The aim of this review is to analyze the literature on the impact of beavers on lakes, summarize their effects, describe consequences for biotic and abiotic components, and highlight unresolved issues and perspectives. Beaver activity changes vegetation structure to the greatest extent, indirectly affecting other ecosystem components. Damming of flowing lakes increases the littoral area, which affects diversity and abundance of invertebrates, amphibians, birds, and mammals. Beavers’ alteration of the water regime and heterogeneity and connectivity of habitats has significant effects on zoobenthos, fish, and amphibians. Changes in hydrochemical properties directly affect phytoplankton and benthos. Unlike river ecosystems, where habitats are altered from flowing to still water, in lake ecosystems, habitat type is not usually changed (from lotic to lentic) but their quality (e.g., heterogeneity, connectivity) is. Beaver activity in rivers leads to increased limnophilic biodiversity, but in lakes, it leads to conservation of pre-existing lentic ecosystems. Therefore, impacts of beavers could be of greater importance to limnophilic complexes in lakes than to streams, especially after long time of beaver absence. Digging activity has a more significant role in lakes (especially floodplain) than in rivers. Beaver alteration of heterogeneity and connectivity of habitats is well studied, but not enough is known about impacts on the water regime of seasonally flowing waters, hydrochemical changes (especially eutrophication), amphibian life cycles, phytoplankton and zooplankton communities, parasitocenoses, and coarse woody debris. Methodological difficulties are noted, which are associated with the correct choice of control lakes. Further studies on riverine lakes are crucial. In considerations of climatic changes and anthropogenic impact, beavers may be an additional aid to conserving small lake ecosystems.
Conservation of aquatic ecosystems is one of the main global problems. Fresh waters are the most vulnerable, with their biodiversity decreasing much faster than that of terrestrial habitats (Vaughn 2010). The main threats are interacting processes of overexploitation, water pollution, degradation of habitats, disturbance of the water regime, and invasion by alien species (Dudgeon et al. 2006). In vast areas of the Northern Hemisphere, one of the additional factors affecting the sustainability of freshwater ecosystems is the activity of beavers (Castor fiber Linnaeus, 1758 and Castor canadensis Kuhl, 1820), which have recovered their range after centuries of absence. Therefore, the possible role of beavers in biodiversity conservation is being actively discussed (Law et al. 2019).
The alteration of watercourses by beavers is well described in the literature, and reviews of various aspects of the species’ influence on aquatic ecosystems have been published regularly (Naiman et al. 1988; Pollock et al. 1995; Gurnell 1997; Collen and Gibson 2000; Rosell et al. 2005; Gibson and Olden 2014; Janiszewski et al. 2014; Stringer and Gaywood 2016; Ecke et al. 2017). In most cases, attention is paid to changes caused by the building activities of beavers, which lead to transformation of lotic ecosystems into lentic ones. Beaver digging activity involving construction of canals and improvement in connectivity between habitats (Grudzinski et al. 2019) and the role of foraging activity in ecosystems are studied to a lesser extent (Nummi and Kuuluvainen 2013; Zavyalov 2014).
Despite many studies devoted to beavers in lakes, scientists have rarely discussed the specificity of the effects of the animal on lentic ecosystems. Such lake-specific effects are caused by significant differences between the two main types of freshwater ecosystems. In addition to obvious differences in flow velocity, lake ecosystems are characterized by a higher level of stability of morphometric characteristics, abiotic parameters, and heterogeneity of the environment, as well as higher intensity of autochthonous processes, than rivers (Odum 1983; Wetzel 2001). Lakes are also characterized by progressive succession, in which internal processes, including biological ones, play a key role (Protasov 2008). Rivers and lakes are different in their sensitivity to climatic changes. Although the biodiversity of lotic ecosystems is more vulnerable, disturbance of biotic exchange is more important for lentic waters (Sala et al. 2000). Therefore, differential effects of beaver activity are assumed for the two types of aquatic ecosystems. In rivers, additional influences may lead to changes in allochthonous processes and increased stability, for example, decreased flow velocity and reduced impact of fluctuations in flooding. In lakes, autochthonous processes (for example, sedimentation and eutrophication) and connectivity between water bodies may be significantly influenced, and vectors of stability changes could be different.
This review aims to analyze the available data on beaver activities in lakes to show what lakes beavers inhabit, what impacts they have, how the abiotic conditions of water bodies change, and what the consequences are for the biota of water bodies. This will allow determination of the specific features of the influence of these key species on lentic ecosystems and identification of unresolved issues and perspectives for further research.
The structure of this review is as follows. After the introduction, I will briefly describe the methods, introduce the literature used for the review, and provide some statistical descriptors. This will be followed by an analysis of the types of lake inhabited by beavers and their distribution in lakes. Next, I will discuss types of beaver activity and their consequences for abiotic and biotic conditions of lakes. I will discuss beaver alteration of the water regime, morphometry, and hydrochemistry and the reactions of lake biota (algae, vegetation, plankton, macroinvertebrates, fish, amphibians, reptiles, birds, and mammals) to the consequences of beaver activity. I will conclude by summarizing the specifics of beaver impact on lake ecosystems and highlighting unresolved issues and perspectives.
Methods and statistical descriptors
For analyzing data from the literature, I searched the Web of Science, Russian Science Index (elibrary.ru), Library for Natural Sciences of the Russian Academy of Sciences databases, and Google Scholar. Subsequently, another search was carried out based on bibliographies and cited publications. The publications reviewed included the following: studies on beaver impact on lake ecosystems and their components; studies on lake ecosystems inhabited by beavers (without assessing the influence of the species); studies on beaver impact on river and riparian ecosystems; and ecological studies of lakes within the range of beavers. The publications used in the analyses of the effects of beavers on lake ecosystems are shown in Table 1.
From the 58 papers shown in Table 1, 48 were about beaver impact on lake ecosystems and its characteristics, and 10 had information only about population characteristics. These studies cover 36 territories in eight countries: 18 studies (31%) were carried out in North America, 39 (67.2%) in Europe, and one in Asia (Western Siberia). The two beaver species featured almost equally in the review—53.4% of the studies were on C. canadensis and 46.6% were on C. fiber. Beavers were one of the main research topics in 53 (91.4%) publications, and the beaver effect was mentioned in papers on hydrology (4 papers) and botany (1 paper).
For analysis, I divided the lakes mentioned in the literature into four groups based on their size—large lakes (more than 1 million m2 in size) in 8.6% of the publications; medium lakes (more than 100,000 m2) in 8.6% of the publications; small (20,000–100,000 m2) in 51.7% of the papers; and extra small, including natural ponds (less than 20,000 m2) in 15.5% of the papers. Other studies did not include morphometric data for lakes; therefore, a more detailed classification of lakes was impossible. In addition, the information about lake types drew on 35 publications (61.3%) on lakes of postglacial origin (including bog and thermokarst lakes), 21 publications (37.9%) on riverine lakes (including floodplain lakes and oxbows), and one study based on an artificial reservoir.
