J. Ahnstrom & J. Bengtsson
Swedish University of Agricultural Science, Dep. Ecology and Crop Production Science, Sec. Landscape Ecology, Box 7043, 75007 Uppsala, Sweden.
Abstract
Organic farming has been suggested to enhance biodiversity. In our literature review and meta-analysis we show in general a 30 % higher species richness on organic compared to conventional units. The results were variable between studies and in 16 % of the studies the species richness was higher on conventional units. We then divided the data into different organism groups and spatial scale of the studies. Birds, insects and plants showed a higher species richness in organic farming. Studies where landscape characteristics were considered showed a significant but a highly heterogeneous result of higher species richness in organic farming systems compared to plot comparisons with highly significant positive species richness.
Abundances were also studied and a 50% higher abundance was found on organic units. Significant positive response was found in birds, predatory insects, soil organisms and plants. However, at the end of the day it is the farmer who decides what to grow and how to grow it on the farm. No matter what farming system, his/her attitudes, knowledge and interest will strongly affect the farms effect on the surrounding environment. Thus to fulfill the Swedish environmental objective ‘A varied agricultural landscape’ we all, farmers, researchers, extension specialist and administrators, have to respect each other and acknowledge the knowledge and interest of the different groups.
Keywords: Meta-analysis, attitudes, farming system, species richness
Introduction
The loss of biodiversity in the agricultural landscape is well known (Fuller et al. 1995, Benton et al. 2003). Organic farming generally increases biodiversity (Paoletti et al. 1992, Bengtsson et al. 2005). Thus organic farming has been suggested to partly replace the loss of biodiversity seen in agricultural landscapes. However, this has also been questioned (Trewawas 1999, Goklany 2002)
Meta-analysis is a method for analyzing and synthesizing the results of several independent studies examining the same question (van Zandt & Mopper 1998, Gurevitch & Hedges 2001). The statistical procedures allow quantitative analyses of treatment effects, and account for the fact that all studies are not equally reliable (Bengtsson et al. 2005). The method will weigh different studies differently depending on number of replicates and the standard deviation within each study. A study with high number of replicates and a low standard deviation will have a higher impact on the general results compared to a study with few replicates and high standard deviation.
Biodiversity issues in the agricultural landscape are much more than merely a biological issue. The manager of the land, often the farmer, will of course affect biodiversity. The effect will differ depending on attitudes, knowledge and interest towards nature, economical situation, and social acceptance for change in the farmer community etc. Thus it is crucial to communicate and cooperate with farmers to be able to conserve biodiversity.
Materials and methods
We conducted a literature research for comparing studies of biodiversity on organically and conventionally managed units. We included all studies published before December 2002. All together we found 66 publications comparing diversity on organic and conventional systems. From the included studies we tabulated the mean species richness or abundance, the n- values and calculated the respective SD, for the two farming systems. This extracted data made it possible to calculate the effect size (Q; Hedges’g) and the pooled SD of each study. Studies that did not provide information on n or SD were included in a sign-test.
The effect size was tried if it was heterogeneous (Q significant) or homogenous (Q not significant) i.e. whether or not the effect size differed greatly between studies. When results were heterogeneous we divided the studies in taxonomical or functional units. We also divided the comparative studies by scale, plot, field on farm and fields/farms in matched landscapes. For more information about the method used see Bengtsson et al. 2005.
During spring and summer 2004 sixteen Swedish farmers from Uppsala and Heby were interviewed (open-ended and semi-structured) about their feelings for, knowledge of, interest in, nature, nature conservation, nature conservationists and the life as a farmer. The interviews will be fully transcribed and analyzed with qualitative methods. Their farms were also studied in terms of different crops, management of crops, landscape characteristics, and biodiversity (birds, bumblebees, Carabidae, solitary wasps and weeds).
Results
Organic farming usually increased species richness (Table 1) and abundances (Table 2) with 30 % and 50 %, respectively. Divided into organism groups birds, insects and plants species richness was higher and the abundance of birds, predatory insects, soil organisms, and plants, was higher in organic units.
Studies made in smaller areas, plots, showed in general a bigger separation of species richness and abundance between the farming systems. Organic fields/farms in matched landscapes had a higher biodiversity but showed no difference in abundance, compared to conventional units, in our meta-analysis.
The interviews and field work from 2004 are not yet processed. The interview answers will be used briefly in the discussion.
