Dense-flowered lupine (Lupinus densiflorus) COSEWIC assessment and status report: chapter 10
Limiting Factors and Threats
Habitat loss presents a serious and urgent threat to Lupinus densiflorus in Canada. The specialized coastline habitats in Victoria and surrounding areas have been extensively developed for residential and commercial purposes and recreation facilities. Facility development almost certainly caused the loss of the Clover Point population.
Development of a sludge treatment plant at the site of population 1, a proposal under consideration by the Capital Regional District board of directors, would eliminate part or all of the largest Canadian populations of Lupinus densiflorus as well as the largest Canadian population of Sanicula bipinnatifida (designated threatened by COSEWIC). Two other sites are also under consideration. One of these alternative sites would eliminate part or all of a large population of Limnanthes macounii (designated threatened by COSEWIC) and a high-rank occurrence of an imperiled plant community.
Even if the sludge plant is not developed at the site of population 1, ongoing activities by the Department of National Defence (DND), the owner of the land, present a threat to the population. A portion of population 1 was destroyed in 2003 when DND widened and paved an existing track.
Habitat degradation compounds this threat. All three populations are threatened by the encroachment of exotic grasses and shrubs, most notably Cytisus scoparius, Hedera helix, Ulex europaeus, Dactylis glomerata, Anthoxanthum odoratum, Lolium perenne, Bromus sterilis and B. hordeaceus.
The warm dry sites that support Lupinus densiflorus were probably burned frequently by First Nations groups seeking to improve Camassia spp. production on the adjacent uplands. Fire has been almost completely suppressed on coastal sites for numerous decades, which has favoured ingrowth by introduced shrubs as well as native species including Rosa nutkana, Symphoricarpos albus, Populus tremuloides and Pteridium aquilinum. L. densiflorus was not found within dense patches of native or exotic shrubs or thick swards of introduced grasses.
Land management practices have also reduced site capability for Lupinus densiflorus. Landscaping, lawn fertilizing, de-thatching and mowing are all common practices on one or more populations. Lawn-mowing at Trial Island (to reduce the threat of fire) is deferred each year until after seed set has begun, in order to favor the perpetuation of Lupinus densiflorus. This informal agreement has undoubtedly had a positive effect, but the majority of plants are still mowed before seed set is complete.
Landform processes also influence lupine populations. The unstable slopes where lupines occur are susceptible to mass wasting and micro-slumping. The persistence of populations on these sites demonstrates that the plants can successfully survive a degree of slope instability. In fact, micro-slumping exposes numerous small fissures which expose mineral soil where seedling establishment is most successful. Currently, mass wasting at sites of populations one and two greatly exceeds historic levels. Slope damage has increased with visitor traffic over the past century to a point where several sub-populations lie within a matrix of deeply worn footpaths and associated sheet erosion.
Summing up, it appears that less than 5% of the sites capable of supporting Lupinus densiflorus at the turn of the century currently provide suitable conditions.
Seed dispersal and rescue effects present a complex problem. At the broad scale, seed dispersal over distances greater than 10 m is probably extremely rare. The widely separated populations (including those on islands in nearby Washington state) have no potential for re-colonizing former sites. The potential for a rescue effect between sub-populations is also slight, as most are separated by well over 10 m of unsuitable habitat.
Within populations one and two, replenishment of up-slope elements is problematic. Seeds are gravity-dispersed. Stochastic events and increased human trampling might be expected to deplete up-slope elements. The former have not eliminated the second population first observed by Macoun in 1887, though we have no information from the Macoun collection as to where on the slope it was found. Thus, we have no way of telling whether or not the occurrence of Lupinus densiflorus first found in 1887 has been slowly migrating downslope to its current position barely above the high-tide line.
In populations one and two, human trampling has increased sharply over the years and many of the up-slope populations appear to be heavily impacted (particularly in the vicinity of trails and park benches). The loss of any up-slope elements cannot be balanced by recruitment into new down-slope habitats because the slopes all tail into the ocean. Severe winter winds may enable some ‘rescue effect’ within sub-populations by blowing seeds upslope, but this is unlikely to counterbalance up-slope human impacts in many populations.
Oil spills could also be potential risks, especially during autumn high tides.
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