Land‐use intensity and the effects of organic farming on biodiversity: a hierarchical meta‐analysis

Abstract

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. 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. 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. 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. 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. 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.

Organic fields sustain weed metacommunity dynamics in farmland landscapes

“…OF fields in general harbour more insect-pollinated plants [54], forbs [55], and rare or threatened weeds [56,57], and fewer nitrophilous species [31,56], while conventional fields have fewer broad-leaved species due to the use of auxin herbicides to control them [58], and more herbicide resistant weeds, in particular grasses [59]. Though in some cases OF may not increase weed species richness [60,61], our results agree with most previous studies [11,12,33], indicating a positive effect on weed species richness (roughly þ30% in the latter studies compared with þ48.9% in the field core and þ30% in the field margin in this study). We also found that magnitude of the difference between field core and margin was higher in CONV than in OF, in accordance with Gabriel et al [33] and other studies that demonstrated that field boundaries can act as refugia for many weeds species including species threatened by agricultural intensification [29,62].…”

“…Many studies have assessed the potential biodiversity benefits of OF in comparison with conventional farming (CONV), but a general consensus is still lacking [11,12]. At the field level, an overall positive effect of OF was detected on plant species richness [13,14], though the response is highly taxon dependent [12].…”

“…At the field level, an overall positive effect of OF was detected on plant species richness [13,14], though the response is highly taxon dependent [12]. However, due to lower yields, larger surfaces are needed to maintain food production under OF, hence the net balance between positive and negative impacts is still debated [12,15,16]. OF effects at the field level may further depend on surrounding landscapes [4,17,18].…”

“…Many weed species occur in both crop and non-crop areas [28,29], especially field edges where management practices are less intensive [30]. At the field scale, weeds strongly respond to OF: species richness may be about 70% higher and abundance doubled compared with CONV [12,31]. Furthermore, weed communities respond also to landscape-scale processes [32][33][34][35].…”

Organic Agriculture 3.0 is innovation with research

“…The latest meta-analysis by Rahmann (2011) and Tuck et al (2014) revealed an overall positive effect of organic agriculture on biodiversity, with an average 30% increase in species richness. Results have been robust over the past 30 years.…”