Approved by COSEWIC November 2020

Preamble:

The Species at Risk Act defines “wildlife species” as “a species, subspecies, variety or geographically or genetically distinct population…”. This definition gives COSEWIC a mandate to assess units within a recognized taxonomic species. COSEWIC recognizes a unit below the level of a recognized taxonomic species as a DU (i.e., as a “wildlife species”) if it has attributes that make it both "discrete" and "evolutionarily significant.

Approach to the identification of DUs for status assessment:

A DU is a unit of Canadian biodiversity that is discrete and evolutionarily significant, where discrete means that there is currently very little transmission of heritable (cultural or genetic) information from other such units, and evolutionarily significant means that the unit harbours heritable adaptive traits or an evolutionary history not found elsewhere in Canada. For example, it may be possible for individuals from the same taxonomic species to move (or be moved) into an area formerly occupied by the lost unit, but these individuals and their descendants would belong to the DU from which they originated until they became eligible for assessment as a new, independent DU.

Where there are many putative DUs, or DU structure is complex, a stand-alone DU structure report can be prepared for approval by COSEWIC prior to a status assessment or reassessment. A summary of this pre-approved DU structure should be presented in the species status report and any new information that could modify the pre-approved DU structure must be included in the DU section of the report.

Guidelines for the identification of DUs:

1) Subspecies or varieties:

A named subspecies or variety identified in accordance with COSEWIC’s guidelines for naming subspecies and varieties should be recognized as a DU if it meets the criteria for both Discreteness and Significance.

2) Discrete and evolutionarily significant units of biodiversity:

COSEWIC recognizes a DU (i.e., recognizes a unit below the level of a taxonomic species as a “wildlife species”) if it has attributes that make it both "discrete" and "evolutionarily significant."

Discreteness

A putative DU may be considered discrete based on one or both of the following criteria (Figure 1), each of which indicate little or no transmission of heritable information between it and other DUs:

D1. Evidence of heritable traits or markers that clearly distinguish the putative DU from other DUs (e.g., evidence from genetic markers or heritable morphology, behaviour, life history, phenology, migration routes, vocal dialects, etc.), indicating limited transmission of this heritable information with other DUs.

D2. Natural (i.e., not the product of human disturbance) geographic disjunction between putative DUs such that transmission of information (e.g., individuals, seeds, gametes) between these "range portions" has been severely limited for an extended time and is not likely in the foreseeable future. “Extended time” is intended to mean that sufficient time has passed that either natural selection or genetic drift are likely to have produced discrete units, given the specific biology of the taxon.

Significance

If a putative DU is found to be discrete, its significance can be assessed (Figure 1). A DU is considered significant based on one or more of the following criteria. These criteria capture two types of significance that can operate together or separately: (i) an evolutionarily significant period of isolation that is expected to generate an evolutionary history that cannot be reasonably expected to be practically reconstituted (criterion 1); and/or (ii) the presence of specific adaptive, heritable traits (phenotype) that may develop in a shorter time-frame but that nevertheless cannot be reasonably expected to be reconstituted (criterion 2).

S1. Direct evidence or strong inference that the putative DU has been on an independent evolutionary trajectory for an evolutionarily significant period, usually intraspecific phylogenetic divergence indicating origins in separate Pleistocene refugia.

S2. Direct evidence or strong inference that can be used to infer that the putative DU possesses adaptive, heritable traits, that cannot be practically reconstituted if lost. For example, persistence of the discrete, putative DU in an ecological setting where a selective regime is likely to have given rise to DU-wide local adaptations that could not be reconstituted. See Practical Considerations/Best Practices.

Some Practical Considerations / Guidelines on Best Practices

The Discreteness and Significance Guidelines described above are the principles that form the basis for DU delineation. The practical considerations and best practices described below provide guidelines for report writers and reviewers. These practical considerations and best practices constitute a living document that COSEWIC will update as needed.

General considerations

1. DUs should be designated following a weight of evidence approach where different lines of evidence (e.g., genetic, behavioural, morphological, geographic distribution) are evaluated for discreteness and then significance (see Figure 1). Consider each line of evidence for the relevant criterion and then evaluate the full body of evidence together for discreteness and significance.
2. Where more than one DU per named species is proposed, it is important to present all available evidence for all criteria for each putative DU, including evidence that might contradict the proposed DU structure. The section (or report) discussing the proposed DU structure should clearly identify the criteria used for supporting Discreteness and Evolutionary Significance among all putative DUs considered in the report. The criteria should be referenced directly in the text and italicized. At minimum, this section of the report should have:
   
   1. Subheadings for Discreteness and Evolutionary Significance;
   2. a short paragraph for each of the corresponding arguments in F5 and a table summarizing evidence for discreteness and significance between pairs of proposed DUs (see Table 1 for an example) and,
   3. a short conclusion statement.
   Report writers should avoid extensive and potentially distracting discussion of the scientific evidence unless that evidence is directly relevant to the DU structure, or is controversial or contradictory among sources.
3. Genetic population structure does not necessarily indicate discreteness, as populations can be genetically structured in the presence of ongoing gene flow. However, it is not necessary to establish genetic discreteness between putative DUs if other evidence meets the criteria. Where genetic information is available, the report should summarize genetic evidence supporting the putative DU structure, but also present and acknowledge any available evidence that does not support the proposed structure. The report should also clearly acknowledge the strength of the evidence. For example, variation in sample sizes, geographic coverage, or the type and quantity of genetic markers used among studies all affect the certainty of results.
4. Significance of a DU reflects the fact that if it were lost, it could not be practically reconstituted. If its entire range had been in Canada, it would be deemed extinct. If it were lost from its Canadian range but still present elsewhere, it would be deemed extirpated.
5. DUs should not be identified on the basis of threats or relative conservation status. Similarly, DUs are distinct from management units, of which there may be more than one within or across DUs.

