Evenness alters the positive effect of species richness on community drought resistance via changing complementarity

Biodiversity loss may decrease the resistance of ecosystems to disturbance. While many studies have tested the effects of species richness on the ability of communities to resist disturbance, few have considered the role of species evenness. We examined whether species evenness affects the relationship between species richness and the drought resistance of plant communities. We constructed experimental plant communities with four levels of species richness (1, 2-, 4- and 8-species) and three levels of species evenness (high, medium and low) and subjected them to two drought treatments (with vs. without drought). Species evenness significantly increased the drought resistance of the communities and also complementarity effects. Moreover, a positive relationship between species richness and the drought resistance of plant communities was found at high and medium evenness but not at low evenness. This result corresponded well with the fact that complementarity effects increased with species richness at high and medium evenness but not at low evenness. However, neither species evenness nor species richness significantly affected selection effects for the drought resistance of the communities. We conclude that species evenness can alter the relationships between species richness and the resistance to disturbance (e.g., drought) in plant communities by enhanced complementarity among species. Thus, in addition to species richness, evenness should also be considered an important indicator of the community resistance to future global changes, such as increased drought intensity and frequency.