The literature was also categorized based on the level of beaver impact: local (influence only on some part of the lake; 6.7% of publications), insufficient (effect is unclear or slight; 11.1% of publications), and significant (ecosystem consequences; 82.2% of publications). Negative effects were also considered (they were mentioned in 12 papers (25%)).
For analyses of beaver activities in lakes, I used information from studies on preferred dwellings, digging activity, dam and lodge building, and foraging. Other statistical data and descriptors are discussed below in relevant sections.
For statistical analysis, I used the phi coefficient (φ) and logistic regression for determining associations between characteristics of lakes and beaver activities. Microsoft Excel, Past 3, and R software were used for the analyses.
Lakes inhabited by beavers
As mentioned in the previous section, the activity of beavers is described mainly for two types of lake: postglacial and riverine. The distribution of beavers is limited to the temperate zone with some expansion into the subarctic and subtropical regions. As the overwhelming majority of lakes in this territory are of postglacial origin, most of the information in the literature about beaver populations and their influence is specifically for such lakes; 61.3% of the studies are on lakes of postglacial genesis, largely due to numerous works on two territories, the Cooking Lake Moraine in Canada and the Evo area in Finland (in total, 40% of all studies analyzed).
Riverine lakes are also widespread within the range of beavers, which are often considered in the literature in conjunction with the entire wetland of floodplain complex. Such lakes inhabited by beavers are mostly studied in European Russia (81.8% of studies on riverine lakes); detailed research has been carried out in the valleys of the Khoper and Oka rivers.
In comparison with rivers, the preferential selection of lakes by beavers is facilitated by the broader development of aquatic vegetation that appears as greater plant coverage (e.g., Milligan and Humphries 2010). The perimeter of the water body and total macrophyte cover determine the stability and density of beaver settlements on lakes (Bergman et al. 2018). In floodplain landscapes, an additional factor may be flood disturbance; beavers are less likely to inhabit lakes in low floodplains, which are the most affected by floods (Pankova and Pankov 2018). Lake size also affects settlement of beavers; in oxbows, large permanent settlements exist on water bodies with an area of more than 30,000 m2 (Glushenkov 2018). However, extra small and temporary lakes can also be inhabited by beavers, and in some cases, they are able to winter in completely dry water bodies (Pankov and Pankova 2016). Selection of lakes by beavers is determined, among other things, by the presence of an outflowing watercourse that can be dammed (Vehkaoja et al. 2017).
According to the literature review, ecosystems of lakes located on drainage divides may show low occurrence of beavers and may have an insignificant effect on them. This is due not only to their remoteness from river networks, but also to the surrounding vegetation, which is often represented by zonal communities, for example, nonedible conifers. This is shown in studies based on the water bodies of Karelia, northwest Russia (Fyodorov 2017) and on swamps (Zavyalov 2017) where beavers inhabit lake shores only in the clearings made by felling or fires, which are overgrown with small-leaved edible trees. Small, individually located lakes with high anthropogenic pressure (for example, ponds in city parks and fire and water reservoirs) may be least populated by beavers.
Distribution of beavers in lakes
Beavers are found equally in watercourses and lakes of various types; therefore, it is difficult at present to compare the proportion of settlements on lakes with that on rivers. In some parts of the range, beaver settlements on lakes are much rarer than stream settlements. This may be due to the presence of such habitats in landscapes. For example, in the south of the range in the steppe and the water-poor Rostov region in southwest Russia (with lakes making up only 0.2% of territory) only 12.8% of the beaver population live in lakes (Stakheev et al. 2018). However, a similar situation can be observed north of the range in areas with a wide distribution of lakes, for example, in the Kostomuksha Nature Reserve, Karelia, northwest Russia (with lakes making up 20% of the territory), where only 12.2% of the beaver settlements are on lakes (Fedorov and Danilov 2018). In the river basins of the Arkhangelsk region, northwest Russia (with lakes making up 0.9% of the territory), only 3.7% of the settlements are located on lakes (Solovjev 1991). On Isle Royale, an island in Michigan, US, colonies on streams and bogs are twice as numerous as colonies on lakes (Bergman et al. 2018). In Poland, 23% of beaver settlements are located on lakes (Janiszewski et al. 2009). In the Czech Republic, beavers prefer river habitats to standing water bodies (Vorel et al. 2008).
In some areas, lakes are the most preferred habitat of beavers. In Finland, beavers predominantly inhabit small lakes over most of the territory (Lahti and Helminen 1974). In a number of areas in Norway, beaver populations on pre-existing standing waters predominate (Myrberget 1967; Parker et al. 2001). In the eastern part of Lithuania, more than half (38–57%) of beaver settlements are located on lakes (Bluzma 2003), especially in plains landscapes (Pupininkas 1999). In Elk Island National Park (southwest Canada), beavers live almost exclusively in lake settlements, because rivers are absent on the territory or are represented by intermittent streams (Hood and Bayley 2008). Oxbow lakes are the most preferred, and sometimes exclusive, places for beaver settlements in the basins of medium and large rivers of European Russia, for example, Voronezh (Mishin 2018), Sura (Glushenkov 2018), Oka and Pra (Pankova and Pankov 2018), and Khoper (Marchenko 2018).
The abundance and density of beavers on lakes depend on many factors, primarily the size of the water body, availability of food resources, and total beaver abundance on neighboring territories. For example, studies in the Oksky Nature Reserve showed that oxbows with a length of less than 1 km were populated by one family and larger lakes by two families (Pankova and Pankov 2018). Settlements of one family per lake are also seen in the systems of shallow postglacial lakes (Hood and Larson 2014b; Hyvönen and Nummi 2008). In the upper flow of the Khoper River, beaver families inhabited several extra-small oxbows (Bashinskiy and Osipov 2018). Beavers introduced into Scotland also used several small lakes (Law et al. 2014a). On the other hand, even large lakes can be used only by one family or a small group of beavers (Raffel et al. 2009).
To summarize, beavers may occupy lakes primarily when such water bodies are widespread on the territory. The most attractive factors for beavers are wider vegetation cover, the stability of water bodies, and the opportunity to build dams on outlets.
Beaver activities in lakes
The types of beaver activity in lakes are obvious and typical for all water bodies: building (construction of dams and lodges), digging (creating burrows and canals), and foraging (eating edible species and import of woody debris into water). However, in lake ecosystems, these activities can appear in slightly different forms. Construction of dams is only possible on outlets and inlets; therefore, its impact is most often expressed locally due to increase in the littoral zone and hydrochemical influences at the junction of the watercourse and water body (Fig. 1a). Construction of dams helps maintain lakes at a constant state, due to the damming of seasonal watercourses (Fig. 1d). Dams on flowing lakes and their outlets are similar in their characteristics to those on streams. The size of dams on channels between riverine lakes is usually small: 3–5 m in length (Pankova and Pankov 2018). Dams can also be built at the junction of oxbows and the river and on large bays, where they will be extended (Fracz and Chow-Fraser 2013). Dam building is the most studied activity in the “lake beaver” literature—it is mentioned in 29 publications (60.4%).
