Introduction

Formerly widespread throughout much of the Palaearctic region, Eurasian beaver Castor fiber populations were reduced through overhunting to c. 1200 animals, in eight isolated populations, by around the end of the 19th Century (Figures 1 & 2). Since the 1920s, effective protection of these remnants, the resultant natural spread, and widespread reintroductions (at least 154 to 16 countries, not including the former Soviet Union), have led to a powerful recovery in both range and population (Figures 1 & 2; Table 1).

Population status

The minimum population estimate in 2003 is 613,000 individuals (Table 1). There are also c. 12,500 North American beaver C. canadensis established in Finland and Russian Karelia; other populations of canadensis introduced to Austria, Poland and France appear to be extinct.

Free-living populations are now established in most European countries of the former natural range, excluding Britain, Portugal, Italy, and the S. Balkans (Table 1). Habitat occupied ranges from wilderness areas to intensively managed landscapes with dense human populations, and from warm temperate to subarctic climates. Reintroductions are continuing. Considerable further expansion of both range and population, especially in western Europe and the lower Danube basin, can be expected. If current trends continue, C. fiber will within a few decades be a fairly common mammal in much of Europe.

Figure 1. Distribution of beavers in Europe, excluding Russia. Locations of relict populations are marked in black: 1 Castor fiber fiber; 2 C. f. albicus; 3 C. f. galliae; 4 C. f. belarusicus. Darker shading represents the present range of C. fiber; lighter shading represents the range of C. canadensis in Finland. Squares are reintroduction sites where range has not yet spread significantly; crosses represent planned reintroductions, with date where known.

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Figure 2. Distribution of beavers in European Russia, western Siberia, Kazakhstan, Mongolia and Xinjiang (China). Locations of relict populations are marked in black: 4 Castor fiber belarusicus; 5 C. f. osteuropaeus; 6 C. f. pohlei; 7 C. f. tuvinicus; 8 C. f. biruli. Darker shading represents the present range of C. fiber; lighter shading represents the range of C. canadensis in Russian Karelia and adjacent regions of Finland. Range in eastern Siberia is little known, although (re)introduced populations of both C. fiber and C. canadensis are established in Kamchatka and on the Amur. Map updated from Savalyev (pers. comm.) and Nolet & Rosell (1998). 

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Table 1. The history and present status of Eurasian beavers (Castor fiber) by country.

1No data. Beavers have probably immigrated from Croatia along the Sava, where beaver are present to the Bosnian border. 2Based on subfossil remains. Philological evidence from placenames suggests a remnant may have survived as late as the 11th century. 3Natural spread from Belgium. 4In final planning stages. 5Natural spread from Croatia.

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Patterns of spread and population development

Following initial establishment on a watershed, populations typically show a pattern of rapid range extension (Figure 3), followed only later by rapid population growth (Figure 4). There is strong selection for the highest quality habitat at this stage, which appears to explain the rapidity of range extension. Infilling occurs thereafter, initially at quite low densities in relatively high quality habitat, but culminating in exponential population growth as relatively poorer quality habitat is taken into use, and densities reach levels where encountering a potential mate is highly probable. Later, on average 34 years after colonisation of a district (see Figure 4), populations go into decline as marginal habitat is occupied and then exhausted (Figure 5). The period of peak and declining populations often coincides with a peak in conflicts with human land use interests, as marginal habitats usually require more extensive modification by beavers.

Figure 3. Patterns of spread of beavers recolonising Värmland province, Sweden. Terrain is flat to mildly hilly and heavily wooded. Watershed divides are shown by bold lines. Dates and locations of reintroductions are indicated. Beaver spread very rapidly throughout watersheds after initial recolonisation, with infilling thereafter. Watershed divides, however, significantly slowed range expansion. Figure adapted from Hartman (1994).

Figure 4. Comparison of rate of spread and rate of population increase in Värmland, Sweden. 'Areas' colonized are Swedish Game Administration hunting districts (c. 10-20 sq. km. in extent). c.85% of the population increase occurred in the final 10 years to 1986. Figure adapted from Hartman (1994).

Barriers to spread

Even in areas with short overland distances between suitable habitats and favourable terrain, watershed divides form a clear barrier to population spread (Figures 3 & 5). Where a significant physical barrier between suitable habitat on different watersheds exists, such as mountains (e.g. Switzerland) or intensive farmland (e.g. Elez River, Brittany), it may be strongly isolating.

Man-made dams and barrages have also been shown to constrain or prevent population spread within a river system, e.g. on the Rhône and Danube (Figure 6). Solutions include the construction of "beaver ladders" around barrages.

Figure 5. Changes in local beaver population densities in Värmland, Sweden, related to time since colonization of the hunting district. Population growth turned negative (i.e. populations began to decline) on average 34 years after initial colonization. Figure adapted from Hartman (1994).

Figure 6. Distribution of beavers in Trondheim Byneset in 2003. Filled black circles represent the centres of active home ranges; hollow circles indicate abandoned home ranges. Beaver were reintroduced in 1975 to Theisendammen (top right filled circle) and by 1998 had colonised three of the six major stream systems. Each of the three largest uncolonised stream systems contain sufficient habitat for several beaver colonies, but as yet remain unoccupied, although clearly marginal sites on the other systems are in use or have been abandoned.

A note on beavers and angling

There is very little concern among anglers in areas of Europe with established beaver populations, regarding the effect of beaver dams on migratory fish. This includes Norway, where beaver share the same watersheds with important salmon sport fishing industries. Evidence from Norway indicates that salmon can pass a series of beaver dams, up to 1.6m high, to spawn (Halley & Lamberg 2001; Figure 7). In addition, beaver ponds are generally regarded as good fishing spots for trout and coarse fish.

Figure 7. Populations of juvenile salmon and brown/sea trout in Litlelva, Aursundet, Norway in 2000, in relation to beaver damming. Litlelva is a typical shallow, fast flowing spawning stream 1-3m wide. There are 4 beaver dams on the stream, 1m, 0.5m, 0.5m and 1.6m high (in succession upstream), over a distance of 600m. Fish populations were sampled by electrofishing at three sites, below all dams (bel); between dams 3 & 4 (btw); and above the upstream (and highest) dam (abv). 0+ and 1+ juveniles (ie, hatched in 2000 and 1999 from eggs laid in 1999 and 1998, respectively) of both salmon (light grey) and trout (dark grey) were found at all sites. See Halley & Lamberg (2001) for a fuller account.

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Age and sampling site

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Management implications

Beaver populations and distribution should normally be managed at the watershed scale. A major exception is that sections of watersheds may be manageable in isolation where man-made dams act as barriers to beaver dispersal. Most benefits for, and conflicts with, human land use goals will occur later in the process of population development.

Populations can be regulated, and conflicts with human land uses minimised, through the introduction of a controlled harvest (e.g. through sport hunting). This should be introduced during the rapid increase phase of population growth. Alternatively, or in addition, amelioration measures such as dam drainage pipes and wire netting around the boles of trees, are effective.

Early provision of interpretation and public viewing opportunities has been a feature of many recent