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Macoun’s meadowfoam (Limnanthes macounii) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

There are ecological parameters that are essential to successful growth of Macoun's meadowfoam:

  1. availability of open soil, and
  2. sufficient moisture during its growing season, i.e., from October to March.

Since the 1988 COSEWIC report, 16 subpopulations have been lost representing about one third of subpopulations listed in the original report. The causes of extirpation give a very good picture about threats to the plant.

There are three categories of threats:

  1. human activity
  2. competition of introduced plant species
  3. animal pests

1) Human activity

Human activities resulted directly in the disappearance of several populations. A sundeck was built over a depression with Macoun’s meadowfoam [27], a site was subdivided and built over [15]. One of the largest subpopulations known at the time of writing the1988 COSEWIC report [3.2] was almost eliminated and reduced to less than 50 plants when the remnants of a two-story building were burned in a depression that hosted this population. Recently, a sub-population on Gordon Head with a protective covenant [26.4] was eliminated when the wet depression was filled with gravel. The site was restored in November 2003 and is being monitored. All subpopulations on private land west of the Devonian Park [11.2 – 11.5] were lost when the area was built over.

Besides the direct destruction of the sites with Macoun's meadowfoam, modification of water regime (such as interruption of a seepage on which the plant depends) results in decline of meadowfoam on a site.

Human activity, on the other hand, is beneficial at quite a few sites in the Greater Victoria area where Macoun’s meadowfoam grows in heavily trampled areas [18] and [24]. In this case, trampling keeps the areas with Macoun’s meadowfoam open, and helps to control the competition of introduced plants. In winter, wet areas are avoided by park visitors and populations of Macoun's meadowfoam are not disturbed. In summer, however, these areas are heavily used and common lawn grasses (e.g., Lolium perenne, Agrostis capillaris) and other grasses (e.g., Dactylis glomerata, Anthoxanthum odoratum, Cynosurus echinatus) are suppressed.

Trampling also seems to be beneficial to the sub-populations in Ruckle Provincial Park [28] and at the Yellow Point Lodge site [30]. This may change, however, if trampling will benefit those competitors that are adapted to it. Poa bulbosa and Trifolium subterraneum have spread into meadowfoam habitats in the Victoria area in the last two decades and Ruckle Provincial Park meadowfoam populations are threatened by introduction of Soliva sessilis brought in by human traffic.

2) Competition with introduced plant species

Introduced plants represent one of the major threats to Macoun’s meadowfoam. The major dominants of Garry oak plant communities and open areas associated with them are European introductions. The major components of meadowfoam sites are introduced species (Tables of associated species are on file with COSEWIC).

Two introduced shrubs, Cytisus scoparius and Ulex europaeus have major impact on coastal vegetation and may overshadow wetter areas with Macoun’s meadowfoam. Cytisus scoparius (30%) is present in almost one third or releves; Ulex europaeus (3.3%), on the other hand, is restricted to the DND areas of Rocky Point and Gonzales Point.

Ivy (Hedera helix) contributed to the decline and disappearance of two sub-populations in Glencoe Cove [26.1] and [26.3].

Competition with introduced tufted grasses and human activities were the main causes of disappearance of Macoun’s meadowfoam from populations that have been lost. All the populations that were lost due to the competition of introduced plants were originally small, i.e., they did not have more than 50 plants. At the same time, the sixteen populations that showed the decreasing trend and that are now listed as “small” may vanish within next ten or fifteen years.

Introduced grasses pose direct competition to Macoun’s meadowfoam and the area of open soil decreases in wet depressions and seepages. At the same time, tufted grasses produce biomass that accumulates in the depressions and make them drier and less suitable for the growth of Macoun’s meadowfoam and other native annual species.

Competition of introduced species was responsible for the loss of 7 (out of 18) sub-populations. Competition also played a very significant role in the decline of populations. Almost all populations that declined since the 1988 report declined because of the competition of introduced species.

