Correlates of rarity in the flora of North America: Life histories, habitats, and geographic distributions

Many people have searched for patterns in rarity in attempt to find traits that are associated with rare species. Rare species are defined by geographic range, ecological breadth, and local population size. We used a large dataset of the native plant species of the United States and Canada to determine if the frequency of rarity is correlated with (a) geographic range size, (b) habitat, (c) elevation, or (d) life history traits (growth form, life span and plasticity in either, and flowering period). We test the hypothesis that rarity is independent of these attributes. Species with life span plasticity tend to have larger geographic ranges, and tend to be more common than those without plasticity. However, geographic range size is not always related to rarity. We asked how rarity is distributed among plant families and among geographic regions of North America. We found that small families have consistently low proportions of rare species. Rarity in California and the Western Mountains appears to be partially influenced by characteristics of families in that region, whereas other regions appear to have regional traits that influence the incidence of rarity in each region. Two categories of rare species, suffusively rare (broad distribution but small local populations) and endemic species, have interesting geographic distributions. Rarity and endemism increase with an increase in species diversity, and decrease with increasing latitude, whereas suffusively rare species show the opposite trend. The two groups also differ in life history traits. While not all rare species have limited geographic distributions, most have limited ecological ranges. Rare species occur in fewer habitats than do common species, have narrower elevation ranges, and are more likely to occur at high elevations in North America. Rare species have shorter flowering periods than common species, but the timing of flowering is dependent on the plant family and geographic location. The information in this study is valuable because it may suggest improved conservation strategies for rare species or point toward preemptive conservation efforts on behalf of species that are not yet, but likely to become, rare or endangered.