Another building activity is construction of burrows (mentioned in 27.1% of all publications) and lodges (mentioned in 53.4% of all publications). On the lake systems of Elk Island National Park in Canada, beavers live almost exclusively in lodges (Hood et al. 2007). On the large anthropogenic Alum Creek Lake (13 km2, maximum depth 21 m) in the US, beavers always build bank lodges without populating burrows, which is apparently due to periodic fluctuations in the water level (Raffel et al. 2009). In Canada, 2–7 abandoned beaver lodges have been observed on lakes 260,000–570,000 m2 in size (8–12 lodges per km2 of water area; France 1997). On the small Black Lake (20,000 m2, depth up to 8 m), beavers built five lodges (250 lodges per km2) over time, with two of them used at the same time (Reddoch and Reddoch 2005). On the much larger Płotycze Lake (160,000 m2) in Poland, beavers built only one lodge (6 lodges per km2), which was inhabited by only one family (Peczula and Szczurowska 2013).
However, in other regions, burrows are the main type of beaver habitation on lakes. Of the habitations on the riverine lakes of the Pra river (in central European Russia), 13.4% were lodges and 22% were bank lodges, whereas on the lakes of the Oka river floodplain (in the same region), 18.1% were lodges and 5.5% were bank lodges (Pankova and Pankov 2018). In the floodplains of the Khoper and Kadada rivers in the forest–steppe zone of European Russia, there were no lodges on lakes (Osipov and Bashinskiy 2018). However, in the same nature zone in the floodplain of the Voronezh River, 75% of the beaver habitats supported lodges (Mishin 2018). Despite the variety of preferred dwelling types in different water bodies, the analysis suggests that lodges are more typical of postglacial lakes (74.3% of the studies show that it is the preferred dwelling type) and burrows of riverine water bodies and oxbows (40.9% of the studies show that it is the preferred dwelling in this habitat). The dependence of the type of dwelling on the category of lake is confirmed by statistical analysis (Chi-square = 18.8; df = 1; p = 0.0000149). The association of the preference for burrows with riverine lakes and the preference for lodges with postglacial lakes is also statistically significant (φ = 0.66, p = 0.002). Riverine lakes inhabited by beavers are usually former riverbeds with a common stream valley morphometry and topography (e.g., high slopes and narrow shorelines); therefore, burrows are the usual dwelling in such habitats.
In general, digging activity is capable of significantly influencing specifically lake ecosystems, because, in contrast to lithogenic processes in rivers, those in lakes occur primarily due to sedimentation rather than sediment transport (Protasov 2008). Burrowing activity can change the shoreline relief and the whole water body, improving connectivity within the lake system (Fig. 1b, c). Soil input as a result of beaver activities changes lake bottom topography, which can lead to a significant change and even disappearance of water bodies (Dyakov 1975). In the studies reviewed, digging activities were associated mostly with beaver alteration of morphometry (φ = 0.40, p = 0.009) and connectivity within the lake system (φ = 0.45, p = 0.003).
Beavers build complex canal networks on lake systems, some of which can exceed 100–200 m in length, and usually more than 0.7 m in depth (Hood and Larson 2014a). Digging activity is facilitated by the presence of soft soil, which is usually observed in floodplain alluvial landscapes (Pankova and Pankov 2018), as well as in moraine valleys (Anderson et al. 2015). The construction of canals in shoreline wetlands significantly increases the number of habitats for aquatic invertebrates and increases the length of thickets along the banks (Hood and Larson 2014a). By building canals between water bodies and watercourses, beavers interconnect lake habitats, thereby supporting the diversity of aquatic ecosystems (Nummi et al. 2018). Connectivity is a highly important property of wetlands as it reduces the risk of extinction and increases the likelihood of ecosystem restoration after catastrophic events (Smith et al. 2019); therefore, its loss leads to negative consequences for various components of ecosystems. Within floodplain complexes, connectivity allows biota to move between different habitats and water bodies, which enables recolonization of habitats, avoidance of inbreeding, escape from stressors, location of mates, and acquisition of resources (Schofield et al. 2018). After creating channels, beavers use them to transport woody material, thereby deepening the channels even more (Hood and Larson 2014b). Canals are mentioned in 25% of all publications.
Foraging also has its own specific characteristics in lakes, since there is a broad distribution of edible aquatic vegetation in lentic habitats (Barabash-Nikiforov et al. 1961; Milligan and Humphries 2010). Depletion of macrophytes leads to alteration of the spatial distribution of vegetation in the water body, and, accordingly, to alteration of spatial heterogeneity (Pankova and Pankov 2010). Beavers significantly reduce the abundance of some aquatic plant species, change the plant species composition of the community, and reduce plant litter (Parker et al. 2007). In Ukraine, it was noted that beavers’ consumption of edible woody vegetation (birch, aspen, and willow) along the shores supported a high-light regime, which prevented relict swamps from overgrowing the interfluve of the Merla and Merchik rivers (Brusentsova and Ukrainskiy 2015). Foraging is mentioned in 22.9% of all publications.
Foraging with building activity leads to deposition of large amounts of woody debris in the water body and along banks (Thompson et al. 2016), which creates new habitats for lake biota, and is of great importance for food chains (Marburg et al. 2006). Input of deadwood into lakes is mentioned only in 6 (12.5%) publications.
Beaver activities are different in the two main types of lakes. According to the literature review, in postglacial lakes, dam building is the predominant factor (63.6% of the publications), and other activities, such as foraging (18.2% of the publications), digging (15.1%), and lodge building (12.1%), are less important. In riverine lakes, dam building (53.3% of the publications), digging (53.3%), foraging (33.3%), and lodge building (26.7%) are described more often.
All these forms of beaver activity together lead to changes in ecosystem attributes, such as the hydrological regime and hydrochemical conditions, as well as habitat heterogeneity and connectivity. Clearly, the impact of beaver activity is most intense in small water bodies. The review shows that the significance impact of beaver activity was significantly positively associated with small lakes (φ = 0.52, p = 0.0004), and negatively with large lakes (φ = − 0.67, p = 0.0003). In large lakes, activity takes place locally in shoreline zones and backwaters, as well as at the confluence of rivers dammed by beavers. Despite a number of examples of the effects of dams on lake tributaries, damming has little effect on large lakes (Westbrook et al. 2013; Robertson et al. 2015). Below, various aspects of the impact of beaver activity on abiotic and biotic components of lake ecosystems are discussed.
Consequences of beaver activities for abiotic conditions of lakes
Water regime and morphometry
As in the case of river systems, alteration of the water regime of lakes occurs primarily due to the building activities of beavers (41.7% of all publications). This is observed most in floodplain systems (in 53.3% of the studies compared to 36.4% for postglacial lakes), where there are temporary pools with seasonal flow. By constructing dams on spring streams, beavers change the status of lakes from temporary to permanent, and also significantly increase the area under water (Pankova and Pankov 2018).
The construction of dams on flowing lakes leads to an increase in their area and the occurrence or an increase in the littoral zone (Nummi and Hahtola 2008; Pankova and Pankov 2010; Timofeeva 2014). At the same time, the expansion of the littoral zone is facilitated not only by construction of new dams, but also by destruction of old ones. Therefore, on the Black Lake (20,000 m2, depth 8 m) in Canada, the destruction of the dam on the tributary led to the formation of lowland bogs on the lake (Reddoch and Reddoch 2005). The dammed channels flowing from thermokarst lakes in Alaska, USA, led to a 26% increase in the surface water area, as well as to amplification of the natural expansion occurring along thermokarst lake margins due to permafrost degradation (Jones et al. 2020).
