An evolutionary case for plant rarity

Abstract

Abstract Species rarity is a common phenomenon across global ecosystems that is becoming increasingly more common under climate change. Although species rarity is often considered to be a consequence of environmental or ecological constraints, we examined the hypothesis that plant rarity is a consequence of natural selection acting on performance traits that affect a species range size, habitat specificity, and population aggregation; three primary descriptors of rarity. Using a common garden of 25 species of Tasmanian Eucalyptus, we find that the rarest species have 53% lower biomass than common species. There is also a negative phylogenetic autocorrelation underlying the biomass of rare and common species, indicating that traits associated with rarity have diverged within clades as a result of environmental factors to reach different associated optima. In support of our hypothesis, we found significant positive relationships between species biomass, range size and habitat specificity, but not population aggregation. These results demonstrate repeated convergent evolution of the determinants of rarity across the phylogeny in the Tasmanian eucalypts. These results also suggest that if rarity can evolve and can potentially be related to plant traits such as biomass, rather than a random outcome of environmental constraints, we may need to revise conservation efforts in these and other rare species to reconsider the distributions of rare plant species and their ecosystem impacts.