How beavers affect riverine aquatic macroinvertebrates: a review

Abstract

Background: As ecosystem engineers, the construction of dams by beavers alters stream habitat physically and biologically, making them a species of interest for habitat restoration. Beaver-created habitat changes affect a wide range of aquatic invertebrate species. However, despite numerous individual studies of how beavers affect aquatic macroinvertebrate assemblages, there has been no evaluation of the consensus of these effects across studies.

Methodology: We collated and examined studies comparing beaver-created ponds to nearby lotic reaches to determine general trends in aquatic macroinvertebrate richness, density, biomass, and functional composition between habitats. From this evidence, we highight knowledge gaps in how beaver activity affects aquatic macroinvertebrates.

Results: Overall, in the majority of studies, aquatic macroinvertebrate richness was higher in nearby lotic reaches compared to beaver-created ponds, but richness at coarser scales (gamma diversity) increased with the addition of beaver ponds due to increased habitat heterogeneity. Functional feeding group (FFG) patterns were highly context-dependent, though predator taxa were generally more abundant in beaver ponds than adjacent lotic reaches. Site-specific geomorphological changes, coupled with dam or riparian zone characteristics and resulting differences in basal food resources likely shape other FFG responses.

Conclusions: We identify a lack of long-term studies at single or multiple sites and conclude that fine-scale approaches may improve our understanding of the dynamics of macroinvertebrates within the freshwater realm and beyond. Due to the context-dependent nature of each study, further systematic studies of beaver engineering effects across a wider variety of environmental conditions and wetland types will also help inform land and species management decisions, such as where to prioritize protection of beaver habitats in the face of a global freshwater biodiversity crisis, or where to restore beaver populations to deliver maximum benefit.

Keywords: Beaver pond; Functional feeding group; Lotic reach; Species richness.

Figure 1. Study locations included in the review and an example of a beaver-altered stream.

Map of the location for each study reviewed. Study locations spanned three continents: (A) North America (Castor canadensis), (B) South America (Castor canadensis, nonnative), and (C) Europe (Castor fiber). The numbered locations correspond to each study’s site, as listed in Table 1. (D) Studies included were for streams containing beaver ponds, such as the ponds in Scotland from study #10, pictured here.

Figure 2. Trends in species richness, density, and biomass of aquatic macroinvertebrates in beaver-altered streams.

Number of studies reporting highest aquatic macroinvertebrate richness, density and biomass per habitat.

Figure 3. Trends in functional feeding group abundances in beaver-altered streams.

Number of studies reporting higher abundance of each functional feeding group (FFG) in each habitat.