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.
Loss of habitat seems to be the most important factor in many cases.
Herbaria are the best source of information documenting the presence of any species within an area. From this information, gathered from all herbaria in the region, the researcher can get a good understanding of how the species is distributed and how common it is. This label information is also helpful in identifying what kind of habitat is necessary for the species. A herbarium is also an essential resource for the training of botanists who will study natural ecosystems and native plants, including endangered species. They are also often important sources of the published literature relevant to endangered species.