Organic farming and biodiversity
A British literature review finds that for the most part, organic farms support more diverse populations of desirable plants, birds, mammals, insects, and soil microbes

By Adam Montri

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:

  1. Prohibition/reduced amounts of chemical pesticides and inorganic fertilizers
  2. Sympathetic management of non-crop habitats and field margins
  3. 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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