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Effects of beaver dams on invertebrate drift in forest streams
Utjecaj dabrovih brana na drift beskralješnjaka u šumskim potocima
Andreas Redin and Göran Sjöberg
Abstract
We aimed to assess the effects of beaver dams on the invertebrate drift fauna in five central Swedish boreal forest streams. Each stream was sampled once during the autumn, with drift traps placed upstream and downstream of the beaver dams. Drift densities (numbers/100 m3 water) were calculated. The invertebrates were determined, dried and weighed. No significant differences were noted in total drift densities or in the drift densities of pelagic species. The drift densities of benthic species were higher upstream of the dam, mainly because Ephemeroptera were more abundant in the upstream part. No significant difference was observed in diversity or dry weight. The functional feeding group ratio: filtering collectors/gathering collectors was significantly higher downstream of the dam.
Key words: stream invertebrates, drift fauna, beaver dam, beaver pond
Introduction
Uvod
Eurasian beaver was reintroduced to Sweden beginning in 1922. By early 1990´s the population had increased to over 100 000 individuals and beaver again occurred over a large part of the country’s landscapes (Hartman 2011).
The construction of dams by beavers alters the ecosystem in many ways (Naiman et al. 1988; Rosell et al 2005; Baskin et al. 2011, Zav’yalov 2011). Also, the cutting and decay of trees causes a substantial input of wood to the water, and zones with a more open canopy are created (Naiman et al. 1986). The alteration of the ecosystem by beaver is larger in low order streams (Naiman et al. 1986). In beaver ponds, typical running-water invertebrate taxa may be replaced by a community more similar to lakes or slow-running water (Sprules 1940; McDowell & Naiman 1986; Naiman et al. 1988; Rosell et al 2005). In central Sweden, typical pond taxa like Dytiscidae and Corixidae were abundant in high numbers in the pond (Rosell & Pedersen 1999). Sprules (1940) in Ontario, Canada, and Nummi (1989) in Finland found that larval densities of Ephemeroptera, Trichoptera and Plecoptera decreased in the over-dammed river bed, but the number of Chironomidae increased. The site immediately downstream of a beaver dam in USA exhibited lower Plecoptera and Trichoptera densities than upstream, but the total invertebrate, Diptera, Ephemeroptera and predator densities were higher immediately downstream of the beaver dam (Smith et al. 1991).
Beaver activities influence community functioning by increasing the absolute importance of collectors and predators, while decreasing the importance of shredders and scrapers at impounded sites. The dams can be an important

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substratum for certain taxa. In Alberta, Clifford et. al. (1993) found larger proportions of simuliid larvae on the dam than at the main stream sites indicating that beaver dams may be important for maintaining a lotic (flowing water) fauna in slow moving, low gradient streams. McDowell & Naiman (1986) found greater density and biomass in beaver ponds compared to riffles in spring and summer, but found no difference in autumn. Sprules (1940) found fewer insects (both number and taxa) emerging from a beaver pond compared to the same area when it was a preimpounded riffle. Naiman et. al. (1988) on the other hand found that the total number of species in beaver ponds appear to be similar to those in the natural stream channel.
Stream-dwelling organisms are often transported downstream in the water column in substantial numbers, and this phenomenon has been called drift (Allan 1995). The drift is composed of benthic animals, emerging or emerged insects on the water and planktonic species (Bailey 1966). The high numbers of drifting planktonic animals from lake outlets is well known (Allan 1995). The input of terrestrial animals in the drift can be substantial. Of the benthic fauna, Ephemeroptera, some Diptera and some Plecoptera and Trichoptera are the most common, in roughly that order. At a given time there is only a small percentage of the bottom fauna (less than 0.01 %) found in the water column above a unit area of bottom (Allan 1995; Elliott 1967). Drifting invertebrates are important food for some fish species (Allan 1995). Since pools serve as depositional regions, one might think that pools (and ponds created by beaver) could serve as a major brake on drift. Bailey (1966) claimed that pools trapped drift. Waters (1962) could see that drift out of the pool was about 20% less than drift of the riffles, suggesting some trapping in the pools, but other studies (e.g. Elliott 1967) give very little support to the proposition that pools trap drifting individuals.
Our purpose was to study how beaver activity affects the invertebrate drift of small boreal streams in central Sweden. This is an aspect of beaver activity that so far has been given very little attention. The following predictions were made:

