Comparisons of related rare and common species in the study of plant rarity

Conservation of rare plant populations is an important dimension of the preservation of biodiversity. Yet the database underlying both the theoretical understanding and management of rare-in contrast to common-plant species is scattered, unfocused, and conceptually weak. Our review of studies comparing attributes of rare vascular plants with those of closely related, more common relatives found fewer studies than expected (n = 38) and no consistency in the attributes measured which precluded further assessment of pattern. Only 10% of 71 response variables compared were reported in more than four studies and most of the traits (79%) were compared in only one or two studies. Improvement of comparisons of rare and common species requires standardization of attributes measured among studies. Although experiments evaluating hypothesized limiting factors are desirable, at a minimum all studies should include population densities and life table schedules. Demographic parameters provide detailed data, required for scientific management of specific rare plants, and extensive data, required for meta-analyses and detection of general patterns in rarity.