Tamarisk, willow and beaver in Grand Canyon National Park: a preliminary landscape-scale analysis

Weisberg, Peter

A preliminary spatial analysis, using existing data sources, was conducted to assess whether beaver have influenced the relative distributions of willow and tamarisk. Beaver have been increasing in Grand Canyon National Park since construction of the Glen Canyon Dam in 1963 resulted in flow stabilization and enhanced development of riparian vegetation. The native coyote willow is a staple food species of beaver, leading to the concern that overabundant beaver populations may lead to an increase in exotic tamarisk (Tamarix ramosissima) over native riparian plant species. A preliminary spatial analysis, using existing data sources, was conducted to assess whether beaver have influenced the relative distributions of willow and tamarisk. The investigation was considered preliminary because the data utilized were either still at a preliminary stage (vegetation database) or were spatially imprecise (beaver database). Beaver, willow and tamarisk co-occur at the broadest spatial scales because they prefer similar riparian habitat, particularly geomorphic reaches of low and moderate resistivity. However, the occurrence and relative cover of willow were independent of beaver distribution, once the preferences of the two species for particular reaches were accounted for. Willow is limited to geomorphic settings with greater sinuosity and greater availability of vegetated terraces, while tamarisk also occurs on straighter sections of river channel (cliffs, channel margins) where it dominates the woody species composition. The relative cover of tamarisk was greatest for sites where beaver were present, after accounting for covariates representing river geomorphology. This indicates either that tamarisk and beaver prefer similar habitats even at fine spatial scales, or that beaver contribute to tamarisk dominance through selective use of its native woody competitors. Given these preliminary results, the hypothesis that beaver herbivory contributes to tamarisk dominance should be considered further, through more mechanistic studies of beaver foraging processes and long-term plant community response. The Park should act promptly to establish a network of monitoring plots for beaver effects, including exclosure studies and appropriate controls, and to actively protect the few remaining groves of large native trees along the river corridor.

Tamarisk, willow and beaver in Grand Canyon National Park: a preliminary landscape-scale analysis

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