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

4. Threats

4.1 Threat Assessment

Table 2. Threat Assessment Table. Accessible version of Table 2
ThreatLevel of ConcernaExtentbOccurrencecFrequencydSeverityeCausal Certaintyf
Exotic, Invasive or Introduced Species
Invasive alien species
Leafy Spurge (Euphorbia esula)
HighWidespread (Routledge, Lauder, Elbow)CurrentSeasonalModerateHigh
Baby’s-breath (Gypsophila paniculata)
Crested Wheatgrass (Agropyron cristatum)
MediumLocalized (Pakowki, Elbow)CurrentSeasonalLowLow-Medium
Changes in ecological dynamics or natural processes
Alteration to, or suppression of, grazing and/or fire regimesMediumWidespread (All)CurrentSeasonalLow-ModerateMedium
Over-grazing by wild or domestic animalsLow-MediumWidespread (Lauder, Routledge, Elbow)CurrentSeasonalModerateLow-Medium
Habitat Loss or Degradation
CultivationMediumLocalized (Lauder, Pakowki)Historic, CurrentOne-timeModerateHigh
Climate and Natural Disasters
Prolonged wet climatic periodsLow-MediumWidespread (All)Historic, CurrentUnknownUnknownMedium
Habitat Loss or Degradation
Sand and Gravel ExtractionLowWidespread (Lauder, Pakowki)Unknown, AnticipatedOne-time, RecurrentUnknownHigh
Road construction or maintenanceLowLocalized (Elbow, Lauder)Current, AnticipatedOne-time, Seasonal, RecurrentLowMedium-High
Oil and Gas ActivitiesLowLocalized (Pakowki)AnticipatedContinuousLowMedium-High
Disturbance or Harm
Recreational activitiesLowLocalized (Routledge, Lauder)CurrentSeasonal, One-time, RecurrentLowHigh

a Level of concern is defined as to whether managing the threat is an overall high, medium, or low concern for recovery of the species, taking into account all of the above factors.
b Extent – Defined as widespread, localized, or unknown across the species range.
c Occurrence is defined as historic (contributed to decline but no longer affecting the species), current (affecting the species now), imminent (is expected to affect the species very soon), anticipated (may affect the species in the future), or unknown.
d Frequency is defined as a one-time occurrence, seasonal (either because the species is migratory or the threat only occurs at certain times of the year), continuous (on-going), recurrent (re-occurs from time to time but not on an annual or seasonal basis), or unknown.
e Severity is defined as high (very large population-level effect), moderate, low, or unknown.
f Causal certainty is defined as whether the best available knowledge about the threat and its impact on population viability is high (evidence causally links the threat to stresses on population viability), medium (correlation between the threat and population viability, expert opinion, etc), or low (assumed or plausible threat only).

4.2 Description of Threats

Any additional loss of habitat among the known populations of Western Spiderwort would adversely affect the species' survival in Canada (Smith 2002, Remarchuk 2006). Future loss of habitat will most likely be as a result of direct activities like cultivation or oil and gas activities, or through habitat degradation from processes like invasion by invasive alien species and factors affecting dune stabilization (climate, grazing and fire regimes). Threats to Western Spiderwort are not the same across the range, but rather more pervasive in some populations than others. Threats are listed in order of decreasing level of concern.

Invasive Alien Species

Some invasive alien plant species may be relatively unpalatable to livestock and wildlife, or alter fuel properties and fire regimes (Brooks et al. 2004). As a result, an influx of these invasive alien plants could stabilize sand dunes and represent an indirect threat to Western Spiderwort habitat. Direct threats through competition may be posed by invasive alien plants as well which can displace native species, and decrease species diversity or richness through their superior competitive ability and negative effects on ecosystem functioning (Wilson 1989, Wilson and Belcher 1989, Reader et al. 1994, Christian and Wilson 1999, Bakker and Wilson 2001, Henderson 2005, Henderson and Naeth 2005).

