Can beaver dam analogue restoration support amphibians?
An introduction to Julianna and her dissertation research.
Frequently Asked Questions
Who is Julianna?...................................................................................
Why are beavers considered ecosystem engineers?.....................................
What is the connection between beavers and other wildlife?.........................
What is a beaver dam analogue?..............................................................
Who builds beaver dam analogues?...........................................................
How can we measure the impact of beaver dam analogues?...........................
Where did we conduct surveys?...............................................................
What did an amphibian survey entail?.......................................................
What amphibians live in BDA and beaver ponds?.......................................
Do all BDAs create habitat for amphibians?...............................................
Do salmon and amphibians both use BDAs?...............................................
What kinds of habitat do BDAs create?......................................................
Where can I find primary literature on this topic? (Sources)..........................
A single long-toed salamander egg attched to grass underwater. This is the sort of thing I'm looking for!
Hello! My name is Julianna Hallza, and I'm a PhD student at Washington State University. I'm interested in understanding how beavers impact other wildlife by damming rivers and creating ponds. Specifically, I'm studying the impacts of beaver habitat on amphibian breeding and trying to understand whether restoration that mimics beavers can facilitate amphibians in the same way that beavers do.
I'm also curious about the way that beaver habitat varies depending on climate, and I'm using the Cascade mountains as a natural laboratory to study beaver ponds across a gradient of high annual
Decrease downstream water temperature.1
Why are beavers considered ecosystem engineers? It's because their dams...
Create stillwater ponded habitat.4
Accumulate fine sediments.3
Increase flow and temperature heterogeneity.2
Increase lateral and hyporhiec* water flow.1
Beavers transform their environments from free-flowing (or lotic) stream habitats to still-water (or lentic) ponded habitats by building dams. Dams made of mud, sticks, and rocks allow some water to percolate through while holding much of the water back in a pond. 4 Ponded water has time to seep underground and recharge the groundwater; later, it will resurface downstream at cooler temperatures. 1 The ponded water also gets pushed toward the stream banks by the dam, which connects the stream mainstem to its floodplains. Floodplain depressions fill with water, creating side-pools and off-stream channels. This increases habitat complexity for wildlife, creating a diverse landscape that multiple organisms can coexist within. 4
*Hyporheic flow refers to ground water beneath the stream's surface.
5
Aerial foragers like birds and bats feed on insects above ponds.6,7
How does wildlife benefit from beaver ecosystem engineering?
Invertebrates that depend on still water thrive in beaver ponds, and overall invertebrate abundance increases.
Ungulates wallow in ponds and feed on vegetation that grows in beaver habitat.8
Amphibians lay their eggs in still water created by beaver dams.9
Salmonids benefit from cool waters and increased habitat complexity.10
What is a beaver dam analogue?
Willows or conifer bows are woven between the posts.
Beaver dam analogues (BDAs) are structures that mimic beaver dams.12 They are semi-permeable barriers to water flow, and they differ from weirs or hydropower dams because BDAs are made of natural materials like sticks and tree bows rather than cement. The result is that some water still flows through the structure, and fish are able to pass without problems. Just like true beaver dams, BDAs are expected to promote healthy hydrology that supports a variety of wildlife.
Multiple structures are placed in aseries to mimic a natural beaver "complex."
Water pools behind the structures, and slower moving water accumulates fine sediments.
Posts are driven into the stream bottom using a hydraulic pounder.
Who builds BDAs?
A variety of organizations build BDAs, often with a focus on creating salmonid habitat. The groups that build BDAs often only have capacity to rigorously monitor hydrology and can't monitor every wildlife species that might benefit from restoration sites. Even where monitoring is possible, it is impractical for a single organziation to constructenough BDA sites for a robust scientific study of the impacts. To address this issue, I'm working with four organizations that all do similar restoration work in the Okanogan-Wenatchee region...
Do BDAs really work as well as beaver dams?
