“Beaver dam capacity of Canada’s boreal plain in response to environmental change”

Nichole-Lynn Stoll & Cherie J. Westbrook

Scientific Reports volume 10, Article number: 16800 (2020) Cite this article

Abstract

“Environmental changes are altering the water cycle of Canada’s boreal plain. Beaver dams are well known for increasing water storage and slowing flow through stream networks. For these reasons beavers are increasingly being included in climate change adaptation strategies. But, little work focuses on how environmental changes will affect dam building capacity along stream networks. Here we estimate the capacity of the stream network in Riding Mountain National Park, Manitoba, Canada to support beaver dams under changing environmental conditions using a modelling approach. We show that at capacity, the park’s stream network can support 24,690 beaver dams and hold between 8.2 and 12.8 million m3 of water in beaver ponds. Between 1991 and 2016 the park’s vegetation composition shifted to less preferred beaver forage, which led to a 13% decrease in maximum dam capacity. We also found that dam capacity is sensitive to the size of regularly-occurring floods—doubling the 2-year flood reduces the park’s dam capacity by 21%. The results show that the potential for beaver to offset some expected climatic-induced changes to the boreal water cycle is more complex than previously thought, as there is a feedback wherein dam capacity can be reduced by changing environmental conditions.

Introduction

“Canada’s boreal forest has long been important beaver, Castor canadensis, habitat1. Intensive trapping of beaver in the 1700s through the 1800s caused near extirpation of the species in the boreal forest2. Population recovery began in the 1930s via re-introduction and conservation programs3. Beaver are keystone in supporting food security of Indigenous people residing in the boreal forest through providing a source of meat and being ecologically influential4,5. They are viewed as an ecologically influential species as they profoundly alter the aquatic ecosystems they occupy2 with benefits for freshwater biodiversity6 and terrestrial wildlife7. Beaver alter aquatic ecosystems primarily through engineering activities—dam and canal building—which modifies ecosystem-forming processes8. For example, beaver dams raise and stabilize water tables9,10, alter stream hydrographs11,12, enhance channel and riparian area sediment retention13,14 and create hydrologically complex, multi-channel networks9,15. The suite of changes beaver dams make to aquatic ecosystems creates a multitude of desired ecological feedbacks that collectively enhance ecosystem resilience to disturbance16.

“Environmental changes resulting from the cumulative impacts of disturbance-recovery dynamics and climate change17 are expected to considerably alter the ecological and hydrological character of the boreal forest over time18. Beaver are likely an important part of enhancing the resiliency of the boreal forest to environmental change19,20. Yet, recent and continued environmental changes will also impact the beaver population. Increases in the occurrence and severity of wildfires, for example, are expected in the future which will change the vegetation composition of boreal forests21. Fires renew beaver food sources, especially willow and aspen which they prefer, providing quality habitat for beaver 5–30 years afterwards22,23,24,25. Further, the growing oil and gas extraction industry in the western portion of Canada’s boreal forest26 is causing loss of beaver habitat but creation of new habitat in engineered landforms27. The way in which the beaver population will respond to the set of ongoing environmental changes is likely complex. For example, beavers may respond to climate change by modestly expanding their range, especially northward28, and densifying in their current range29.

“Many studies have compared beaver site occupancy with habitat characteristics; recent examples include Smeraldo et al.30 and Scrafford et al.31. But, few studies have asked how changing beaver habitat characteristics owing to environmental change will affect how many and where beaver dams are built along stream networks, information critical to understanding the effects of climate change on water resources. In this study we assess the landscape capacity of the boreal forest to support beaver dams under changing environmental conditions. We use the example of Riding Mountain National Park in Canada (Fig. 1) because the park is located at the southeastern limit of the boreal plain which is most vulnerable to climate change18; the park also has long-term records of beaver caches. Beaver were extirpated from the park and adjacent areas via trapping before 1930 and reintroduced in 194732. Ongoing aerial surveys of food caches since 1973 are used by the park to track beaver population dynamics33. To achieve our aim, we ran the BRAT model34 (see “Methods”) across Riding Mountain National Park under various scenarios to assess: (1) the capacity of the park’s 2604 km of streams to support beaver dams; (2) how changes in forest composition over the past three decades have affected beaver dam capacity; and (3) how climatic-induced changes in streamflow will influence beaver dam capacity. We find that the park can support a large number of beaver dams, and that beaver ponds store a considerable volume of water. However, changes to park vegetation cover and the size of regularly occurring floods will reduce the number of beaver dams the park can support. The approach used should hold significant value for use elsewhere, not only for understanding changes in beaver habitat and dam capacity under previously recorded change, but also for estimating changes under future land use and land cover change scenarios…”

Beavers play an every more important role in critical habitat, and to maintaining water supply for all waterfowl and many species fo fish. They maintain woods and forests, as any natural user co-evolving over millenia with their environment will do – a view at odds with many hunters, and agencies.

This article, appearing in Nature Scientific Reports is OPEN ACCESS, which means it is available to public viewing, and a PDF is also available for download. However, it is still copyrighted by Nature. We give you a sense of the urgency behind this article by supplying the URL, below,

https://www.nature.com/articles/s41598-020-73095-z

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