In addition to the construction of dams, digging activities—deepening of the lake bottom and construction of underwater canals—contribute to changes in the water regime of lakes (Pankov and Pankova 2016). Such activity leads to the situation of water being stored only in canals and burrows created by beavers when the water body dries out (Brusentsova and Ukrainskiy 2015). Studies on Elk Island National Park (Canada) showed that the presence of beavers leads to a ninefold increase in open water in wetlands (Hood and Bayley 2008). The depth of lakes and canals populated by beavers is significantly greater than that of abandoned water bodies; channels increase both the perimeter of water bodies and the area of open water (Hood and Larson 2014a, b). With the help of digging, beavers can actually create new water bodies, by deepening canals in marshy areas and constructing dams from a mineral substrate (Rebertus 1986; Westbrook et al. 2017). On the other hand, active burrowing by beavers can have negative consequences. For example, over the five years of a beaver family inhabiting a lake with an area of about 10,000 m2 in the floodplain of Moksha River (in central European Russia), the maximum depth decreased from 2.3 m to 0.5–0.6 m, which led to the beavers subsequently abandoning the water body (Babushkin 1993).
In addition, a significant change in morphometric characteristics of lakes occurs as a result of complete flooding of oxbows by beaver ponds in the floodplains of small rivers (Bashinskiy 2014). After beavers leave, due to the subsequent dam destruction and water discharge, such lakes actually become completely new water bodies, with altered hydrochemical characteristics, a different lake bottom topography, a new aquatic and shoreline vegetation structure, and a different level of illumination (Fig. 2).
Alteration of the water regime by beavers also leads to changes in the hydrochemical characteristics of water bodies (as described in 35.4% of all publications). When beavers dammed outlets of lakes (average area of 43,000 m2) in Finland, the concentration of dissolved organic carbon increased and dissolved oxygen decreased (Vehkaoja et al. 2015). Lakes inhabited by beavers on Isle Royale (Lake Superior, USA) had low dissolved oxygen concentrations and showed a decrease in temperature and electrical conductivity and an increase in acidity of the water (Bergman and Bump 2015). The construction of a dam by beavers at the outlet of Lake Płotycze (160,000 m2, mean depth of 2.2 m) in Poland also led to a decrease in electrical conductivity of the water and a decrease in concentrations of phosphorus and calcium (Peczula and Szczurowska 2013). The increase in water level caused by beavers in flowing lakes in Karelia (northwest Russia) led to increased concentrations of ammonium, phosphorus pentoxide, and potassium oxide in the shoreline soil. Moreover, in comparison with settlements on streams, the ammonium content was almost three times higher, and the potassium oxide content was twice higher in settlements on lakes. However, these parameters in the control sections without beavers in rivers and lakes differed very little (Danilov et al. 2007).
Digging activity, as a result of which large amounts of soil are released into water bodies, also leads to changes in the chemical characteristics of water. In small oxbow lakes (in central European Russia), positive correlations were observed between phosphorus concentrations in suspended matter and the number of burrows along the shores (Bashinskiy and Osipov 2019). The changes in hydrochemical conditions due to burrowing activity are confirmed by studies on the distribution of macrozoobenthos in microhabitats in central European Russia (Khitsova et al. 2010). The number of indicator species and saprobiological indices showed a high degree of biogenic load in the littoral zone of riverine lakes around beaver burrows and canals.
In addition, the input of excretory products of beavers into the water could be an important factor for small lakes. Defecation by a family of 3–4 beavers contributes 160–200 g of dissolved organic matter per day to the water, which subsequently affects its buffering properties (Legeyda and Sergyienko 1981). Zoogenic contribution to eutrophication was observed in beaver settlements on small oxbow lakes (in central European Russia) more than in settlements on small rivers (Katsman et al. 2020).
The hydrochemical properties of water in the littoral zone are also changed due to construction activity and contribution of coarse woody debris, resulting in a significantly increased share of δ13C in particulate organic matter, as shown for small lakes (260,000–570,000 m2) in Canada (France 2000).
The effect of beavers on the hydrochemistry of water in large lakes is less significant. However, local influences can be observed at the confluence of rivers dammed by beavers. The destruction of dams in tributaries leads to discharge of pond water into lakes, which leads to a change in hydrochemical parameters. Studies on Lake Vishtynets (18.3 km2, average depth 20 m) in western Russia showed that the destruction of beaver dams led to the appearance of “tongues” of elevated permanganate oxidation in the lake, which in some cases extended from the mouth of the flowing stream to the entire lake (Bernikova et al. 2013). Similar results were noted for Lake Borovno (lake area 10.03 km2, average depth 10 m) and the adjacent system of Razliv–Beloe lakes (Valdai National Park, northwest Russia), where beavers locally influenced the flow of water into the lake system (Frolova et al. 2015).
On large lakes, the influence of beavers is observed not only through dam construction on tributaries, but also through active building activity in backwaters and swampy shores. Construction of dams on coastal marshes in Georgian Bay on Lake Huron (Canada) leads to lower pH levels and higher concentrations of phosphorus, suspended solids, and chlorophyll a (Fracz and Chow-Fraser 2013).
Beaver activity in floodplain lake systems is important for the movement of reduced metal ions, due to formation of transport relationships between river and valley complexes (Briggs et al. 2019). Canal construction also influences hydrochemistry. The constant passage of beavers between water bodies along the channels leads to movement of the water, which facilitates movement of nutrients and bioturbation through disturbance of the substrate (Hood and Larson 2014a).
Reaction of lake biota to beaver activities
Currently, there are only a few studies that show a significant effect of beavers on the algal component of lake ecosystems. Research on small riverine lakes (4000 m2, maximum depth of 2–3 m) in Belarus showed that in water bodies inhabited by beavers, there is high species richness and diversity of algae. The algal flora of lakes subject to beaver activity has a higher proportion of periphyton than phytoplankton, since the substrate area increases due to beaver activity (Makarevich et al. 2016). In Lake Płotycze (Poland) dammed by beavers, an increase in the water area and changes in chemical parameters led to alteration of the phytoplankton community structure, mainly shown as a decrease in cyanobacteria and green algae and an increase in flagellate species (Pęczuła and Szczurowska 2013). The effect of beavers on algae may appear indirectly through alteration of aquatic vegetation structure (Celewicz-Goldyn and Kuczynska-Kippen 2017) and as a consequence of zoogenic eutrophication, which occurs due to the input of, among other things, beaver excretory products into the water (Krylov et al. 2016).