Some research indicates that BDAs have similar effects to true beaver dams hydrologically, 13 but there is very little research concerning biological impactsof BDAs for wildlife (but see [14, 15]). Both true beaver dams and BDAs are temporary and eventually wash out, allowing the stream to fluxuate between a ponded and flowing state throughout time, although BDAs tend to be even more temporary than true beaver dams since there is not always a beaver to maintain the BDA. However, BDAs can have substantial impacts on degraded stream systems and help to prepare a stream for a real beaver to move in.
Do beaver dam analogues create breeding habitatfor
amphibians?
STUDY DESIGN: compare the percent of beaver dam analogue sites with pond-breeding amphibians to the percent of unrestored reference and true beaver dams with amphibians.
Unrestored Reference
Paired with an existing BDA site
5-1000 m upstream of the paired BDA
Matches the stream length of the paired BDA
No beaver influence
No restoration done
Beaver Dam Analogue (BDA)
BDAs installed in the last 4 years
BDAs are functioning (water has not overtopped structures, floods have not blown structures out)
Between 2 to dozens of structures
Between 30-1100 m stream length
No beaver influence
True Beaver Dam
Dam is holding water
Interspersed with BDA sites (not paired)
May be actively inhabited by beavers or abandoned
Between 100-500 m stream length
May include multiple dams or a single dam
No restoration done
Replication is key! We surveyed for amphibian breeding at 25 BDA sites, 25 paired unrestored sites, and 17 beaver ponds (67 sites total)! Where did we find all these sites? Sites are located on a mix of public and private property. Many sites are located in the Okanogan-Wenatchee National Forest, but lots ofbeaver dam analogue restoration is happening on private land, which is whywe needed landowner help to get the amount of replication we need for robust scientific results. To get these results, we surveyed every restoration site we could find and calculated the percent of those restoration sites with breeding amphibians. Then, we compared this percentage to the percent of unrestored reference and true beaver dam sites with breeding amphibians.
Working with restoration partners who create the BDA structures and landowners who allow access for surveying is the only way I can find enough sites to do a statistically robust study. THANK YOU to everyone who has helped put these sites together!!
Chelan
Omak
What does an amphibian survey look like?
At each site, our team of 2-3 people walked the water's perimeter, searching for amphibians.
As we searched, we recorded environmental parameters that help us assess habitat suitability and understand why amphibians lay eggs at some sites but not others.
When we found amphibian egg masses, larvae, or adults, we will recorded the species and number of individuals.
We surveyed each site twice between April and August..
Egg masses
What are we searching for?
salamanders and tadpoles
Young of year (frogs and salamanders that hatched this year)
Adult frogs and salamanders
We discovered that amphibians breed at higher rates in BDA sites than unrestored sites (24% vs. 0%). However, amphibians breed at BDAs at lower rates than at true beaver sites (24% vs. 88%). This means that beaver dam analogues provide intermediate quality habitat for amphibians - they're not quite as good as a true beaver pond, but they're better than no restoration at all!
* p < .05
p < .001
Why are there amphibians at some BDAs but not all of them? Not all BDAs create areas of still-water habitat, and amphibians need this completely ponded habitat to breed. While all BDAs slow the flow of water, only some of them fully impound water to create true ponds rather than pools. In contrast, most true beaver dams create still-water ponds. When there is still water present in reasonably large areas (~3 square meters) in either BDA or true beaver sites, amphibians usually breed.
Why aren't all BDAs made to create still water? Not all animals prefer the same kinds of habitat: salmonid (salmon and related fish) abundance is higher at BDAs on faster flowing streams than BDAs on slower flowing streams! Different BDAs are likely to support different widlife. This means that building BDAs in different kinds of streams will diversify the kinds of wildlife likely to benefit. Restoration can be targeted to support different species depending on the kind of habitat created.
We measured the unrestored flow just upstream of BDA restoration as a proxy for what the stream flow would be like without restoration. Then, we compared this stream flow to the abundance of both amphibians and fish (salmon and related species). Amphibian abundance is lower in BDAs on faster flowing streams, while fish abundance is higher in BDAs on faster flowing streams.