Leafy Spurge, an invasive Eurasian species, is present at, or near, all Western Spiderwort populations in Saskatchewan and Manitoba (Smith 2002). It reduces the abundance of native species in areas where it occurs, and is capable of turning sites into a stabilized monoculture (Wilson and Belcher 1989). In Manitoba, it was found that 95% of spurge occurrences were associated with human disturbances such as fireguards or vehicle tracks as it was easier for Leafy Spurge to establish in areas with more exposed soil (Wilson and Belcher 1989); active sand dunes may be particularly susceptible to establishment of Leafy Spurge. Crested Wheatgrass has been observed in the Pakowki and Elbow Sand Hills, and Baby’s-breath in the Pakowki Sand Hills; long term impacts of these species on Western Spiderwort or its habitat are not known. There is also the potential for Western Spiderwort to be killed, or its habitat negatively altered, by indiscriminate use of herbicides intended to control invasive species.

Alteration to, or suppression of, grazing and/or fire regimes

Western Spiderwort prefers dune slope habitat only partially or recently stabilized (Godwin and Thorpe 2006) and could decline without disturbance that creates partially active to active sand patches (Smith 2002, Godwin and Thorpe 2006). Dunes in the southern Canadian prairies have been stabilizing over the last century through a combination of climate and changes in land-use practices since European settlement (Epp and Townley-Smith 1980, Wallis 1988, Wallis and Wershler 1988, Geological Survey of Canada 2001). Changes in land-use practices contributing to dune stabilization primarily includes eradication of Bison (Bison bison), a reduction in the frequency and extent of prairie fires, as well as a more homogenous pattern of grazing (Higgins et al. 1989, Frank et al. 1998, Brockway et al. 2002, Samson et al. 2004, Hugenholtz and Wolfe 2005). It is estimated that less than 1% of dunes are still active in the prairies compared with 10-20% a few hundred years ago (Wolfe et al. 2001); rates of stabilization have been estimated at 10-20% per decade over a 40 year period (Manitoba, Wolfe et al. 2000), although the rate may be as high as 40% per decade over a 50 year period (Middle Sand Hills, Bender et al. 2005) or 30-90% since the 1940’s (Wallis 1988). In the absence of natural disturbances like grazing and fire which interact with cycles of drought and disrupt vegetation growth, natural succession can stabilize and cover sand dunes with vegetation (Potvin and Harrison 1984, Hulett et al. 1966). The stabilization of open sand patches may result in reduced colonization or spread of Western Spiderwort in new and existing areas, respectively, through seed dispersal.

Prairie plants evolved with the ecological processes like fire and grazing. Historically, natural disturbances occurred frequently, randomly, and at different scales and magnitudes across the landscape, and have contributed to plant community composition and structure, and the overall ecological integrity of the prairie (Daubenmire 1968, White 1979, Lesica and Cooper 1999). It is possible that, historically, fires in the summer or fall created lush vegetation the following spring which attracted large herds of grazing animals like bison (Higgins 1986, Vinton et al. 1993) and resulted in reactivation of sand dunes. A combination of fire and grazing likely destabilizes sand dunes and disrupts vegetative succession more effectively than either disturbance independently (Lesica and Cooper 1999). Dunes have been stabilizing in some areas where there have been repeated fires but little grazing, while in other areas dunes have stabilized where there has been grazing but few fires (Wallis and Wershler 1988). Historically, the stabilization of active dunes was thought to be good conservation practice and land managers attempted to stabilize dunes by extinguishing fires, actively reseeding, altering grazing patterns, and placing objects, such as tires or bales, on blowouts (David 1977, Wallis and Wershler 1988). It is only recently that people have realized the benefits of having active dunes for wildlife.


In general, remaining sand dune areas that support Western Spiderwort are not considered high quality land for agriculture due to low soil moisture, low soil fertility, and high risk of wind erosion (Geological Survey of Canada 2001). Nevertheless, these areas are surrounded by Mixed Prairie grasslands which are commonly converted for cultivation resulting in sand dunes becoming islands in a landscape dominated by crops (Neufeld and Henderson pers. obs.). In addition, within sand dune complexes where there is relatively level topography it is possible to irrigate those soils for potato, sugar beet, and corn production (Neufeld, pers. obs.). Much of the Pakowki Sand Hills in Alberta contain nearly level sand plains between dune features. Sand dune complexes in Alberta and Manitoba with similar landscapes have already been converted to irrigated crops (Neufeld and Henderson, pers. obs.); the relative threat is likely limited by economics of irrigation infrastructure development and water supply. Historical conversion of native sandy grassland habitats to cultivated cropland likely contributed to the loss of historical spiderwort habitat as well as to habitat fragmentation. Cultivation will permanently result in habitat loss for which there is no mitigation (Alberta Western Spiderwort Recovery Team 2004).

