Beaver herbivory on aquatic plants

Abstract

Herbivores have strong impacts on marine and terrestrial plant communities, but their impact is less well studied in benthic freshwater systems. For example, North American beavers (Castor canadensis) eat both woody and non-woody plants and focus almost exclusively on the latter in summer months, yet their impacts on non-woody plants are generally attributed to ecosystem engineering rather than herbivory. Here, we excluded beavers from areas of two beaver wetlands for over 2 years and demonstrated that beaver herbivory reduced aquatic plant biomass by 60%, plant litter by 75%, and dramatically shifted plant species composition. The perennial forb lizard's tail (Saururus cernuus) comprised less than 5% of plant biomass in areas open to beaver grazing but greater than 50% of plant biomass in beaver exclusions. This shift was likely due to direct herbivory, as beavers preferentially consumed lizard's tail over other plants in a field feeding assay. Beaver herbivory also reduced the abundance of the invasive aquatic plant Myriophyllum aquaticum by nearly 90%, consistent with recent evidence that native generalist herbivores provide biotic resistance against exotic plant invasions. Beaver herbivory also had indirect effects on plant interactions in this community. The palatable plant lizard's tail was 3 times more frequent and 10 times more abundant inside woolgrass (Scirpus cyperinus) tussocks than in spatially paired locations lacking tussocks. When the protective foliage of the woolgrass was removed without exclusion cages, beavers consumed nearly half of the lizard's tail leaves within 2 weeks. In contrast, leaf abundance increased by 73-93% in the treatments retaining woolgrass or protected by a cage. Thus, woolgrass tussocks were as effective as cages at excluding beaver foraging and provided lizard's tail plants an associational refuge from beaver herbivory. These results suggest that beaver herbivory has strong direct and indirect impacts on populations and communities of herbaceous aquatic plants and extends the consequences of beaver activities beyond ecosystem engineering.

The role of forage availability on diet choice and body condition in American beavers (Castor canadensis)

“…We did not estimate biomass of some possible diet sources, such as submersed aquatic vegetation, nor some species of emergent aquatic vegetation (e.g., Sparganium spp. ; Parker et al, 2007). We also did not control for aquatic vegetation browsed by beavers before sampling occurred.…”

Including wildlife habitat in the definition of riparian areas: The beaver (Castor canadensis) as an umbrella species for riparian obligate animals

“…Studies documenting how beavers create or enhance habitat for a multitude of species include effects on the occurrence, number and diversity of aquatic plants (Ray et al 2001;Parker et al 2007), invertebrates (Collen and Gibson 2001), fish (Collen and Gibson 2001;Pollock et al 2004;Rosell et al 2005), birds (McKinstry et al 2001;Aznar and Desrochers 2008;Nummi and Hahtola 2008), amphibians (Cunningham et al 2006;Stevens et al 2007), and mammals (Anthony et al 2003;LeBlanc et al 2007). As such, the beaver can be considered an umbrella species for conservation purposes (Beazley and Cardinal 2004;Roberge and Angelstam 2004) throughout much of North America, as its geographic range extends from coast to coast and from the arctic tundra to northern Mexico (Müller-Schwarze and Sun 2003;Butler 2006).…”

“…Beavers find shelter and food (aquatic plants) in water but also eat terrestrial vegetation (forbs, grasses, bark) and cut woody vegetation on land for building lodges, dams and food caches (NRC 2002;Müller-Schwarze and Sun 2003;Parker et al 2007). Through these activities, the beaver (i) changes the flow regime of sections of a river and thus erosion and sedimentation patterns (Gurnell 1998;Butler and Malanson 2005;Butler 2006); (ii) shifts the aquatic environment from lotic to lentic upstream of the dam (Collen and Gibson 2001;Rosell et al 2005); (iii) increases wetland area through flooding (Naiman et al 1988;Cunningham et al 2006;Hood and Bayley 2008a) with attendant shifts from terrestrial to hydrophilic or aquatic vegetation (Ray et al 2001;Rosell et al 2005); and (iv) affects the composition, structure, and succession of the terrestrial vegetation in the riparian area (Martell et al 2006;Donkor 2007;Wright 2009).…”

Forage Preferences of the European Beaver Castor fiber: Implications for Re-introduction

“…In addition to the reintroduction activities of statutory agencies, re cent efforts in Europe have also involved 'private' indi viduals undertaking release programmes under licence. Beavers are known to be able to feed on a very broad range of nonwoody vascular plants (Parker et al 2007), and so will have different diets at different sites. This study therefore focused on the use of tree species.…”