Four reasons why beaver wetlands are paradise for pin lichens

Beaver activity enhances the occurrence and diversity of pin lichens (Caliciales). Both the number of species and individuals is much higher in beaver-created wetlands than in other types of boreal forest landscapes. There are four reasons behind this:

1. High amounts of deadwood. Pin lichens grow on both living trees and deadwood. Decorticated deadwood in particular is preferred by pin lichens. Beaver-induced flooding kills trees in the riparian zone and produces high amounts of decorticated snags.

Pin lichen on decorticated stump. © Mia Vehkaoja

2. Diversity of deadwood types. Beaver activity produces snags, logs and stumps. Snags are created by the flood, whereas logs and stumps are also produced by beaver gnawing. The diversity of deadwood tree species is also wide, containing both deciduous and coniferous tree species. The diversity of deadwood types maintains a high diversity of pin lichen species.

3. High humidity conditions. High humidity conditions are favorable for many pin lichen species. Old-growth forests are usually the only places in the boreal forest belt that contain high humidity conditions. There the shading of trees creates a beneficial microclimate for pin lichens. Lighting, on the other hand, becomes a limiting factor for pin lichens in old-growth forests. Most snags in beaver wetlands stand in water, where steady and continuously humid conditions are maintained on the deadwood surface.

Snags produced by a beaver flood in Evo (southern Finland). © Mia Vehkaoja

4. Sufficient lighting conditions. Because most of the deadwood in beaver wetlands stands in water, it is concurrently in a very open and sunny environment. Many boreal pin lichens are believed to be cheimophotophytic (cheimoon=winter), meaning that they are able to maintain photosynthesis also during winter at very low temperatures. The algae member of pin lichens requires enough light for photosynthesis. Open beaver wetlands make photosynthesis possible for pin lichens during both summer and winter. Snow also enhances light availability during winter.

More information: Vehkaoja, M., Nummi, P., Rikkinen, J. 2016: Beavers promote calicioid diversity in boreal forest landscapes. Biodiversity and Conservation. 26 (3): 579-591.


Crawlers and fliers – how to study forest insects

Studying insects is interesting yet challenging. Determining individuals to the species level


The presence of birch bark beetles can be detected by their unique eating patterns. ©Stella Thompson

nearly always requires capturing them first, although some species, such as the birch bark beetle (Scolytus ratzeburgi), can be identified by the unique pattern they leave on tree trunks. However, it is almost always necessary to use various types of traps to capture individuals if identifying the insect species present at a certain site is the main objective of a study. For example, butterflies are trapped during the night using light traps, and the occurrence of certain protected species can be confirmed using feromone traps that use synthetic lures as bait. Traps can be dug into the ground, lifted high up into tree canopies, or attached to the insides of hollow tree trunks.

As my PhD research I am assessing how beavers affect forest beetle populations. I have several research questions:  do beaver-induced flood zones have different beetle species assemblages than other areas, do the increased moisture and sunlight conditions in the flood zone affect species assemblage, and do beaver areas advance or hinder potential forest pest or protected species. My research combines a game species with widespread effects on its surroundings and forest beetles, several species of which have become scarce and require protection. Beaver-induced flooding and the species’ habit of felling tree trunks may locally disturb forest owners, but my study is looking into whether beavers’ actions facilitate or disturb forest pests. Combining game and insect research is cool, and generates new information on which to base decision-making for future protection measures, beaver population management, and even for using beavers as a natural tool for restoring degraded wetlands and forests.

Window traps are widely used for determining the insect assemblages of sites. Window traps cannot be used to capture specific insect groups, because all sorts of invertebrates ranging from flies to pseudoscorpions and wasps to beetles creep or fly into them. Window traps are very simple: the trap is attached to a tree trunk or set to hang between two trees. Insects crawl or fly into the plastic plexiglas frame and then fall through the funnel into a liquid-filled container at the bottom. The container is filled halfway with water, dishwashing fluid, and salt. The dishwashing fluid prevents the insects from regaining flight, consequently drowning them. The salt helps preserve the insects until the trap is emptied out, which happens about once a month. I have 120 traps spread out at several sites, so every summer I collect about 600 samples.

