Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

The original status report indicated that threats to the survival of western spiderwort in Canada were:  conversion of tame pasture to cropland, dune stabilization, grazing and fire control, invasive weeds, and petroleum production and extraction (Smith and Bradley 1990).  Any loss of habitat within the known locations of the species would adversely affect the species' survival in Canada.  Loss of habitat is most likely as a result of agricultural practices such as grazing and introduction of weedy species such as the leafy spurge. 

Goulet and Kenkel (1997) list the following as limiting factors:  cattle grazing, invasion by leafy spurge, shading and human disturbance.

* Total population count based on incomplete data for most years

Cattle grazing

Overgrazing is highly detrimental to western spiderwort habitat and should be discouraged.  Low to moderate grazing levels may help prevent vegetation encroachment in sand dune habitats.  Shoot regeneration was observed in plants damaged by cattle grazing and trampling.  If the plant is not grazed to the ground, regeneration can occur from the remaining stem tissue (Goulet  and Kenkel 1997).  The lower level of flowering (30% as opposed to 50% in ungrazed sites) was attributable to cattle grazing.  Cattle appear to be attracted to the colourful flowers, as many flowering shoots are nipped off (Goulet  and Kenkel 1997).

All Canadian western spiderwort sites, except the MHHC site in Manitoba, have been subject to grazing.  Grazing of western spiderwort by both cattle and native herbivores (mainly white-tail deer) has previously been reported (Hohn 1994).  The majority of the grazing pressure appears to be attributable to cattle.  The effects of grazing pressure on western spiderwort may be both positive and negative.  Grazing-trampling of plants may reduce population size and fecundity.  However, cattle may prevent dune stabilization by their movements, and by their browsing of woody plant species and dune-stabilizing grasses (Goulet  and Kenkel 1997).  In areas where grazing pressure is light to moderate (Hellman pasture, Loutit sites),light to moderate cattle grazing does not appear to be a serious threat to the long-term persistence of western spiderwort populations.  However, cattle grazing is known to decrease the relative diversity of communities.  Heavy grazing is highly detrimental to western spiderwort populations (and sand dune habitats generally) and should be actively discouraged (Goulet  and Kenkel 1997).

The ESA in the Routledge Sand Hills is now protected from cattle grazing.  The area south of the ESA is grazed by about 25 head of cattle.  Grazing by cattle was particularly common in the southern portion of the main Routledge site.  All the areas surrounding the ESA, with the exception of the Crown land area, were subjected to cattle grazing (Goulet  and Kenkel 1997).  Cattle no longer graze the main Routledge site found on private property (pers. com., Elizabeth Reimer, Manitoba Cons. Data Centre).

Although the MHHC site is protected from grazing the site is surrounded on all sides by pasture land and the area is used as a winter feeding station for white-tail deer.  Hohn (1994) noted heavy grazing damage to western spiderwort populations at this site indicating the species is grazed by native herbivores.  Since the area is used as a winter feeding station the populations of white-tail deer may be artificially high. 

About 168 head of cattle graze in the area of the Eastern Population on the Lauder Sand Hills property site during the summer months, affecting mainly the north and south portions of the property.  The less accessible steeper slopes in the central region remain relatively undisturbed thus affording some protection to the western spiderwort populations.  Cattle grazing and trampling have destabilized these sand dunes, resulting in open areas dominated by the cactus Opuntia fragilis (Goulet  and Kenkel 1997).

Ungrazed sand dunes have a 'typical' profile of tall grasses on the south-facing slope, where the majority of western spiderwort plants are found.  In grazed areas, south-facing slopes are used by cattle as 'pathways', resulting in open sand 'bowls' and shrub-dominated areas.  On grazed sand dunes, western spiderwort is largely restricted to shrubby areas and to the northern dune face.  Shrub cover may provide protection from herbivory and adverse microenvironmental  conditions (Goulet and Kenkel 1997).

At the Routledge Sand Hills, western spiderwort is most abundant in the ungrazed areas (ESA), and is less abundant in the grazed pasture lands.  At the Eastern Population site (Lauder Sand Hills), western spiderwort is most abundant on the high, steep dunes (inaccessible to cattle) in the central portion of the property.  These observations suggest that long-term grazing by cattle may reduce western spiderwort population sizes.  It is important to note, however, that western spiderwort has persisted in these habitats despite many years of light to moderate cattle grazing (Goulet  and Kenkel 1997).

The Manyberries site in Alberta is held under a grazing lease.  No indications of active grazing of the site were noted during fieldwork (Smith 2001B). 

The Douglas Provincial Park site in Saskatchewan has had yearly grazing for the past several decades with apparently little impact although direct grazing of plants has been noted to occur (Godwin and Thorpe 1991).

