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Rethinking Avian Response to Tamarix on the Lower Colorado River: A Threshold Hypothesis
Authors:Charles van Riper III  Kristina L Paxton  Chris O'Brien  Patrick B Shafroth  Laura J McGrath
Institution:U.S. Geological Survey, Southwest Biological Science Center SDRS, 125 Biological Sciences East, University of Arizona, Tucson, AZ 85721, U.S.A.;
Department of Biological Sciences, University of Southern Mississippi, Hattiesburg, MS 39406, U.S.A.;
U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO 80526, U.S.A.;
Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, U.S.A.
Abstract:Many of the world’s large river systems have been greatly altered in the past century due to river regulation, agriculture, and invasion of introduced Tamarix spp. (saltcedar, tamarisk). These riverine ecosystems are known to provide important habitat for avian communities, but information on responses of birds to differing levels of Tamarix is not known. Past research on birds along the Colorado River has shown that avian abundance in general is greater in native than in non‐native habitat. In this article, we address habitat restoration on the lower Colorado River by comparing abundance and diversity of avian communities at a matrix of different amounts of native and non‐native habitats at National Wildlife Refuges in Arizona. Two major patterns emerged from this study: (1) Not all bird species responded to Tamarix in a similar fashion, and for many bird species, abundance was highest at intermediate Tamarix levels (40–60%), suggesting a response threshold. (2) In Tamarix‐dominated habitats, the greatest increase in bird abundance occurred when small amounts of native vegetation were present as a component of that habitat. In fact, Tamarix was the best vegetation predictor of avian abundance when compared to vegetation density and canopy cover. Our results suggest that to positively benefit avian abundance and diversity, one cost‐effective way to rehabilitate larger monoculture Tamarix stands would be to add relatively low levels of native vegetation (~20–40%) within homogenous Tamarix habitat. In addition, this could be much more cost effective and feasible than attempting to replace all Tamarix with native vegetation.
Keywords:birds  habitat restoration  invasive species  lower Colorado River in Arizona  riparian habitat              Tamarix
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