Clarification of taxonomy issues for western chorus frog and the Species at Risk Act
Note: At its November 2015 Wildlife Species Assessment Meeting (November 22-26, 2015) COSEWIC considered the document entitled "Western Chorus Frog (Pseudacris triseriata) and Boreal Chorus Frog (P. maculata): clarification concerning the wildlife species listed under SARA in light of recent taxonomic interpretations" written by J.P. Bogart, E.B. Taylor and R. Boles. COSEWIC subsequently produced its own statement dated November 26, 2015, and it has also been published on the Species at Risk Public Registry.
This document was produced in response to a request by Environment Canada to clarify recent taxonomic advances on the Western Chorus Frog and Boreal Chorus Frog and their implications for the chorus frogs listed as Threatened under the Species at Risk Act in 2010. COSEWIC will discuss the document at their upcoming meeting on 22-26 November 2015, and it will be proposed to make it available to the public through the Species at Risk Registry.
In 2008, COSEWIC assessed the status of Western Chorus Frog (Pseudacris triseriata) in Canada. It assessed the Carolinian population as Not at Risk and the Great Lakes/St. Lawrence – Canadian Shield population as Threatened. The latter population was listed under the Species at Risk Act (SARA) in 2010. The geographic distributions of both populations are shown in Figure 1, which is reproduced from COSEWIC’s status report (COSEWIC 2008a).
Some authorities (Crother 2012; Dodd 2013; Frost 2013) have recently allocated chorus frogs in southeastern Ontario and in southwestern Québec (i.e., those frogs in the Great Lakes/St. Lawrence – Canadian Shield population) to a different species, Boreal Chorus Frog (Pseudacris maculata), whose range in Canada had been previously limited to the prairie provinces and western Ontario. This revision to the geographic range of both species is based on data from Lemmon et al. (2007) and supplemental genetic data, which identified two genetic lineages within southern Ontario and Québec based on mitochondrial DNA. The genetic lineage found in southeastern Ontario and southwestern Québec corresponded to that of Boreal Chorus Frog, while the lineage from extreme southern Ontario corresponded to Western Chorus Frog (P. triseriata), which is also found in the adjacent U.S.A. (see Figure 2).
The findings of Lemmon et al. (2007) were considered in COSEWIC’s 2008 assessment. However, COSEWIC argued that the discovery of populations possessing a mitochondrial DNA lineage similar to that of Boreal Chorus Frog was not necessarily sufficient to consider those populations as Boreal Chorus Frogs. Other amphibian species are known to possess multiple distinct mitochondrial DNA lineages, some of which may be similar to completely different species as a result of, for instance, historical hybridization between species. In such situations, changes to the species boundaries may not follow, and other data such as from nuclear DNA, morphology, behaviour, etc, are generally also considered to more fully understand taxonomic relationships. In the case of chorus frogs from southeastern Ontario and southwestern Québec, the mitochondrial DNA evidence was relatively recent in 2008 and corroborating evidence was lacking at that time to support the recognition of Boreal Chorus Frog there. As such, COSEWIC considered it premature to consider the chorus frogs in southeastern Ontario and southwestern Québec to be Boreal Chorus Frog (P. maculata). COSEWIC instead retained the taxonomic interpretation from Platz (1989), which considers all chorus frogs in southern Ontario and southwestern Québec as the Western Chorus Frog (P. triseriata). Notwithstanding taxonomic uncertainty, COSEWIC did use the geographic distribution of the mitochondrial DNA lineages, in part, to establish the boundary between the two populations for Western Chorus Frog that were assessed, namely the Carolinian population and the Great Lakes/St. Lawrence – Canadian Shield population.
