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

Background/Question/Methods   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 do common species; 2) to determine if the effect of rare species differs based on the definition of rarity employed.

Results/Conclusions   Using grassland data from Cedar Creek Minnesota, we ran two different models to assess the importance of rare species on community trait space based on two different extinction scenarios. First, we ran a model where species were lost randomly. This analysis found that rare species and common species were both equally likely to contribute to community trait space; furthermore, this similarity remained when using different definitions of rarity to rank the species. Second, we ran a model where species were lost non-randomly, from most rare to most common. Using this simulation, we find that rare species appear to contribute more to community trait space than do common species when ranked by geographic range and habitat specificity. This suggests that the random loss of rare species does not have a large impact on ecosystem function, given that rare and common species contribute equally to trait space. However, when considering non-random species loss, where rare species are lost first, it appears as if the cumulative loss of rare species does play an important role in ecosystem function.