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1. The drift densities of benthic animals are expected to be lower downstream than upstream of the pond. The amount of drifting invertebrates decreases with decreasing water velocity (Elliott 1967; Everest & Chapman 1972). Reservoirs effectively block the drift of benthic species which cannot survive in a lentic (still water) environment (Ward & Stanford 1980). Gönczi et al. (1986) found that the drift of benthic animals below hydro-power dams was reduced compared to rivers with no dams.
2. It is likely that the amount of pelagic invertebrates in the drift is higher downstream than upstream of the pond. When a stream is dammed, the lotic taxa of the benthos are expected to be replaced by organisms preferring a lentic environment (Baxter 1977). In beaver ponds lotic taxa may be replaced by lentic (Sprules 1940; Macdowell & Naiman 1986; Naiman et al.1988).
3. The drift densities of Plecoptera should be lower downstream of the pond. Plecoptera have been shown to decrease in the original stream bed after impoundment (Sprules 1940; Baxter 1977; Nummi 1989). In the site immediately downstream of the dam, Smith et. al. (1991) found lower densities of Plecoptera. Plecoptera are typically reduced or absent below dams (Ward and Stanford 1980).
4. It is likely that the functional feeding group ratio of filtering collectors to gathering collectors is higher downstream the dam. Clifford et al. (1993) found that the fauna of the wood and debris dams shows similarities to the fauna of lake outlets, and filter-feeding Simuliidae were abundant in high densities. Streams below reservoirs are characterised by a predominance of filter-feeding Trichoptera and Simuliidae (Ward & Stanford 1980). The pond accumulates large amount of fine particulate organic matter in the sediment so the amount of FPOM deposited in benthos (the food for gathering collectors) downstream of the dam probably is reduced (Naiman et. al 1986).
Study area
Područje istraživanja
The study was performed in central Sweden (63°N, 15,30–16°E), in the eastern parts of the county of Jämtland (Fig. 1). The beaver dams are located in the drainage basins of river Indalsälven and river Ljungan, at 270–380 m above sea level. The landscape is hilly with the highest peaks reaching an altitude of over 500 m. Mean temperature in January is –10 °C and in July +15 °C. The precipitation is 700 mm per year. The dominant tree species are Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and birch (Betula spp). Along the streamside there is also alder (Alnus incana) and small numbers of rowan (Sorbus aucu­paria), bird-cherry (Prunus padus) and sallow (Salix caprea). There is intensive forestry in the region, and the amount of old-growth stands is thus very low. According to local hunters, the first beavers were observed in the area during the 1950s. Today beaver is very abundant in the area, and most of the permanently flowing streams have or have had beaver present since late 1970’s, i.e. 20 years before the study (Andreas Redin, personal observation). Fish fauna present in small streams in this area is generally brown trout (Salmo trutta) and minnow (Phoxinus

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phoxinus). In Norrån also pike (Esox lucius) and bullhead (Cottus gobio) occurred (Sjöberg & Hägglund 2011).
Samples were taken from five first- to fourth-order streams, with quite different drainage areas and medium year flows (Table 1). The sites where field work was undertaken had to fill certain criteria. First, they should have a beaver dam that raised the water level substantially (Fig. 2B). Streams with small beaver dams raising the water level less than 0.5 m were not considered. The area downstream of the dam had to have a certain minimum water velocity for the functioning of the drift samplers (Fig. 3). To avoid dealing with large lake effects (which would make any possible effects of the dam more difficult to detect), we avoided beaver dams close to lake outlets. The amount of beaver activity was estimated by looking after signs of foraging activity or tracks

