Recently I wrote about a study on organic vs synthetic pesticides on sustainability, which suggested that organic pesticides are not always more efficacious against pests nor as selective (not killing natural enemies of pests) than synthetic pesticides.
Earlier this month a study was published on a related area: organic farming on natural enemy evenness. Different farming techniques can alter the balance of the natural enemy species’ and cause a reduction to only a select few species. Existing evidence suggests this may limit the ability to control pest populations.
Natural enemy diversity includes richness (number) and evenness (abundance of species). The latter has not been studied as well as the former. The authors analyzed survey data from Washington potato fields and found no significant impact on richness from different pest-management techniques, but organic fields were more even on the distributions of natural enemy species compared to conventional fields, which were ruled by largely 1 species. To extend the findings to other crops, they performed a meta-analysis on 38 studies and found that organic farms had greater evenness in organic fields.
However, as I count on the graph below, counting individual increases or decreases when going from conventional to organic reveals approximately 50/50 split; it seems that the magnitude of evenness on organic farms is sometimes much greater. This could be for several reasons. The studies that they analyzed had 40 predator and 8 insect pathogens, 23 crops, in 16 different countries.
So there are a lot of different variables to consider here (as goes with meta-analyses), and perhaps sub-analyses with different groups would show advantages for conventional with some crops or species or areas. But in general, the data suggest organic currently has the advantage.
Next, they performed field enclosure tests to see if natural enemy evenness suppressed pests. They setup 42 predator-pathogen evenness combinations (7 predators and pathogens) based on the previous potato field survey data with total densities remaining equal (richness constant).
They found that increasing evenness in predators and pathogens increased plant biomass. Plant mass correlates with potato tuber yield, suggesting that yield is also increased when evenness is increased. More evenness also increased pest (potato beetle) mortality. Predator and pathogen evenness were additive in these findings. Also, they found that natural enemy species better survived when evenness was greater (a little competition is good).
I particularly liked the way they visually presented these results, here they are:
a. lighter shading = higher plant weight with increasing evenness; the plane angle suggests both pathogen and predator evenness are independently related to plant weight:
c. this shows that more natural enemies survived when predator (but not pathogen) evenness was greater:
The authors discuss the findings of this study, that pest densities were ~18% lower and plant mass 35% larger, which could mean that a greater evenness with organic farming could offset the losses in yield from reduced pesticide use. They note that field enclosure experiments have limitations though, and state a need for more studies.
They suggest that organic farming may improve evenness through (generally) reduced usage of broad-spectrum (non-selective) pesticides. The study I recently discussed suggests that organic pesticides are not always more selective against pests compared to natural enemy species over synthetics. Perhaps the development of more selective pesticides will give the best of both worlds: an increased evenness seen with organic farming and an even greater yield characteristic (in general) of conventional farms.
Edit: Note the good points on some weaknesses in this paper here.
Crowder DW, Northfield TD, Strand MR, & Snyder WE (2010). Organic agriculture promotes evenness and natural pest control. Nature, 466 (7302), 109-12 PMID: 20596021