Beaver and Invertebrates

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

By building dams and lodges, beaver (Castor canadensis and Castor fiber) indirectly affect the abundance of a wide variety of insects and other invertebrates.  Before the beaver dam is built, streams may be dominated by lotic (fast-water) invertebrates, such as stoneflies (Plecoptera), mayflies (Ephemeroptera) and caddisflies (Trichoptera) (Collen and Gibson 2001).

After the beaver dam is built and an impoundment (pond) of water forms behind it, lentic (slow-water) invertebrates replace the lotic invertebrates.  Examples of lentic invertebrates that increase after beaver dam a stream include leeches (Hirudinea), dragonflies (Odonata), mussels (Pelycopoda), and  oligochaete worms (Oligochaeta)(Collen and Gibson 2001). 

In the nearby forest where beaver cut down trees and browse other plants, invertebrate life is also affected by the beaver.  In the following review, we look in more detail at the effects that beaver engineering has on the distribution and abundance of invertebrates.

Mosquitos

In the mountains of New York state, most pest mosquitos belong to the genus Aedes, and breed in temporary, snowmelt pools (Means 1979).  In this region, near the historic village of Cooperstown, a swamp forest and low area with remnant bog had large numbers of pest mosquitos before beaver impoundment.  After beaver colonized the area and built dams that created two ponds 15 and 30 acres in size, pest mosquitos that laid eggs in temporary pools could no longer breed in the wetland (Butts 2001; 2004). 

Thus, at this particular locality, beaver engineering reduced the number of pest mosquitos.  However, a weakness of this research was the low number of study plots examined both before and after damming. 

Butts (2001) further cautions that in regions where most pest mosquitos are species that breed in permanent ponds and lakes, beaver engineering might produce different results.  Also, 25 years after beavers first built dams at his study site, there is now evidence that a pest mosquito species of permanent water is slowly establishing itself along the periphery of the beaver ponds, among rooted, emergent vegetation (Butts 2004).

Benthic Invertebrates

In northeastern Quebec, McDowell and Naiman (1986) found that the following benthic invertebrates were most abundant at stream sites not impounded by beaver:

Black Flies (Simuliidae)
Tanytarsini Chironomids
Scraping Mayflies (Ephemeroptera)
Net-Spinning Caddisflies (Tricoptera)

In contrast, stream sites with beaver impoundments were dominated by:

Tanypodinae and Chironomini Chironomids
Predaceous Dragonflies (Odonata)
Tube Worms (Tubificidae)
Filtering Bivalves (Pelycopoda)

Thus, by building dams and creating impoundments, beaver increased the absolute importance of collectors and predators, while decreasing the relative importance of shredders and scrapers (McDowell and Naiman 1986).

Clifford et al. (1993) report that the Bigoray River in Alberta, is "almost a continuous series of beaver dams for long stretches;" making the stream appear more lentic than lotic.  The only fast-moving water occurs right at  the beaver dams, where water flows swiftly "over and through the wood-sediment substratum of the dam." 

Invertebrates associated with these beaver dams are those typical of  fast-flowing water, such as blackflies, Amphinemura stoneflies, Hydropsyche caddisflies and Baetis mayflies, while those of the slow-moving river are typical of lentic waters (e.g. Pisidium spp. Leptophlebia cupida, Caenis mayflies and Cladocerans (Clifford et al. 1993). 

During the summer, blackfly larvae and pupae comprise eighty percent of the beaver dam fauna and occur in great numbers, similar to what one finds at lake outlets (Clifford et al. 1993).  These researchers conclude that in slow-moving boreal streams, "beaver dams appear to be major sources of these economically important biting flies." 

During autumn, winter and early spring, when most blackflies are in the egg stage, chironomids are the most abundant invertebrates at beaver dams (Clifford et al. 1993).  Different taxa of chironomids (i.e. Chironimidae and Pisidium) are abundant in the slow-moving stream, where they comprise over 80% of the invertebrate community (Clifford et al. 1993).

Rolauffs et al. (2001) studied insect emergence from a mountain brook in Germany.  Three different aquatic habitats (undammed stream, beaver dam, beaver pond) of the brook all had different insects associated with them, but the insect fauna of the beaver dams was more similar to that of the undammed stream than that of the beaver pond.