Changes to lacustrine vegetation caused by beavers is of key importance, because, unlike for watercourses, vegetation is a decisive factor for lentic ecosystems and their structure and functioning. For example, the trophic status of water bodies is determined by vegetation (Scheffer 2001). Macrophytes, as well as phytoplankton, zooplankton, and benthos, affect abiotic parameters and trophic webs in aquatic communities (Blindow 1987; Cyr and Downing 1988; Laugaste and Reunanen 2005; Špoljar et al. 2018; Kolar et al. 2019); they form a mosaic microhabitat structure (Celewicz-Goldyn and Kuczynska-Kippen 2017; Clemente et al. 2019). Beavers affect vegetation by consuming it directly, changing the water regime, and digging activity. Beavers include many macrophytes in their diet (Dyakov 1975); they can make up 60–80% of the total beaver diet, while in lentic ecosystems, preferential consumption of aquatic vegetation in the winter is observed (Milligan and Humphries 2010). In the Kiev Reservoir (Ukraine), half of beaver families did not stock wood for winter and foraged for only grassy vegetation year-round (Panov and Legeyda 1981).
Consumption of edible plant species leads to a change in the structure of communities. Selective foraging leads to a decrease in the number of dominant species and to a threefold increase in species richness of macrophytes (Law et al. 2014b). Studies from Ukraine have shown that beavers thin out Nymphaea thickets, thereby contributing to the introduction of other species, primarily submerged and attached plants (Dubyna 1982). Beavers’ prolonged habitation in small water bodies is likely to lead to the complete disappearance of Nymphaea thickets (Pankova and Pankov 2010). In the lakes of the Oksky Nature Reserve (usual size 10,000 m2, depth 1.5–2 m), due to beaver activities, plant communities changed from water lilies and pond weed to communities dominated by Stratiotes aloides. Helophyte thickets were thinned and decreased due to rising water levels and direct consumption by beavers, and Potamogeton perfoliatus completely disappeared from lakes (Pankova and Pankov 2010). In some cases, selective consumption of Nymphaea has no consequences for the plant community; in lakes in Scotland (4000–165,000 m2), species richness of grazed and ungrazed areas did not differ and new species did not colonize after Nymphaea alba was consumed, either due to the oligotrophic conditions of the water bodies, or because the effect of beavers was slight (Law et al. 2014a).
In addition to feeding on plants, beavers affect vegetation by changing the water level and increasing the littoral area, which contribute to overgrowth of helophytes and hydrophytes. The 15-year damming of outlets of the small lake Talaya Lamba (21,000 m2, depth up to 3 m) in northwest Russia led to a large deposition of silt in the littoral that negatively affected plant communities, which are now represented by monospecific patches of hydrophilic mosses and groups of 1–3 species of vascular plants (Timofeeva 2014). The increase caused by beavers in the water level of Lake Purwin (13,800 m2, max depth 4 m) in northeast Poland, and its enrichment with dissolved organics led to the overgrowth of the coastal peatland by Phragmites australis (Gałka and Apolinarska 2014). After the beavers flooded and subsequently left the small lakes of Karelia (northwest Russia), swamp communities dominated by Sphagnum russowii developed on abandoned settlements (Danilov et al. 2007). In Finland, damming of flowing lakes did not cause significant changes in species structure; however, macrophyte stands became significantly thinner (Nummi 1989). Even if beavers have an insignificant effect on the water regime of flowing lakes, their activity leads to fluctuations in the water level, which are of great importance to the vegetation (Van Geest et al. 2005; Bornette and Puijalon 2011). Moreover, even small changes in water depth play a key role (Roznere and Titus 2017) and affect germination of seeds of various species (Coops and van der Velde 1995; Fraser and Karnezis 2005).
Construction of burrows leads to topographic changes in the lake bottom, which alters the distribution of different groups of aquatic vegetation. Studies on the riverine lakes of the Oksky Nature Reserve showed that large numbers of burrows reduced the amount of aquatic vegetation, with emerged plants spreading mainly along shores unsuitable for digging activity, or on water bodies with soft shores where beavers lived mainly in lodges (Pankova and Pankov 2018).
Also beavers’ activities lead to changes in lacustrine and terrestrial vegetation due to trampling. Studies on the Big Tatkul (2.5 km2, depth up to 3.5 m) and Bolshoi Ishkul (2.7 km2, depth up to 14 m) lakes in the Southern Urals, Russia, showed that along beaver paths, the species composition of plant communities changed, plant cover and height decreased, the stratification was disturbed, and the proportion of dead phytomass reached 63–73% of the plant communities (Korobeynikova and Dvornikova 1983).
Beaver dwellings also affect vegetation. Studies of beaver lodges in northeast Poland (including lake settlements) showed that the biodiversity of the flora on lodges is significantly greater than that of the flora in adjacent habitats. These differences are ensured by the presence of therophytes and synanthropic plant species that are resistant to adverse conditions (Obidziński et al. 2011).
Dead wood deposition in lakes as a result of the foraging and construction activities of beavers increases the diversity of fungi and lichens (Vehkaoja et al. 2017).
Planktonic invertebrates are mentioned only in three studies (Nummi 1989; Hood and Larson 2014a; Malison et al. 2014), and the consequences of beaver activities for these organisms are not well described and discussed. Beavers may affect lake zooplankton by total alteration of the water body by changing it from a temporary to a permanent one, or just by changing its habitat structure. The literature on zooplankton in small water bodies suggests a significant effect of beavers on zooplankton through alteration of vegetation. As noted above, selective foraging for macrophytes, creation of the littoral, and topographic changes in the lake bottom lead to heterogeneous and mosaic lake vegetation. Mosaicity has been shown to be a key factor for zooplankton—the availability of shelters leads to redistribution of these organisms in the water (Celewicz-Goldyn and Kuczynska-Kippen 2017).
In addition, plankton may be affected by significant changes in the hydrochemical characteristics of water. In the literature reviewed, references to the impact of beavers on plankton were associated slightly with mentions of hydrochemical alterations (φ = 0.35, p = 0.04). The process of zoogenic eutrophication contributes to a change in zooplankton communities. According to Nummi (1989), the number of Cladocera significantly decreased after the second year of damming, and became similar to the number in an eutrophic pond, which was connected to organic loadings from flooded meadows. In addition, experimental studies have shown that excretory products of beavers can lead to a change in the structure of zooplankton communities—the change caused by beavers to the environment creates favorable conditions for large Daphnia species, which in turn determine the conditions for the existence of small-sized species (Krylov et al. 2016).
The direct impact of beavers is expressed in the creation and maintenance of unique microhabitats for macroinvertebrates (Hood and Larson 2014a). According to Khitsova et al. (2010), in riverine lakes changed by beavers, maximum densities of benthic invertebrates are observed in the area of beaver building activity (91 species versus 22–39 in other parts of the lake). This is due to the input of biogenic compounds by beavers and the appearance of new microhabitats. The literature review showed that hydrochemical changes by beavers and improved heterogeneity were significantly associated with the impact of beavers on these organisms (φ = 0.37, p = 0.005 and φ = 0.44, p = 0.0003, respectively). Invertebrate competition increased in places of beaver movements and junction parts of canals and around beaver burrows (Khitsova et al. 2012). On lakes in Canada (260,000–570,000 m2), beaver lodges serve as oases for invertebrates in the conditions provided by rocky and sandy shores, which prevent the growth of macrophytes. Therefore, higher input of woody debris by beavers leads to greater species richness and density of benthic and epibenthic organisms (France 1997).
