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- Materials and methods
- Supporting Information
The global demand for food and farmland is rapidly growing due to a variety of factors including rising human population numbers, increased meat consumption, urbanization, competing land uses for non-food crops and the alteration in the suitability of land to grow crops due to climate change (Tilman et al. 2009; Beddington 2010). While a reduction in food waste, improvements in infrastructure and transport, a change in human diets and expanding aquaculture are important mitigation strategies against increased demand (Godfray et al. 2010), it has been argued that agricultural production has to increase globally to supply the food required for the estimated over nine billion people by 2050 (Foresight 2011; Tilman et al. 2011). Increasing supply logically has two axes: either via intensification (increasing output over the same area) or via extensification (bringing more land into agricultural production). With agricultural intensification and land-use change being the major drivers for biodiversity loss, this will undoubtedly have a heavy impact on wildlife and the environment (Tilman et al. 2001).
Currently, two contrasting landscape-level scenarios are widely discussed with regard to preserving biodiversity while maintaining food production: wildlife-friendly farming (‘land sharing’) vs. land sparing (Green et al. 2005; Fischer et al. 2008). In a land-sparing scenario, the available land in a landscape is subdivided into some areas specialized for producing mainly agricultural produce, and others are devoted mainly to maintaining biodiversity and ecosystem services. This allows the agricultural land to be farmed intensively for high yields, while the spared land can be managed specifically for other services. There is no necessity for the ‘spared land’ to be spatially separated from the agricultural land; indeed, there are arguments that support it being a landscape-wide network of wildlife areas formed by field margins, small farm woodlands, water courses, etc. (Benton 2012). In the wildlife-friendly, land-sharing scenario the available land is under lower-intensity agriculture. The increased area of land in production compensates for its lower yield, and the decrease in intensity allows biodiversity to be conserved across the whole landscape. The optimal scenario depends on the shape of the yield vs. population density (or biodiversity) function (Green et al. 2005). If, from a high-yield baseline, a small reduction in yield causes a marked increase in biodiversity (a concave-down shape), then land sharing, or wildlife-friendly farming, is the better option. If, however, significant biodiversity gains require a very large reduction in yields (a concave-up shape), then land sparing is the better strategy. These contrasting scenarios should be considered as the endpoints of a continuum; it is not a question of ‘either/or’, but of how much of each strategy shall be applied and under what circumstances (Fischer et al. 2008). The solution is likely to depend on the peculiarities of populations, species groups or ecosystem services and the landscapes, regions or countries in focus (Hodgson et al. 2010).
In this study, we are interested in quantifying the trade-off between agricultural production and biodiversity. We recognize that impacts of farming are broader than biodiversity (e.g. environmental pollution and reduction in soil quality), and in theory, the sparing vs. sharing analysis could have a broader ‘impacts vs. yield’ trade-off. However, many reductions in ecosystem services are, by definition, mediated through species abundance and diversity, so examining this relationship in this instance is valuable. In the European context, different models have been used to describe the relationship between yield and biodiversity. Kleijn & Sutherland (2003) predicted that biodiversity will decline in a concave-up curve with agricultural intensity, a prediction recently supported by a study of farmland plants (Kleijn et al. 2009). This shape suggests that significant biodiversity is supported only when agricultural production is very low. A negative linear relationship between wheat yield and farmland bird species has been observed by Donald et al. (2006) and Geiger et al. (2010), which suggests that a reduction in agricultural intensity is equally effective at any yield. Hoogeveen, Petersen & Gabrielsen (2001) suggested a unimodal relationship, where biodiversity first increases and then declines as intensity increases. Under this scenario, the disturbance created by low-intensity farming leads to increased biodiversity relative to unmanaged land, but beyond an intermediate level of intensity biodiversity will decline.
Here, we focus on two farming systems, conventional intensive agriculture and organic farming – a specific example of wildlife-friendly agriculture. Organic farming is widely regarded as a more sustainable farming system than conventional agriculture because it produces food while conserving soil, water, energy and biodiversity (Pimentel et al. 2005), although ‘sustainability’ is a concept defined in many ways and with multiple currencies (e.g. greenhouse gas emissions, synthetic inputs, land use and biodiversity). Organic yields are globally on average 25% lower than conventional yields according to a recent meta-analysis (Seufert, Ramankutty & Foley 2012), although this varies with crop types and species and depends on the comparability of farming systems. Hence, it is questionable whether the environmental performance of organic farming is still better if related to the unit output per area.