Figure 1. Flow chart to guide determination of DU structure. See text for complete wording of criteria.

Table 1. Example of how to summarize pairwise differences between putative DUs. The goal is to provide all the information available, including evidence that would support the putative DUs, and available evidence that might not support the proposed DU structure. In the case of genetic data, the table would ideally distinguish between evidence from neutral genetic markers (that might support discreteness between DUs), and functional markers (that can inform evolutionary significance). This example provides some examples of evidence that might inform DU recognition. It is not intended to be exhaustive, recognizing that the available evidence and the relevant types of evidence will vary among taxa.

1. Genetics: moderate to highly sig diff at particular loci (+ = FST>**; GST>**)
2. Isotopes (+ = Significant difference at P<0.05, indicating focus on different prey species) (An example of possible, significant phenotypic variation among DUs).
3. Spatial separation (An example of evidence that could inform discreteness)
4. Length at age (+ = >5%) (An example of possible, significant phenotypic variation among DUs).
   Blank = No data

Specific best-practice examples

1. Some lines of evidence may be relevant for both discreteness and significance. For example, deep phylogenetic divergence across the genome (e.g., where reconstructed Pleistocene refugia were non-overlapping and there is no evidence for subsequent gene flow) may indicate discreteness (D1) and significance (S1).
2. Eco-geographic boundaries can generate isolation (discreteness due to natural range disjunctions, D2), particularly for freshwater species. They also may provide evidence that can be used to clearly infer evolutionary significance because they often reflect long-term isolation (S1) and because the different ecological conditions among eco-geographic zones have been demonstrated to promote adaptive differences in the taxon (S2). The key for DU designations using geographical evidence to infer significance is that each zone has a distinct environment that can be argued to represent different selective regimes that drive "significance". Reports should rationalize the relevance of the zones in this way if they are used to support DU criteria. Some brief explanation is required in terms of the relevance of the zones to capture both past isolation and distinct selective regimes. Use the most appropriate map for the Wildlife Species under consideration (examples shown in Figures 2, 3, and 4).
3. Examples of “evidence of adaptive heritable traits” relevant to S2 could include fixed differences in alleles at multiple functional nuclear loci, or clear distinctions in fixed DU-wide, DU-specific traits, including stable, culturally transmitted, functional behaviour, such as foraging methods or migration routes. Summary documents outlining the various types, uses, and limitations of genetics data and culturally transmitted behaviours are available upon request.
4. Evidence and inference regarding heritable components of traits are necessary when assessing highly plastic species that can exhibit strong morphological variation in response to environmental conditions.
5. The Manipulated Wildlife Species Guidelines (Appendix E7) provide further advice on DUs that may or may not be considered “reconstituted.”

Practical Considerations on Sympatric Forms and Designatable Units

1. Sympatric forms of a species may occur within an area when the species exhibits sympatric (co-occurring), multiple forms differentiated by variation in heritable traits (e.g., morphology, behaviour, life history, phenology, migration routes, vocal dialects) with little to no information flow between forms. Evidence, or inference, of isolation (i.e., lack of information flow) between putative DUs fulfills a criterion (D1) for discreteness for each form of the species. Evidence, or inference, that each putative DU possesses trait(s) that are adaptive, heritable, and cannot be reconstituted if the DU were lost, fulfilling criteria (S2) for significance for each form of the species. If both discreteness and significance criteria are fulfilled, then each sympatric form should be considered a DU.
2. Species that exhibit multiple forms of traits, but that have substantial natural information flow (e.g., gene flow) between forms should not be considered sympatric forms, but rather polytypic species. In these cases, the species should be considered a single DU. Sympatric forms of a species may occur when the species exhibits sympatric (co-occurring), multiple forms differentiated by variation in heritable traits (e.g., morphology, behaviour, life history, phenology, migration routes, vocal dialects) with little to no information flow between forms in at least one mode of inheritance of functional traits (e.g., nuclear genes or cultural transmission).
3. In cases where sympatric forms exhibited little to no information flow between forms prior to threats, followed by information transmission subsequent to the threat(s), each form that existed prior to the threat(s) shall be considered a DU. Forms existing following the threats and consequent increase in information flow should be not considered to be the original DUs as they no longer fulfill the criterion (D1) for discreteness.

Glossary:

Reconstitute: To replace a lost DU by individuals that would be identified as belonging to the same DU. That is, the individuals would have the same heritable adaptive traits or hallmarks of the same evolutionary history as those they are replacing.

Figure 2. COSEWIC National Ecological Areas

Figure 3. COSEWIC National Freshwater Biogeographic Zones

Figures 4. COSEWIC Terrestrial Amphibians and Reptiles Faunal Provinces.

Appendix

Archived Figure 4. COSEWIC Terrestrial Amphibians and Reptiles Faunal Provinces in use since at least 2007. See the Amphibian and Reptile Faunal Provinces of Canada report to COSEWIC (dated March 2016), which was part of the November 2016 discussion, for differences between this map and the current map.