A large amount of woody debris in the water creates additional shelters, and contributes to the appearance of new taxa in the pond. Deadwood can increase the diversity of saproxylic beetles (Mourant et al. 2018). Accumulation of building material and increase in littoral plant stands lead to increased supply of substrate for periphyton (Makarevich et al. 2016). An increase in surface area due to the appearance of additional vegetation contributes to an increase in biofilm area, which improves the food supply of periphyton (Wolters et al. 2019).
Beaver channels also contribute to the increase in biodiversity of invertebrates. Transport of woody material through the channels leads to the release of plant debris and associated terrestrial invertebrates into the water, which increases the availability of prey for aquatic organisms (Hood and Larson 2014a).
Dams created by beavers in flowing lakes affect the shoreline invertebrate fauna. After beavers moved elsewhere, the biodiversity of the former pond at the edge of Lake Petilambi (Karelia, northwest Russia) increased and the number of soil faunal species and terrestrial invertebrates doubled. Moreover, in stream settlements, beaver activity did not lead to changes in the abundance of soil fauna (Fyodorov and Yakimova 2012). In cases when beavers affect the hydrological regime of lakes, a change in the biodiversity of macroinvertebrates may occur similar to changes in watercourses after construction of beaver ponds, which is well described in the literature (Bush et al. 2019; Washko et al. 2019). In addition, changes in the structure of invertebrate communities may be under the influence of indirect effects through vegetation and zooplankton, which improve the availability and diversity of food resources.
The consequences for fish of beaver activity on watercourses are quite obvious—construction of dams leads to transformation of lotic ecosystems into lentic ones. Creation of littoral zones and habitat alteration in lakes described above also cause a significant redistribution of fish in water bodies. Moreover, these changes may have extremely negative consequences: as noted by Dyakov (1975), burrowing activities of beavers in small lakes with sandy shores lead to a shallow and unsuitable water body for fish.
In oxbow lakes, beavers are a highly important factor for the existence of fish. Floodplain complexes are significant for reproduction of many fish species (Naus and Adams 2018), while being connected to a riverbed is of key importance (Glińska-Lewczuk et al. 2016). Maintaining connectivity between ponds in floodplains, creating dams in seasonally flowing waters, and deepening are all aspects of beaver activity that may be necessary for fish. Changes in vegetation structure and improved connectivity in peatland bog ecosystems favor the existence of multi-species fish communities (Ray et al. 2004). Malison et al. (2014) showed that by changing connectivity and hydrological dynamics of riverine lakes, beavers have the ability to significantly affect populations of juvenile salmonids, and this impact changes from positive in early-successional beaver impoundments to negative in late-successional ones. By redistributing the volume of water within floodplain systems, beavers provide a choice of different feeding habitats for fry. By conserving water through the construction of dams, beavers provide wintering habitats for the salmonids Oncorhynchus tshawytscha and O. kisutch.
In addition, beavers’ digging activity may lead to the creation of new habitats for some fish species. Barabash-Nikiforov (1950) in studies located in the Voronezh Nature Reserve (including lake settlements) found Misgurnus fossilis, Lota lota and Silurus glanis in beaver burrows.
According to France (1997), Chrosomus spp., Phoximus spp., Cottus cognatus, Culea inconstans, and Catostomus commersoni prefer the adjustments of beaver lodges in lakes, because of the input of woody debris. Submerged tree remains significantly change the biodiversity and structure of fish communities, creating shelters and habitats for food organisms (Werner et al. 1983; Angermeier and Karr 1984; Schneider and Winemiller 2008; Sterling and Warren 2018). In addition, the changes in lacustrine plant communities described above, as well as the related changes in the structure of invertebrate communities, undoubtedly affect the composition and quality of food supply for fish.
As in the case of fish, the impact of beavers on streams has significant consequences for amphibian fauna since new habitats are created for spawning. Natural lakes serve as the usual habitats and spawning grounds for most species of amphibians. In many cases, beaver activity changes their habitats in lakes to a lesser extent than those in rivers. However, beaver activity can also positively affect amphibians in lakes. A study by Anderson et al. (2015) on the Cooking Lake Moraine system in Canada (2400–88,600 m2, less than 2.5 m in depth) showed that creation of channel systems promoted resettlement and redistribution of amphibian juveniles after metamorphosis. The availability of additional littoral areas increases the number of spawning grounds for the species that spawn in shallow waters. In addition to generating shallow water, beavers improve the quality of lake habitats for amphibians by creating light windows in the forest canopy, which improves heating and increases the concentration of organic materials and nutrients, affecting the food supply of adults and tadpoles (Vehkaoja and Nummi 2015; Vehkaoja 2016).
Maintenance of the water body and its transformation from temporary to permanent may allow tadpoles to undergo metamorphosis. Studies of small riverine lakes in the forest–steppe zone of European Russia showed that a significant number of tadpoles of Pelobates vespertinus died when the floodplain water bodies dried up (Bashinskiy et al. 2019). Creation of dams by beavers on temporary flowing channels could save parts of the water bodies.
Studies on small river valleys in northwest Russia (Bashinskiy 2014) showed that beavers significantly changed small riverine lakes, which at first were completely flooded by beaver ponds, and after the beavers left, they represented a completely different water body (Fig. 2). Before damming by beavers, these lakes had less light and, accordingly, were less attractive to spawning amphibians in the spring. After successive flooding and draining of beaver ponds, heating of the water in such lakes improved due to removal or loss of tree canopy cover, and lake depth increased due to digging activity. This led to alteration of plant communities and greater diversity of habitats for spawning. As a result, the diversity of amphibians, abundance of their larvae, and success of metamorphosis increased in these water bodies. However, in some cases, beaver-transformed oxbows dried up more intensively, which led to the death of amphibian larvae (Bashinskiy 2014).
The appearance of lodges and burrows can also be a positive factor for amphibians—they serve as shelters and places for wintering. Tadpoles of Lithobates clamitans and adults of Notophthalmus viridescens were found significantly more often near beaver lodges compared to other areas of the littoral zone of lakes (France 1997). Barabash-Nikiforov (1950, 1959) found large numbers of frogs (Rana sp. and Pelophylax sp.), and periodically encountered Bufotes viridis, Bombina bombina, and Lissotriton vulgaris, in beaver burrows and passages, including flooded ones. In the beaver population in the Khopersky Nature Reserve, where the overwhelming majority of settlements (79%) are located in lakes, amphibians were found in 89.76% of the beaver burrows (Dyakov 1975). Since tadpoles need periphyton for nutrition, increased surface area due to the appearance of woody debris and overgrowth of the littoral zone can also be considered a favorable factor associated with beaver activity.
The effect of beavers on reptiles is not obvious and not well understood. It is clear that beaver ponds, instead of watercourses, contribute to increased diversity of aquatic species (for example, terrapins and grass snakes); this is well demonstrated for streams (Russell et al. 1999). Changes in woody vegetation lead to the appearance of “beaver windows” that improve illumination and provide new places for basking, which is favorable for reptiles. However, in lake ecosystems, this effect is less apparent. An exception is oxbows of small rivers at sites of abandoned ponds, where, due to the loss of flooded trees, illumination and temperature change; such an effect is described in the previous section.
