In July 2008, after reading the article by Malcolm Gray in MacLean’s Magazine: "Ivan, what's that chewing sound? Canadian beavers are chomping their way across Russia", I became interested to explore Canadian beaver habitat in Finland and the neigbouring Republic of Karelia.
The history of beaver in Europe is as interesting as it is in North America. There are many parallels: over hunting to satisfy demand for its fur in the European fashion market and its recovery through conservation, reintroduction and fortunately, disappearance of the market demand for its fur.
We know now that two different species of beaver are involved in this story, but in some of the early restoration initiatives in Europe, North American beaver (Castor canadensis) was thought to be genetically identical to the European beavers and were introduced in Sweden, Finland, and Karelia. They are now considered as invasive species in the areas originally inhabited by the Eurasian Beaver (Castor fiber). In areas where the two species occur together a struggle for dominance have seem to end in favor of the North American intruders. This struggle for dominance has been of much interest to scientists as well as newspaper media in Canada.
Google Earth has proven to be an effective tool in mapping and monitoring changes in beaver habitat in Canada and locating exceptional beaver communities, beaver landscapes and locating long beaver dams, and high density populations. The objective of this study is to see if Google Earth can be used in Europe to locate and monitor beaver activity in a similar way.
In addition it has been reported that North American beaver tend to build longer dams than their European hosts. This raises the question: Could the longest beaver dam in Europe be build by a Castor canadensis?
Figure 1: Oblique Google Earth Overview of Karelia with about 100 place marks showing possible beaver dams and other beaver impacts. The proximity to the the border with Finland reflects the migration of the Canadian Beaver from earlier reintroduction in Finland. Canadian Beaver around lake Onega were introduce in Karelia in 1964. Concentration of place marks are the result of availability of high resolution satellite images in Google Earth.
Figure 2. The longest dam found in this systematic Google Earth survey using high resolution images. The dam has a length of about 230 meters: Possibly the longest beaver dam in Karelia, perhaps the longest one in Europe. This dam is located North East of lake Onega.
Over hunting reduced Eurasian Beaver (Castor fiber) populations to c. 1200 animals, in eight isolated populations, around the end of the 19th century.  Protection, natural spread and reintroductions led to a powerful recovery in both range and populations during the 20th century, which continues at a rapid pace. The minimum population estimate is 593 000. They also reported that there are also c. 12'500 North American Beaver (C. canadensis) established in Finland and Russian Karelia; however, other populations of C. canadensis introduced in Austria, Poland and France appear to be extinct.
During 1935 and 1937, seven C. canadensis from the USA were released in Finland together with the 17 Scandinavian beavers. Descendants from the North American beavers at Sääminki were subsequently translocated to other places, including Lapland. At present, C. Canadensis numbers 3300-5200 in Finland (Lahti, 1995). North American beavers immigrated into Russia from Finland in the 1950s. This immigration was boosted with the release of six C. canadensis near lake Onega in 1964 (Safonov, 1975). In 1989 their number in Karelia was estimated at c. 2000 (Ermala et al., 1989).
In 2002, Geoffrey York reported in the Globe and Mail : "The Canadian beavers were introduced in the 1950s and 60s into Finland and Sweden, where no native beaver population existed. With no natural predators, they swiftly expanded their area. Beginning about 25 years ago, they spread from Finland into the northern Russian region of Karelia, where they continued to expand. Up to 20,000 Canadian beavers are believed to be thriving in northwestern Russia today, and scientists predict they will soon march further south, rudely shoving out European beavers as they go. The Canadian beavers seem to have more stamina and flexibility, they are more active and they can survive better.
One of the main differences between the two is that Canadian beavers build dams -- sometimes huge structures up to hundreds of meters in length -- while European beavers generally don't"
A 2008 article in Canada’s MacLean’s Magazine by Malcolm Gray[3)]drew again attention to the problems that the North American beaver are causing in Russia. To quote part of the article:
“Not only do the Canadian interlopers have sharper teeth and more energy, those eager beavers have better engineering skills than the Russian rodents they are displacing. They build bigger, higher dams — some as long as 100 m — reproduce more quickly than the locals, and have few natural predators to keep their numbers in check. Disgruntled officials grumble that beaver dams drown forested areas, killing commercially valuable trees, and complain that the animals' tree-felling activities often leave adjacent farmland vulnerable to erosion. As well, say the officials, the 20,000 Canuck beavers in northwestern Russia are a health hazard as their water-borne feces contains such diseases as giardiasis, or beaver fever, an intestinal illness spread by a microscopic parasite.”
The two newspaper articles inspired a closer look at beaver habitat in Finland an Karelia. Surely with the reported damage that beaver seem to have caused it should be possible to see this with Google Earth. At the same time it would be interesting to find out how the North American beaver was building its dams in Europe.
An early reconnaissance of Finland was not very promising. Although Harkonen (1999) reported a 2.2 ha average size of forest damage by Canadian beavers, very few beaver impacts areas could be found with Google Earth. The study also noted that most of the damage occurred in peat land forest.
Halley and Rosell provide a small scale map of beaver distribution in Eurasia . It shows the location of the aerial extent of the European beaver as well as the North American beaver. The most authoritative maps for the distribution of beaver around the world are provided by the IUCN Species Survival Commission through its 'Red List' web site. However, the Red List site does not provide distribution maps for invasive species, like the North American Beaver in Europe.
