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Organic Farms Support More Species, Greater Biodiversity

A meta-analysis of 184 observations from 94 studies has found that on average, organic farms support 34% more plant, insect and animal species (‘species richness’) than conventional farms. This result was relatively robust across 30 years of data, mostly from developed countries. The research team further found that organic farms had a greater positive effect on biodiversity in intensively farmed regions, providing rich habitats free from pesticides.

“Organic farming…. can yield significant long-term benefits for biodiversity. Organic methods could go some way towards halting the continued loss of diversity in industrialised nations,” says Sean Tuck of Oxford University's Department of Plant Sciences, lead author of the study.

The researchers call for more studies on organic farming to be conducted in tropical, sub-tropical and Mediterranean regions, where very few such studies exist, in order to provide a globally relevant analysis of the effects of organic farming.

The study, entitled “Land-Use Intensity and the Effects of Organic Farming on Biodiversity: A Hierarchical Meta-Analysis” has been published in the Journal of Applied Ecology. The summary, synthesis and recommendations (Item 1) as well as a media release on the paper (Item 2) are reproduced below. The full study can be downloaded from: http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12219/full


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Item 1

Land-Use Intensity and the Effects of Organic Farming on Biodiversity: A Hierarchical Meta-Analysis

Sean L. Tuck, Camilla Winqvist, Flavia Mota, Johan Ahnstrom, Lindsay A. Turnbull, and Janne Bengtsson

http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12219/full
 

Summary

1.         The benefits of organic farming to biodiversity in agricultural landscapes continue to be hotly debated, emphasizing the importance of precisely quantifying the effect of organic vs. conventional farming.

2.         We conducted an updated hierarchical meta-analysis of studies that compared biodiversity under organic and conventional farming methods, measured as species richness. We calculated effect sizes for 184 observations garnered from 94 studies, and for each study, we obtained three standardized measures reflecting land-use intensity. We investigated the stability of effect sizes through time, publication bias due to the ‘file drawer’ problem, and consider whether the current literature is representative of global organic farming patterns.

3.         On average, organic farming increased species richness by about 30%. This result has been robust over the last 30 years of published studies and shows no sign of diminishing.

4.         Organic farming had a greater effect on biodiversity as the percentage of the landscape consisting of arable fields increased, that is, it is higher in intensively farmed regions. The average effect size and the response to agricultural intensification depend on taxonomic group, functional group and crop type.

5.         There is some evidence for publication bias in the literature; however, our results are robust to its impact. Current studies are heavily biased towards northern and western Europe and North America, while other regions with large areas of organic farming remain poorly investigated.

6.         Synthesis and applications. Our analysis affirms that organic farming has large positive effects on biodiversity compared with conventional farming, but that the effect size varies with the organism group and crop studied, and is greater in landscapes with higher land-use intensity. Decisions about where to site organic farms to maximize biodiversity will, however, depend on the costs as well as the potential benefits. Current studies have been heavily biased towards agricultural systems in the developed world. We recommend that future studies pay greater attention to other regions, in particular, areas with tropical, subtropical and Mediterranean climates, in which very few studies have been conducted.

Synthesis and Recommendations

This analysis affirms that organic farming usually has large positive effects on average species richness compared with conventional farming. Given the large areas of land currently under agricultural production, organic methods could undoubtedly play a major role in halting the continued loss of diversity from industrialized nations. The effect of organic farming varied with the organism group and crop studied, and with the proportion of arable land in the surrounding landscape. We found larger effects in cereals, among plants and pollinators, and in landscapes with higher land-use intensity. Despite the fact that organic farming has been suggested to have large effects on soil conditions, its effects on soil organisms were ambiguous and in general understudied. Finally, it is clear that three decades of studying the effects of organic farming on biodiversity have been heavily biased towards agricultural systems in the developed world, especially Europe and North America. We therefore recommend that other regions and agricultural systems are given much greater attention. In particular, more studies are needed in tropical, subtropical and Mediterranean climates. Studies at any scale would be beneficial: at the farm scale because this is the economic unit of farming, and at the landscape scale because this is the scale at which many organisms respond. This would allow a more balanced and globally relevant assessment of organic farming effects on biodiversity, ecosystem services, food production and agricultural sustainability.


Item 2

Organic farms support more species

http://www.ox.ac.uk/media/news_stories/2014/140204.html

On average, organic farms support 34% more plant, insect and animal species than conventional farms, say Oxford University scientists.

Researchers looked at data going back 30 years and found that this effect has remained stable over time and shows no signs of decreasing.

'Our study has shown that organic farming, as an alternative to conventional farming, can yield significant long-term benefits for biodiversity,' said Sean Tuck of Oxford University's Department of Plant Sciences, lead author of the study. 'Organic methods could go some way towards halting the continued loss of diversity in industrialised nations.'

For pollinators such as bees, the number of different species was 50% higher on organic farms, although it is important to note that the study only looked at 'species richness'.

'Species richness tells us how many different species there are but does not say anything about the total number of organisms,' said Mr Tuck. 'There are many ways to study biodiversity and species richness is easy to measure, providing a useful starting point. Broadly speaking, high species richness usually indicates a variety of species with different functions. Taking the example of bees, species richness would tell us how many different species of bee were on each farm but not the total number of bees.'

The study, published this week in the Journal of Applied Ecology, looked at data from 94 previous studies covering 184 farm sites dating back to 1989. The researchers re-analysed the data using satellite imagery to estimate the land use in the landscape surrounding each farm site to see if this had an impact on species richness. The study was carried out by scientists at Oxford University and the Swedish University of Agricultural Sciences, and was partly funded by the Natural Environment Research Council (NERC).

Organic farms had a bigger impact on species richness when the land around them was more intensively farmed, particularly when it contained large tracts of arable land. Arable land is defined as land occupied by crops that are sown and harvested in the same agricultural year, such as wheat or barley.

'We found that the impacts of organic farms on species richness were more pronounced when they were located in intensively farmed regions,' said Dr Lindsay Turnbull of Oxford University's Department of Plant Sciences, senior author of the study. 'This makes sense because the biodiversity benefits of each organic farm will be diluted in clusters of organic farms compared to an organic "island" providing rich habitats in a sea of pesticide-covered conventional fields. This effect was weakest in pollinators, which may be because pollinators are likely to visit neighbouring farms and could be affected by pesticides there.'

The impact of organic farming on total species richness varied significantly across the data, with the average gain in species richness varying between 26% and 43%. This variation could be down to a number of factors relating to regional variation in farming practices and definitions of 'organic'.

'Some conventional farms will intensively spray pesticides and fertilisers whereas others will use mixed methods of crop rotation and organic fertilisers with minimal chemical pesticides,' said Dr Turnbull. 'There are also regional differences in farming practices, and the majority of the studies in our data were in developed nations with long histories of farming such as those in Western Europe. There, some wildlife have thrived in extensively managed farmland but are threatened by agricultural intensification. However, in developing nations there is often great pressure on the land to provide enough food for local people, resulting in the conversion of natural habitat to farmland. In such cases the benefits of organic farming are less clear, as this may require more land to achieve the same yield as conventional farming.

'More research is needed on the impact of organic farming in tropical and subtropical regions. For example, there are no studies on organic bananas or cocoa beans, two of the most popular organic products found in European supermarkets. At present, we simply cannot say whether buying organic bananas or chocolate has any environmental benefit.'

 


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