Wood-poppy (Stylophorum diphyllum) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat destruction and modification

Habitat destruction (by fill) was a major cause of population decline in 1994 at the London population. This was a “once only” disturbance, but lingering effects include erosion, sedimentation, and change in the woody species strata as dead and dying trees are replaced by saplings. There has also been an increase in weedy species that may be competing with the Stylophorum plants.

The Ilderton population is at the edge of an active sugar bush and adjacent to a municipal drain. Some activities in the sugar bush, such as logging or creating access roads could damage the population. Several outlying plants have established on the spoil from the drain. Future management of the drain would likely affect these individuals.

Development

The London population is now within the City of London, and it will only be a matter of time before the adjacent lands are converted for residential development. Although Stylophorum diphyllum and its habitat are protected under the Ontario Endangered Species Act (ESA) and the Provincial Policy Statement (PPS), some plants are growing very close to the edge of the woodland. Adjacent development is certain to have some effects on the site, through increased levels of trampling, increase in invasive species and other disturbances. Due to the very steep slopes at the site, any human traffic poses a threat. Traversing, climbing or descending the slopes is very difficult, and damage to the substrate and to plants growing on the slope is almost inevitable.

Invasive species

Alliaria petiolata is abundant at Fanshawe, and may compete directly with Stylophorum plants, especially seedlings. Rosettes close to established plants have been removed each spring as part of recovery activities. Disturbance from pulling garlic mustard here may have created germination sites for Stylophorum seedlings. The number of garlic mustard plants appears to have decreased in 2006. At the London and Ilderton populations, garlic mustard is increasing.

Japanese knotweed (Polygonum cuspidatum) is present at the London site and has been treated to control its spread, but it is not yet eradicated. Herb Robert (Geranium robertianum) is also of concern at this site because large numbers of seedlings have been noted under and around mature Stylophorum plants, and could be in competition with Stylophorum seedlings.

Seed predation

Seed predation has been observed, both through browsing of capsules by white-tailed deer and woodchucks, and of fallen seeds by mice. Browsing appears to be limited and is not likely to be a significant limiting factor at the London or Ilderton populations. Conversely, seed predation by miceat Fanshawe, where the population is quite small, may be a significant limiting factor affecting recruitment (Bowles, 2000).

Trespass and recreation

The London population is in an area where there is a high level of off-road bicycle traffic. Unauthorized trails are created from time to time and some of these are close to the Stylophorum stand.

At Fanshawe a trail close to the population was relocated in 1998 and the threat from trampling has diminished.

Genetic contamination

Stylophorum diphyllum is an attractive plant and is sold in garden stores in southern Ontario. The origin of this commercial stock is unknown, but is most certainly not Canadian. Knowing the plant is native, some well-meaning enthusiasts have introduced the plants into ravines and other natural areas. Records are not usually kept of these endeavours. Preliminary results from genetic analysis (Galbraith, 2005, pers. comm.) show that Canadian populations may be genetically distinct from American populations. Not only could Canadian plants be genetically contaminated by American stock, but there is a real possibility that recovery, rather than introduction, may be hard to track because native and non-native populations may be indistinguishable in appearance.

Other natural or human-made factors

Because of the striking appearance of Stylophorum diphyllum, especially in flower, there are potential threats from wildflower gardening enthusiasts, either transplanting individual plants or collecting seeds, and from photographers or naturalists trampling the populations.

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