Species judged to be less immediately in danger are said to be threatened . A third class of plants which are rare and appear to be at risk of becoming threatened or endangered are said to be of special concern . A list of endangered, threatened and special concern species of vascular plants for Wisconsin is available on the web.
Plant species become rare for several reasons.
Some species may naturally be present at low numbers even if all conditions are favorable. It is also possible that a given species can become less common due to the constantly changing interplay between the physical features of the Earth and the other plants and animals. However, we are most concerned in those cases where the activities of people have exserted a strong negative effect on other species. In this regard, we have contributed to the rarity and possible extinction of individual species by our collective actions. Common causes of rarity for plant species include conversion of their natural habitat into other, unsuitable habitats by agriculture, forestry, construction of home sites or roads, introduction of invasive exotic species of plants or animals, and intentional harvesting of plants.