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in the mud. Combined with the condition of the dams and observations of any recent construction activity on them provided a good picture of whether beaver were present or not.
Materials and methods
Metoda rada
Drift samples were taken during the autumn of 1997, from August 29 to September 4. Each stream was sampled once during 4 hours, from 19.00 to 23.00 (daylight savings time). Six drift samplers (Figs. 2A, 3) were used. Three were placed downstream of the beaver dam within a distance of 1,5–4 m from the dam, depending on where most of the water had returned to the main stream bed. The other three were placed immediately upstream of the pond (Fig. 4). The samplers were held in position with iron rods driven into the stream bed.
All samples were preserved in 70% alcohol and then hand-sorted in the laboratory. Samples were weighed on a Mettler AD 160 balance after drying at 60° for 24 h. Drift density was calculated as the number of individuals in 100 m3 of water. Animals already dead at the time of the catch were not included. The animals were divided into a benthic and a pelagic group, with both free-swimming and  planktonic animals. Emerging water insects and terrestrial animals were placed in separate groups.
Benthic insects were divided into functional feeding groups according to keys in Merritt & Cummins (1996) and Merritt & Cummins (1978). Functional feeding group ratios are responsive to changes in food resource base (e.g., algae, litter, fine organics, prey) and are a useful ecological tool, indicative of various ecosystem parameters (Table 2). The following ratios were calculated: predators/total of all groups, filtering collectors/gathering collectors, shredders/total collectors and scrapers+filtering collectors/shredders+ gathering collectors.
Habitat features were measured at each site. Water temperature was measured with a digital thermometer. To measure pH, indicator paper was used (Merck Spezialindikator pH 4,0–7,0). The height, width and length of the beaver dam were measured. Water velocity was measured by timing a weighted float over a marked distance. The bottom substrate was also categorised by ocular inspection at each site.

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The drift sampler (Fig. 3) was made of a metal tube with an opening diameter of 12 cm, which gave an effective sampling area of 113 cm2. The tapered net had a mesh aperture of 0.5 mm (Elliott 1970). At the back of the net, a 500 ml plastic bottle was fixed with a net at the bottom.
The residuals were tested for normality (Shapiro-Wilk W Test) and for equality of variance (O’ Brien test) to see if conditions were met for using Anova. A Two Way Anova test was used for the statistical analysis of all parameters except the functional feeding group ratios, where the residuals were not normally distributed. Drift densities were transformed with the fourth root (Allan & Russek 1985). The Friedman test was used for the functional feeding group ratios and before testing, the data from the three drift traps in each site were combined. Data from Lötbergstjärnbäcken were excluded from the functional feeding group ratios because there were so few individuals in this stream.
Results
Rezultati
Temperature and pH were only measured in the upstream part in every stream. Both pH and temperature were highest in the largest stream, Norrån (Table 3). The bottom substrate and water velocity varied both between different streams, and between the upstream and downstream sampling points (Table 3).
A total number of 3882 and 1758 animals were caught in the upstream and downstream parts respectively. In Kroktjärnbäcken, which had the highest catches (Fig. 5), the

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total drift densities exceeded 1000 individuals/100 m3 in two of the drift traps in the upstream part. In Lötbergstjärnbäcken, which had the lowest catches, no animals were caught in one of the traps in the upstream part. In Kroktjärnbäcken the family Leptophlebiidae was very abundant. The Ephemeropteran dominance was not that pronounced in the other streams. Among the pelagic group, Cladocera on average was very dominant, but most of them were found in Rismyrbäcken. In the other streams the number of Cladocera was low.
The drift differed significantly between the streams for most of the variables, and that the interaction between stream and the part analyzed was significantly different in most cases (Table 4). No significant difference between the upstream and downstream area was noted in the total drift densities (Fig. 5). The drift densities of benthic species were however significantly higher at the sites upstream of the dams. In this study no significant difference in the drift density of pelagic species was observed.
The drift densities of Ephemeroptera (Fig. 6) were significantly higher upstream of the beaver dams. For Diptera, Trichoptera and Plecoptera no significant difference was observed.
Diversity was studied as the number of aquatic genera identified. A total of 45 taxa were identified across all sites (table 5). No significant difference was observed in total diversity or in diversity of benthic and pelagic species. There was no significant difference in dry weight (Fig. 5).