We examine the impact of farming on biodiversity and ask two questions. First, in comparison with conventional farming, is organic farming beneficial for all biodiversity or differentially beneficial for different taxa and/or across different landscapes (Bengtsson, Ahnstrom & Weibull 2005; Hole et al. 2005)? Both the management of the farmland in the landscape, such as areas dominated by organic land, and the proportion of farming in the landscape, such as areas dominated by arable crops, can enhance or detract from the benefits of organic farming for different species groups (Holzschuh et al. 2007; Rundlof, Bengtsson & Smith 2008; Diekötter et al. 2010; Gabriel et al. 2010). Second, as crop yields are typically lower in organic compared with conventional farming systems (de Ponti, Rijk & van Ittersum 2012; Seufert, Ramankutty & Foley 2012), is the increase in biodiversity on organic farms sufficient to offset the necessary increase in total agricultural land that will be needed to increase the required crop yield? To our knowledge, few studies have contrasted crop yields of organic farming with biodiversity [see Ostman, Ekbom & Bengtsson (2003) for pest–natural enemy dynamics and Clough, Kruess & Tscharntke (2007) for staphylinids]. Thus, knowledge is very limited for the costs, in terms of yield loss, that are associated with biodiversity gains through organic farming in a wildlife-friendly farming scenario.
The aim of this study was to assess the trade-off between yield and biodiversity in both organic and conventional farms in lowland England. To reduce variation due to crop species, we focus in particular on winter cereal as Europe's most widespread arable crop. Biodiversity was assessed on a total of 165 fields of 29 farms in two regions over 2 years and measured as abundance and species density of plants, earthworms, insect pollinators (hoverflies, bumblebees and solitary wild bees), butterflies, epigeal arthropods (abundance only) and birds. Our expectation was that the shape of the negative relationship between biodiversity and yield might differ between taxa and farming systems. One might expect that taxa with limited mobility that use crop fields as their main habitat should respond more strongly than mobile multi-habitat users to crop yield. Furthermore, this response should follow a concave-down curve in organic fields if organic farming should be regarded as a wildlife-friendly farming system.
- Top of page
- Materials and methods
- Supporting Information
The relationship between farming intensity, farming methods and their impact on wildlife is hugely important given the projected demand for increased global food production (Tilman et al. 2001; Foley et al. 2011; Foresight 2011; Tilman et al. 2011). Additionally, to guide effective conservation management, it is crucial to know how much agri-environmental management practices benefit biodiversity and how much they ‘cost’ in terms of reduced yield. We examined the relationship between diversity of important farmland taxa and crop yield on organic and conventional farms. Of eight species groups examined, five (farmland plants, solitary bees, bumblebees, butterflies and epigeal arthropods) responded negatively to crop yield. With the exception of plants, there were generally low or no diversity gains through organic farming when compared with conventional farming at similar yields. These results indicate that an increase in biodiversity comes about largely through a considerable reduction in yield independent of the farming system. The higher biodiversity levels in organic compared with conventional farming observed in many studies (Bengtsson, Ahnstrom & Weibull 2005; Hole et al. 2005) may simply reflect the lower production levels rather than more wildlife-friendly farming methods per se. These wildlife benefits accrue in low-yielding conventional farms as much as they do in organic ones, and conversely, they disappear in the most intensive organic farms whose yields rival those of conventional practices.
The shape of the yield vs. biodiversity relationship varied between taxa. Hence, our results indicate that there is no single solution to the debate concerning sparing vs. sharing, suggesting instead that the solution may differ depending on the species group and the productivity of the agricultural landscape. Taxa that require yields to be reduced to very low levels before a biodiversity benefit is realized were typically mobile taxa, such as epigeal arthropods, and flower-visiting insects, such as solitary bees and butterflies (abundance only). These groups typically utilize a range of habitats, using crop fields to some extent as foraging habitat, but most also require undisturbed (semi-)natural habitats as nesting and hibernation sites to fulfil their life cycles. These species groups are often more abundant on organic farms due to their higher floral diversity (Holzschuh et al. 2007; Rundlof, Bengtsson & Smith 2008; Gabriel et al. 2010), but they can be even more abundant in field margins and nature reserves, such as grasslands of high nature conservation value (Ockinger & Smith 2007; Hodgson et al. 2010). For these taxa, our results suggest that effective conservation may require very extensively ‘wildlife-friendly’ shared land or specifically ‘spared’ conservation land. This may be (semi-) natural land outside agricultural production and/or uncultivated field margins, such as wildflower strips (Aviron et al. 2009), which are managed for biodiversity and also provide nesting habitat (Benton 2012).