Discussion
Organic farming generally enhance biodiversity. The reason for higher biodiversity in organic management can be the lack of pesticides, the higher variation of crops, the ley in the crop rotation and the use of farm and green manure. All farmers can of course have a varied crop rotation with ley and use farm manure but the incentives to do so is higher in organic farming. We want to highlight the fact that at the end of the day it is the farmers who decide what to grow and how to grow it on the farm.
There are systemic differences between the farming system on the theoretic level but not always on farm level. Many studies try to overcome this by using farm pairs. There is a risk of erasing the differences between the systems working with farm pairs. Finding an organic matching neighbour in the most intense agricultural area in Sweden is hard. Thus it is an obvious risk for underestimating the differences between the systems.
Table 1. A meta-analysis of the effects of organic management on species richness. Positive effect sizes indicate higher species richness in organic farming systems. The studies in the meta-analysis are available from the web site http://www.cul.slu.se. The number of positive studies column and the associated n column include all studies, and those in which no quantitative effect size, only increases or decreases in species richness, was given. When Q (heterogeneity of effect sizes among studies) is significant, the results are shown for a random effects model. *P < 0·05 for average effect size ≠ 0, for heterogeneity of effect sizes (Q) and for binomial test of the number of positive studies; CL, confidence limit.
Average effect size (Hedges’g)(± 95% CL) n Q No. of positive studies n Total 1.152(±0.524)* 32 170* 53* 63 By organism group Birds 1.495(±1.236)* 2 0 3 3 Arthropods 0.929(±0.589)* 19 71.7* 21* 28 Predatory insects 0.843(±0.590)* 15 43.8* 15 21 Carabidae 0.941(±0.861)* 11 34.7* 10 13 Non-predatory arthropods 1.046(±1.982) 4 26.2* 6 7 Soil organisms 0.306(±0.559) 5 3.3 7 10 Plants 2.684(±1.976)* 6 81.6* 22* 22 By scale of study
Plot or single field 2.917(±1.769)* 8 55.8* 15* 17 Field on farm 0.703(±0.550)* 11 19.3* 24* 27 Field/farm in matched
landscape
0.818(±0.791)* 13 79.6* 14 19
Table 2.A meta-analysis of the effect of organic management on the abundance of organisms. Average effect size
(Hedges’g)(± 95% CL) n Q No. of positive studies N Total 0.700(±0.272)* 71 522* 96* 117 By organism group Birds 0.708(±0.868)* 7 18.5* 12* 12 Insects 0.122(±0.300) 30 85.4* 29* 42 Predatory insects 0.486(±0.457)* 14 27.4* 16* 21 Carabidae 0.799(±0.865) 9 19.2* 9 12 Non-predatory insects -0.133(±0.373) 16 47.4* 13 21 Pest species -0.398(±0.441) 7 10.4 3 7 Soil organisms 1.022(±0.551)* 26 144* 44* 49 Earthworms 0.286(±0.362) 8 6.94 12* 13 Plants 1.305(±0.358)* 5 15.4* 7* 7 By scale of study
Plot or single field 0.567(±0.308)* 16 23.3 30* 33 Field on farm 1.278(±0.358)* 30 122* 43* 51 Field/farm in matched landscape 0.029(±0.273) 25 93.6* 23* 33
With our multidisciplinary project with interviews and ecological/agronomical data we want to highlight the farmer and that we cannot ignore his/her knowledge, interest and feelings
when designing subsidy programs and management strategies. A diverse landscape with a diversified management will promote biodiversity. Thus it is not good if management is uniform over big areas or if the same habitat is uniformly managed over the whole country. This has been the case with the semi-natural pastures in Sweden. The rules have in general mostly focused on enhancing the botanical diversity not biodiversity (Söderström et al. 2001). The appearance of the semi-natural pastures has thus been a product of the management requirement of the subsidy. Local knowledge of management and tradition and farmer interest has not been considered. To be able to reach the Swedish environmental objectives we all, farmers, researchers, extension specialist and administrators, have to respect each other and acknowledge the knowledge and interest of the different groups.
Conclusion
Organic farming enhance biodiversity but this effect varies between organism groups and landscapes. Positive effects of organic farming on biodiversity will be more pronounced in intensive than in small-scale heterogeneous landscape. We suggest that more work is focused on the socio-economic unit; the farm. Measures to preserve biodiversity ought to be more landscape-, farm-, and farmer specific than is presently the case.
References
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Acknowledgement
This work was funded by the Ekhaga foundation, Oscar & Lili Lamm’s foundation and FORMAS. This paper was written while Johan was a visiting PhD-student at University of Nebraska-Lincoln and Kansas State University-Manhattan. We would like to thank Dr. Francis at UNL and Dr. With at KSU for their hospitality.