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The ratio filtering collectors/gathering collectors (Fig. 7) was significantly higher downstream the dam, but for the other ratios no significant differences were noted.
Discussion
Rasprava
Beaver activity certainly alters the habitat and makes it potentially very unique (McDowell and Naiman 1986). The drift of the downstream sites is likely to receive animals from all these different beaver altered habitats – pond, dam, and downstream reach – and perhaps also from the area upstream the pond since drift distances can be quite long (Allan 1995).
Drift density of benthic species: Our first prediction, that the density of drifting benthos would be lower downstream the dams, was upheld. The significantly higher drift density of benthic species in the upstream part comes largely from more Ephemeroptera. This could be a result of: 1) the ponds trapping drifting individuals 2) reduced entry to the drift from the beaver pond or 3) reduced entry to the drift from the area downstream the dam. McDowell & Naiman (1986), Nummi (1989) & Sprules (1940) found that larval densities of Ephemeroptera decreased in the over-dammed river bed. There is no absolute correlation between the benthic community and the drift fauna. Certain genera can be present in high numbers but contribute very little to the drift, and vice versa (Bailey 1966). Sjöberg (unpublished.data) found Ephemeroptera in the ponds, but of the genera Cloëon that prefers slow-running waters and possibly is not a drift-prone taxon. The third explanation is not likely since Smith et al. (1991) found higher densities of Ephemeroptera downstream the pond. Sjöberg (unpublished data.) could detect no difference in Ephemeropteran density at the downstream site. Reduced entry from the pond caused by reduction and/ or change to genera which drift little is a possible explanation for the lower Ephemeropteran drift densities. If, however, all the Ephemeroptera that enter the pond would pass through, there would be no difference in drift density. Some trapping and maybe more importantly, predation from fish in the pond is also likely to occur and contribute to the result.

