Polski
I was inspired to write about beavers by the text about keystone (keystone) species and my consideration of whether such a thing exists at all. Modern ecology and conservation biology introduce concepts and terms, the consequence of which is a tendency to consider some species more important than others (such as keystone/key species, charismatic, flagship, umbrella, etc.). Any attempt to rank or polarize species arouses my skepticism, as I believe (in line with a holistic approach close to me) that everything affects everything. Nevertheless, it is hard to deny that some species seem to have a stronger impact on their environment than others.
In our biogeographical conditions, the canonical example of such a species is Castor fiber, or the European beaver. This terrestrial lifestyle rodent, the largest in Europe, plays an unquestionable role as an environmental engineer. We so define a species (usually belonging to the animal kingdom) that creates, significantly modifies, maintains or destroys a habitat, so that it has a significant impact on the biotope and biocenosis of an area. As a result, ecosystem engineers are important for maintaining the health and stability of the environment in which they live. And since all organisms affect the environment in which they live, it has been proposed that the term “ecosystem engineers” should be used only for keystone species, whose behavior strongly influences other organisms. And the beaver is definitely one of them.
Beavers arouse strong, often extreme emotions in various social and professional groups. The dispute over its role in the environment continues in earnest, and the species has both numerous opponents, calling for culling, and (I’m afraid, far fewer) defenders, advocating more effective protection. On the one hand, we recently celebrated International Beaver Day on April 7, on the other hand, the media every now and then report on demands for their elimination, because local governments cannot cope with the damage done by these animals. The argument is that there are too many beavers. Or is it that there is not enough space for beavers?
First deplete, then restore, that is, the effects of disrupting natural population dynamics
For many centuries, beavers were a widely distributed species in Europe, including in Poland. However, in the 19th century. Their population has been almost completely depleted for their valuable fur, used mainly for collars and hats, castoreum used as a painkiller and an ingredient in perfumes, or meat. Interestingly, the scaly tail contributed to the Catholic Church’s classification of beavers as fish, and therefore allowed to be eaten on Fridays and holidays. In the early 20th century. European beaver numbers were already limited to about 1,200 individuals, living in eight isolated populations. After World War II, the species was considered extirpated in Poland.
Around the middle of the 20th century, as a consequence of initially less, with time more successful reintroductions, and also as a result of the migration of individuals from the east, the European beaver extended its range to the area of almost all of Poland, excluding the high mountains (although we have fresh media reports of a beaver colony in the Tatra Mountains, above Morskie Oko). By the end of the last century, the population, according to various sources, reached from a dozen to more than 20 thousand individuals, and over the next 20 years it increased several times (according to various estimates, from 5 to even 10 times).
The precise number of individuals currently alive is impossible to determine, but CSO data from 2020. say about 142,500. individuals, which shows the overall scale of the phenomenon. Also in other parts of Europe, reintroductions of the species carried out in recent decades have contributed to the natural expansion and return of the beaver to much of its Eurasian range. A recent review of studies indicates that the population in Europe is estimated to be at least 1.5 million individuals [1], which is 3.5 times more than in the 1990s. In the 1970s. and 2.5 times more than in 2002.
Do we really know everything about the beaver yet?
Research and papers on beavers have been published in abundance, and it seems that everything has been written about them, although many issues still remain unrecognized, as evidenced, for example, by our helplessness and mismanagement of environmental resources in the face of their growing population. A comprehensive review of the issue was published in 2020. In the pages of WIREs Water. Based on well over 200 scientific publications, Brazier and co-authors [2] have analyzed in detail the impact of these beneficial animals on various components of the environment, including humans, also identifying areas that still need to be added to our knowledge. Due to the enormity of the publications, I take the liberty of using this review (and several other cross-sectional works, including a comprehensive 2010 study by Chekhov. [3]) without citing the authors of each study. However, for anyone interested in exploring the topic, I recommend reading the sources.
