for Naturforskning has very kindly written this facinating piece for Iberianature on the Navarran beavers.

“The reasoning of the Spanish authorities, that allowing illegal reintroductions to take place sets a bad precedent, is quite understandable. Conservationists often complain when others take the law into their own hands, e.g. by illegal shooting of protected species; the same principle applies here.

In 2005 I was contacted by scientists working for the Navarrese regional government, who sent some pictures of unmistakable beaver signs they had come across while working on European mink. I later visited the site, and saw from a video that the animals are definitely breeding. The exact site I saw was the most favourable for beavers I think I have ever seen, oxbows off a main river with thick, shrubby growth of poplars (easily the beaver’s favourite tree genus), and lots of herbaceous growth as well. Combined with the very benign climate and the very large amount of suitable habitat available on the Ebro system, I have no doubt the species, left to itself, will become firmly established. Unmanaged the river system could support a large population, probably over a thousand individuals. After at least four breeding seasons, the population, allowing for natural increase, is probably well in excess of 50.

As regards the technical aspects of a removal effort, it’s worth recalling how extinction occurred. Beavers were hugely reduced in numbers in most places by mediaeval times, but in most of Europe, including Spain, small populations hung on for well over 500 years until the 17th-19th centuries, the 17th century seeing the arrival of accurate firearms and efficient steel traps. These last beavers tended to be in the best beaver habitat, marshes or large rivers with plenty of cover and no need to dam, or even fell trees much; beavers were therefore hard to find there. The conditions for final extinction also included the very high value of beaver fur combined with the value of the castoreum gland (medicine, perfume), literally worth more than its weight in gold. One beaver was worth more than a Norwegian farmhand’s annual wage in the 18th century, to give perspective. Every man’s hand was turned against them, and by the C17th most people could get hold of a gun or a trap if a beaver was noticed.

Under modern conditions, to remove beavers from a watershed as large as the Ebro requires a professional effort, as the animals no longer have significant commercial value (except for sport hunting in some places). In the case of the Ebro, there are two factors further complicating a removal effort. At low populations beavers typically select strongly the very best habitat patches, living in burrows and felling few trees; in areas with much bushy growth by the waterside, nothing larger than twigs. This will make finding all the sites difficult in a big river system with so much suitable habitat. Interpretation of signs can also be problematic: are they caused by an established family or just a passer by? All this takes time, and so money. Secondly, at least four breeding seasons have occurred since reintroduction. At low population densities, beavers disperse as yearlings and very much evidence shows they will travel long distances – many cases of over 50km are known – to find the very best habitat rather than settle close to their natal territory.

It seems the trapping will be carried out in La Rioja and Navarra only; however, the Ebro runs into Aragon just downstream from the known reintroduction area, the north bank is in the Basque Country not far above it, and the river runs on into Castile y Leon another 20km or so upstream. All these are well within the recorded dispersal distance of beavers in unsaturated habitats, and it is highly probable that some beavers are established there by now. A trapping effort confined to Navarra and La Rioja with therefore probably leave breeding groups both up- and downstream, from which young animals can redisperse into those provinces.

It appears that live trapping will be the preferred method. There are two effective methods, dazzle netting using a trained trapping crew operating at night, effective but expensive given the manpower requirements; or Hancock traps, which are set partly in the water on the riverbank, and baited with territorial scent or poplar twigs. When triggered, they scoop the animal up from below so that it ends up sitting in a basket above water level.

To summarise, an effective removal is technically feasible. However, it will have to carefully check many tens of km of riverbank for beaver signs, a considerable way up and downstream from the known reintroduction area, including stretches beyond Navarra and La Rioja, determine where the settled groups are, and trap them all. The Ebro is a particularly difficult river system for the purpose, being so large and containing so much good habitat; the resources put into the effort would have to be commensurate with this. If the effort is confined to Navarra and La Rioja, the prospects of complete success depend on whether beavers are established elsewhere on the river. Given their known behaviour and the local political geography, this must be regarded as highly probable.”

By Dr Duncan Halley, Norsk Institutt for Naturforskning

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