Keystones from the bottom up: Loss of rare species disproportionately impacts higher trophic levels

Background/Question/Methods

Keystone species—organisms with large impacts on community structure that are disproportionate to their abundance—that have been described to date are virtually all consumers. Here, we show that the loss of rare basal species can result in disproportionately large declines in mobile consumers in an intertidal community. We marked and surveyed 30 plots on a rocky shoreline and found that both the abundance and richness of mobile consumers were positively related to basal species richness. We ranked the plots according to basal richness and assigned experimental treatments as follows: (1) 10 low-richness un-manipulated controls, (2) 10 high-richness un-manipulated controls, and (3) 10 high-richness plots from which we removed basal species so that their composition mimicked the low-richness controls. These experimental removals thus simulated a realistic loss of sessile species from plots. The species removed were rare, collectively representing <10% of basal biomass.

Results/Conclusions

Consistent with the rarity of basal species that were removed, basal cover, a good surrogate for biomass, did not differ among the three treatments over the course of the 1-month experimental manipulation. In contrast, mobile species richness and biomass declined in removal plots so that values for both richness and biomass were indistinguishable from those in low-richness control plots after 1 month. Thus, loss of an insignificant amount of basal biomass resulted in a 29% reduction in mobile invertebrate richness and a 44% reduction in mobile invertebrate biomass. The organisms removed from plots, including 11 different species of sessile invertebrates and seaweeds, collectively act as a keystone guild from the bottom up, with large impacts on organisms at higher trophic levels. Our work highlights the surprising importance of realistic losses of rare species and contributes new perspectives on the roles that keystone species can play—acting not only from the top-down but also from the bottom-up—in mediating community structure and dynamics.