As in the case of amphibians, the burrows constructed by beavers play an important role by serving as shelters and wintering places for reptiles. The studies of Barabash-Nikiforov (1950) and Dyakov (1975) in the Voronezh Region, central Russia, showed that six species of reptiles sheltered in beaver dwellings—Emys orbicularis, Natrix natrix, Vipera berus, Coronella austriaca, Anguis fragilis, and Lacerta agilis. Moreover, when these two studies are compared, the occurrence of reptiles in burrows is significantly higher in the “lake” Khopersky beaver population. Beaver burrows are used to the highest extent by grass snakes (51.97% of burrows), adders (11.81%), and sand lizards (9.45%). Pond terrapins and grass snakes were also found in lodges on wetlands and lakes of the Slobozhansky National Natural Park, Ukraine (Brusentsova and Ukrainskiy 2015). In addition, indirect changes in the habitats of invertebrates, fish, and amphibians caused by beavers may significantly improve the food supply of reptiles living in lakes.
The free movement of birds between water bodies makes it difficult to assess the specific role of beavers in relation to them in lake ecosystems. Nevertheless, an increase in the littoral area and growth of helophytic and hydrophytic vegetation, as well as related increases in food supplies, can attract a number of bird species to water bodies with beavers. A series of studies on dammed lakes in Finland (Nummi and Pöysä 1997; Nummi and Hahtola 2008; Nummi and Holopainen 2014; Nummi et al. 2019b) showed that the increased abundance of invertebrates associated with alteration of habitat structure is of great importance for abundance and biodiversity of birds. Studies on Elk Island National Park (southwest Canada) showed that beavers affect nesting by Branta canadensis due to the earlier appearance of open water in the spring in the area of winter stocks and outcrops and the smaller snow cover in active beaver settlements (Bromley and Hood 2013).
Studies on flooded lakes in Finland have shown a significant increase in the abundance of mammals in beaver habitats (Nummi et al. 2019a). Beavers have an impact on lake ecosystems by altering vegetation along shores and creating new shelters for mammals. In particular, the role of beaver burrows is well known—they are used by many mammals as shelters and habitats (Ulevičius and Janulaitis 2007; Samas and Ulevicius 2015). According to data from the Voronezh region, in the predominantly “lake” Khopersky beaver population (Dyakov 1975), the occurrence of mammals in burrows is significantly higher than in the more “riverine” beaver population of the Voronezh reserve (Barabash-Nikiforov 1950). The incidence of Ondatra zibethicus was 16.54 and 1.6%, Arvicola amphibius was 80.32 and 40%, and small rodents were 80.32 and 16.8% in the “lake” and “riverine” beaver burrows, respectively. The foraging activities of beavers affect herbivorous mammals that have similar food preferences, e.g., Lepus timidus and Alces alces, which are attracted to beaver feeding grounds (Danilov et al. 2007). Mammals feel the impact of beavers indirectly, through changes in food supplies. For example, lakes inhabited by beavers have a positive effect on bats, which is determined by insect abundance (Nummi et al. 2011).
Beaver digging activity could have negative consequences. The previously mentioned decrease in the depth of Lake Moksha (central European Russia) caused by beavers led to the disappearance of Ondatra zibethicus (Babushkin 1993). It is worth mentioning in this section the study on the Masurian lakes, Poland (Sroka et al. 2015), which showed the potential role of beavers in parasitic infection of water bodies—the neighborhood of beaver dwellings was consistently contaminated with cysts of Giardia duodenalis and Cryptosporidium spp., which are dangerous to domestic animals and humans. The role of beavers in the spread of beaver fever is well known (Appelbee et al. 2005).
Conclusions and perspectives
In comparison with the consequences of landscape transformation by beavers for biotic components of river and lake ecosystems, the main differences are clear. In flow systems, large lentic habitats appear, which did not exist before beavers, or were only present locally, for example, as backwaters. Therefore, in sites with beaver activity, biodiversity is altered due to changes in species and communities: new species appear that prefer pond ecosystems, whereas rheophilic species disappear. In lake ecosystems, beaver activities do not lead to changes in type of habitat; therefore, the species and communities remain the same. However, ecosystem structure changes due to possible significant changes in the quality of the environment, which are described above. Thus, beaver activity in rivers leads to an increase in biodiversity, and beaver activity in lakes leads to conservation of the biodiversity of limnophilic components of aquatic ecosystems. Therefore, to preserve existing species, the activities of beavers are of greater importance specifically to lakes, rather than to watercourses where beaver abandonment of ponds and their subsequent destruction disrupt lentic habitats. Besides, beaver ponds on rivers influence limnophilic species in a more negative way. Colonization of beavers’ former stream habitats by these organisms could last for some years, and they may face difficulties as pioneer populations.
An example is amphibians, most of which do not breed in flowing waters. For this reason, construction of a beaver pond leads to gradual arrival of new species in the section of the river inhabited by beavers (Bashinskiy and Osipov 2016). At the same time, these species have always spawned on nearby oxbow lakes; therefore, beaver settlements and even significant changes in habitats are not able to increase their numbers. However, the improved water regime and heterogeneity and connectivity of habitats significantly improve food supplies and the survival of tadpoles, leading to successful metamorphosis.
A similar example is benthic invertebrates, especially gastropod mollusks dominant in lentic habitats, which benefit from beaver-created microhabitats (Khitsova et al. 2010). Therefore, the conclusion that beaver activities are of high importance for conservation of limnophilic complexes is valid for all biotic components of aquatic ecosystems, except for edible aquatic vegetation. The differences in effects of beavers between lakes and rivers are less for terrestrial mammals and birds.
Those conclusions mainly refer modern situation when on many territories ecosystems faced impact of anthropogenic activities and climate changes along beaver recolonization. During natural successions of beaver habitats effects on biodiversity could be less appeared because aquatic communities adapted to cyclic occurrences of activities of the key species throughout their evolution. Also, at the landscape level the situation could be also different. Though biodiversity of individual lakes or group of them do not increase due to beaver activities, changes of heterogeneity and connectivity could promote various successions of habitats that could impact biodiversity in the future.
In addition to positive impacts on biota, that was mentioned in 23 papers (48% of all publications), in some cases authors describe negative consequences of beaver activities in lakes. Such effects were documented in 12 studies (25% of all publications). Negative effects were observed in riverine lakes more often (46.7% of the publication) than in postglacial lakes (18.2%). All these studies were conducted in small and extra-small lakes. Most of the negative effects concern vegetation (5 publications) due to beavers foraging for edible species and flooding of the shores. The association between plants and negative impacts in the studies analyzed was statistically significant (φ = 0.48, p = 0.002). In addition to vegetation, some fish populations faced disturbance after beaver building and digging activities (mentioned in 3 studies). Although, a strong negative impact on fish is debatable, because beavers and native fish coevolved and shared the same habitats for centuries, after the long absence of this key species from habitats and great alteration of ecosystems by humans, the return of beavers could be critical in some cases. Other negative impacts of beavers in lakes were found for plankton (decrease in abundance), amphibians (dying of larvae), and mammals (degradation of habitats and increase in parasite numbers). In some cases, e.g., vegetation and amphibians, the negative effects appear after beavers abandon territory and dams break down.
