Human activity can degrade ecosystem function both by reducing species number (richness) and by skewing the relative abundance of species (evenness). Conservation efforts often focus on restoring or maintaining species number, reflecting the well-known impacts of richness on many ecological processes. In contrast, the ecological effects of disrupted species evenness have received far less attention, and developing strategies for restoring evenness remains a conceptual challenge. Here, in a large-scale field experiment we examined how evenness among two ecologically-distinct groups of natural enemies, predators and pathogens, affected herbivore suppression and plant growth.

Results/Conclusions

We found that very even communities of predator and pathogen natural enemies strengthened herbivore suppression and yielded the largest plants. In contrast, herbivore densities were high and plant biomass was low when enemy evenness was disrupted. Our results were independent of the numerically dominant predator or pathogen species, and so resulted from evenness per se. Moreover, evenness effects among the two ecologically-distinct enemy groups were both independent and complementary. Our results strengthen the argument that rejuvenation of ecosystem function requires restoration of species evenness, and not just richness.