The beaver – our wetland rescuer

The beavers (Castor canadensis and Castor fiber) have recovered from near extinction, and come to the rescue of wetland biodiversity. Two major processes drive boreal wetland loss: the near extinction of beavers, and extensive draining (if we exclude the effects of the ever-expanding human population). Beaver dams have produced over 500 square kilometers of wetlands in Europe during the past 70 years.


The wetland creation of beavers begins with the flood. As floodwaters rise into the surrounding forest, soil and vegetation are washed into the water system. The amount of organic carbon increases in the wetland during the first three impoundment years, after which they gradually begin reverting back to initial levels. The increase in organic carbon facilitates the entire wetland food web in stages, beginning with plankton and invertebrates, and ending in frogs, birds and mammals.

The previous shoreline is very evident from an aerial photograph. Also the beaver flooded area shows clearly. © Antti Nykänen

The previous shoreline is very evident from an aerial photograph. Also the beaver flooded area shows clearly. © Antti Nykänen

Beaver-created wetlands truly become frog paradises. The wide shallow water area creates suitable spawning and rearing places. The shallow water warms up rapidly, and accelerates hatching and tadpole development. Beaver-created wetlands also ensure ample nutrition. The organic carbon increase raises the amounts of tadpole nutrition (plankton and protozoans) in the wetland, along with the nutriment of adult frogs (invertebrates). Furthermore, the abundant vegetation creates hiding places against predators for both tadpoles and adult frogs.

Beaver-created wetlands are perfect rearing places for frogs. The warm water accelerates hatching and the abundant aquatic vegetation gives cover against predators. © Mia Vehkaoja

Beaver-created wetlands are perfect rearing places for frogs. The warm water accelerates hatching and the abundant aquatic vegetation gives cover against predators. © Mia Vehkaoja

The flood and beaver foraging kill trees in the riparian zone. Deadwood is currently considered a vanishing resource. Finnish forests have an average 10 cubic meters of deadwood per hectare, whereas beavers produce over seven times more of the substrate into a landscape. Beaver-produced deadwood is additionally very versatile. Wind, fire and other natural disturbances mainly create two types of deadwood: coarse snags and downed logs. Beavers, on the other hand, produce both snags and downed logs of varying width, along with moderately rare deciduous deadwood. The more diverse the deadwood assortment is, the richer the deadwood-dependent species composition that develops in the landscape.

Beaver-created wetlands produce  especially standing deadwood. © Mia Vehkaoja

Beaver-created wetlands produce especially standing deadwood. © Mia Vehkaoja

Deadwood-dependent species are one of the most endangered species groups in the world. The group includes e.g. lichens, beetles and fungi. Currently there are 400 000 to a million deadwood-dependent species in the world. Over 7000 of these inhabit Finland. Pin lichens are lichens that often prefer snags as their living environment. Beaver actions produce large amounts of snags, which lead to diverse pin lichen communities. Snags standing in water provide suitable living conditions for pin lichens; a constant supply of water is available from the moist wood, and the supply of light is additionally limitless in the open and sunny beaver wetlands.


The return of beavers has helped the survival of many wetland and deadwood-associated species in Finland, Europe and North America. Only 1000 beavers inhabited Europe at the beginning of the 20th century. Now over a million beavers live in Europe. I argue that this increase has been a crucial factor benefitting the survival and recovery of wetland biodiversity. Finland and the other EU member states still have plenty of work to do to achieve the goals of the EU Water Framework Directive. Both the chemical conditions and the biodiversity of wetlands / inland waters affect the biological condition and quality of wetlands.


The whole research published here


Hot spots of boreal landscape

The beaver (Castor spp.) is a known ecosystem engineer that modifies its environment quite drastically. It builds a dam and raises floodwaters into surrounding forests, killing trees, and releasing organic material into riverine systems and lakes. The rising water level changes both the abiotic and biotic conditions of a wetland. Many organisms, from water lice to water birds, benefit from these changes. Beavers facilitate these species by offering both nesting and sheltering areas in the form of low bushes and trees by the water’s edge, increased aquatic plant communities for nutrition, and ice-free water areas for extended periods.

Beaver-created wetlands are cyclic ecosystems. Beavers usually inhabit a site for one to three years and then move to the nearby site, where the whole process starts again. After the beaver has left the site, the abandoned site reverts quite slowly back to the original. So the beaver’s actions endure much longer than they occupy the site, and commonly they return to former sites within 10 years.


Beaver-created wetlands can be seen as a biodiversity hot spots. This pic is from eastern Finland. © Mia Vehkaoja

Beaver-created wetlands can be seen as a biodiversity hot spots. This pic is from eastern Finland. © Mia Vehkaoja

The beavers’ actions can be seen as quite sharp shifts in an ecosystem, but the very nature of the changes that the beavers create tends to be rather stable. As the beavers transform the ecosystem they also enable resilience in landscapes. Beaver-created wetlands increase the heterogeneity of the landscape, and can be seen as biochemical and biodiversity hot spots. They maintain several declining species, especially in the northern Boreal Hemisphere, where eutrophic wetlands are relatively rare.
The EU has an ongoing project called the Return of Rural Wetlands. The size of the EU funding in this project in Finland is a little over a million Euros. The other million Euros come from the Finnish Government and the rest from the Finnish Wildlife Agency. The aim of the project is to create a new frame and a good start for the future nationwide program for wildlife habitat conservation, restoration and re-creation. So people are creating new wetlands using tractors and diggers, and by bringing soil and water from elsewhere.

