Small-flowered sand-verbena COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
1. Dune stabilization
The main limiting factor affecting small-flowered sand-verbena is its narrow preference for unstabilized sites within dune fields. Indeed, the single greatest threat to the survival of this species in Canada is habitat loss due to the process of dune stabilization resulting from the encroachment of vegetation on active dunes. This process is apparently occurring throughout the prairie provinces. In several sandy sites in Alberta there had been a reduction of between 30% and 90% in active sand. Large areas of once active sand have been stabilized over the last forty years (Wallis 1988). If the current trends continue, rare native plants, such as small-flowered sand-verbena, which now have dangerously low populations, could be eliminated entirely (Wallis and Wershler 1988). This long-term process could be reversed with climatic changes or through human interference with selective dune destabilization.
In 2001, a severe drought year, there was an ever increasing and substantial loss of habitat over the Canadian range of small-flowered sand-verbena. The Wolf Island and Lost River sites have most likely suffered a total stabilization of habitat resulting in the species being extirpated from these two sites. The South Saskatchewan River site in Saskatchewan has also lost most of its sandy habitat. Also, there has been substantial habitat loss as a result of dune stabilization in the remaining sites at Bow Island, Purple Springs and Bull Pound. There has apparently been an overall dramatic loss of sandy habitat over the Canadian range of small-flowered sand-verbena.
It would take several years of drought and active dune destabilization to restore necessary habitat for the survival of small-flowered sand-verbena. Habitat has simply been lost, dune surfaces stabilized by various other species in what appears to be a natural extension of the process resulting from the absence of fire and browsing by buffalo over the Canadian prairie. In the author's opinion it would not be overstating the situation to suggest that small-flowered sand-verbena will disappear over its Canadian range in probably a relatively short time period if the process of dune stabilization is not somehow reversed.
A variety of researchers have noted the ongoing process of dune stabilization over the past fifty years in both Alberta and Saskatchewan (Epp and Townley-Smith1980, 1982, Pylypec 1989, Wallis 1988, Wallis and Wershler 1988). Most dune areas in Alberta and Saskatchewan have experienced severe loss of open sandy habitat as a result of dune stabilization. Documented loss of habitat has been established in the Middle Sand Hills in Alberta as well as the Dundurn, Harris and Great Sand Hills in Saskatchewan (Smith and Bradley 1992).
Ironically, stabilization of the active dunes was seen as good conservation practice. Land managers went to great lengths to stabilize active blowouts, extinquishing fires, modifying their grazing patterns, and even placing old tires in the blowouts (Wallis and Wershler 1988).
2. Changes in land use
Historically, suitable habitat has been lost as a direct result of farming practices. Several major sand plains have been almost completely cultivated and a major threat exists to the remaining habitats (Wallis 1988). The process of cultivation of areas surrounding the known small-flowered sand-verbena sites disallows the spread of the species to surrounding areas.
The Bow Island site, previously containing the largest Canadian population of small-flowered sand-verbena, is fully engaged as an oil field with extensive road, transmission lines, well sites and active exploration and oilfield activities. In effect, this activity has destabilized the sand in several dune areas allowing for creation of suitable habitat for the species. This potentially positive effect must be countered with the physical loss of habitat, within the dune field, now used for facilities and roads.
3. Invasion of weedy species
There is also the potential for invasion of non-native species as a result of vegetation reclamation along oil and gas access roads and well-sites. The Lower Bow dune at one site is being stabilized by Russian thistle (Salsola kali), bee plant (Cleome serrulata) and foxtail barley (Hordeum jubatum) (Wallis 1987).
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