Using beaver dam analogs to restore riparian ecosystems influenced by large ungulates: a review for the southern Rocky Mountains
Year: 2020
Type: article
Abstract: Beaver (Castor canadensis) are recognized as ecosystem engineers and a keystone species due to their ability to modify landscapes to suit their needs.Beaver engineering activities are focused around constructing dams that alter both abiotic and biotic environmental components.Over millennia, the altering of system hydrology, geomorphology, and species compositions by beavers and dams change the shape and structure of landscapes by lowering stream gradients and increasing habitat heterogeneity.Although beaver, and the effects they create, were once ubiquitous across most of North America, centuries of over-harvesting and habitat loss led to steep beaver population declines.Concurrently, the removal of top-level predators, such as the gray wolf (Canis lupus), released large ungulates, including elk (Cervus elaphus) and moose (Alces americanus), from top-down control, allowing these ungulates to flourish and reach unprecedented densities throughout their ranges.As elk and moose increased in abundance, they competed with beaver for forage by consuming the smaller size classes of woody riparian vegetation (e.g., willow, Salix spp.; aspen, Populus tremuloides; birch, Betula spp.; and alder, Alnus spp.).In many places, elk and moose ultimately drove beaver from the landscape by preventing willow and aspen from regenerating.With beavers functionally extirpated, many ecosystems lost the ecological conditions associated with beaver meadow habitats including high water tables and complex, braided streams.In areas iii dominated by elk, beaver meadows transitioned to elk grasslands, wherein streams became incised and water tables receded across the floodplain, allowing upland grasses and shrubs to invade the former riparian zone.In moose dominated systems, beaver meadows transitioned to spruce-moose savannas, wherein woody riparian species favored by moose and beaver (e.g., willow, birch, and aspen) declined in abundance leaving grasses and unpalatable woody species (e.g., spruces, Picea spp.).As these new habitats are relatively simple compared to beaver meadows, biodiversity declined across all taxonomic levels.In recent decades, land managers and researchers have been mimicking beaver impoundments via structures called Beaver Dam Analogs (BDAs) to reverse these effects and restore beaver meadow habitats.In this review, I evaluated over 300 publications that examined the effects of natural and artificial beaver impoundments and of elk and moose browsing on abiotic and biotic ecosystem components as well as how these herbivores compete.I also reviewed detriments of beaver and their activities, including flooding, property damage, and disease.I concluded with a case study on Rocky Mountain National Park (RMNP) which is home to beaver, elk, and moose and has seen beaver-less ecosystem transitions play out since the 1910s.With regards to abiotic components, beaver dams tended to increase stream temperature, nutrient retention, water quality, and sedimentation above the dam as well as overbank flows, groundwater, and hyporheic exchange on the floodplain.Beaver dams tended to decrease stream flow rate as well as dissolved oxygen in the pond.Some of the literature indicates that beaver dams also decrease stream pH.With respect to biotic components, willows, aquatic macrophytes and invertebrates, fish, amphibians, birds, and small mammals all had larger total biomass in stream reaches with beavers than in non-impounded stream reaches.Beaver dams also tended to increase species vi ACKNOWLEDGMENTS An endeavor of this magnitude is never merely the work of one person and, while there are many to thank, I will restrict myself to the largest contributors.I would like to thank the Lord God for making my paths straight throughout this process.Truly, the many necessary pieces fell together far too easily to be the result of only my work and I am grateful.I am also grateful to my graduate committee, Dr. Paul Doherty, Jr., Dr. Dana Winkelman, and Dr. Ellen Wohl, for their support, guidance, and constructive criticism throughout this process.I am especially thankful for my advisor, Dr. Paul Doherty, Jr., for his undying patience and for giving me a chance when few others would have
Author: Nicholas C. Bartush
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
SDG: Life on land
Open Access status: green
OpenAlex ID: W3198441976