Beaver and hydro-geomorphological conditions, or environmental engineer
The impact of beaver activity on the geomorphology and hydrology of waters is visible to the naked eye, obvious and relatively well recognized. Beavers build dams with which they regulate the extent of their territory, while modifying the entire environment. They usually require waters of moderate flow and adequate depth to carry out their ecological functions. Low stream energy with reduced frequency of floods favors the construction and maintenance of dams, so the activity of these animals is most often limited to watercourses no larger than those classified as fifth order.
Beaver dams block and slow the movement of water, contributing to the formation of reservoirs and floodplains with increased retention of water, sediment and nutrients. By slowing the flow of water, they reduce peak flows. They regulate hydrological conditions, storing water in periods of surplus and releasing it in times of drought. Hydrological changes modify the processes of erosion and deposition, affecting the longitudinal and transverse profile of the river, leading to changes in the slope of the river bed, increasing its winding (meandricity) and improving its connectivity with flood plains. In order to gain access to food and building material, beavers dig cross-channels through flood plains, improving their connectivity and geomorphological structure and dynamics.
In general, due to the complexity of geomorphological and hydrological processes in catchments, the modifying effect of beaver activity is multifactorial and diverse. It is also difficult to predict or model because the effects are strongly dependent on the initial conditions. Studies conducted in different areas of the globe, in different types of ecosystems and in different biogeographic settings indicate different scale, scope and range of impacts of beaver dams. However, this does not change the fact that these animals have a remarkable ability to restore degraded habitats, transforming small, often human-modified streams into cascading systems of dams, floodplains and wetlands, contributing to a more mosaic environment, often reversing the effects of its anthropogenic homogenization.
The Beaver and water quality, or a dam like a treatment plant
A less obvious but very important function of dams is the retention of huge amounts of matter, including sediment and nutrients, which contributes significantly to improving water quality in the lower parts of the catchment. The amount of matter retained in beaver structures varies and depends on a great many factors, such as the nature and abundance of the watercourse, water flow, as well as the size or age of the dam (with old dams usually showing greater retention efficiency than new ones).
Studies on Canadian beavers indicate that the rate of sediment accumulation in their dams can, depending on the design, range from 2 to 40 centimeters per year [4], which is consistent with the average accumulation of 5.4 cm/year reported for the European beaver in the UK [5]. A single dam can hold up to 100 tons of sediment containing 16 tons of carbon and 1 ton of nitrogen. Estimates conducted for a sequence of dams on the Chevral River in Belgium put the value at nearly 500 tons of sediment deposited there, averaging more than 70 kg of sediment perm2.
Along with the deposition, huge amounts of elements, mainly nitrogen, phosphorus and carbon, accumulate in the dams, although here the quantitative ratios are even more difficult to estimate. Beaver ponds, for example, have been shown to be particularly effective at trapping nitrates, hence it is thought that they may be a good way to reduce nitrogen compounds from diffuse sources in the agricultural landscape.
Beaver dams and ponds can also act as sinks for sediment-bound phosphorus. In a review on the effects of beavers on nitrogen and phosphorus content in ponds and downstream of beaver habitats, Rozhkova-Timina and co-authors [6] cite conflicting findings and their strong dependence on individual conditions (context), suggesting the need for further research into the mechanisms controlling nutrient retention in beaver dams. The dams, regardless of their effectiveness, act somewhat like large water filterers and are a kind of storehouse of organic and inorganic matter, which is released back into the environment if the dam is destroyed.
Beaver vs. other aquatic organisms, or keystone species
The impact of beavers on geomorphological and hydrological conditions entails the remodeling of habitats and, consequently, the biocenoses that inhabit them. The habitat-forming activity of beavers is one of their more important functions, allowing it to be counted among the so-called keystone (keystone) species. Such a species determines the existence of other species, and is necessary for the proper functioning of the entire ecosystem. If, as defined, the removal of a keystone species causes a significant transformation of the ecosystem, it can be assumed that it will be no different in the situation of its restoration.