Unfortunately other creatures may sometimes end up caught in the window traps. So far I have inadvertently captured a few common lizards and a bat. This is always disappointing, because an individual dying for nothing does not advance research or science in any way. In the same way it is frustrating if you unintentionally set up a trap on a tree trunk that an ant colony uses as its route. Hundreds or even thousands of ants may drown in the window trap. As my own study focuses on beetles, I cannot utilize the ants in any way. At least this does not happen very often.

After the trap container has been emptied the gathered sample is sifted through using tweezers and a microscope, to separate the insect groups that I am interest in. Next the individuals are determined to the necessary level. Sometimes determining the family level is enough, but if making conservation decisions or gaining new information on certain species is the goal, it is usually necessary to determine individual insects to the species level. How this is done depends on the order in question, e.g. beetles are often recognized by their ankles and genitals.

Occasionally you come across data deficient species, i.e. species that are not well known or understood. Species, genera, and families are determined using identification keys, which are sometimes incomplete. For example, currently the best key for identifying Finnish rove beetles is in German, and for several families the most complete keys are in Russian. So I’m currently kind of happy that I studied German in middle and high school. I guess next I should begin uncovering the secrets of Russian vocabulary.

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

Beavers restore the dead wood of boreal forests

Dead wood is a necessary element for numerous species living in the boreal zone. It functions as a food resource, nesting space or growth substrate for several mammals, fungi, insects, and birds. Dead wood is produced through two main mechanisms: senescence and disturbances e.g. forest fires or wind damage. A controlled forest has less ageing trees and disturbances, and currently up to 90% of Fennoscandian forests have been influenced by forest management. The recent drop in dead wood levels due to intensive forest management across the globe has concurrently led to dead wood-dependent (= saproxylic) species becoming rare as well, which weakens food webs and ecosystem functionality. Managed forests may only contain a few cubic meters of dead wood per hectare, while dead wood levels in old-growth forests and forests influenced by disturbances can rise up to hundreds of cubic meters per hectare.

Beaver, the ecosystem engineer © Sari Holopainen

Beaver, the ecosystem engineer © Sari Holopainen

Strong disturbances are less frequent in moist lowland areas of the boreal zone, where dead wood is mainly created as single trees die due to competition and ageing. However, beavers act as wetland ecosystem engineers, raising floodwaters through the damming of water systems. These floodwaters kill surrounding shore forests due to oxygen deprivation, thus creating significant amounts of dead wood into the habitats. In certain cases the flooding may kill entire forest stands. Beavers can therefore be considered the main natural disturbance factor of lowland forests.

Beavers require wood for food and as a building material for their nests and dams. Foraging for woody materials causes the resource to run out within a few years, forcing the beavers to move location. The process of flooding and dead wood creation begins again in a new area, thus producing a continuation of dead wood hotspots into the landscape. Eventually after several years the beavers can return to a previously inhabited location, which will be then be repeatedly subjected to their engineering. These hotspots may be very important to dead wood -dependent species, especially as they uphold a network and continuous supply of different-aged dead wood.

Calculating dead wood levels at a beaver flood - spot the researchers! ©Mia Vehkaoja

Calculating dead wood levels at a beaver flood – spot the researchers! ©Mia Vehkaoja

Despite an overall decrease in dead wood levels, certain types of dead wood have become rarer in the boreal forest than others. Currently the rarest forms are standing dead trees (snags) and deciduous dead wood. Both have declined more rapidly than other types due to forest management actions and attitudes. Beavers create a broad range of dead wood types (e.g. downed wood, stumps and coniferous dead wood), but they particularly aid in the production of snags and deciduous dead wood. This is good news for many saproxylic species, as these organisms are often strongly specialized, utilizing very specific dead wood types.

