ISIS Report 05/07/10

Organic Agriculture for Biodiversity and Pest Control

Scientists find organic fields have more even distribution of natural enemy species, thereby providing significantly better pest control than conventional fields and promoting plant growth. Dr. Mae-Wan Ho

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Intensive industrial agriculture has resulted in great losses of biodiversity due to the destruction of natural habitats, the displacement of indigenous varieties by green revolution monocultures, the massive diversion of water for irrigation, and the heavy inputs of chemical fertilizers and pesticides.

Pesticides are non-selective, they kill pests as well as the natural enemies that devour the pests and keep pest populations down. Organic agriculture reduces the damage due to pesticides by eliminating or limiting their use, and is generally acknowledged to result in protecting and increasing biodiversity (see [1] Food Futures Now: *Organic *Sustainable *Fossil Fuel Free , ISIS report). But does organic agriculture give better pest control? Ecologists have been challenged to provide real evidence for that [2].

Two different measures of biodiversity

There are two measures of biodiversity, species richness - the number of species - and species evenness - the relative abundance of those species. Species richness and evenness can vary independently. Communities dominated by a few common species and many rare species have low evenness; whereas those with species more equally represented have high evenness.

Comparisons on biodiversity have focussed largely on species richness. Similarly, conservation efforts are concentrated on restoring or maintaining the number of species without regard for evenness.

Researchers led by David Crowder at Washington State University, Pullman, and University of Georgia, Atlanta, in the United States have published new research clearly demonstrating that organic farming promotes evenness among natural enemy species, and it is species evenness, rather than species richness that is more important for pest control [3]. This new result not only confirms the overriding benefit of organic agriculture over industrial farming, but also has far-reaching implications both for conservation and the practice of biological control.

Three different investigations yield the same answer

The researchers investigated the problem in three different ways: comparing organic and conventional potato fields in Washington State, comparing organic and conventional farms in general, and finally setting up their own experiment in field enclosures. All give the same answer, pointing to the even distribution of natural enemies and pathogens of pests as the key to effective pest control in organic farms.

The team began by analyzing data from surveys of predators and pathogens of the Colorado beetle, a serious pest of potato fields in Washington State. They found no impact on species richness between organic and conventionally managed fields. However, evenness of natural enemies was significantly greater in organic than conventional potato fields. But was it true of organic management itself or just a feature of potato fields?

So they scoured the literature for studies that reported abundances of at least three taxonomic groups of natural enemies in organic and conventional fields of the same crop. They found 38 studies providing comparisons for 40 predators and eight insect pathogens. Again it revealed significantly greater evenness in organic fields than conventional fields.

To put their finding to a further test, the researchers carried out an experiment in which they changed the distribution of predators and pathogens in field enclosures, and recorded the effects on plant growth and potato beetle mortality. The distributions of natural enemies and pathogens used in the experiment reproduced those that they have found in field surveys - seven different predator distributions and six different pathogen distributions - making a total of 42 combinations of predator-pathogen evenness. The total densities and richness of pathogen and predators were held constant in all the combinations.

They found that increasing natural enemy evenness in the field enclosures “triggered a powerful trophic cascade beneficial to plants and harmful to herbivores [pests]”. Evenness among predators and pathogens acted in concert to increase plant biomass, so that the largest plants occurred when both predators and pathogens were evenly distributed. Above ground biomass correlated with potato tuber yield. Consistent with this effect, greater natural enemy evenness increased potato beetle mortality, with effects of predator and pathogen evenness again acting independently to complement each other completely.

Another bonus is that predators survived better with predator evenness, and were unaffected by pathogen evenness. In the potato system, the evenness increase is accompanied by an 18 percent lower pest density and 35 percent larger plants. It appears that evenness may also promote resilience to disturbance, acting as a buffer against environmental stress, and enabling the community to recover faster.

The researchers added [3]: “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.”

Scientists at the University of Zurich, Switzerland, who commented on the work agreed. They wrote [2]: “The control of pests by their natural enemies is a valuable ecosystem service: unpaid, and often overlooked.” In their view, the study also demonstrated the damaging effects of pesticides on natural enemies. Even subtle damage such as changes in species evenness can have large and measurable effects on crop performance. The reduction in natural control associated with pesticide use is probably one reason why the yields from organic farming are often comparable to those from conventional farming (see evidence reviewed in [1]).

References

1. Ho MW, Cherry B, Burcher S and Saunders PT. Green Energies, 100% Renewables by 2050, ISIS/TWN, London/Penang, 2009, http://www.i-sis.org.uk/GreenEnergies.php

2. “How to get even with pests”, Lindsay A. Turnbull and Andy Hector, News & Views, Nature 2010, 466, 36-37.

3. Crowder DW, Northfield TD, Strand MR and Snyder WE. Organic agriculture promotes evenness and natural pest control. Nature 2010, 466, 110-3.

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