Whatever name is ascribed to them in the literature, chorus frogs within southeastern Ontario and southwestern Quebec are listed under the Act as Threatened. As COSEWIC clarified in a supplement to its assessment (COSEWIC 2008b), the southern boundary of the Great Lakes/St. Lawrence – Canadian Shield population is demarcated explicitly by the Terrestrial Amphibians and Reptiles Faunal Provinces line shown in Figure 4 of COSEWIC’s status report (Figure 1 in this document). The northern boundary extends just beyond the northern boundary of the Great Lakes St. Lawrence Faunal Province, to include a narrow southern fringe of the adjacent Canadian Shield Faunal Province where there are a small number of occurrences (Figure 1). NatureServe (2015), a non-profit organization that provides high-quality scientific expertise for conservation, currently considers the chorus frogs assessed as Threatened by COSEWIC (2008a) to be Boreal Chorus Frog. These frogs have been assigned a NatureServe rank of imperiled (S2) in Québec.
When COSEWIC reassesses the status of the Great Lakes/St. Lawrence – Canadian Shield population, it will take new available chorus frog research into account. For example, recent genetic and acoustic data support the contention that the Great Lakes/St. Lawrence – Canadian Shield population of chorus frogs is the Boreal Chorus Frog, P. maculata (Rogic et al. 2015). A possible outcome of COSEWIC’s upcoming reassessment (anticipated around 2017-2018) is the adoption of the name Boreal Chorus Frog for this population. If this occurs, COSEWIC will examine the entire Canadian distribution of that species, which includes large parts of mid-western Canada (see Figure 2), to determine whether population components therein warrant separate status assessments in accordance with COSEWIC guidelines (COSEWIC 2014).
Based on its geographic distribution and other factors, it would be highly unlikely that COSEWIC would assess the entire Canadian distribution of Boreal Chorus Frog as a single unit. In particular, a large distance separates the area in southeastern Ontario and southwestern Québec from the more western portion of the species distribution, likely reflecting different post-glacial histories of chorus frogs in the two regions. They also occupy almost entirely separate Amphibians and Reptiles Faunal Provinces. Consequently, these differences would qualify as both “discrete” and “evolutionarily significant” (COSEWIC 2014). As such it is anticipated that the Great Lakes/St. Lawrence – Canadian Shield population currently listed under SARA would continue to be assessed separately from the mid-western population in Canada.
COSEWIC. 2008a. COSEWIC assessment and update status report on the Western Chorus Frog Pseudacris triseriata Carolinian population and Great Lakes/St. Lawrence – Canadian Shield population in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 47 pp.
COSEWIC. 2008b. Supplemental Information on Boundary Lines and Designatable Units for the Western Chorus Frog Pseudacris triseriata Carolinian population Great Lakes/St. Lawrence – Canadian Shield population in Canada. Accessed December 2013.
COSEWIC 2014. Guidelines for recognizing designatable units. The Committee on the Status of Endangered Wildlife. Accessed November 2015.
Crother, B. I. (ed.). 2012. Scientific and Standard English Names of Amphibians and Reptiles of North America North of Mexico, with Comments Regarding Confidence in Our Understanding. SSAR Herpetological Circular 39:1-92.
Dodd, C.K. Jr. 2013. Frogs of the United States and Canada. Volume 1. The Johns Hopkins University Press, Baltimore, USA.
Frost, Darrel R. 2013. Amphibian Species of the World: an Online Reference. Version 5.6 (9 January 2013). Electronic Database accessible at American Museum of Natural History, New York, USA.
Lemmon, E.M., A.R. Lemmon, J.T. Collins, J.A. Lee-Yaw, D.C. Cannatella. 2007. Phylogeny-based delimitation of species boundaries and contact zones in the trilling chorus frogs (Pseudacris). Molecular Phylogenetics and Evolution 44:1068-1082.
NatureServe (2015). NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington Virginia. (Accessed: November 1, 2015).
Platz, J.E. 1989. Speciation within the chorus frog Pseudacris triseriata: morphometric and mating call analyses of the Boreal and western subspecies. Copeia 1989: 704-712.
Rogic, A., N. Tessier, S. Noël, A. Gendron, A. Branchaud, F-J. Lapointe. 2015. A “trilling” case of mistaken identity: call playbacks and mitochondrial DNA identify chorus frogs in southern Québec (Canada) as Pseudacris maculata and not P. triseriata. Herpetological Review 46: 1-7.
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