Short-rayed alkali aster (Symphyotrichum frondosum) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

As a shoreline species of beach habitat that is dependent upon natural lake cycles and disturbance regimes, and recruitment and recovery in bad years from a seed bank, this species faces several key limitations and threats. Specifically, the following threats and limitations have been identified:

• Lake level management at three sites has imposed a continuing limitation on this species since 1927 because it has resulted in removal of much of the natural ecological dynamics of this species. The impacts from this management regime, however, likely have been primarily historic in action. Specific effects of lake management include:
  • Reduction of the narrow shoreline habitat occupied by this species, which is needed for population maintenance and expansion.
  • Reduction of the potential exposed seed bank in a given year resulting in reduced potential recruitment in years of reduced abundance and associated reduced annual recruitment. The effective seed bank for this species has been reduced; pre-management size of the seed bank was no doubt significantly larger.

As an ecological safety net that contributes to shoreline species populations on a yearly basis, the seed bank is critical for this species. Loss of seed bank availability alone could result in declining populations, as recruitment from the seed bank is critical in low reproductive years. 

This species was described by Eastham (1947) as abundant at Osoyoos Lake, but this may be a relative abundance as compared with pre-lake management levels. Since 1947, the species has declined, possibly as a result of lower recruitment potential in ‘off’ (i.e., poor seed set) years.

The artificial management of the water levels of Osoyoos Lake is controlled by the Osoyoos Lake Dam in Oroville by the US under the International Joint Commission. Water levels of Skaha Lake are artificially managed at the Skaha Lake Dam by the Municipality of Okanagan Falls. Vaseux Lake is artificially managed by the McIntyre Dam by the Municipality of Oliver (Jubb pers. comm. 2003).

• Reduction of the narrow shoreline band of suitable habitat means that persisting suitable sites are fragmented: this has ecological implications.
  
• Smaller and fragmented populations of a rare species face other inherent problems and limitations, including reduced rescue affect, potentially declining genetic strength and potential for catastrophic loss.
  
• Site management activities represent a major current threat to this species at three sites, these activities include roto-tilling, sand sifting, lawn mowing, beach cleaning.  Specifically:
  • Roto-tilling as part of beach management may pose a major threat to this species, yet could be considered a viable management tool.
  • The Skaha Lake site was roto-tilled twice in the summer of 2002 and was subjected to a sand sifter in the early spring. This was done to keep the weeds down, to keep the lawn from encroaching onto the beach and to loosen compacted sand, thus maintaining the positive aesthetic qualities of the beach (Gunoff pers. comm. 2003). In spite of this management activity, the population persisted.
  • Roto-tilling in 2003, however, severely impacted the population because of the timing and scope of the activity and caused a severe decline in population numbers.
  • In some instances, however, roto-tilling and sand sifting may be mimicking natural disturbance regimes, like wave and storm action, and generating the same shifting of the seed bank. This seed bank may be accessed and replenished during roto-tilling and sand sifting events, which bring the seeds back to the top few centimetres of sand where they can germinate. Dyer (pers. comm. 2005) has indicated that roto-tilling has continued at this site, but that timing was adjusted to benefit the plants.
    
• Site use is a major threat at 3 sites:
  • Excessive trampling by people and livestock is a definite threat. In 2002 and 2003, the writers observed that peripheral areas of the beaches at Vaseux Lake and Skaha Lake that had little trampling had higher densities of S. frondosum. Central, heavily used areas had low densities of plants to none. Trampling appears to severely affect the survivorship of S. frondosum.
  • ctive use of the beach habitats by sunbathers, boaters, and children directly impacts the critical habitat for this species. Direct impact is caused by digging in the sand, boat launching and storage, trampling and compaction.
  • Encroachment of lawns over drawdown areas in recreation sites may be a concern. Lawns have been seeded in adjacent shoreline areas at Skaha, proximal to our populations. Lawn grass grows mixed with S. frondosumon along the periphery of the beach.
    
• Intensive site use and associated disturbance can increase threats from invasive species and competition from alien species suited to disturbed habitats. Invasive species and competition are a threat to this species, especially in the absence of naturally high seasonal water levels that can provide natural controls. A large component of associated species at three sites is alien species. Additionally, a particularly dense cover of the invasive white sweet clover at the Vaseux Lake site was observed in 2003. This competition may negatively affect the fecundity and survivorship of S. frondosum at these sites. Importantly, a plant community with an undisturbed, native cover is more resistant to invasive species than one with a disturbed, non-native component (Anderson and Inouye 2001, Levin 2000). Therefore, maintaining (or recovering) a native plant community to allow resistance to invasive species is important.
  
• Proposed development on the eastern shores of Osoyoos Lake poses a serious potential threat at this site and will affect any remaining sub-populations.
  
• Competition from introduced alien plants poses a major threat.
  
• The potential for nutrient loading in these heavily used lakes may be a concern. This species’ preferred habitat is low in nutrient availability. Increased nutrients will result in increased competition. No specific tests, however, have been documented that demonstrate such nutrient loading has occurred and impacted the plants.