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


The Red List of Ecosystems: assessing the extinction risk of ecosystems

Alps and Krim waterfall ©Mia Vehkaoja

Alps and Krim waterfall ©Mia Vehkaoja

Most people are familiar with the concept of extinction, and are aware of the IUCN Red List of Threatened Species (RLTS), a classification of the Earth’s organisms into different categories based on their population levels and dangers facing their survival. RLTS was founded in 1964, and was initially considered the best way forward in global conservation. Unfortunately, the plan of documenting all living organisms of the planet has proven cumbersome and slow, and currently the project’s aim is to classify 160 000 species by 2020. That accumulates to only 8% of all species currently known to exist. The task seems daunting and never-ending. Although certain individual species have been brought back from the brink of extinction thanks to having their “specs” measured, the RLTS has been unable to curb or stop the increasing population decline of countless species.

As a list of individual species has proved inadequate to counter biodiversity loss, the next step forward was taken during the 2000s with the formation of the Red List of Ecosystems (RLE). The idea behind this is fairly simple; each ecosystem is compared to a set of criteria, assessed in terms of its risk of collapsing (meaning the disintegration of its functioning leading to collapses in biodiversity), and finally categorized according to its current functioning and stability. It is in fact a hazard assessment for the extinction risk of individual ecosystems. The criteria used in evaluating each ecosystem includes assessing how much of their original flora and fauna have been converted, degraded or destroyed, and how much of their original size remains.

Classification is pretty similar to that of the RLTS: ecosystems can currently be at no risk of collapse (least concern), at three different levels of being threatened (vulnerable, endangered, or critically endangered), or they may already be approaching a final state of degradation (collapse). We may also have too little information (data deficient) to make an assessment, or their health may not have been appraised yet (not evaluated).

Once classified, appropriate planning and management measures can be taken to enhance or restore ecosystem functionality, which often also improves the stability of societies living within the influence of these ecosystems. For example, wetland restoration benefits societies by providing cleaner water for household use and by preventing or lessening flood and drought damage.

The RLE is an important tool for communicating between ecologists, decision-makers, and developers. The system provides robust and straightforward guidelines that are applicable to all types of ecosystems around the globe. One noteworthy goal of the IUCN is to assess and showcase ecosystems that are currently doing well, not just those that are at risk of collapse. Such well-to-do ecosystems are important, so as to pinpoint the reasons and most efficient management practices that have led to their current health.

Desert in Dubai ©Mia Vehkaoja

Desert in Dubai ©Mia Vehkaoja

However, improvement is always necessary. One such avenue for improvement lies in getting nations to collaborate together in conserving ecosystems crossing borders. This would greatly improve the connectivity of landscapes, improving habitats for migratory species, species with large habitat area requirements etc. But when used jointly with other conservation measures, such as the assessment of ecosystem services, spatial management planning, and the IUCN RLTS, this new method seems very promising. Currently the aim of the IUCN is to have all the Earth’s ecosystems assessed by 2025. This will be carried out at the national and regional level, and results will be freely accessible in an online database.

For more info and case studies, go to

For a practical assessment guide, visit

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.