Anaxyrus boreas

The genus Anaxyrus was split from Bufo by Frost et al. (2006). However, taxonomy within the genus Bufo remains controversial and many references still use the long-established Bufo. The two remaining subspecies (Anaxyrus boreas boreas and Anaxyrus boreas halophilus) are still recognized by some authors, but these are physiologically strongly divergent and there may be cryptic species masquerading under this binominal (Frost 2017).

Bufo (Anaxyrus) boreas is closely related to Bufo (Anaxyrus) canorus and Bufo (Anaxyrus) exsul. "Stephens (2001) examined mitochondrial DNA from 8 Yosemite toads (selected from the samples examined by Shaffer et al. (2000) to represent the range of variability found in that study) and 173 western toads. Stephens' data indicate that Bufo in the Sierra Nevada occur in northern and southern evolutionary groups, each of which include both Yosemite and western toads (i.e., toads of both species are more closely related to each other within a group than they are to members of their own species in the other group). Further genetic analysis of Yosemite toads sampled from throughout their range, and from other toad species surrounding their range is needed to fully understand the evolutionary history and appropriate taxonomic status of the Yosemite toad (Stephens 2001)." (USFWS 2002).

Molecular data indicate that exsul is phylogenetically nested within canorus; further data are needed to determine whether exsul should be subsumed with canorus (Shaffer et al. 2000).

The subspecies of Bufo (Anaxyrus) boreas, as traditionally defined (e.g., Stebbins 2003), appear to be not valid as distinct evolutionary lineages. The ranges of these subspecies (boreas, halophilus) do not correspond very well with geographic patterns of mtDNA variation as found by Goebel et al. (2009). The geographic distribution of the haplotype clade that includes most of the traditionally recognized range of boreas includes not only the type locality of boreas but also that of subspecies halophilus (Goebel et al. 2009).

Phylogenetic analyses of mtDNA data from throughout the range of the Bufo (Anaxyrus) boreas species group (including boreas, canorus, exsul, and nelsoni) by Goebel et al. (2009) identified three major haplotype clades. The Northwest clade (NW) includes both subspecies of boreas (boreas and halophilus) and divergent minor clades in the middle Rocky Mountains, coastal, and central regions of the west, and Pacific Northwest. The Southwest (SW) clade includes exsul, nelsoni, and minor clades in southern California. Bufo (Anaxyrus) canorus, previously identified as paraphyletic, has populations in both the NW and SW major clades. The Eastern major clade (E) includes three divergent lineages from southern Utah, the southern Rocky Mountains, and north of the Great Basin at the border of Utah and Nevada. Goebel et al. (2009) tentatively suggested that some or many of the clades might warrant recognition as distinct species. However, the authors refrained from delineating new species circumscriptions, noting that additional research might suggest different taxonomic outcomes (e.g., recognizing the traditionally defined Bufo canorus as two distinct species or, conversely, combining it with other minor groups and thus broadening its scope).

This species is widespread through much of the western United States and western Canada. It is still locally common, but rapid losses and declines have occurred in many populations across the range. It is threatened by disease, pollution, habitat degradation, and climate change.

The range extends along the Pacific Coast from southern Alaska (Wiedmer and Hodge 1996) to Baja California, and eastward through the Rocky Mountains to west-central Alberta, Montana (Werner et al. 2004), Wyoming (Baxter and Stone 1985), Utah (Ross et al. 1995, Thompson et al. 2004), Colorado (Hammerson 1999), (formerly) northern New Mexico (Degenhardt et al. 1996), and Baja California, México. The species is absent from most of the desert Southwest (Stebbins 2003). Elevational range extends from sea level to more than 11,000 feet (3,355 meters) in some areas in the Rocky Mountains. Range extent is estimated to be 5.45 million km² (GBIF 2025, RARECAT 2025).

Represented by many and/or large occurrences throughout most of the range.

This species is greatly affected by multiple diseases. Chytrid fungus (Batrachochytrium dendrobatidis), which attacks keratinized tissue and is especially detrimental to recently metamorphed toadlets, reduce survival rates and has been attributed to multiple population declines throughout the range (Daszak et al. 2000, Dodd Jr. 2023). Chytrid fungus has a temperature preference for moderately cold environments, thus is less prevalent at the coldest and warmest temperature extremes of this species’ range (Dodd Jr. 2023). Toad eggs and skin are highly susceptible to the pathogenic fungus Saprolegnia ferax (which may be introduced during fish stocking), and mortality of eggs due to this fungus has been documented in Oregon (Blaustein et al. 1994, Kiesecker and Blaustein 1997, Kiesecker et al. 2001, Dodd Jr. 2023). Immunosuppression, coupled with the apparent effect of cold body temperatures on the ability of the immune system to fight disease, may lead to infection by Aeromonas hydrophila bacteria (which causes "red-leg") or other infectious agents and subsequently to death of individuals and extirpation of populations (Carey 1993). Limb malformations in toads have been linked directly to trematode infections by Ribeiroia ondatrae, although the impacts seem to be very limited (Johnson et al. 2001, Johnson et al 2002, Dodd Jr. 2023).

A warming climate will almost certainly result in population decline and a northward range contraction, as survival of this species is greater in areas with high levels of snowpack during winter and ponds deep enough to support complete the cycle of metamorphosis (Dodd Jr. 2023). Habitat fragmentation from urbanization also causes local declines (Dodd Jr. 2023). Vehicle strikes, especially during movements to and from breeding ponds, can cause significant localized mortality rates (Dodd Jr. 2023).

Environmental contamination is another serious, but generally localized, threat. This species does not tolerate acidic aquatic environments with pH less than 4.0, and can be entirely eradicated by acid mine runoff (Corn and Vertucci 1992, Dodd Jr. 2023). Numerous pesticides reduce survival or cause direct mortality even at concentrations less than those recommended (Dodd Jr. 2023). Nitrites, but not nitrates, can be lethal to tadpoles (Dodd Jr. 2023).

Due to decreased levels of the enzyme photolyase, which is known to be involved in photo-damaged DNA repair, tadpoles of this species are more susceptible to UVB radiation than other anurans (Blaustein et al. 1994, Kiesecker et al. 2001, Dodd Jr. 2023). This is likely not solely due to UV radiation, but a complex interrelationship of pathogen susceptibility, temperature tolerance, water clarity, habitat quality, and UVB exposure (Corn and Muths 2002, Palen et al. 2002, Dodd Jr. 2023).