Showy phlox (Phlox speciosa): COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
The factors limiting the size and distribution of Phlox speciosa populations in Canada are unknown. The taxon’s habitat requirements within the biogeoclimatic regions BGxh1, IDFxh1, and PPxh1 and restricted elevation range (700-1100 m) may, in part, be limiting factors in the plant’s distribution. Within these biogeoclimatic restrictions and elevational limits, the plant occurs in a variety of site and vegetation types. The amount of apparently suitable habitat greatly exceeds the extent of actual occurrence (57 km²) of the taxon in Canada. Perhaps, another limiting factor may be the plant’s inability to disperse readily to adjacent areas of suitable habitat, although no specific information has been found to support this contention.
A potential threat to Phlox speciosa populations is the lack of protection of natural plant communities throughout the majority of the plant’s range in Canada. In general, the habitats that support Phlox speciosa are subject to a variety of anthropogenic threats, including livestock grazing, range re-seeding, off-road recreation, agricultural cultivation and development, recreational property development, prescribed burning, forest encroachment, road and trail development, and alien plant species introductions (Royal British Columbia Museum 1995). Although field observations indicate that the plant tolerates a certain degree of anthropogenic disturbance, greatly increased levels of disturbance would likely result in loss of habitat for the taxon.
Recreational property development represents the most important trend in habitat loss. Within the current extent of occurrence, 41% of the area is privately owned land. The remainder of the extent of occurrence is within provincial crown land, but none of that crown land occurs within a protected area. The following populations (or portions) occur on private land: Park Rill (majority), Twin Lakes (majority), White Lake (majority, part on federal), Yellow Lake East (minority), and Yellowlake Creek. Current private land uses in and around the extent of occurrence include rangeland, agricultural cultivation, hobby farms, golf course, residential properties, and recreational properties.
The potential threat of property development is illustrated by cases where Phlox speciosa occurs in yards directly adjacent to homes. For example, a portion of the Twin Lakes population occurs in a yard between a public road and a private driveway and large new home. Given the site topography, it is likely that the home and driveway occupy former Phlox speciosa habitat, and that a portion of the population was lost during construction. This represents a relatively small loss of habitat for the relatively large population, but such incremental losses could become significant if the habitat of the population experiences significant development pressures. Another example is the small McKay Creek population that occurs on a private property within tens of metres of a home, the construction of which may have resulted in loss of Phlox speciosa plants. In that case, a significant portion of the population may have been lost. At the time of writing, at least 32 ha of property are for sale within the extent of occurrence of Phlox speciosa (Canadian Real Estate Association 2003). The properties for sale are being marketed as undeveloped sites to build homes on and for keeping horses. Both of those land uses have the potential to result in loss of habitat for Phlox speciosa.
Another potential threat is the loss of populations due to demographic and environmental stochasticity and loss of genetic variability. These factors are a concern for geographically restricted populations (Primack 1998). Populations of Phlox speciosa are vulnerable given their relatively small area of occupancy, only 0.9-1.4 km².
A final potential threat is weed control activities. In the future, Phlox speciosa populations could potentially be threatened by weed control activities required by the Weed Control Act. Various legally designated noxious weeds, including hound’s-tongue (Cynoglossum officinale) and diffuse knapweed (Centaurea diffusa), occur at some of the phlox locations. Use of marginally specific chemical weed control substances that kill broad-leaved plants would likely kill Phlox speciosa. Under the Weed Control Act, an occupier must control noxious weeds growing or located on land and premises.
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