March of the Beavers

When most people think of "rodents", they most likely think of rats or mice. This is fair enough, as the great majority of rodent species pretty much fit that description. But there are, of course, many others. Voles, hamsters, gerbils, and so on are probably fairly obvious, and somebody once asked me if squirrels were "members of the vermin family", by which I assume they meant "rodents" - which they certainly are. One suspects, however, that beavers might come fairly low on most people's list.

Having said which, it's probably not so surprising when you think about it. They do, after all, have huge gnawing teeth that look like those of rats (and which, indeed, help define the rodents as a group). Granted, they are pretty enormous as rodents go, and bulky with flat, paddle-shaped tails, but rodents they are, with animals like gophers and kangaroo rats among their closest relatives.

The beaver family, ignoring the extinct forms, contains a grand total of two species, both of which are found in Europe. One of them is the animal familiar to Americans (Castor canadensis). Descended from a population that originally lived in Wyoming and Canada, these were shipped across to Finland in 1937, and subsequently spread across the country, crossing the eastern border into Russia in 1952. A year later, some of them were moved again, this time to Austria, and in the 1970s, some were shipped all the way to eastern Siberia. All of these populations survive, although attempts to introduce the animals to France, Ukraine, and Belarus all ultimately failed.

The other species, however, is the Eurasian beaver (Castor fiber), which is the native, home-grown sort. Physically, it's almost exactly the same, although the tail and snout are both a bit narrower, and it tends, on average, to be ever-so-slightly paler. In fact, the people who brought American beavers to Finland probably didn't even realise they were a different species from the local sort, just a subspecies. As it turns out, though, the two can't interbreed, and they have different numbers of chromosomes, so any young would be sterile even if they could.

Given their similarity, though, and the fact that they're equally useful for making fur hats out of, or whatever else one might want to do with them, why bother introducing the American sort to Finland in the first place? Quite simply because, at the time, the Eurasian beaver was virtually extinct.

They had once lived across almost the whole of Eurasia north of the tropics, from Britain to Siberia. But over a thousand years of hunting had taken their toll. It wasn't even just their fur, since the contents of their anal scent glands (marginally bigger in the Eurasian than in the American species) were also valuable. Indeed, we still use this "castoreum" today to make perfumes, although at least we've now realised that it's of no use medicinally, as was once thought.

Or consider this passage from German zoologist Karl Vogt, writing in the 19th century:

"The beaver is fortunately quite eradicated all over cultivated Europe. No doubt some authors have set up sentimental lamentations over its extermination. However, it must be admitted that the beaver is one of the most harmful animals to the woods and we need indeed wood materials more than beaver skins and castoreum!"

So not much support there, then. It's thought that, by 1900, the worldwide population of Eurasian beavers numbered only about 1,200, and they had disappeared altogether from much of their original range. Those that did survive in Europe were found only in five isolated pockets in France, Germany, Norway, and the Pripet Marshes, and there were likely even less east of the Urals.

Since then, there has been a remarkable recovery. Conservation efforts have succeeded wildly, to the point that the species is now about as safe as it's possible to be in the modern world. Today, there are well over half a million Eurasian beavers alive - there are even some in western Finland, although the American sort are still by far the more common in that country. Since 2001, beavers have also been re-introduced to Britain, where they were last reported in the wild in 1525, when Henry VIII was still on the throne.

That represents a population increase of about 500-fold over the course of a century. Which is pretty good going - and it isn't really over yet, with beavers continuing to advance into new territory. This gives us an opportunity to see how a successful species goes about colonising previously empty land, as we follow the expansion of the beavers across Europe.

A recently published study looked back over decades of reports to see how beavers had colonised Western Bohemia in the Czech Republic, an area devoid of the animals for much of the 20th century. One feature of beaver expansion that makes this easier than it might be for some other re-introduced animals is that, whereas most animals will spread out in all directions from wherever they started, beavers are largely restricted to just two. That is, they can either go upstream, or downstream, but, either way, they're going to be following the rivers. So, to watch their progress, we can do the same.

In fact, beavers were never deliberately introduced into Western Bohemia at all. The most likely source for the current population there is a group of re-introductions along the River Danube in Bavaria, Germany, starting in 1965. From there, they would have had to have travelled up the Naab and Regen Rivers, tributaries of the Danube, to head towards the Czech border. It's an area of scattered farmland and forest, almost entirely devoid of things like wolves, and therefore ideal for beavers. So, to start with, all would have gone well.

But then, the beavers would have hit a snag. In this region, the German/Czech border is essentially a watershed, with German rivers flowing to the west down towards the Danube, while the rivers on the other side of the border flow east, towards the Elbe. In other words, at some point, they ran out of river.

Beavers understandably don't like travelling away from rivers - there's nowhere comfortable for them, and they have difficulty hiding if they do come across a predator. But they will if they must, and, in this case, they evidently did. Braving the wild hills between the two river basins, a few adventurous (or desperate) beavers must have crossed the watershed and travelled over the border into Bohemia. It did, however, take them a while, with the first report of beavers in the region dating from 1980. Notably, this was in a stream not far from the border itself, and quite close to its source.

For ten years, very little happened. In the early '90s, a few more beaver territories were spotted, still fairly close to the border. At this time, population expansion, such as it was, seems to have been slow and haphazard. This likely represents a problem that the first pioneers of a new land will have. The good news is that they have the pick of the landscape; they can settle down wherever they want, choosing the very best habitats for themselves. Which probably feels great, until they realise that there isn't anyone else to mate with.

Young beavers typically leave home at the age of two years, by which time they are more or less adults. They won't travel more than three or four miles if they don't have to, before finding a place to settle down. But when there are few, if any, unattached beavers of the opposite sex around, that really doesn't help much. To begin with, then, the advantages of greater freedom are offset by the inability to breed and establish any long-term territory.

But then, at some point, essentially down to sheer random luck, the population crosses a threshold, perhaps due to the arrival of sufficient newcomers (in this case from Germany). Until 1995, there were only two beaver territories in Bohemia. In that year, one of those sites experienced a sudden burst of population growth, with new territories popping up along the river. From then on, expansion was rapid, with new rivers being colonised as well as new territories forming along the established ones. Each new river eventually had its own population explosion, followed by a rapid, and relatively predictable, rise as the area became more and more settled.

By around 2010, many of the rivers were supporting as many beavers as they could, and population growth slowed down again, or even stopped. Beavers leaving home were forced to accept relatively poor habitats to establish their dens, everything else already having been taken, and they weren't so successful as their parents had been. Some went further downstream, with more distant parts of the river system experiencing their own population explosions later.

The front of colonisation moved onward, generally towards the east, approaching the Elbe. But now it moved slowly, and steadily, the consequence of continuous population expansion, each new colony not wanting to be too far from its fellows, or risk running out of breeding partners.

The hardy pioneers were no longer needed, and the beavers marched across Bohemia.

[Photo by Bohuš Číčel, from Wikimedia Commons.]