Patterns of rarity in the Oregon flora: implications for conservation and management

Threatened and endangered species are a significant component of Oregon’s flora, comprising 0.2% to over 15% of the state’s plant taxa, depending on one’s definition of endangerment. With such a potentially large group of plants to conserve, generalizations regarding types of rarity and threats to these species would help improve our basic understanding and management of them. To evaluate the types of rarity in Oregon’s threatened and endangered flora, we classified all Oregon Natural Heritage Program species (lists 1 and 2) according to geographic range (wide vs. narrow), habitat specificity (broad vs. restricted), and local population size (large vs. small). This 2×2×2 classification results in seven possible forms of rarity (and one type that is common). We also listed primary and secondary threats to these rare species and identified each plants’ habit (annual, biennial, perennial, etc.). The distribution of rare plant populations in Oregon appears to be positively correlated with the distribution of biological diversity in the state, as well as the distribution of individual rare species and zones of collecting by botanists. The majority of Oregon’s rarities have a narrow geographic range, restricted habitat, and small populations. There is also a large group of widely distributed species with small populations in restricted habitats. As might be expected, taxa with wide ranges that occur in several habitats are poorly represented among Oregon’s threatened and endangered species. Local planning and conservation efforts will be important to the survival of local rarities, while widely distributed taxa will require the coordination and awareness of many land managers. The primary threats to threatened and endangered species in Oregon are the following, ranked (from greatest to least): livestock grazing, logging, recreation, urbanization, agriculture, mining, natural causes, “other,” horticultural collection, fire suppression, dams, and scientific collection. Herbaceous perennial species form the highest percentage of Oregon’s rare flora, followed by annuals, shrubs, biennials, and trees. The percentage of annual species appears to increase as categories of rarity increase from “endangered in Oregon but more common elsewhere,” to “endangered everywhere,” to “possibly extirpated.” Therefore, annuals may be especially vulnerable and understudied. Our results support the assertions that rare plants are idiosyncratic and management actions, including ecosystem management, should incorporate species-specific biological information.