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).
Plants
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 farms.
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 fertilizers
- 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 systems.
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 2005): 113-130.
Adam Montri is pursuing a master's degree in horticulture
at Pennsylvania State University.
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