Provisioning of breeding habitat by beaver and beaver dam analogue complexes within the Great Salt Lake catchment
Authors: Wolf J. Marshall , Hammill Edd 
Published: February 2023

Abstract

Beaver are ecosystem engineers capable of converting free‐flowing lotic habitats into a series of lentic ponds, thereby enhancing the wetland area of a riverscape. Process‐based riverscape restoration using beaver reintroductions and mimicry (beaver dam analogues, BDAs) are increasingly used to restore functions, the provisioning of services, and improve the resiliency of ecosystems across North America and Europe. Beaver can create breeding habitat for a wide range of species within the highly imperilled class Amphibia by increasing wetland area, increasing emergent vegetation, prolonging wetland hydroperiod, and creating deep ponds. However, it remains unclear whether BDAs are creating suitable breeding habitat for amphibians by adequately emulating crucial ecosystem processes.

We investigated the relationships between beaver dam and BDA complexes with amphibian breeding occupancy across four catchments feeding the Great Salt Lake, U.S.A. We surveyed 24 beaver dam complexes and nine BDA complexes during the 2019–2021 breeding seasons with varying levels of water availability. We compared environmental characteristics between beaver and BDA complexes, including elevation, depth, and fish relative abundance among others. We then used occupancy models to evaluate factors affecting breeding occupancy of the most frequently encountered amphibian species, barred tiger salamanders (Ambystoma mavortium).

Without accounting for imperfect detection, at natural beaver ponds we recorded barred tiger salamanders in 58% of sites, boreal chorus frogs (Pseduarcris maculata) at 17% of sites, and northern leopard frogs (Lithobates pipens) at 4% of sites. Tiger salamanders were the only amphibian found at BDA sites, occupying 11% of those sites. We found that beaver complexes had 3 times less fish relative abundance, were 250m higher in elevation, 32 years older, and 0.43 m deeper than their BDA counterparts. Meanwhile, emergent vegetation height, water temperature, and hydroperiod demonstrated no significant differences between site types. BDAs modified ecosystem processes in a way that only partially replicated modifications by natural beaver dams.

Occupancy models suggested that fish relative abundance, altitude, age, and being at the top of the drainage network were the most important variables for predicting tiger salamander breeding occupancy. Overall occupancy rate of tiger salamanders was 60%, while BDA complexes had a modelled occupancy rate of 14% compared to 78% in natural beaver complexes. Survey year did not appear to be an important factor in tiger salamander occupancy despite varying levels of water availability.

Beaver and BDA complexes located near the top of their effective drainage or tributary junctions appeared to support amphibian along with fish species, thus boosting local diversity relative to a stream with only fish species. We therefore suggest future beaver restoration efforts target the top of effective drainages to benift both taxa. Overall, we found that while BDAs have the capacity to provide amphibian habitat, their younger age and higher fish abundance may explain why they are less effective than natural dams. Managers should therefore consider that the effects of restoration with BDAs may not be immediate, and future maintenance may be required.