Beavers facilitate Teals at different scales

Wetland ecology group_University of Helsinki_Teal

Teal broods utilize beaver ponds.

Blog text by Petri Nummi, Eeva-Maria Suontakanen, Sari Holopainen and Veli-Matti Väänänen “Beavers facilitate Teals at different scales” is now available on the Ibis website.


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.

Reintroduction of extinct keystone species – which ecosystems should we restore?

Extinctions are a natural part of species history, but for example habitat fragmentation, habitat loss, alien species, hunting and competition with humans have accelerated the rate of extinctions. The extinction of some species is more crucial than that of others, because it may jolt the whole food web or ecosystem. In some cases the whole ecosystem has been maintained by an animal, the keystone species. Without reintroducing the keystone species the ecosystem’s structure and stability might not re-evolve at all or the ecosystem is not complete and self-managing.

How long is too long?

Quite often the main question when talking about restoration is which ecosystems should be restored. Ecosystems have fluctuated through time and so the target is not stationary. Replacing a current ecosystem may not result in the same pure historical ecosystem, but in a mixture from current and historic. Some researchers think that restoration should be aimed at ecosystems existing just after the last ice age before the extinction of big mammals. This was the time before humans greatly effected speciescompositions. With this base, some researchers have proposed the reintroduction of megafauna in the prairies of North America. Megafauna were lost about 13 000 years ago, but some have remained in Africa. Large carnivores and herbivores could work as keystone species and create rare temperate grasslands.

The question is what happens to the current fauna if the old fauna is restored, i.e. what is the value of these actions for the current fauna? In the USA a large share of current fauna is threatened and wildlife is fragmented. Megafauna would need large areas and even current environments are fragmented. Quite many African megafauna species are also endangered and every individual is needed for making the gene pool bigger. In this case restoration would be controversial, because there is a threat that the current African fauna will suffer because of shrinking gene pools and the current American fauna would suffer when losing space.

Megafauna was extinct from North America about 13 000 years ago due to humans. In Africa they survived.  African elephant in Basel zoo. © Sari Holopainen

Megafauna was extinct from North America long ago due to human actions. In Africa they survived. African elephant in Basel zoo. © Sari Holopainen

Good bad wolves

Top predators can be keystone species, but many areas are lacking them because hunting and persecution have driven them to local extinctions. Top carnivores have an important impact in controlling herbivore populations and as a result also vegetation. For example the wolf (Canis lupus) is a keystone species that was killed to extinction from Scotland almost three hundred years ago. There is now an attempt of woodland restoration in Scotland and it has been supposed that also wolves could be reintroduced. Woodlands are not regenerated in Scotland, because of strong herbivore pressure, mainly by sheep and red deer (Cervus elephus). The reintroduction of wolves could decrease deer populations in a natural way and allow forest regeneration.

Apart from ecological aspects, there are sociological aspects that must be considered before reintroductions. In the Scottish example, economic aspects associate with sociological aspects via costs to the sheep farmers. A study about public attitudes against wolf reintroductions revealed that the rural population was concerned about loss of livestock, but still had less negative views to the wolf reintroductions that was expected. Rural population saw deer control as a major benefit brought by wolf reintroductions. Farmers actually had less negative attitudes than the organizations representing them; this might be due to the low price of sheep. But tolerance might not depend on economic costs, and so also emotional consequences should be regarded. The urban population was more concerned about the harm wolves could cause to humans, but they also saw that tourism would be the major benefit. As a conclusion, the general attitude of the Scottish public had a positive idea of reintroducing wolves.

American beaver full fills the ecofunction of the extinct european beaver © Sari Holopainen

American beaver full fills the ecofunction of the extinct european beaver © Sari Holopainen

Restoration of ecofunction

The quality and number of wetlands is decreasing in all of Europe. One of the reasons is that beavers have been extinct from some parts of Europe for a long time and this has had a remarkable effect on wetland availability. In the Kabetogama peninsula of North America the recovery of beaver populations caused strikingly large changes in the landscape by flooding. It can be supposed than also in Europe the effect of beavers has historically been larger. Beavers have been reintroduced in some areas and in many cases they have been successful.

If species have only become extinct locally and remained elsewhere, the knowledge about population biology is available. Genetic evidence must be used reintroducing species, to ensure that the species is the same as the extinct one. This is critical, because usually the target of reintroductions is to restore the original ecosystem. Before advanced gene technology some mistakes were made. For example, in Finland the locally extinct European beaver (Castor fiber) was partly replaced with the American beaver (Castor canadensis). But sometimes the problem is to decide whether a genetic or functional relationship is more important, especially if the original species is extinct. This can concern especially keystone species, because the functioning of a keystone species is in the main role. The target of restoration might be to return the ecofunction instead of the actual original species. Ecofunction was returned although the species was changed with the case of beavers in Finland.