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

5. Population and Distribution Objectives

Substantial increases in the abundance, occurrence and distribution of Western Spiderwort in Canada are unlikely given that the suitable habitat for the species is limited in extent and highly fragmented with large extents of unsuitable habitat between populations, and the Canadian populations exist at the northern limit of the species’ range. Because of these factors, even if threats are reduced or mitigated, it is possible its status will always remain as threatened. Considering habitat loss and degradation (mainly due to invasive species encroachment and changes in fire and grazing regimes) are likely the greatest threats to Western Spiderwort, recovery should focus on maintaining, and if possible increase, existing populations over the long term and reversing or preventing further declines in quality of habitat through beneficial management practices and stewardship arrangements.

Therefore, population and distribution objectives for Western Spiderwort are to:

Maintain and, if possible, increase the current estimated distribution of the existing naturally occurring populations and to similarly maintain and, if possible, increase the distribution of any newly-discovered naturally occurring populations.

Specifically, for each population[9]:

  1. Lauder Sand Hills Population: Maintain mature individuals in at least 6 quarter sections.
  2. Routledge Sand Hills Population: Maintain mature individuals in at least 7 quarter sections.
  3. Elbow Sand Hills Population: Maintain mature individuals in at least 18 quarter sections.
  4. Pakowki Sand Hills Population: Maintain mature individuals in at least 8 quarter sections.

Rationale:

At this time, it is not feasible to define quantitative population objectives for a species for which there are no accurate estimates of population size, long-term data on population trends, or understanding of the range of natural variability for the populations. Once consistent and accurate baseline data are obtained, and a reliable monitoring program is established, quantitative population objectives may be able to be defined.

A decrease in the distribution of Western Spiderwort could reflect a loss of both habitat and individuals. Maintaining the species distribution at the current levels will help ensure the extent of occurrence and index area of occupancy are maintained, preventing the species from being evaluated as “declining” in future COSEWIC assessments, as per the COSEWIC criteria in either category A, “decline in total number of mature individuals”, or category B, “small distribution range and decline or fluctuation” (COSEWIC 2010). Therefore, until reliable and accurate population estimates are available, establishing distribution objectives is a more meaningful approach for recovery planning purposes for this species.


9 Occurrences included in the current estimated distribution (quarter section) needed to meet the following criteria: occurrences were reported using precise and accurate geographic referencing systems; habitat still exists at the location to support the species; occurrences have been confirmed at the location within the past 25 years. As some of the occurrences within populations have only recently been discovered, complete area of occupancy data at the square metre scale is only available for some occurrences. Additionally, where area of occupancy has been measured in multiple years at an occurrence, it appears to fluctuate naturally from year to year, sometimes joining or splitting adjacent occurrences. Western Spiderwort populations may also act as metapopulations with colonizations and extinctions occurring dynamically within a dune system. These factors make it difficult to use a count of occurrences for population objectives or a measure of area in m2 for distribution objectives. It may be useful for determining natural range of variation and trends, however.