Subordinate plant species enhance community resistance against drought in semi-natural grasslands, Mariotte et al. 2013

This study examines how subordinate species^[8] influence community insurance against drought in semi-natural grasslands of the Swiss Jura. The insurance hypothesis proposes that an increase in community diversity corresponds to an increase in the range of potential species responses to environmental stress. The authors tested the role of subordinate species in community resistance to drought, recovery and resilience, and on productivity. They induced summer drought conditions for two months by covering the test plants with raincovers.

The drought simulation reduced soil water content by 67%, relative to comparable watered land plots. Drought, removal of subordinate species, and their interaction, all had dramatic adverse impacts on community resistance. In contrast to dominant and transient species, subordinate species showed significantly stronger resistance in drought plots than in control plots. Additional findings supported the conclusion that the plant community was more resistant and produced more biomass after drought when containing high biomass of subordinate plants.  

Plant community resilience was not affected by drought but was decreased by the subordinate removal treatment. Species composition was also affected by drought and removal conditions; most dominant and transient species^[9] were associated with watered plots. Some transient species (such as the ox-eye daisy) were associated with plots in which subordinate removal had occurred.

The authors conclude that, in general, dominant species fared poorly in response to drought, whereas the proportion of subordinate and transient species increased under these conditions.  They also noted that the decline in resistance was about 10 times higher in plots where subordinates had been removed than in plots without removal. Thus, the subordinates facilitated the regrowth of dominants and transients during drought. They proposed that the reduced competition among dominants during drought conditions afforded the subordinates the opportunity to accumulate more biomass.

The authors demonstrate that: “in species-rich grassland communities, subordinate species, a key component of plant diversity, are a main driver of community resistance to drought. Our findings show the importance of ecosystem-level impacts of these low abundant plants” [Mariotte 2013: 771]. They further speculated that the role of subordinates in resisting drought for the whole community may lie in their ability to increase water availability through greater interaction with the soil microbial community, such as mycorrhizal fungi. This article adds credence and specificity to our understanding of the key role of biodiversity in ecosystem functioning.