Comparative studies investigating relationships between plant traits and species rarity and commonness were surveyed to establish whether global patterns have emerged that would be of practical use in management strategies aimed at the long-term conservation of species. Across 54 studies, 94 traits have been examined in relation to abundance, distribution and threatened status at local, regional and geographical spatial scales. Most traits (63) have yet to be the focus of more than one study. Half of the studies involved less than 10 species, and one-quarter did not replicate rare–common contrasts. Although these features of the literature make it difficult to demonstrate robust generalizations regarding trait relationships with species rarity, some important findings surfaced in relation to traits that have been examined in two or more studies. Species with narrow geographical distributions were found to produce significantly fewer seeds (per unit measurement) than common species (in four of six studies), but did not differ with respect to breeding system (five of five studies). The majority of traits (including seed size, competitive ability, growth form and dispersal mode) were related to rarity in different ways from one study to the next. The highly context-dependent nature of most trait relationships with rarity implies that application of knowledge concerning rare–common differences and similarities to management plans will vary substantially for different vegetation types and on different continents.
A comparative analysis of distribution patterns in relation to several life-history and ecological traits among 700 Australian eucalypt species was then performed. A significantly disproportionate number of tall species and species with long flowering durations had wide geographical ranges. Trait relationships with distribution were explored further through the development of a methodology incorporating multiple spatial scales. Eight theoretical categories were described illustrating variation in distribution patterns (and hence rarity and commonness) across small, intermediate and large spatial scales, based on the spatial structure of species occurrence across the Australian landscape. Each eucalypt species was placed into a category, and trait variation was explored across all species in relation to distribution patterns across multiple spatial scales. This approach yielded important information about trait relationships with distribution among the eucalypts, linking the spatial structure of points-of-occurrence with patterns of rarity and commonness. With the pressing need to protect increasing numbers of threatened species and slow rates of extinction, the development and refinement of a broadly usable methodology for rarity studies that encompasses multiple spatial scales, which can be used for any geographical location, will be useful in both conservation and management.