Introduced perennial grasses are the most serious threat to Macoun’s meadowfoam populations. In the following tabulation they are listed in decreasing constancy (numbers are percentages of the species’ occurrence in 89 sampled sites):

Grasses that form either large tussocks (Dactylis glomerata) or dense swards (namely Agrostis capillaris, A. gigantea and Lolium perenne) have the greatest negative impact on Macoun’s meadowfoam populations. Dactylis glomerata and Agrostis spp. are distributed through the general area where Macoun's meadowfoam occurs. Dense stands of Holcus lanatus and Anthoxanthum odoratum contribute to the build up of organic matter. Lolium perenne is a problem in urban areas. Poa bulbosa has started to spread through the urban areas around Victoriaabout five to ten years ago and its population in the meadowfoam site has become denser in the last few years. Whereas Lolium perenne is usually suppressed in meadowfoam sites that are trampled by people in the municipal parks, Poa bulbosa can tolerate trampling or even increase in trampled areas and it is a potential threat to the sites in municipal parks.

Annual grasses are extremely common in the meadowfoam sites. Vulpia bromoides, Vulpia myuros, Cynosurus echinatus and Bromus hordeaceus are ubiquitous in almost all sites. Their vegetation peak is when Macoun’s meadowfoam is already in fruit and they don't represent any spatial competition to this plant. Aira praecox and Aira caryophyllea are also common, but because of their small size, they do not pose a problem.

In addition to these annual introduced grasses, some other introduced grasses, such as Bromus racemosus, are abundant at some localities, but because they start to germinate later, they are not listed in the releves and because of this phenological shift, they do not represent a direct competition to Macoun’s meadowfoam.

All the introduced grasses, however, contribute to the build-up of organic matter in the soil and filling up vernal pools. This, besides the direct competition, contributes to demise of Macoun’s meadowfoam populations, since the vernal pools get drier and less suitable for Macoun’s meadowfoam growth.

Several populations of Macoun’s meadowfoam are in parks, in areas that are trampled by visitors. Cattle Point, Uplands Park and Saxe Point - are good examples. At such sites, Macoun’s meadowfoam flourishes because the grass competition is reduced in the area frequented by people. During the growing season for Macoun’s meadowfoam, these areas are usually wet and avoided by people. Similarly, the only two known sub-populations at bird roosting sites [7.1] and [14.1] are enhanced by birds that graze the perennial grasses and disturb the soil surface.

The following introduced herbs are common in meadowfoam sites during its growing period:

Exotic species with rosettes of basal leaves (such as Plantago lanceolata, Hypochaeris radicata, Bellis perennis, Leontodon taraxacoides and Erodium cicutaria) are, together with introduced tufted grasses, major competition to Macoun’s meadowfoam.

Two, relatively new introductions are Burrowing clover (Trifolium subterraneum) and Lawn burrweed (Soliva sessilis):

Burrowing clover (Trifolium subterraneum) was introduced to British Columbia about forty years ago and the first records came from the Becher Bay area (Ceska 1975). Today it is a common plant in the DND properties on Rocky Point and intrudes in several populations of Macoun’s meadowfoam. In the Victoria area, Burrowing clover has become a weed in the city lawns; it is present at several Macoun’s meadowfoam localities (e.g., [3] and [20].)

Lawn burrweed (Soliva sessilis) is at present known only from the Ruckle Park on Saltspring Island, where it was first collected by Frank Lomer in 1996 (Lomer 1997). In several subpopulations in Ruckle Park [28] it occurs in mixed populations with Macoun’s meadowfoam.

Both Trifolium subterraneum and Soliva sessilis have similar ecology and phenology as Macoun’s meadowfoam. They are all winter annuals that germinate in winter and they have similar water requirements, growing in wet depressions. Both these species grow in drier conditions than Macoun’s meadowfoam and they don’t represent a great threat on wet sites. On the other hand, they both represent a competition to Macoun’smeadowfoam in sites that are drier than optimal for its growth. In drier sites, they can outcompete Macoun’s meadowfoam and reduce its populations.

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