The dead wood produced by beaver-induced flooding is also very moist, which may affect the wood-decay fungi species that begin colonizing the dead wood. For example, sac fungi are more tolerant of wet conditions, and may therefore outcompete Basidiomycetes at beaver sites. This in turn will lead to differing invertebrate communities that utilize sac fungi instead of Basidiomycetes. Very different dead wood –dependent species assemblages may therefore be formed at beaver sites compared to fire areas of clear-cuts. The interactions of these species are currently poorly understood.

The beaver offers a possibility for all-inclusive ecosystem conservation compared to the conservation of single species. The species could be used to produce dead wood and restore the shore forests of wetlands.

Our research group has recently published an article concerning the impacts beavers have on boreal dead wood. The article can be accessed from

Calculating dead wood levels at a beaver flood - spot the researchers! ©Mia Vehkaoja

Calculating dead wood levels at a beaver flood – spot the researchers! ©Mia Vehkaoja

Frogs love beaver-created wetlands

Amphibian and wetland loss is occurring globally at an increasing rate. Since the 1900s, approximately half of the world’s wetlands have been destroyed. During this time up to two-thirds of European wetlands were lost at a regional scale. This trend is reflected by the fact that 23% of Europe’s amphibians are threatened.

Wetlands in the boreal region are frequently constructed through the damming activities of an ecosystem engineer, the beaver (Castor sp.). They create and maintain special habitats by constructing dams. Beaver-created wetlands are open and sunny due to tree felling and flooding-induced tree mortality. They produce large quantities of woody debris and detritivorous invertebrates, e.g. chironomids and Asellus. Beaver ponds contain structurally heterogeneous vegetation.

Beaver-created wetlands contain structurally heterogeneous vegetation, as well as open water areas. © Mia Vehkaoja

Beaver-created wetlands contain structurally heterogeneous vegetation, as well as open water areas. © Mia Vehkaoja

According to our new study beaver-created wetlands increase frog species heterogeneity and abundance. There are only three native anuran species in Finland (the common frog, the moor frog and the common toad), and all of them were found in beaver ponds. The moor frog (Rana arvalis) was only found in beaver ponds, where the common frog (Rana temporaria) was also most abundant. The common toad (Bufo bufo) prefers deeper wetlands than the other two species, but because beaver ponds contain both shallower and deeper parts, it was also found from the beaver ponds.

Beaver wetlands offer high quality habitats for anurans and facilitate the occurrence of moor frogs. The shallow and warm water areas accelerate the hatching and metamorphosis of tadpoles. The rich aquatic vegetation provides attaching places for spawn and protection against predators. The abundant vegetal detritus, zooplankton and aquatic invertebrates offer nutrition for both larvae and adults. In addition, beaver-created wetlands create overwintering habitats that are less likely to freeze down to the bottom.

Both the moor frog (Rana arvalis) and the common frog (Rana temporaria) are the most abundant in beaver ponds. © Mia Vehkaoja

Both the moor frog (Rana arvalis) and the common frog (Rana temporaria) are the most abundant in beaver ponds. © Mia Vehkaoja

Beaver facilitation includes both habitat amelioration and resource enhancement. Frogs are not the only group that is benefitted by beaver activity. Other such groups are ducks, bats, woodpeckers and many invertebrates. These ecosystem engineers could be used in wetland restoration, and furthermore the beaver clearly promotes amphibian conservation.

Eradicating species – an occasional necessity

If Finland is to obey the EU strategy on Invasive Alien Species (IAS), 10 000 North American beavers (Castor Canadensis) are to come under the trigger. Why is this eradication necessary?

Although invasive alien species, e.g. the American mink (Neovison vison), the ruddy duck (Oxyura jamaicensis) and the Himalayan balsam (Impatiens glandulifera), may seem adorable and interesting novelties, they nearly always threaten the survival of native species. Invasive alien species are harmful to agriculture and forestry, and at their worst can even threaten human health. Sometimes we face the inevitable: the eradication of an animal or plant species.

The most efficient way to minimize the risks is to prevent it spreading to an area in the first place. Australia is probably the most famous example of preventing the spread of alien species, as they even clean the shoes of tourists at the airport before allowing them into the country. Unfortunately no nation has been successful in averting the spread of IAS.

