Organic agriculture promotes evenness and natural pest control

Journal name:
Nature
Volume:
466,
Pages:
109–112
Date published:
DOI:
doi:10.1038/nature09183
Received
Accepted

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 number8, 9, reflecting the well-known impacts of richness on many ecological processes1, 2, 3, 4. In contrast, the ecological effects of disrupted evenness have received far less attention7, and developing strategies for restoring evenness remains a conceptual challenge7. 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 outbreaks6, 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.

At a glance

Figures

  1. Evenness of natural enemies across cropping systems.
    Figure 1: Evenness of natural enemies across cropping systems.

    Lines connect evenness values in conventional (circles)/organic (diamonds) pairs from each comparison in the meta-analysis. In the overall meta-analysis, natural enemy evenness was significantly greater in organic than in conventional fields (median 7.2% increase: SR+ = 177.0 (signed-rank test statistic), N = 48, P = 0.044). Similarly, predator evenness in Washington potato fields (blue symbols and line) was significantly greater in organic than in conventional fields (t = 2.28, d.f. = 18, P = 0.035); pathogen evenness (red symbols and line) was also greater in organic potato fields, although this difference was not significant (N = 19, P = 0.19).

  2. Predator and pathogen evenness in the field-enclosure experiment.
    Figure 2: Predator and pathogen evenness in the field-enclosure experiment.

    The top row shows the seven levels of predator evenness included in the field-enclosure experiment (Supplementary Table 2), and the leftmost column presents the six levels of pathogen evenness (Supplementary Table 2) that were included. These were fully crossed to yield 42 unique predator–pathogen evenness compositions (shown in the box). Predators were G. bullatus (yellow), N. alternatus (green), H. convergens (red) and P. melanarius (blue). Pathogens were H. megidis (black), S. carpocapsae (dark grey) and B. bassiana (light grey).

  3. Effects of natural enemy evenness on multiple trophic levels.
    Figure 3: Effects of natural enemy evenness on multiple trophic levels.

    Cascading effects of predator and pathogen evenness on final plant weight (log10 transformed; a), final densities of herbivorous potato beetles (log10 transformed; b) and final predator retrieval (ratio of number recovered to number released; c). In each panel, the response of each of the 42 experimental arenas is indicated with a circle, and the plane indicates the two-dimensional trend in the data. The shading of the plane darkens as the response values decrease along either evenness axis and lightens as the response values increase.

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Author information

Affiliations

  1. Department of Entomology, Washington State University, Pullman, Washington 99164, USA

    • David W. Crowder,
    • Tobin D. Northfield &
    • William E. Snyder
  2. Department of Entomology, University of Georgia, Athens, Georgia 30602, USA

    • Michael R. Strand

Contributions

D.W.C. and T.D.N. designed the field experiment, with contributions from W.E.S. and M.R.S. The field work was conducted by D.W.C. and the literature review was done by D.W.C. and T.D.N. The manuscript was written principally by D.W.C. and W.E.S., with input from T.D.N. and M.R.S.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (309K)

    This file contains Supplementary Tables S1-S8, References and Supplementary Figures S1-S5 with legends.

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