In the littoral zone of headwater lakes of northwestern Ontario, France (1997) found that almost 100% of the large diving beetles and hemipterans in these lakes were associated with beaver lodges (France 1997).  Most invertebrate taxa, including, Amphipods, aquatic beetles (Coleoptera), large aquatic bugs (Hemiptera), mayflies (Ephemeroptera), flies (Diptera), caddisflies (Tricoptera), leeches (Hirundea), and Oligochaeta were more abundant amid coarse woody debris in shallow waters within 2 meters of beaver lodges than in shallow waters further away that had only a sand or rock bottom.  In contrast, crayfish (Orconectes virilis) were more abundant on rocky bottoms than amid the coarse woody debris of beaver lodges. 

France (1997) concluded that in boreal headwater lakes, where large aquatic plants (vascular macrophytes) are rare, beaver lodges may be important in structuring littoral communities.  This researcher also speculated that most macroinvertebrates were attracted to beaver lodges because there was more food for them there, and the physical structure of the coarse woody debris provided refuge from predators.

Forest Insects

The leaf beetle (Chrysomela confluens) is a forest insect that feeds primarily on leaves of juvenile cottonwood (Populus) trees.  After beaver cut down cottonwood trees, the stumps resprout new shoots that grow to be new trees. 

Martinson et al. (1998) found that Chrysomela confluens was 15 times more abundant on reprouts from beaver-felled cottonwood than on normal juvenile cottonwood sprouts.  When resprouting, the new shoots of beaver-felled cottonwood contain twice the amount of defensive chemicals (phenolithic glycosides) than normal juvenile cottonwoods.  In this way, a cottonwood discourages beavers from eating it again.  Chrysomela confluens larvae eat cottonwood leaves, convert them to salicylaldehyde, and store this chemical in their dorsal glands, where the larvae use it to defend themselves against predators. 

Martinson et al. (1998) showed that Chrysomela confluens preferred cottonwood resprouts because the higher concentrations of phenolithic glycosides and nitrogen there provide more protection and nutrition for the larvae.  In a series of experiments, these researchers found that Chrysomela confluens larvae which they placed on resprouts developed faster and weighed more at maturity, than larvae of the same group that they placed on normal sprouts.  In addition, when Martinson and coworkers fed Chrysomela confluens larvae to ants, the larvae that had eaten resprouts survived longer than the larvae that had eaten normal sprouts.  Thus, by changing the chemistry of the cottonwoods they fed on, beavers indirectly improved the abundance, growth and defense of the leaf beetle Chrysomela confluens.

Click the following links to learn more about the effects of beaver engineering on wildlife:
birds, frogs and salamanders, lizards, turtles and snakes, trees.
Click this link for the introductory review: Ecology of the Beaver.

References

Butts WL  (2001)  Beaver ponds in upstate New York as a source of anthropophilic mosquitos.  Journal of the American Mosquito Control Association 17: 85-86

Butts WL  (2004)  Changes in distribution and abundance of mosquito populations in an ecological research tract over a 35 year history.  Journal of the American Mosquito Control Association 20: 319-320

Clifford HF, Wiley GM, Casey RJ  (1993)  Macroinvertebrates of a beaver-altered boreal stream in Alberta, Canada, with special reference to the fauna of dams.  Canadian Journal of Zoology 71:1439-1447

Collen P, Gibson RJ  (2001)  The general ecology of beavers (Castor spp.) as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish - a review.  Reviews in Fish Biology and Fisheries 10: 439-461

France RL  (1997)  The importance of beaver lodges in structuring littoral communities in boreal headwater lakes.  Canadian Journal of Zoology 75: 1009-1013

Martinsen GD, Driebe EM, Whitham TG  (1998)  Indirect interactions mediated by changing plant chemistry: beaver browsing benefits beetles.  Ecology 79: 192-200

Means RG  (1979)  Mosquitos of New York, Part 1. The Genus Aedes Meigen.  Bulletin 430a.  State Science Service, New York State Museum, Albany

McDowell DM, Naiman RJ  (1986)  Structure and function of a benthic invertebrate stream community as influenced by beaver (Castor canadensis).  Oecologia 68: 481-489

Rolauffs P, Hering D, Lohse S  (2001)  Composition, invertebrate community and productivity of a beaver dam in comparison to other stream habitat types.  Hydrobiologia 459: 201-212

Information about this Review 

The author is:  Dr. Paul D. Haemig (PhD in Animal Ecology)

The photograph at the top of the page was taken by Kevin Williams (UK).  It shows a dragonfly, one of the invertebrate taxa that benefits from beaver engineering.

The proper citation is:

Haemig PD  2012    Beaver and invertebrates.  ECOLOGY.INFO #18.

If you are aware of any important scientific publications about beaver and invertebrates that were omitted from this review, or have other suggestions for improving it, please contact the author at his e-mail address: 

director {at} ecology.info

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