January 27, 2005:
Many proponents of organic agriculture have made claims concerning
the positive effect of organic practices on biodiversity. But are
such claims statistically verifiable? In 2004, a group of UK researchers
sought to find out.
The researchers reviewed 76 surveys comparing the biodiversity
impact of organic and conventional agricultural practices and categorized
the results according to the biological group examined: flora, soil
microbes (bacteria, fungi, and nematodes), invertebrates (earthworms,
beetles and spiders), and vertebrates (mammals and birds).
Ten studies specifically compared non-crop vegetation between systems,
five investigated invertebrate abundance and botanical diversity,
while two focused on grassland systems. All but one of the studies
investigating arable and mixed farming systems recorded higher weed
species richness and abundance in fields under organic management.
Differences were greater for broad-leafed families such as Fabaceae,
Brassicaceae, and Polygonaceae than for grass families. In addition,
fields under organic management held considerably more rare and/or
declining species. Many of the weeds found in the conventional systems
were ones that flourish in nitrogen-enriched environments and are
considered major agricultural pests.
In some circumstances organic systems had far fewer weeds than
conventional ones. The authors attributed these results to mechanical
weeding techniques, undersowing of crops, and the use of clover-ryegrass
leys within the organic crop rotations. Hedgerows showed much higher
biodiversity on organic farms; this was attributed to the absence
of herbicide drift and higher migration rates from larger weed species
in organic fields. Grasslands were similar across organic and conventional
systems, although there was some evidence that organic systems supported
greater grassland species richness.
Soil microbes (bacteria, fungi, and nematodes)
Fourteen studies primarily investigated soil microbes while another
five included an aspect of soil microbes in the research. Results
showed that differences in microbial communities were limited, but
there was a general trend towards higher bacterial and fungal abundance
and activity under organic systems. This was attributed to the addition
of organic matter on the majority of organic farms. Overall, the
nematode population followed the same trend, but genus/group specific
traits largely dictated community composition. These results showed
that microbial communities may be heavily influenced by other factors
such as soil and crop types.
Invertebrates (earthworms, beetles, and spiders)
Six studies specifically focused on earthworm populations while
another seven included them as a component. Most found earthworm
densities in organic systems to be almost twice as high as in conventional
systems. This may result from the use of farmyard and green manures,
which provide a major food source for earthworms, and/or from the
prohibition of pesticides on organic farms. One study, however,
found no difference between systems; while another found more earthworms
in the conventional system--this result was attributed to very excessive
tillage in the organic field being studied.
Eleven of the studies focused solely on beetles with 10 others
recording abundance and richness. Of these, 12 reported higher abundance
and richness of carabids (ground beetles) in organic fields, while
four found the reverse. Greater ground beetle abundance was attributed
to varied vegetative structure in organic fields. The authors also
reported a positive correlation between weed species richness/cover
and beetle species richness.
A total of 10 studies looked at spider populations. All found a
higher abundance of spiders under organic production, although results
were not statistically significant in every case. Some studies reported
higher species richness and abundance of surface-active spiders
in organic wheat fields. Higher spider richness and abundance in
organic systems were attributed to greater structural complexity
of the field understory, creating a more suitable climate and a
higher supply of prey.
Vertebrates (mammals and birds)
Two studies focused specifically on mammals. Both found a greater
number of small mammals in organic systems, possibly due to the
food abundance and shelter provided by hedgerows. Bat activity and
foraging were also significantly higher on organic farms in both
studies (61 percent and 84 percent), suggesting that bats may be
actively seeking out organic farms, perhaps because of increased
and more diverse insect populations. Two rare species of bat, the
greater and lesser horseshoe bats, were found only on the organic
Five studies compared bird communities as a whole in each type
of system. All five investigated species richness and abundance,
primarily during the summer, with one investigating nest density
and nesting success. All five reported greater abundance and species
richness on organically managed farms. Both nesting density and
success were higher in organic as well. One study found 31 bird
species—including many that have declined nationally over
the last two decades—to be more abundant on organic farms,
versus three species occurring more often on conventional farms.
Greater invertebrate and plant species diversity on organic farms
was cited as the principal reason for these findings.
In summary, the 76 studies reviewed clearly illustrate that species
richness and abundance across a wide-range of taxa tend to be higher
on organic than on conventional farms. The authors propose three
main reasons this increased biodiversity:
- Prohibition/reduced amounts of chemical pesticides and inorganic
- Sympathetic management of non-crop habitats and field margins
- Preservation of mixed farming
The authors also emphasize that biodiversity components are interdependent.
That is, increased plant species richness and abundance may increase
the numbers of microbes, which may increase invertebrate numbers,
etc., across many levels. The researchers suggest that further work
is required to assess whether the “whole farm” philosophy
underpinning organic systems offers greater biodiversity than the
adoption of specific “key” practices within the conventional
Citation: D.G. Hole, A.J. Perkins, J.D. Wilson,
I.H. Alexander, P.V. Grice and A.D. Evans. Does organic farming
benefit biodiversity? Biological Conservation 122, 1 (Mar
Adam Montri is pursuing a master's degree in horticulture at
Pennsylvania State University.