Home: Species at Risk: Spiderwort

Western Spiderwort

Limiting Factors

The main limiting factors for western spiderwort relate to loss or alteration of its preferred habitat - destabilized sites within dune fields. The Grasslands Natural Region is one of the most threatened natural regions in Alberta and many sand plains have been destroyed by human development. In Alberta, about 20% of the rare plants in the grassland and parkland regions are found in sandy soils principally in sand hill areas.  Some of the more common limiting factors for western spiderwort are Conversion of Tame Pasture and Cropland, Dune Stabilization, Grazing and Fire Control, Invasive Weeds and Petroleum Exploration and Extraction.

Conversion of Tame Pasture and Cropland:  More than two-thirds of the Mixed Prairie Grassland has been destroyed by cultivation. The proportion of farmland occupied by rangeland declined from 53% to 41% between 1956 and 1981 in Alberta. The area of uncultivated grasslands in Saskatchewan and Alberta is declining at a rapid rate. As time goes by, the surviving untilled area contains a smaller proportion of typical grassland and a large proportion of azonal types (saline flats, sloughs, sandhills, badlands). This is because the typical upland areas are being converted to cropland.

Dune Stabilization:  Although the exact mechanisms are unclear, dune stabilization has been noted at several dune fields in Alberta since 1950. The active sand surface of some dunes in the Pakowki Lake area has been reduced by 50% to 75%. Continual stabilization of the dunes at Pakowki Lake would likely be detrimental to the long term survival of western spiderwort.

Grazing and Fire Control:  While the dynamics of dune destabilization are poorly understood, experts generally agree that a combination of fire and grazing keeps blowouts active. Dunes have been stabilizing in the Middle Sand Hills where there have been repeated fires but little grazing and in other areas where there has been grazing but few fires. The Pakowki Lake site is leased for grazing but the condition of remaining Mixed Grassland is deteriorating because of increased grazing. This presents a management dilemma. The positive or negative impacts of grazing at various times of the year are unknown. A current theory is that late summer or fall fires formerly created lush green areas the following spring. These green patches attracted large herds of grazing animals like bison and resulted in reactivation of the sand dunes. The sand hills were also apparently used as sheltering areas by bison during the winter and this could have been significant in keeping dunes active. Fire control and changes in grazing patterns have completely changed the factors that shape sand dune environments.

Invasive Weeds:  All western spiderwort locations in Canada, except Alberta, are experiencing habitat loss from the invasion of leafy spurge. The small population at the Alberta location is particularly susceptible to destruction as a result of weed introduction. The Alberta site is currently free of leafy spurge but the site should be closely monitored.

Petroleum Exploration and Extraction:  Any attempt to develop oil resources or pipeline right-of-ways in the area northeast of Pakowki Lake could eradicate the species in Alberta. This would, of course, be true for a development project on or near the site. 

Reprinted from Alberta Wildlife Status Report No. 31 (2001), with permission from Alberta Sustainable Resource Development.

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