Invasion by leafy spurge

Western spiderwort  habitat in Manitoba has been colonized by leafy spurge  (Euphorbia esula), a Eurasian invasive plant.  This species is particularly common in the Lauder Sand Hills (Western and Eastern Populations) where it occurs at high cover (Goulet  and Kenkel 1997).  The introduction of leafy spurge to North America (sometime in the 1900's) has resulted in extensive loss of habitat and biodiversity in the western United States and southern Canadian provinces.  Once established, leafy spurge is difficult if not impossible to eradicate (Harris 1988).  Chemical and natural tillage cannot be used to control the invasion of leafy spurge in natural communities.  Biological control, using European spurge beetles, was undertaken at the Lauder Sand Hills (Western Population) and at the Routledge Sand Hills in 1993.  Results, to date, are unknown (Hohn 1994).  The transport of hay bales containing leafy spurge seed is thought to be a major contributing factor to the spread of the species in the Canadian provinces (Schmidt 1989).

The species is locally abundant at the Routledge Sand Hills, particularly in areas grazed by cattle.  In the ESA, leafy spurge is only occasionally encountered (three small populations), and it usually occurs at relatively low cover.  Leafy spurge covers almost 75% of the sand dunes at the MHHC site in the in the Lauder Sand Hills.  There is some uncertainty as to whether spurge beetles have been released at this site.  Leafy spurge has completely covered large sections of the dunes within the Eastern Population in the Lauder Sand Hills.  The land owner has plans to release spurge beetles on the property (Goulet  and Kenkel 1997).

Leafy spurge did not appear to adversely affect fecundity, abundance, or phenology of the western spiderwort in the summer of 1996 (Goulet and Kenkel 1997).  It is important to note that information on community composition and structure prior to the invasion of leafy spurge is not available.  It is also possible that leafy spurge might be capable of strongly competing with western spiderwort if adverse climatic conditions occur (Goulet and Kenkel 1997).

Leafy spurge has invaded several thousand acres of land in the Elbow Sand Hills.  The leafy spurge within the sand hills is currently concentrated in the eastern half of the area.  The location where the western spiderwort is found lies just west of the extensive leafy spurge invasion (Godwin and Thorpe 1991).  In 2001, leafy spurge now occurs to a limited extent amongst the western spiderwort populations at the Saskatchewan site (Douglas Lake Provincial Park, 2001).

Leafy spurge does not occur at the Manyberries site at the Pakowki Lake Sand Hills in Alberta (Smith 2001B).

Shading/dune stabilization

Fire suppression policies implemented in the early 1900's may have promoted woody shrub-tree encroachment into sand-dune communities resulting in dune stabilization and loss of western spiderwort habitat.  The full effects of dune stabilization on western spiderwort populations are unknown.  The presence of shrubs may have a positive effect, since shrub cover protects the species from herbivores and creates a more favorable microclimate (Goulet and Kenkel 1997).  Plants growing in semi-shaded habitats (amongst shrubs or under a canopy of bur oak) often appeared healthier and had high levels of seed production.  Low shrub cover may be critical to the survival of western spiderwort in more exposed habitats (Goulet  and Kenkel 1997).

An examination of a time-series of aerial photographs of the Routledge Sand Hills (1940's-present) indicate that minimal shrub-tree encroachment has occurred.  The south-facing dune slopes are very hot and dry in summer, and this may preclude the establishment of long-lived woody species.  In addition, low to moderate browsing by cattle and native ungulates may be important in keeping woody vegetation 'in check'.  The species will grow also in meadows and shaded habitat as well as open, partially stabilized dunes.

Dune stabilization and cover by shrubby species has resulted in a negative impact on the Alberta population in the Pakowki Sand Hills (Manyberries site).  There has been a substantial loss of more open grassy/sandy habitat in favour of shrubby habitat (Smith 2001B).

Human disturbance

Western spiderwort is restricted to ecologically-sensitive sand dune habitats that are easily destroyed by human activity.  Human disturbance to the fragile sand dune communities may take many forms.  Pedestrian and motorized vehicles are a major problem on the ESA and MHHC sites. At MHHC, a sign on the highway encourages exploration on foot (pers. com., Elizabeth Reimer, Manitoba Cons. Data Centre).   Motorized vehicles are particularly destructive to the dune habitats.  Sand-gravel excavation and oil exploration are potential threats to western spiderwort habitats.  Removal of plants from their natural habitat for transplanting into gardens is also a problem (Goulet  and Kenkel1997).

The Routledge Sand Hills is relatively undisturbed, although trespassers on motorized dune bikes and snowmobiles occasionally disturb the ecological integrity of the area (Goulet  and Kenkel 1997).

There is little impact from human disturbance in the Manyberries site in Alberta.  The Saskatchewan site lies in Douglas Provincial Park and is subjected to typical park usage such as campground operations, hiking and picnicing.  The population is apparently stable at present (Douglas Provincial Park 2001, Smith 2001B).