Skip booklet index and go to page content

Recovery Strategy for the Western Spiderwort (Tradescantia occidentalis) in Canada – 2013

7. Critical Habitat

Critical habitat is defined in SARA (section 2(1) as “the habitat that is necessary for the survival or recovery of a listed wildlife species and that is identified as the species critical habitat in the recovery strategy or in an action plan for the species”.

7.1 Approach to Identifying Critical Habitat

Identification of critical habitat for Western Spiderwort is based upon the best available occurrence information known to Environment Canada up to the end of 2010, and reflects what is needed to achieve the population and distribution objectives. The approach used for identifying critical habitat for Western Spiderwort is based on a decision tree developed by the Recovery Team for Plants at Risk in the Prairie Provinces, as guidance for identifying critical habitat for terrestrial and aquatic prairie plant species at risk, and is summarized below.

Primary habitat required by Western Spiderwort consists of moderately sloped, partially stabilized sand dunes with patches of bare sand, as well as more stabilized dune slacks, rolling sand hills and level grasslands in parts of its range (Section 3.3). Although it is possible that areas of shrubs and trees interspersed within the sand dune matrix may also be beneficial by Western Spiderwort as secondary, and perhaps transitory, habitat for reasons described in Section 3.3, occurrences within this type of habitat are likely not critical to the survival of a population. Therefore, only occurrences within primary habitat were considered in the critical habitat identification process.

Primary habitat usually has distinct boundaries and appears as well-defined habitat patches in land cover classification analysis using remote sensing technology[10]. Therefore, because the species occupies well-defined and easily delineated habitat patches, and because the species is a medium-sized perennial, reliably present and easy to detect during the flowering period, critical habitat for Western Spiderwort is identified as the occupied primary habitat patches and all natural landforms, soils and native vegetation within a 300 m distance of each habitat patch[11], as per the criteria in the decision tree. The 300 m distance around the habitat patch represents the minimum distance needed to maintain the habitat required for long term survival of the species. This specific distance is based upon a detailed literature review that examined edge-effects of various land use activities that could affect resource availability for native prairie plants generally, and could contribute to negative population growth.

7.2 Identification of Critical Habitat for Western Spiderwort

Maps showing the location of areas containing critical habitat are provided in Appendix A. The total size of the areas containing critical habitat is 2851 hectares, with 633 hectares identified in Manitoba, 1047 hectares in Saskatchewan, and 1171 hectares in Alberta. This occupies or overlaps into 103 quarter sections of land in the Dominion Land Survey System (28 in Manitoba, 44 in Saskatchewan, 31 in Alberta; Appendix B).

In order to locate critical habitat, only generalized geographic locations at the scale of quarter sections are provided (Appendix B). All jurisdictions and landowners who are controlling surface access to the area, or who are currently leasing and using parts of this area, will be provided with geo-referenced spatial data or large-format maps delineating the boundaries of critical habitat displayed in Appendix A, upon request.

7.3 Activities Likely to Result in Destruction of Critical Habitat

Destruction is determined on a case by case basis. Destruction would result if part of the critical habitat were degraded, either permanently or temporarily, such that it would not serve its function when needed by the species. Destruction may result from a single or multiple activities at one point in time or from the cumulative effects of one or more activities over time (Government of Canada 2009).

Examples of activities that may result in destruction of critical habitat include, but are not limited to:

  1. Compression, covering, inversion, or excavation/extraction of soil – Examples of compression include the creation or expansion of permanent/temporary structures, trails, roads, repeated motorized traffic, and objects that concentrate livestock activity and alter current patterns of grazing pressure such as spreading bales, building new corrals, or adding more watering sites. Compression can damage soil structure and porosity, or reduce water availability by increasing runoff and decreasing infiltration, such that critical habitat is destroyed. Examples of covering the soil include the creation or expansion of permanent/temporary structures, spreading of solid waste materials, or roadbed construction. Covering the soil prevents solar radiation and water infiltration needed for germination and survival of plants, such that critical habitat is destroyed. Examples of soil inversion and excavation or extraction include new or expanded cultivation, sand and gravel extraction pits, dugouts, road construction, pipeline installation, and stripping of soil for well pads or fireguards. Soil inversion or extraction can alter soil porosity, and thus temperature and moisture regimes, such that vegetation communities change to those dominated by competitive invasive species, and the critical habitat is therefore destroyed.