Slowing the flow of water, increasing the area of aquatic and wetlands, transforming lotic ecosystems into lentic ecosystems, increasing the lateral connectivity of the river and floodplains, changing the characteristics of the substrate – all this leads to an increase in the mosaic of habitats with a huge number of new ecological niches, and consequently to an increase in biodiversity. Beaver ponds and stagnant ponds develop vegetation typical of standing water ecosystems.
In marshy areas, marshy vegetation develops as a result of slowing down the flow and increasing the floodplain. This is facilitated by increased light access as a result of stand thinning and greater ground stabilization with slowed flow. Over time, sediment accumulation associated with the construction of dams can lead to the disappearance of beaver ponds and their transformation into meadows with variable moisture content and high species richness, colonized by grasses, shrubs and sedges, and at later stages of succession also woody vegetation.
An increase in the variability of flow conditions, water depth, the nature of the bottom substrate, the availability of the so-called “bottom”. woody debris (woody debris) and macrophytes promote increased diversity of benthic macroinvertebrates. Dams and feeding grounds, thanks to their complex design, provide shelter from predators, offer breeding and development sites, as well as foraging areas, especially for species that feed on woody material (xylophagous) and epixylic organisms. Increased retention of organic particles promotes the emergence of pickers and shredders. The structures also provide attachment sites for filter organisms.
As a result, beaver structures are characterized by higher abundance, biomass, density, productivity, richness and taxonomic diversity of invertebrate macrofauna than, for example, ponds or slow-moving streams. It can be said that such a dam or feeding ground is boiling with biological life, providing a kind of micro-hotspot of taxonomic diversity. In addition, benthic macroinvertebrates provide food for many species of fish, amphibians, reptiles and birds, which, with their increased abundance, makes the area particularly attractive.
A positive impact of beavers was also observed for fish. Beaver structures, by increasing habitat heterogeneity, provide greater availability of spawning sites, breeding and juvenile development sites or resting sites for migratory species. They offer refugia against lows, high flows, extreme temperatures and predators. The inhabitation of the dams by an abundant invertebrate macrofauna ensures the availability of a food base for fish.
Studies indicate an increase in survival rate, increased growth rate, biomass, density, productivity, species richness and diversity of ichthyofauna assemblages in sites transformed by beaver activity. The abundance of fish makes the beaver habitat highly populated and visited by otters, mink, herons, storks and other fish-eating species.
Research on the impact of beaver dams on migratory fish migration routes is of interest. There is evidence that the presence of dams modifies salmonid migration pathways within river networks, although the impact is location-dependent and is mainly marked in systems with low gradient and low flow energy.
In writing forWater Matters, I focus primarily on aquatic organisms, but it is important to keep in mind that just as much, if not much more, research and evidence is available on the enormous impact beavers have on mammals, birds or amphibians. This is, nomen omen, a river topic, annotated with a whole lot of fascinating literature. As a summary, one can cite the results of Stringer and Gaywood’s 2016 meta-analysis of 63 scientific publications on the impact of beavers on biodiversity. In the pages of Mammals Review [7].
The authors analyzed publications that showed positive, neutral or negative effects of Canadian and European beavers on species abundance or diversity of organisms, including plants and various groups of animals (aquatic and terrestrial macroinvertebrates, fish, amphibians, reptiles, birds and mammals). For the European beaver, a negative impact was found in only 6% of cases and involved one publication describing a decline in the local population of the white-tailed eagle(Sylvia communis) after beaver reintroduction in Denmark. In contrast, a positive impact was found in as many as 88% of cases, confirming the huge role of the beaver as a keystone species.
Beaver vs. man, or pest or benefit?