Prolonged Wet Climatic Periods

Climate historically has played an important role in the stability of dunes as it can impact the vegetation cover on the dunes through periods of drought or moisture, exposing more or less of the sand to wind erosion (Thorpe et al. 2001, Wolfe et al. 2001). An increase in wet climatic cycles over the last 100-150 years, despite short periods of drought, has increased vegetation growth and dune stabilization (Wolfe et al. 1995, Vance and Wolfe 1996, Wolfe et al. 2000, Wolfe et al. 2001). Recent climate change models and predictions for the prairie sand dunes in Canada predict eventual reactivation of some dune crests due to increased aridity and temperatures, despite increased precipitation in winter and spring (Thorpe et al. 2001, Wolfe et al. 2001). However, due to the uncertainty surrounding these models, it is difficult to evaluate the impact climate change will have on inhabitants of dune ecosystems and impacts may vary depending on the land use. Therefore, conservation of this unique habitat and its species through proper management should be the focus to ensure the sand dunes and species inhabiting them are maintained.

Over-grazing by domestic livestock and wildlife

Western Spiderwort evolved with ungulate grazing as a natural disturbance, and therefore should tolerate some amount of grazing pressure. However, it is possible that the timing, duration, location and diet selection of cattle and ungulates today is unlike what occurred naturally with ungulates prior to European settlement. Grazing may benefit Western Spiderwort habitat by decreasing vegetation cover, increasing soil disturbance and reactivating more stabilized sand dunes (Lesica and Cooper 1999, Hugenholtz and Wolfe 2005). Conversely, heavy grazing may be harmful if plants are trampled or repeatedly grazed during the sensitive flowering period and not allowed to set seed, as this could affect fitness and productivity. In Saskatchewan, deer herbivory reduced flowering by nearly half in 3 out of 4 years (Godwin and Thorpe 2007). In Manitoba, herbivory by both deer and cattle was directly observed, and significant differences in flowering were observed between cattle grazing and exclusion areas (Goulet and Kenkel 1997). Plants resprout following herbivory and thus long-term survival of parent plants may not be affected by occasional grazing (Hohn 1994, Goulet and Kendel 1997). However, Remarchuk (2006) observed more non-reproductive stems on grazed Western Spiderwort plants. Also, Western Spiderwort occurred at significantly lower frequencies in a 50-year moderately-grazed sand dune pasture versus the adjacent ungrazed dunes in Oklahoma (4.6% versus 18.9% frequency, respectively; Sims et al. 1995).

Other Threats

Sand and gravel extracted from sand dunes is used for road construction, oil and gas activities (e.g., fracking), agriculture (e.g., potato farming), and personal use. Currently, there are active borrow pits near the Lauder Sand Hills population, and sand has been removed from a dune at Pakowki recently and in the past. Although there are no occurrences under immediate threat from large-scale extraction, with the continued need for aggregate it is possible these sand dunes also will be considered as sources.

Western Spiderwort occurs along a few right-of-ways in Manitoba and Saskatchewan. Road maintenance or upgrading activities, such as road widening or repair, mowing and herbicide applications intended to control weeds and woody vegetation are potential threats to these occurrences.

Oil and gas activities have been considered for the Pakowki Sand Hills in the past and they may pose a future threat to the Alberta and Manitoba populations (Hohn 1994, Smith 2002, Remarchuk 2005 and 2006); currently only the Pakowki location has any existing wells within 1 km of Western Spiderwort occurrences.

Recreational use of motorized or recreational vehicles (e.g., dune bikes, snowmobiles, all-terrain vehicles, 4 x 4 trucks) is occurring in Manitoba. In the Lauder Sand Hills, part of the dunes is used as a motocross race track. Some damage has been observed in the Routledge Sand Hills from snowmobiles or dune bikes (Goulet and Kenkel 1997, Krause-Danielson and Friesen 2009). The removal of western spiderwort plants for use in gardens may have occurred in the past at Routledge (Goulet and Kenkel 1997), but is now prohibited under the provincial Endangered Species Act.