Do Beavers Promote the Invasion of Non-native Tamarix in the Grand Canyon Riparian Zone

• Published in: Wetlands (Wilmington, N.C.) Vol. 28; no. 3; pp. 666 - 675
• Main Authors: Mortenson, Susan G., Weisberg, Peter J., Ralston, Barbara E.
• Format: Journal Article
• Language: English
• Published: 1313 Dolley Madison Boulevard, Suite 402, McLean, Virginia, USA 22101 The Society of Wetland Scientists 01.09.2008; Springer Netherlands; Springer Nature B.V
• Subjects: Aquatic mammals; Beavers; Biomedical and Life Sciences; Canyons; Castor canadensis; Coastal Sciences; colonizing ability; Colorado River; Competition; correlation; Dominance; Ecology; Environmental Management; feeding preferences; foraging; Freshwater; Freshwater & Marine Ecology; Geomorphology; Grand Canyon National Park; Habitats; herbivores; Herbivory; Hydrogeology; interspecific competition; invasive species; Landscape Ecology; Life Sciences; National parks; Plant communities; plant competition; Plant species; plant-animal interactions; REGULAR SUBMISSIONS; Relative abundance; riparian areas; Riparian land; riparian vegetation; Rivers; Salix; Salix exigua; Salix gooddingii; selective foraging; Sinuosity; Spatial analysis; spatial distribution; Species composition; tamarisk; Tamarix; Tamarix ramosissima; Terraces; topography; trees; vertebrate pests; wildlife food habits; Willow; Colorado River; Grand Canyon; USA, Arizona, Grand Canyon; USA, Arizona, Grand Canyon Natl. Park; tamarisk; Salix; riparian vegetation; plant-animal interactions; Colorado River; herbivory; Castor canadensis
• ISSN: 0277-5212; 1943-6246
• DOI: 10.1672/07-142.1

Beavers (Castor canadensis Kuhl) can influence the competitive dynamics of plant species through selective foraging, collection of materials for dam creation, and alteration of hydrologic conditions. In the Grand Canyon National Park, the native Salix gooddingii C.R.Ball (Goodding's willow) and Salix exigua Nutt. (coyote willow) are a staple food of beavers. Because Salix competes with the invasive Tamarix ramosissima Ledeb., land mangers are concerned that beavers may cause an increase in Tamarix through selective foraging of Salix. A spatial analysis was conducted to assess whether the presence of beavers correlates with the relative abundance of Salix and Tamarix. These methods were designed to detect a system-wide effect of selective beaver foraging in this large study area (367 linear km of riparian habitat). Beavers, Salix, and Tamarix co-occurred at the broadest scales because they occupied similar riparian habitat, particularly geomorphic reaches of low and moderate resistivity. Once the affinity of Salix for particular reach types was accounted for, the presence of Salix was independent of beaver distribution. However, there was a weak positive association between beaver presence and Salix cover. Salix was limited to geomorphic settings with greater sinuosity and distinct terraces, while Tamarix occurred in sinuous and straighter sections of river channel (cliffs, channel margins) where it dominated the woody species composition. After accounting for covariates representing river geomorphology, the proportion of riparian surfaces covered by Tamarix was significantly greater for sites where beavers were present. This indicates that either Tamarix and beavers co-occur in similar habitats, beavers prefer habitats that have high Tamarix cover, or beavers contribute to Tamarix dominance through selective use of its native woody competitors. The hypothesis that beaver herbivory contributes to Tamarix dominance should be considered further through more mechanistic studies of beaver foraging processes and long-term plant community response.