The importance of seed characteristics in distinguishing the rare and common plant species in Astragalus (Fabaceae)

Understanding the key important traits responsible for the rarity of plant species is crucial for effective conservation management. This study investigated the functional traits underlying rarity in several Astragalus species by comparing seed morphology, germination, dispersal, and pre-dispersal predation across 12 congeneric rare and common shrub and herbaceous species in Iranian steppe habitats, while controlling for phylogenetic relatedness using Phylogenetic Generalized Least Squares. Our analyses revealed that seed mass showed no consistent relationship with rarity after accounting for phylogenetic constraints, with both rare and common species exhibiting variable mass patterns. Our results also showed that rare species were typically constrained by a combination of low seed output, high proportional seed predation (~ 65%), and limited dispersal capacity (exclusive reliance on abiotic vectors) after controlling for phylogenetic relatedness. These limitations are compounded by poor seedling competitiveness (low Vigor Index II and biomass), creating demographic bottlenecks. In contrast, common species, despite experiencing even higher proportional seed damage (75%), compensate through high fecundity, biotic dispersal, and rapid germination (16.7% faster than rares), ensuring population resilience. In general, rarity emerged from synergistic deficiencies-low seed numbers, high predation impact, and limited dispersal-rather than single-trait limitations whereas abundance reflects compensatory trait combinations. These findings contradict the assumption of a universal rarity indicator and highlight the need for conservation strategies to integrate species-specific trait interactions across life stages. Mitigating demographic bottlenecks should be a priority, particularly for species lacking compensatory adaptations such as high fecundity or biotic dispersal.