Commonness and rarity of species: Does species’ rank influence contribution to ecosystem function?

Across the globe, biodiversity loss is occurring at an unprecedented rate. Rare species are especially susceptible to extinction, given that they typically have small population sizes and restricted geographic ranges, are less adaptable to disturbances, and are greater habitat specialists. However, while rare species may be prone to extinction, it remains unclear whether the loss of rare species is important to ecosystem function. In addition, it is important to consider the way in which rarity is defined, given that there are multiple definitions of rarity based on a species' geographic range, habitat specificity, and abundance in a community. Therefore, to better understand the contribution of rare species to community function, our study has two goals: 1) to determine whether rare species contribute to community trait space, which serves as a proxy for ecosystem function, more than common species; 2) to determine if the effect of rare species differs based on the definition of rarity employed. Using grassland data from Cedar Creek Minnesota, we assessed the importance of rare species on community trait space. We found that rare species have an equal effect on trait space as common species, except in the case of rarity defined as mean abundance. This suggests that when defining rarity by geographic range and habitat specificity, rare species may play an important role in ecosystem functioning due to their unique contribution to trait space. When considering local abundance of species, however, rare species contribute less than common species to trait space, possibly due to habitat filtering. These results suggest that the importance of rare species to trait space, and possible ecosystem function, depend on the definition of rarity employed.