Based on the major differences between lentic and lotic ecosystems (Wetzel 2001; Protasov 2008), two specific impacts of beavers on lakes could be distinguished. Firstly, changes in the structure of aquatic vegetation are of greater importance in lakes. Secondly, digging activity has a significant effect on lakes, leading to high impacts on the topography of the lake bottom and shoreline as well as on connectivity, since most lentic ecosystems are not initially susceptible to alluvial processes and deposits.
Beaver activity changes the structure of aquatic and subaquatic vegetation to the greatest extent, which in turn affects other components of ecosystems—algae, invertebrates, birds, and mammals. The digging activity of beavers lead to the appearance of additional habitats for invertebrates, fish, reptiles, and small mammals. The direct impact of beavers in the form of changes in the water regime, structure, and connectivity of habitats has a significant influence on fish and amphibians. A direct change in the hydrochemical properties of water has a direct effect on phytoplankton and macrozoobenthos.
In terms of various aspects of beaver impact, there is a large number of studies on beaver alteration of environmental heterogeneity (39.6% of the studies analyzed) and habitat connectivity (22.9%). More than half of the publications (60.4%) describe effects of beavers on hydrological and hydrochemical characteristics of lakes. Of the biotic components, vegetation changes are most reported (22.9% of all articles). There is a good understanding of changes in macroinvertebrate communities in beaver lakes (16.7% of all articles). Beaver impacts on mammals (14.6% of all publications), amphibians (10.4%), and fish (8.4%) are topics addressed to a lesser extent in the literature. Algae and phytoplankton (4.2% of the publications), zooplankton (6.3%), reptiles (4.2%), and birds (6.3%) are least likely to be mentioned.
There are obvious methodological difficulties in studying lakes inhabited by beavers. In studies on lotic ecosystems, non-beaver streams are chosen as control habitats. However, selection of lakes not populated by beavers is more difficult. While beaver activities in river systems result in obvious differences between affected and unaffected habitats, beaver action in lake systems does not have such obvious consequences. The differences between inhabited and uninhabited lakes may be related to initial differences between water bodies, which always exist even in closely located water bodies with the same origin. Therefore, it is difficult to distinguish the role of beavers from other natural processes, especially those with indirect influence. In addition, even after beavers leave, the consequences of their activities persist for many years (for example, due to the presence of woody debris or succession of aquatic vegetation). In this case, the currently unoccupied control lake might have previously been changed, and its existing features would be a consequence of past beaver activity. To overcome these methodological difficulties, many years of monitoring of lake habitats are required. Research on the effects of beavers on lakes could be planned with correct controls in territories with pioneer beaver populations, where it is possible to survey pre-beaver conditions. In other cases, a proper approach may be choosing altered and control habitats within one lake and studying beaver-created microhabitats.
In addition to methodical issues, this review found some weaknesses in data in the literature. Many studies do not have information on specific characteristics of habitats and lakes. For the authors, the features of the study territory may be clear, but for the readers, especially from other countries, more detailed descriptions are necessary. In addition, some population studies do not specify the habitats (lake or stream) in which beaver settlements are present. If this information had been given, this review could have relied on many more publications.
Based on the results of this review, some unresolved issues and perspectives are identified for further studies on the impact of beavers on lentic ecosystems:
Despite the fact that alteration of the water regime is well studied in processes caused by damming of flowing lakes by beavers, changes caused by beavers to seasonally flowing lakes, as well as drying lakes, have almost no qualitative descriptions and quantitative measurements;
The processes of water pollution by beavers and subsequent zoogenic eutrophication of water bodies, which could potentially have serious negative consequences for small lakes, especially those located in areas with relatively low external water exchange (the southern part of the range, forest–steppe, and steppe regions) are insufficiently studied;
Beaver impact on the stability of the water regime and maintenance of the water body could have a key influence on organisms that depend on lentic habitats during metamorphosis—amphibiotic insects and amphibians—but information on their reproductive success in beaver-altered lakes is lacking;
Few studies have focused on changes in phytoplankton and zooplankton communities caused by beaver activities, which are affected by alteration of habitat and water quality and the trophic structure of water bodies;
The effect of beavers on parasitocenoses in water bodies—occurrence of parasites indirectly associated with eutrophication (McKenzie and Townsend 2007)—is an important topic of study, as well as the indisputably significant influence of beavers on benthic fauna, a significant proportion of which is gastropod mollusks, which are intermediate hosts of many species of trematodes;
Coarse woody debris is of great importance to many components of lake ecosystems, but there are a few studies assessing beaver input of deadwood in water.
In general, all these issues are largely related to the insufficient knowledge of the consequences of beaver activity in riverine lakes. The relevance of studying these water bodies is determined by global environmental changes. At present, disturbance of the hydrological regime is observed throughout Europe, which interrupts water exchange between oxbows and the main river and between oxbows themselves, leading to significant changes in aquatic communities (Paillex et al. 2013; Hill et al. 2017; Bashinskiy et al. 2019). Under such conditions, beaver activity could provide water exchange and connectivity, and even maintain floodplain water bodies. Considering climatic changes and anthropogenic impacts, which lead to reduced flooding and degradation of floodplain ecosystems, beavers could become an additional aid to saving small lakes and their biotic components.
Most studies (67.2%) considered in this review are about lakes with a surface area less than 100,000 m2. If only those studies that discuss beaver impacts are taken into account, 72.9% focus on small water bodies. Such water bodies have been underestimated by researchers for many years; however, more and more studies have recently been published showing the importance of small lakes and ponds (Oertli et al. 2009; Downing 2010). The disproportionately high intensity of many processes in the ecosystems of small lakes allows them to play an unexpectedly large role in global cycles (Downing 2009), especially in nitrogen and phosphorus cycles (Céréghino et al. 2013). Small water bodies perform many important ecosystem functions, increasing not only local, but also regional biodiversity (Lemmens et al. 2013). They are characterized by the highest index of rare species and gamma diversity (Davies et al. 2008). Therefore, small lakes and ponds should be the main object of protection of aquatic biodiversity. This reflects the ability of beavers to preserve limnophilic species and complexes in lakes, especially in territories that lack lentic ecosystems or where these ecosystems are disturbed and vulnerable. Conservation managers and agencies should consider beavers as a possible key factor in preservation of lake ecosystems.
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The author is very grateful to Nikolai Zavyalov for revising the draft manuscript. It is also necessary to express the author’s gratitude to Vitaly Osipov for many years of help and support in field research. Special thanks go to Anton Svinin for valuable advices.
This study was funded by the Russian Science Foundation (Grant Number 16-14-10323).
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Bashinskiy, I.V. Beavers in lakes: a review of their ecosystem impact. Aquat Ecol 54, 1097–1120 (2020). https://doi.org/10.1007/s10452-020-09796-4
- Castor sp.
- Ecosystem engineering
- Freshwater biodiversity