Organic agriculture promotes evenness and natural pest control

Even break for organic crops Declining species number (richness) harms ecosystems, and conservation efforts have largely focused on conserving or restoring particular rare species. However, greater disparity in species relative abundances (evenness) might also do ecological harm, which could only be reversed by altering the densities of many species at once. A new survey of organic and conventionally managed potato fields shows that species evenness is greater under organic management. Replicating these levels of evenness in a field trial shows that the evenness of natural enemies found in organic fields promotes pest control and increases crop biomass. In organic crops many beneficial species (that eat pest insects) are equally common, which in potatoes leads to fewer pests and larger plants. A survey of organic and conventional potato fields shows that species evenness is greater under organic management. Replicating these levels of evenness in a field trial shows that the evenness of natural enemies found in organic fields promotes pest control and increases crop biomass. This is independent of the identity of the dominant enemy species, so is a result of evenness itself. Human activity can degrade ecosystem function by reducing species number (richness) 1 , 2 , 3 , 4 and by skewing the relative abundance of species (evenness) 5 , 6 , 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 , 2 , 3 , 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.