Thirty Years of Hybridization between Toads along the Agua Fria River in Arizona: I. Evidence from Morphology and mtDNA
The Arizona Toad (Bufo [ = Anaxyrus] microscaphus) occupied the entire Agua Fria River drainage in central Arizona until relatively recently. By the 1980s, a close relative, Woodhouse's Toad (Bufo woodhousii), colonized the lower reaches of the Agua Fria and replaced B. microscaphus at some sites. We tested the hypothesis that habitat disturbance drives replacement of B. microscaphus by B. woodhousii, via hybridization, by examining shifts in the distribution of these toads following the expansion of the Waddell Dam on the lower Agua Fria River in the early 1990s. As of 2010, the high elevation headwaters of the Agua Fria River were still occupied by B. microscaphus, the lower reaches near the confluence with the Gila River were occupied by B. woodhousii, and along the middle reaches, hybridization between these two anurans occurred at the same three sites as documented in the early 1990s. Contrary to expectations, evidence of hybridization along middle reaches of the river is largely unchanged: B. microscaphus has not been replaced by B. woodhousii at any additional sites nor is there any evidence of introgression of woodhousii mtDNA into putatively “pure” microscaphus populations upstream of hybrid sites.Abstract
Collecting localities along the Agua Fria River from site #10 (uppermost) to site #2 (lowermost) and the single New River collecting locality (site #1). Map modified from USDA (2007); components of the watershed shown in blue, major roadways in grey.
Sampling sites along the Agua Fria River, near Black Canyon City; site #7 (upper photo) and site #5 (lower photo) during the spring breeding season, 2010 (note the evidence of extensive flooding from previous months). These represent the primary areas of hybridization during the 1990s and 2000s.
Hybrid index scores for individuals from the Agua Fria River, Black Canyon City (site #7), in the 1990s (solid bars; 1990–92; N = 60) and in the 2000s (open bars; 2009–10; N = 57); scores from 0–3 are associated with Bufo microscaphus and 9–12 with Bufo woodhousii. Contrary to predictions, there was not a shift toward scores indicative of B. woodhousii over the past 20 yr (Kruskal Wallis, H = 0.05, P = 0.83).
Hybrid index scores for individuals from Black Canyon Creek, just north of its confluence with the Agua Fria River, Black Canyon City (site #6), in the 1990s (solid bars; 1990–94; N = 34) and in the 2000s (open bars; 2009–10; N = 33); scores from 0–3 are associated with Bufo microscaphus and 9–12 with Bufo woodhousii. Contrary to predictions, there was not a shift toward scores indicative of B. woodhousii over the past 20 yr (Kruskal Wallis, H = 3.37, P = 0.067).
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