Organic agriculture promotes evenness and natural pest control

Human activity can degrade ecosystem function by reducing species number (richness)(1-4) and by skewing the relative abundance of species (evenness)(5-7). Conservation efforts often focus on restoring or maintaining species number(8,9), reflecting the well-known impacts of richness on many ecological processes(1-4). In contrast, the ecological effects of disrupted evenness have received far less attention(7), and developing strategies for restoring evenness remains a conceptual challenge(7). In farmlands, agricultural pest-management practices often lead to altered food web structure and communities dominated by a few common species, which together contribute to pest outbreaks(6,7,10,11). Here we show that organic farming methods mitigate this ecological damage by promoting evenness among natural enemies. In field enclosures, very even communities of predator and pathogen biological control agents, typical of organic farms, exerted the strongest pest control and yielded the largest plants. In contrast, pest densities were high and plant biomass was low when enemy evenness was disrupted, as is typical under conventional management. Our results were independent of the numerically dominant predator or pathogen species, and so resulted from evenness itself. Moreover, evenness effects among natural enemy groups were independent and complementary. Our results strengthen the argument that rejuvenation of ecosystem function requires restoration of species evenness, rather than just richness. Organic farming potentially offers a means of returning functional evenness to ecosystems.