Not that a tough fight isn’t being fought the new EU strategy on invasive alien species took effect earlier this year.

The main aim of the strategy is to aggregate a list of the most pernicious invasive alien species and to repulse them in different ways. Finland has 160 harmful IAS. Whether each of them will be on the EU’s black list remains unclear.

The North American beaver is first in line

The North American beaver is one of the most potential mammal alternatives for the list. The species was brought to Finland in the 1930s to save Finland’s beaver population. Back then the genetic differences between European and North American beavers were unknown, although we now know that the species differ even more than humans and chimpanzees.

The niches of both species are identical. They eat the same nutrition and their damming activities are the same. Both species have identical effects on ecosystems.

There are approximately 12 000 beavers in Finland. Most of them (10 000) are North American beavers, while European beavers comprise only a fifth of the North American beaver population. The North American beaver threatens the existence of the European beaver in Finland. It might eventually competitively exclude the European beaver. Adhering to the precautionary principle and seriously considering eradicating the North American beaver from Finland and Eurasia is essential. An eradication plan has nevertheless been conspicuously absent. An eradication plan for North American beavers would abide to the guidelines of both the IUCN’s and Finland’s National Strategy on Invasive Alien Species.

How to eradicate a species in practice?

A large scale eradication of the North American beaver is possible, at least in theory. Several possible methods could be used simultaneously, such as hunting, live capture, sterilization, reintroduction of the European beaver and population monitoring.

Beaver hunting is also financially tempting. Beaver furs have once again become popular in China, so their markets have a demand for beaver furs. After the sterilization or dead trapping of North American beavers, they should be replaced with European beavers.

But this is not a straightforward process. Although the two species differ genetically, they have a similar effect on the ecosystem. Beavers act as ecosystem engineers and benefit several other species in Finland and elsewhere. The present population size of the North American beaver ameliorates e.g. the green sandpiper (Tringa ochropus), the moor frog (Rana arvalis) and the Daubenton’s bat (Myotis daubentonii). However, if all North American beaver individuals were removed and replaced by European beavers, the eradication would be harmless to Finnish nature. Unfortunately, there is nothing to guarantee the success of the reintroductions.

Finland must begin eradication if the North American beaver is placed on the EU strategy plan on Invasive Alien Species. The activity of citizens and hunters will determine the eradication outcome. The Ministry of Agriculture and Forestry in concert with the Finnish Advisory Board for Invasive Alien Species are in charge of the decisions and eradication procedures.

The beaver is back in town!

The beaver (Castor sp.) is an ecosystem engineer of wetland habitats. Its actions modify both aquatic and terrestrial areas, and several species (e.g. waders, ducks, frogs) are aided by these changes. Beavers are therefore keystone species, and not only do they increase biodiversity through their actions, but can also positively influence humans e.g. mitigating the effects of flooding and drought and by improving water quality. In fact, beavers have such strong effects on their habitats, that their absence from an area where they would naturally occur can actually mean that the area is in a non-natural state and its ecosystem functioning may be (severely) modified.

© Sari Holopainen

© Sari Holopainen

This may well be the case in Britain, where Eurasian beavers (Castor fiber) were hunted to extinction 400 years ago. Some hope maybe on the way though; last week saw the publication of a final report by the Scottish Wildlife Trust and Royal Zoological Society of Scotland, following a licensed release of beavers into Argyll, Scotland. The program, nicknamed the Scottish Beaver Trial (SBT), has looked into the biological, social, and economic effects this release has and will have on Scotland, and results seem promising.