Beavers would do the same work for free. Instead of misspending lots of money on labor, expensive machines and moving earth, we could use some part of the funding to re-introduce the European beaver (Castor fiber) to a wider area. In this way we would save money, get the same results, if not even better ones, and help our original, once extinct species to recover. In addition, Finland would achieve the obligations of EU Inland Water Directive.

The new re-introduction of the European beaver project would involve the same interest groups as the Return of Rural Wetlands project. Some of the re-introductions could be conducted on state-owned lands and some on privately owned land. There are several local landowners involved in the Return of Rural Wetlands project, so there is a good possibility that they would be interested in the same kind of project as well. Regional hunting clubs would want to be involved, as beaver-created wetlands offer improved hunting and fishing opportunities, because their habitat engineering increases the number of game and fish species. It might be easy to get regional authorities and policymakers to engage in the project, because of the EU obligations that abide them. Furthermore, the policymakers would conserve the biodiversity of Finland, and gain the respect of The Finnish Association for Nature Conservation and the public. When all these interest groups are involved in and the role of power is divided to various levels, a revolution in wetland creation is possible. When such a project succeeds in Finland, it should be possible to implement it also in other EU countries.


The Mallard (Anas platyrhynchos) favors beaver-created wetlands, especially during breeding season. © Mia Vehkaoja

The Mallard (Anas platyrhynchos) favors beaver-created wetlands, especially during breeding season. © Mia Vehkaoja

The beaver’s actions extend wider than just creating suitable wetlands for several species. Beaver-created wetlands produce high amounts of dead wood. Dead wood is a decreasing natural source and the species dependent on dead wood are under threat. There are numerous bryophyte, lichen and beetle species that rely on moist dead wood. The resilience of beaver-created wetlands is more general than specified, as its transformability reaches from wetlands into the forest.

Beavers provide also other ecosystem services to humans. They mitigate flood peaks by retaining rainwater and drought conditions by slowly releasing water. Beaver-created wetlands act as buffer zones by filtering impurities, e.g. heavy metals, thus increasing water quality. They facilitate and conserve endangered and declined species, and create interesting hiking and relaxation possibilities for humans. All in all, beaver-created wetlands are one of the key ecosystems in boreal areas to be conserved.

Beavers of Finland

We have written some posts about beaver ponds, but in past months I have realized that most people don’t even know that e.g. Finland has beavers at all, and in particular that Finland has two beaver species: the Eurasian beaver (Castor fiber) and the North American beaver (Castor canadensis). Both species came close to extinction in their native habitats during the 19th century due to overexploitation. In Finland the native Eurasian beaver was hunted to extinction in 1868. In 1935 Finland reintroduced 17 individuals of the Eurasian beaver (9 males and 8 females) and in 1937 seven individuals of the North American beaver from New York (3 males and 4 females). Back in those days it was not known that the Castor-genus has two separate species. As recently as 1973 it was discovered that these two beaver species differ in chromosome numbers (C. fiber = 48, C. canadensis = 40). Compared to humans and chimpanzees, which differ in chromosome number by two chromosomes, the difference between the two beaver species is remarkable.

The North American beaver (Castor canadensis) in Finland. © Sari Holopainen

The North American beaver (Castor canadensis) in Finland. © Sari Holopainen

As a result of this duplicate reintroduction, Finland nowadays has approximately 10 000 North American beavers and just 2 000 Eurasian beavers. Why the North American beaver has excluded the Eurasian beaver is not quite clear. The construction activities and body size of both species are similar, but the North American beaver reproduce somewhat more effectively with bigger litter sizes (C. fiber ≈ 2.5, C. canadensis ≈ 4.5). As no other differences have been detected, this higher fertility seems to be the North American beaver’s advantage.

Finland was one of the few countries introducing the North American beaver to its nature. However as a consequence of being introduced to Finland, the North American beaver has also spread out to Russia. As the North American beaver is not a native species in Finland’s or Russia, it is classified an alien species, and is included in Finland’s alien species strategy. The status of the North American beaver as an alien species is quite tricky. Although it has excluded Finland’s native beaver species for the most part, it benefits rather many other native species, such as the common teal (Anas crecca), the moor frog (Rana arvalis) and the Daubenton’s bat (Myotis daubentonii). If we eradicate the North American beaver from Finland, will the Eurasian beaver spread out to the location where the North American beaver has occurred? We can’t know this for sure, but are we willing to take the chance and place the other species benefitting from beaver at risk? One possibility could be that the spread of the Eurasian beaver would be aided by humans. Originally the Eurasian beaver inhabited the entire country, so it has every prerequisite to be distributed to its original range. What strategies Finland’s policy-makers decide to take remains to be seen.

To read more about the eradication plans and shared history of beaver species in Finland and Russia

Parker, H., Nummi, P., Hartman, G. & Rosell, F. 2012. Should (and can) the invasive North American beaver Castor canadensis be eradicated from Eurasia? — Wildlife Biology 18: 354–365.

Or in Finnish
Vehkaoja, M., Nummi, P., Parker, H., Hartman, G. & Rosell, F. 2013. Amerikanmajava Castor canadensis Suomessa ja Euroopassa: pohdintoja vaikutuksista ja mahdollisesta hävittämisestä. – Suomen Riista 59: 52–61.