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COSEWIC assessment and status report on the Bog Bird’s-foot Trefoil in Canada

Limiting Factors and Threats

Habitat loss presents a serious and urgent threat to Lotus pinnatus in Canada. Harewood Plains is currently under application for a development permit for a trailer park. This site contains the only large population of L. pinnatus in Canada and approximately 25 - 30% of the plants at this location are at risk. The landowner has applied for a Preliminary Layout Agreement and the City of Nanaimo has requested an environmental inventory of the property (Lawrance pers. comm., 2003). The existence of rare species may be easily overlooked unless the vegetation inventory is conducted during the appropriate season when plants are visible.

The population of L. pinnatus at Harewood Plains is also under direct and immediate threat of habitat degradation resulting from intensive use of recreational off-road vehicles. Although this species appears to tolerate some disturbance, the shallow soils at Harewood Plains and at site 7 (Nanaimo south of Extension) are vulnerable to degradation by current recreational uses. In some areas at Harewood Plains, the thin and fragile soils have been rutted to bedrock and plants have been dislodged on to bare rock where they cannot re-establish (Figure 4). There is high potential for further impacts.

Root systems may be damaged or destroyed through compaction. Hydrological disruptions that modify groundwater flow also constitute a threat to this species’ long-term survival. Vehicles continue to access the site on a regular basis, despite efforts to restrict access to off-road vehicles by the installation of several cement barriers along the access road (Thurkill pers. comm., 2003).

Recreational use of off-road vehicles creates ample dust that covers the flowers and hinders pollination and at Harewood Plains has also resulted in the fragmentation of essential habitat. In areas where the vehicles enter the seepages, plants have been damaged and populations have become partitioned. Once a previously contiguous population becomes segregated into a series of isolated smaller patches the smaller populations may no longer interbreed, resulting in restricted gene flow and reduced genetic variability. The ability of species to colonize available habitat is also reduced (McPhee et al. 2002). Information is not available on the number of plants required to maintain viable populations of L. pinnatus; however, an effective population size of about 5000 individuals is considered necessary to maintain sufficient adaptive genetic variability for evolution to occur (Culotta 1995, Lande 1995). Populations that fall below 1000 individuals will experience the accumulation of deleterious alleles that may ultimately result in further population declines and extirpation. Therefore, to maximize genetic and ecological variation and prevent the consequences of small population size, the effective population size needs to be at least 5000 individuals (Culotta 1995, Lande 1995).

Figure 4: Destruction of Habitat Caused by All-terrain Vehicles at Harewood Plains

Figure 4: Destruction of habitat caused by all-terrain vehicles at Harewood Plains (photo courtesy, G.W. Douglas, June 29, 2003).

Photo courtesy, G.W. Douglas, June 29, 2003.

Over 99% of the extant population of L. pinnatus is located on land that is privately owned, making this species vulnerable to habitat loss as a result of urban expansion and residential development. The population of the Regional District of Nanaimo increased from 77 624 residents in 1981 to 127 016 residents in 2001, and is projected to increase to 219 321 residents by 2026 representing an average growth rate of approximately 2.9% per year and a total increase of 73% over 1981 to 2026 (BC Statistics, 2003).

The real estate market within the Regional District of Nanaimo saw above-average growth during 2002 - 2003, due to low mortgage rates and consumer confidence in the prospects of the provincial economy. According to the Canada Mortgage and Housing Corporation (2003), housing starts in Nanaimo were 57.7% higher in the first quarter of 2003, compared to the first quarter in 2002. The momentum of starts is expected to continue through 2003 as low housing inventories in British Columbia and strong consumer demand continue to support sales activity (CMHC 2003). Despite the rocky terrain, Harewood Plains has high potential for residential development due to the broad panoramas provided of the surrounding landscape.

Other than habitat destruction, competitive exclusion from native and non-native vegetation represents the most significant ongoing threat to L. pinnatus at all sites. Encroachment of native shrub species in potential habitats may prevent this species from occupying new sites. Severely invasive alien grass species that threaten the persistence of L. pinnatus include Anthoxanthum odoratum, Dactylis glomerata, Poa pratensis and Bromus sterilis. Cytisus scoparius is the most dominant exotic herbaceous shrub. By aggressively sequestering water and nutrients and reducing light at ground level, many alien species can outcompete native species, reducing their ability to maintain themselves within open meadow sites. As invasive species become more dominant they have the potential to modify ecosystem processes by producing extensive, nitrogen-enhanced litter, by altering fire regimes as a result of their high flammability and by exacerbating soil moisture deficits (D’Antonio and Vitousek 1992). 

Logging operations close to (within 50 m of) populations of L. pinnatus at Harewood Plains increase the potential for the spread of aggressive native and non-native plant species. Invading Douglas-fir trees have become established in the vicinity of several subpopulations of L. pinnatus at Harewood Plains, likely as a result of fire suppression. If permitted to grow, these trees could alter the composition of the grassland meadow complex by reducing the amount of open habitat available for L. pinnatus and other less shade-tolerant species. Logging prescriptions or other future management activities that would result in direct sunlight and rapid evaporation in occupied habitat for extended periods may pose a threat to populations of this species. Off-road vehicles are also a major factor in the spread of invasive non-native plants in sensitive areas. Off-road vehicles make it easier for exotic plants to become established, by disturbing soils and carrying seeds. A Montana study showed that a single off-road vehicle can spread 2000 knapweed seeds over a 16 km [10 mile] area during one trip. (Lacey et al. 1997). 

The specific habitat requirements of L. pinnatus, though not definitively known, clearly indicate the relatively restrictive conditions for the establishment, growth and dispersal of this species. The localized nature of available habitat is thus a strong limiting factor within the currently known range of Lotus pinnatus.