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Recovery Strategy for the Hill's Thistle (Cirsium hillii) in Canada

Appendix A: Effects on the Environment and Other Species

Recovery efforts for Hill's Thistle are not expected to have adverse effects on other species; however, the use of controlled burns as a habitat improvement tool must be studied before any potential effects of this can be fully determined. As discussed in Section 1.3.2, burning may be needed to create suitable habitat but could also destroy Hill's Thistle (where it already exists) and possibly other species, particularly invertebrates. Whether burning is necessary is a knowledge gap.

Mechanical removal of shrubbery, if found to be useful, might have a local, adverse effect on common early successional species such as Common Juniper, Balsam Poplar (Populus balsamifera), Staghorn Sumac (Rhus typhina), or Green Ash (Fraxinus pennsylvanica) if these species were removed. However, the loss of individuals of these widespread, generalist species from a small area of habitat is not considered a long-term adverse effect.

On the other hand, maintenance of open grassland through mechanical or other methods is expected to have a beneficial effect on many other species that are restricted to open grassland habitat. While many of these species are not yet at risk, they are much less common than the shrubs mentioned above, and they face the same problem of habitat loss from succession that Hill's Thistle faces. Therefore, although there may be some local loss of individuals of common woody species, the overall benefit to more uncommon species requiring open habitats is considered to outweigh the loss.

In addition, the open grassland vegetation type is itself considered rare. Naturally open habitats with good quality native vegetation, such as alvars, prairies and sand barrens, are dwindling both due to lack of wild fire (or other ecological processes) and due to conversion of these places to human uses. The overall natural landscape of Southern Ontario is forested, and naturally open habitats are the exception to the norm. Thus, maintaining open grassland will benefit less common elements of Ontario's biodiversity.

All other recovery steps are likely to help other species as they mostly deal with protecting habitat and outreach to landowners.