ISIS Report 05/07/10
Organic Agriculture for Biodiversity and Pest Control
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.
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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  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 .
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.
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 . 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
investigations yield the same answer
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.
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
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
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
researchers added : “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 the University of Zurich, Switzerland, who commented on the work agreed.
They wrote : “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
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
“How to get even with pests”, Lindsay A.
Turnbull and Andy Hector, News & Views, Nature 2010, 466, 36-37.
Crowder DW, Northfield TD, Strand MR and Snyder
WE. Organic agriculture promotes evenness and natural pest control. Nature
2010, 466, 110-3.
There are 1 comments on this article so far. Add your comment
|Bill Young Comment left 16th July 2010 00:12:35|
good work. This is very useful information.