  2. Alteration to hydrological regimes - Examples include temporary or permanent inundation resulting from construction of impoundments downslope or downstream, and accidental or intentional releases of water upslope and upstream. As the seed bank and plants of Western Spiderwort are adapted to semi-arid conditions, flooding or inundation by substances like water or hydrocarbons, even for a short period of time, can be sufficient to alter habitat enough to be unsuitable for survival and re-establishment. Even construction of a road can interrupt or alter overland water flow, altering the conditions of the habitat required for the long-term survival of the species at this occurrence enough to render it unsuitable for growth.

  3. Indiscriminate application of fertilizers or pesticides – Examples of both herbicide and fertilizer effects that change the habitat include altering soil water and nutrient availability such that species composition or the surrounding community changes. These changes in addition to the altered interspecific competition that results from them could render the habitat unsuitable for the species at risk. Additional examples are the single or repeated use of broad-spectrum insecticides that may negatively affect pollinators, an essential part of critical habitat, such that the functioning of critical habitat may be negatively impacted.

  4. Spreading of wastes – Examples include spreading of materials such as manure, drilling mud, and septic fluids. These have the potential to negatively alter soil resource availability, species compositions, and increase surrounding competitor plants effectively destroying the critical habitat. Unlike covering the soil, these liquid or semi-liquid materials can infiltrate the surface in the short-term, but leave little long-term evidence at the surface that could point to the cause of negative changes observed thereafter.

  5. Deliberate introduction or promotion of invasive alien species – Examples of deliberate introduction include intentional dumping or spreading of feed bales containing viable seed of invasive alien species, or seeding invasive alien species within critical habitat. Examples of deliberate promotion include use of uncleaned motorized recreational vehicles on existing race courses, where many of the vehicles arrive contaminated from off-site use and represent significant dispersal vectors for invasive alien species. Once established, these invasive alien species can alter soil resource availability and directly compete with species at risk, such that population declines occur. This effectively destroys the critical habitat. Critical habitat may be destroyed by Leafy Spurge and Baby’s-breath (all Gypsophila species) which were discussed in Section 4.2, as well as any other prohibited or noxious prohibited weeds. It may also be destroyed by the following species which are not restricted by any legislation due to their economic value: Smooth or Awnless Brome (Bromus inermis), Crested Wheatgrass, Yellow Sweet Clover (Melilotus officinalis), White Sweet Clover (Melilotus alba). This form of destruction is often a cumulative effect resulting from the first four examples of critical habitat destruction.

While the human activities listed above can destroy critical habitat, there are a number of activities that can be beneficial to Western Spiderwort and its habitat. These activities are described in Appendix D.


10 Using object-oriented classification of satellite imagery (2.5 m panchromatic Spot 5 imagery for AB and MB; 2.5 m panchromatic Spot 5 and 10 m multispectral Spot 5 for SK), the landscape was classified into the following land cover classes, or habitat patches: dunes (bare sand), grassland (including vegetated dune slopes), shrub, forest, water and cropland following methods outlined in Lowe 2011. Primary habitat patches used for critical habitat were occupied grassland and dunes patches. Post-visual interpretation of the habitat classification using satellite imagery and higher resolution orthophotos, where available, was used minimally to increase accuracy in boundary delineation.

11 For the purposes of identifying critical habitat for Western Spiderwort, rivers, and wetlands are not included in the definition of natural landforms and vegetation as the species does not use these habitats. In addition, large barriers like river channels or cultivated fields (e.g., greater than 150 m wide) can create a discontinuity in the natural habitat. These barriers may overwhelm other edge effects at the distal end of critical habitat, or prevent effective dispersal of the plant at the proximal end closest to the occurrence. In these particular cases, some patches of natural vegetation on natural landforms within a distance of 300 m but discontinuous from the habitat occupied by the plants may not be identified as critical habitat.