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Drift density of pelagic species: The second prediction, that the amount of pelagic invertebrates would be higher downstream of the dam, was not supported. In the ponds, typical running water taxa may be replaced by pond taxa (Sprules 1940; Nummi 1989; McDowell & Naiman 1986). Since we did not take any insect samples from the ponds per se we cannot say for sure that lentic taxa were abundant. In only one of our five streams was the dam made quite recently (Lötbergstjärnbäcken). In the other, older, ponds it is likely that a more lentic fauna has developed. No overall significant difference was, however, observed in drift densities of pelagic species between upstream and downstream sites. There may be suitable habitats for lentic invertebrates in slow-running parts and in lakes further upstream of the sampling point above the dam. Although we tried to find beaver dams far from lake outlets there might still be some lake effects in our results. It is likely that the distance to wetlands and lakes upstream overrides the local production of pelagic fauna in the pond itself (see Table 1).
Plecoptera: The third prediction, that drift of Plecoptera would be reduced downstream of the dam, was not supported by a statistically-significant difference. The results thus do not agree with the idea that the reduction of Plecoptera in the benthos of ponds and reservoirs (Sprules 1940; Ward & Stanford 1980; McDowell & Naiman 1986; Nummi 1989) and in the area downstream the dam (Ward & Stanford 1980; Smith et al. 1991) would reduce the Plecopteran drift densities.
Functional feeding groups: The fourth prediction, that the functional feeding group ratio filtering collectors to gathering collectors should be higher in the sites downstream the dams, was supported. The wood and debris dam created by beaver are often a suitable habitat for filtering collectors (Clifford et al. 1993). Streams below reservoirs are characterised by a predominance of filter-feeding Trichoptera and Simuliidae (Ward & Stanford 1980). FPOM is deposited in large amounts in the benthos of the beaver dam (Naiman et al. 1986). Gathering collectors that feed on FPOM in deposit are likely to have their food resource decreased in the downstream area.
Conclusions
Zaključci
Drift sampling is rapid and relatively easy and appears to be a useful method for studying effects of beaver activities on the stream community. Although it does not directly mirror the composition of benthos, it can be seen as an indicator of the conditions, and also gives a direct picture of food availability for salmonid fish.
Although the numbers of streams we investigated was not large, the functional group approach and analysis of trophic group ratios (Merritt & Cummins 1996) is, we suggest, a promising avenue for future studies, and allows testing of a priori hypotheses as we do here.
There was no general trapping of drifting animals in ponds created by beaver, so the area downstream the dam can have a rather rich drift. The drift densities of benthic animals are however reduced downstream of beaver dams, and beaver ponds in this case seem to have the same general effect on drift as large water power station reservoirs (Gönczi et al. 1986).
The drift differed considerably among the studied streams. The effects on the invertebrate drift are likely to differ considerably with the size of the stream, as well as the size and age of the dam. Finally, the condition of the dam and the distance to the nearest upstream lake may also impact the composition of the drift downstream.
Acknowledgements
Zahvale
We wish to thank Stig and Christine Redin, who helped with parts of the field work.
References
Literatura
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Elliott, J.M., 1967: Invertebrate drift in a Dartmoor stream, Arch. Hydrobiol., 63: 202–37.
Elliott, J.M., 1970: Methods of sampling invertebrate drift in running water, Ann. Limnol., 6: 133–59.
Everest, F.H., D.W. Chapman, 1972: Habitat selection and spatial interactions by juvenile chinook salmon and steelhead trout in two Idaho streams, J. Fish. Res. Board Can., 29: 91–100.
Gönczi, A.P., Henricsson, J., G. Sjöberg, 1986: Fisheries management in river reservoirs. Final report from the project Management of Fisheries in Hydropower Reservoirs, Part 1. Institute of Freshwater Research, Drottningholm, 115 p., Sundsvall (In Swedish)

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Hartman, G., 2011: The beaver (Castor fiber) in Sweden, Restoring the European beaver: 50 years of experience, Pensoft, 13–17. Sofia
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Sažetak
Cilj istraživanja bio je utvrditi imaju li dabrove brane utjecaj na drift beskralješnjaka u 5 šumskih potoka na području središnje Švedske. Uzorci su na svakom potoku prikupljani u jesen, koristeći kečere za hvatanje drifta postavljene uzvodno i nizvodno od ujezerenih dijelova potoka zajaženih dabrovim branama. Ulovi bes­kra­lješnjaka su osušeni, vagani i determinirani te je izračunata gustoća drifta (broj uzoraka/100 m3 vode). Uočeno je da nema značajne razlike u ukupnoj gustoći drifta kao i u gustoći drifta pelagičkih vrsta uzvodno i nizvodno od dabrovih brana. Gustoća drifta bentičkih vrsta bila je viša u dijelovima potoka uzvodno od dabrovih brana, uglavnom iz razloga što su vrste iz reda Ephemeroptera bile jače zastupljene u uzvodnim dijelovima. Nije zabilježena značajna razlika u suhoj tvari i raznolikosti vrsta. Utvrđen je sljedeći omjer organizama prema funkcionalnim hranidbenim skupinama: odnos broja filtratora koji skupljaju sitnu čestičnu organsku tvar u odnosu na brojnost sakupljača detritusa značajno je veći u korist prvih u dijelu vodotoka nizvodno od pozicije dabrove brane.
Ključne riječi: vodeni beskralješnjaci, faunalni drift, dabrova brana, dabrovo jezero

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