Durka et al. 2005  provide a map, based on Halley and Rosell, which shows a bit more detail related to the distribution of the American beaver in Finland and Russia. The map below superimposes their information, a blue overlay, on the IUCN the distribution of the Eurasian beaver map. The red striped areas show the Eurasian beaver distribution , the light blue the area shows the American beaver occurrence.
During 1935 and 1937, 7 C. canadensis from the USA were released in Finland together with the 17 Scandinavian beavers mentioned above. Descendants from the North American beavers at Sääminki were subsequently translocated to other places, including Lapland. In 1995, C. Canadensis numbers 3300-5200 in Finland (Lahti, 1995) . North American beavers immigrated into Russia from Finland in the 1950s. This immigration was boosted with the release of six C. canadensis near lake Onega in 1964 (Safonov, 1975). In 1989 their number in Karelia was estimated at c. 2000 (Ermala et al., 1989).
Nummi (2007) provides an overview factsheet of the American beaver as an invasive species. There is a possibility of competitive exclusion of Castor fiber by C. canadensis (Nummi 2001) due to higher reproductive output, since litter size is bigger in C. canadensis (Danilov 1995). Competitive displacement of C. canadensis by C. fiber can be seen in the South of the Russian side of Karelia (Данилов 2005). C. canadensis seems to be a little more active constructor of dams and lodges than the Eurasian species. Otherwise the ecological engineering by both species have a similar keystone effect on various plant and animal species, including fish, amphibians and birds (Rosell et al. 2005).
C. canadensis and C. fiber do not hybridize, due to the difference in chromosome numbers (Lavrov 1983).
Figure 3. This beaver dam of about 80 meters, is the most northern dam in this selection of Google Earth Place marks. It is fairly long, but not very high, constructed in a ribbed fen type of wetland, not a great source of food. The blue arrow shows the direction of water flow
Figure 4. Two beaver dams a few km south of Lendery, in the Muezersky region of Karelia. The surrounding landscape is heavily logged , some logging trails are visible on this image (LT). Place marks K4 and K5
Figure 5. A 70 meter long beaver dam about 11 km south of Lendery. Place mark K10
Figure 6. Series of six beaver dams in a typical wetland area south of Lenderly. The longest dam is about 80 meters.
West of Lake Ladoga
Figure 7. Small beaver dam (probably near arrow) blocking drainage flow and causing flooding of replanted trees. West of lake Ladoga (north of lahdenpohja)
West of Lake Onega
Figure 8. A typical example of a beaver dam in the heavily forested area south of Petrasavodsk. Blue arrow, direction of water flow, black arrow points to the beaver dam
Figure 9. This photo is accessible via Panoramio on Google Earth. Credit: Kemsky. On the photo the beaver felled trees are quite obvious, on the satellite image they are not visible. The photo was uploaded on Panoramio in May 2010, the satellite image is dated July 2004, so the beaver damage could not have been
North East of Lake Onega
Figure 10. Significant tree kill quite typical of the beaver dams on the north east side of Lake Onega
Figure 11. Significant tree kill quite typical of the beaver dams on the north east side of Lake Onega
2. York, Geoffrey. The Globe and Mail (Canada) January 22, 2002. Tough Canadians shut out feeble Russians. This winter, a cherished symbol of Canada is rolling through Europe with a vengeance
3. Gray, Malcolm, July 16 2008, MacLean’s Magazine. Ivan, what's that chewing sound? Canadian beavers are chomping their way across Russia.
4. Härkönen, S. Forest damage caused by the Canadian beaver (Castor canadensis) in South Savo, Finland. Silva Fennica 1999 Vol. 33 No. 4 pp. 247-259
5. Halley and Rossell 2008 Chicago Poster [+]
6. Durka et al. 2005 Mitochondrial phylogeography of the Eurasian beaver Castor fiber L. Molecular Ecology (2005) 14, 3843–3856
7.Lahti, S. (1995) Bäverns utbredning i Finland från 1980-talet fram till idag. In Proceedings of the 3rd Nordic Beaver Sym- posium, ed. A. Ermala and S. Lahti, pp. 41-43. Finnish Game and Fisheries Research Institute, Helsinki.
8. Ermala, A., Helminen, M. and Lahti, S. (1989) Some aspects of the occurrence, abundance and future of the Finnish beaver population. Suomen Riista 35, 108-118.
9. Safonov, V. G. (1975) Ergebnisse der Wiedereinbürgerung des Flussbibers (Castor fiber L.) in der UdSSR. Beitragezur Jagdund Wildforschung 9, 397-405.
10. Nummi, P. 2001. Alien species in Finland. – The Finnish Environment 466. Ministry of the Environment
11. Lahti, S. (1995) Bäverns utbredning i Finland från 1980-talet fram till idag. In Proceedings of the 3rd Nordic Beaver Sym- posium, ed. A. Ermala and S. Lahti, pp. 41-43. Finnish Game and Fisheries Research Institute, Helsinki.
12. Данилов П. И. 2005. Охотничьи звери Карелии. М.: Наука. 340 с. (Danilov P.I. Game animals of Karelia Moscow Nauka 2005, 340pp.
13. IUCN Red list; Castor Fiber: http://www.iucnredlist.org/apps/redlist/details/4007/0
14. IUCN Red List: Castor Canadensis: http://www.iucnredlist.org/apps/redlist/details/4003/0