Abundance
Amphibians
Fish
Reference
BDA
Beaver Fish
Amphibians
Amphibians and fish both use the kind of habitat that is created by beaver sites and BDAs more than the habitat in unrestored sites. Habitat in beaver ponds is relatively deep and fast-flowing, ponds are wide, and there is often little canopy cover because beavers fell trees. In contrast, unrestored sites are often shallow, fast-flowing, narrow, and have high canopy cover. BDA restoration creates habitat somewere in bewteen unrestored sites and beaver sites. Both fish and amhibians use habitat that is similar to beaver habitat, so transforming unrestored habitat using BDAs can facilitate these animals. The figure above shows multiple habitat variables compressed into two-dimensional space for easy visualization of microhabitat used by each organism (blue and green elipses) and available habitat in each site type (elipses in shades of brown).
Sources
[1] Dittbrenner, B. J., J. W. Schilling, C. E. Torgersen, and J. J. Lawler. 2022. Relocated beaver can increase water storage and decrease stream temperature in headwater streams. Ecosphere 13:e4168.
[2] Majerova, M., B. T. Neilson, N. M. Schmadel, J. M. Wheaton, and C. J. Snow. 2015. Impacts of beaver dams on hydrologic and temperature regimes in a mountain stream. Hydrol. Earth Syst. Sci. 19:3541–3556.
[3] Meentemeyer, R. K., and D. R. Butler. 1999. Hydrogeomorphic effects of beaver dams in Glacier National Park, Montana. Physical Geography 20:436–446.
[4] Larsen, A.,J. R. Larsen, and S. N. Lane. 2021. Dam builders and their works: Beaver influences on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems. Earth-Science Reviews 218:103623.
[5] Barn swallow photo credit to JJ Cadiz.
[6] Ciechanowski, M., W. Kubic, A. Rynkiewicz, and A. Zwolicki. 2011. Reintroduction ofbeavers Castor fiber may improve habitat quality for vespertilionid bats foraging in small river valleys. European Journal of Wildlife Research 57:737–747.
[7] Nummi, P., S. Kattainen, P. Ulander, and A. Hahtola. 2011. Bats benefit from beavers: a facilitative link between aquatic and terrestrial food webs. Biodiversity and Conservation 20:851–859.
[8] Nummi, P., W. Liao, O. Huet, E. Scarpulla, and J. Sundell. 2019. The beaver facilitates species richness and abundance of terrestrial and semi-aquatic mammals. Global Ecology and Conservation 20:e00701.
[9] Romansic, J. M., N. L. Nelson, K. B. Moffett, and J. Piovia‐Scott. 2021. Beaver dams are associated with enhanced amphibian diversity via lengthened hydroperiods and increased representation of slow‐developing species. Freshwater Biology 66:481–494.
[10] Kemp, P. S., T. A. Worthington, T. E. L. Langford, A. R. J. Tree, and M. J. Gaywood. 2012. Qualitative and quantitative effects of reintroduced beavers on stream fish: Impacts of beaver on freshwater fish. Fish and Fisheries 13:158–181.
[11] McCaffery, M., and L. Eby. 2016. Beaver activity increases aquatic subsidies to terrestrial consumers. Freshwater Biology 61:518–532.
[12] Wheaton, J., S. Bennett, N. Bouwes, J. Maestas, and S. Shahverdian. 2019. LowTech Process-Based Restoration of Riverscapes: Design Manual. Version 1.0.
[13] Pollock, M. M., T. J. Beechie, and C. E. Jordan. 2007. Geomorphic changes upstream of beaver dams in Bridge Creek, an incised stream channel in the interior Columbia River basin, eastern Oregon. Earth Surface Processes and Landforms 32:1174–1185.
[14] Pollock, M. M., G. R. Pess, T. J. Beechie, and D. R. Montgomery. 2004. The importance of beaver ponds to Coho Salmon production in the Stillaguamish River Basin, Washington, USA. North American Journal of Fisheries Management 24:749–760.
[15] Wolf, J. M., and E. Hammill. 2023. Provisioning of breeding habitat by beaver and beaver dam analogue complexes within the Great Salt Lake catchment. Freshwater Biology 68:659–673.
[16] Map base layers are from ESRI and USGS.
All photos are my own unless otherwise marked.