The growing beaver population and its increasing presence, and above all the effects of its activities, are not without human impact. And as was pointed out early in the text, attitudes toward the beaver can vary widely. Studies of local people’s attitudes toward beavers indicate that they are often viewed positively in terms of regulatory or cultural ecosystem services, but negatively in terms of provisioning services. The positive perception of the beaver is primarily associated with its restoration and regulatory activities, and is particularly appreciated by naturalists, but also by amateurs of the so-called “beavers”. wildlife tourism, which has been gaining popularity recently.
Negative attitudes towards the beaver, for obvious reasons, are shown primarily by those social or professional groups for whom the activities of these animals bring harm. The two most common problems associated with beavers are flooding and waterlogging (with a whole host of consequences) and tree damage. Therefore, the negative impact of beavers mainly affects agriculture, forestry and water management.
On the one hand, the species is under legal protection (admittedly partial, but nevertheless), on the other hand, there are increasing calls to abolish this protection and even take measures to reduce the population. Less than humane ways of solving the problem through dam destruction or shooting are proving completely ineffective. Destroyed dams and feeding grounds are immediately rebuilt at the expense of more felled trees.
It turns out that leaving old beaver structures alone does less damage to the environment than stimulating their restoration by demolition. In addition, the demolition of the dam involves the release of huge amounts of sediment and nutrients retained in it. Shooting also proves to be ineffective, as the eliminated individuals leave a niche for which many takers are immediately found. Given that annual population growth in Poland is estimated at several to several percent [3], there is no shortage of candidates for the new site. And the whole process starts from the beginning.
Beavers have lived with us and beside us for millennia, changing the environment according to their needs. Some groups of organisms “gained” from this, others “lost” (and I’m intentionally using quotation marks here to emphasize the anthropocentricity of these terms). This is what ecology in the textbook understanding of it is all about. Man first treated beavers as a source of benefit to himself, then exterminated them, and then successfully reintroduced them. And today he is surprised by the unpredictable and ambiguous effects of this reintroduction, trying (rather ineptly) to regulate the phenomena and processes whose dynamics he himself disrupted.
In many countries that have experienced similar problems as a result of excessive beaver population growth, ways are being developed to mitigate the activities of these useful but unceremonious animals. Let’s try to look at beavers not as pests devastating human efforts to subdue nature, but as repairers of what humans have messed up in nature. These animals make a huge contribution to restoring ecosystems to their natural structures and functions, and they do so without additional financial outlays for restoration. Let’s use this capability wherever possible. And where undesirable, we can always use compensation. Costly, perhaps, but probably no more so than the outlays needed for water restoration.
Photo author: Tomasz Modry
In my work, I used, among other things. From the publication:
[1] Halley D.J., Saveljev A.P. Rosell F., 2021. Population and distribution of beavers Castor fiber and Castor canadensis in Eurasia. Mammal Review, 51: 1-24.
[2] Brazier R.E., Puttock A., Graham H.A., Auster R.E., Davies K.H., Brown C.M.L., 2021. Beaver: Nature’s ecosystem engineers. WIREs Water, 2021;8:e1494.
[3] Czech A. 2010. Beaver – Builder and Engineer. Foundation for Supporting Ecological Initiatives, Krakow
[4] Butler D.R., Malanson G.P., 1994. Beaver Landforms. The Canadian Geographer, 38(1): 76-79.
[5] Puttock A., Graham H.A., Carless D., Brazier R.E., 2018. Sediment and nutrient storage in a beaver engineered wetland. Earth Surface Processes and Landforms, 43(11): 2358-2370.
[6] Rozhkova-Timina I.O., Popkov V.K., Mitchell P.J., Kirpotin S.N., 2018. Beavers as ecosystem engineers-A review of their positive and negative effects. IOP Conference Series: Earth and Environmental Science, 201(1): 1-11.
[7] Stringer A.P., Gaywood M.J., 2016. The impacts of beavers Castor spp. on biodiversity and the ecological basis for their reintroduction to Scotland, UK. Mammals Review 46: 270-283.