Four beaver families (a total of 17 individuals) were reintroduced to Argyll in 2009, following a long round of discussions and debates that began in the 1990s. Opposers of the Trial voiced concerns of the possible negative impacts the species may cause, e.g. by flooding land and killing trees. The first license application was rejected, and only after the second application did the Scottish Government grant the SBT the right to execute a trial release, in fact the first reintroduction of a mammalian species in the UK to ever take place. The four beaver families were brought into the country in 2008, and after a six-month quarantine period were released into three separate Argyll lakes. The release area is mainly owned by the Forestry Commission Scotland (FCS), and it is also a working forest. This means that the beavers’ actions and impacts on forestry and forest management areas could be monitored. For this same reason the area already had an extensive road and path network that could be effectively utilized in beaver eco-tourism, an aspect that significantly helped to promote the program to the public and local residents. Parts of the area were already designated Special Areas of Conservation (SAC) or Special Protection Areas (SPA) because of their native flora and fauna, and thus visitor facilities were available. Viewing platforms and floating pontoon walkways were constructed to maximize ease of access.

The source population was chosen from Norway, as Scandinavian beavers were concluded to morphologically most closely resemble their UK cousins, based on samples taken from fossil beaver skulls found in Britain. The IUCN has set a strict protocol for species reintroductions and translocations, necessitating the use of the taxonomically closest population as the source.

After release the beavers were monitored by staff using e.g. radio-tracking, visual sightings, and annual health checks, and most individuals seemed to quickly adapt to their new surroundings. Post-release the separate families have accomplished some impressive feats: in total they have created 13 000 m2 of new freshwater habitat (the equivalent of 10 Olympic swimming pools), and constructed several dams (the largest being 25 m in length) and lodges (the largest nearly the same size of a double garage, 7.7×2.1×11.3 m). Despite the radio-tracking, several released beavers and their wild-born offspring disappeared. Five were later found dead, one was caught outside the relocation area and brought back, but a few were never found and were presumed to have moved outside the FCS area.

The first successful reproduction was recorder in the spring of 2010, with a single kit born to two separate beaver families, and breeding occurred during each year of the SBT.

The Trial included monitoring programs for several species, groups, and habitats, to assess how the beavers influenced their populations and ecology, e.g. woodland vegetation and river habitat assessments, mammal (mink and otter), aquatic macrophyte, and dragonfly and damselfly surveys. The socio-economic effects of the SBT were also assessed. The first visual sign of beaver presence was tree-felling, followed by the building of canals, burrows, lodges, and dams. Dam building was closely monitored, so as to ensure that water levels in the SAC areas did not rise to detrimental levels for organisms protected in the areas. When such situations occurred, the dams were either removed or a device was installed to manage the water flow.

The Trial has been a huge endeavor, resulting in the publication of 24 independent journal articles and conference proceedings, and engaging 23 colleges and universities in addition to the three organizations in charge of the program. Volunteering has been enabled from the onset, and has also produced valuable information concerning the beavers’ actions and movement. An estimated nearly three million people were somehow engaged during the trial, ranging from school visits, education programs, guided walks to BBC’s Springwatch that aired several episodes on the Trial. Residents of the relocation area were involved in program planning well ahead of its onset, and planning was made as transparent as possible. The public (including residents) were again consulted at the end of the trial period. The trial also worked with local businesses to develop a ‘beaver brand’ to encourage nature-based tourism in the area. Over the course of the program, the trial won several awards for enhancing local tourism.

The active engagement of both local residents and entrepreneurs and the general Scottish public led to very positive attitudes towards the relocation and the possible future of beavers in Scotland. An estimated 74% of adult Scots favor beaver reintroductions, and 84% of local residents support wild beavers in their area. Such levels can be considered a major success, as the Trial did after all bring in a species that the British have never seen on their home turf, and additionally a species that causes a lot of changes in its living environment, some of which can be potentially problematic and destructive. The SBT succeeded in dispelling fears of the beaver causing socio-economic and/or ecological problems, and showed that local communities can in fact benefit from their reintroduction.

The Trial serves as an excellent example of how relocations should be handled so as to ensure success. Now it is up to the Scottish Government to decide on the beaver’s future in Britain – hopefully a bright one at that. Until a decision is made, the beavers will remain in Argyll. With luck the population will be aloud to stay and more reintroductions elsewhere in the country are on the way.

For more info, visit