For plants, the response curve between species density and crop yield differed between organic and conventional management. While organic and conventional fields exhibited similar plant numbers at high yields, species density increased dramatically with reductions in yield in organic fields leading to much higher densities compared with conventional fields. Therefore, organic farming at average organic yields will produce reasonable biodiversity benefits and can be a particularly beneficial wildlife-friendly method to promote plants within a production system, but if pushed towards intensive levels (i.e. the average conventional yield), it ceases to produce a benefit. All conventional fields were sprayed with herbicides (Table S1, Supporting information), and this most likely underpins the difference in plant species density from organic fields. Recently, Geiger et al. (2010) confirmed the overwhelming negative effects of pesticides on various farmland taxa. However, beside pesticide use, this pattern may be linked to other management decisions, such as the amount of nitrogen fertilization and the length of crop rotation, which determine crop yield (Table S1 and Fig. S1, Supporting information). As farmers increase inputs, they increase the density of crops and negatively affect plant diversity, specifically promoting nitrophilous and competitive weeds at the expense of other wild species (Kleijn & vanderVoort 1997). In organic fields, the average levels of inputs were much lower than their conventional counterparts (Table S1, Supporting information) and may have promoted plant species density substantially (Kleijn et al. 2009). Moreover, in the conventional fields of our study, short crop rotations were associated with greater total nitrogen fertilizer and crop management passes (especially herbicides), larger farms and a reduced number of farm products (Table S2 and Fig. S3, Supporting information). Hence, beside direct field management effects (i.e. increased inputs), a loss of spatial and temporal heterogeneity occurs at farm scales that may itself have direct or indirect impacts on farmland biodiversity (Benton, Vickery & Wilson 2003).
Species density of bumblebees declined in a concave-down shape with increasing crop yield in margins. Bumblebees may be less sensitive to agricultural intensification compared with solitary wild bees and butterflies because they respond to their surroundings at larger spatial scales due to generally larger foraging ranges (Steffan-Dewenter et al. 2002; Osborne et al. 2008). Moreover, bumblebees most likely enter cereal fields to exploit (non-crop) floral resources of weeds, which themselves displayed a concave-downward response to yield.
Hoverflies were the only species group that responded positively to crop yield. This might be related to their larval food source (Appendix S3, Supporting information). Indeed, if we subdivide hoverfly species into those with aphidophagous, phytophagous and microphagous larvae, we observe a differential response of the hoverfly community to land-use intensity: aphidophagous hoverflies, which are the majority of hoverflies in our study, were positively related to crop yield and conventional farming, while phytophagous and microphagous hoverflies were related to organic farming, where floral resources and organic matter from organic fertilizer (such as manure) are more abundant (Power & Stout 2011).
The organic/conventional yield ratio in our study was lowest in arable-dominated landscapes and highest in mixed landscapes (Appendix S2, Supporting information). de Ponti, Rijk & van Ittersum (2012) showed that organic/conventional wheat yield ratios declined as conventional yields increased, suggesting higher yield gains in conventional compared with organic fields in more productive landscapes or with higher inputs of fertilizer and pesticides. Given these results and the yield vs. biodiversity relationships observed in our study, it is likely that the greatest gains in biodiversity per unit crop yield would occur in mixed and low-productivity landscapes. This result conflicts with the existing consensus that maximal biodiversity gain will occur by promoting organic farms in homogeneous, intensive landscapes (Rundlof & Smith 2006; Holzschuh et al. 2007). However, this pervading consensus does not consider the yield differences and the associated additional area of land necessary for food production. In addition, in the UK, organic farming is more prevalent in low-productivity and mixed landscapes (Gabriel et al. 2009), which creates ‘naturally’ aggregated areas that are beneficial to biodiversity (Gabriel et al. 2010). In highly productive agricultural landscapes, our results suggest that effective conservation may require specifically ‘spared’ land, which is managed for wildlife.
Of course, land-sharing and land-sparing approaches are only the ends of a continuum. Land can be ‘spared’ at very different scales. If sparing is implemented at a coarse scale, spared land would be geographically distinct and very different in character and biodiversity from agricultural land (Phalan et al. 2011). In contrast, if sparing is implemented at fine scales, spared land could be on farms (e.g. margins and non-cropped areas) leaving aside field centres for intensive production. Such fine-scale land-sparing approaches, which are conceptually in the transition to wildlife-friendly farming, are likely to support species associated with and living on the managed farmland and may also potentially promote ecosystem services (a function that has been usually associated with wildlife-friendly farming only, see Fischer et al. 2008; Tscharntke et al. 2012). In a companion study to this one (using the same farms and nearby nature reserves), Hodgson et al. (2010) show that the optimal landscape design to manage butterflies depends on the landscape context, with organic farming being more likely to be favoured in mosaic landscapes, while a combination of conventional land with specifically targeted non-farmed conservation areas is more effective in intensive arable landscapes.
When interpreting our results, some aspects should be borne in mind. First, yield and biodiversity are affected by processes at different scales. Yield may depend more on local conditions, that is, the field management, crop variety, soil and local climate, than does biodiversity, which is also affected by larger spatial scale processes, for example, landscape structure (Schweiger et al. 2005) and longer temporal scales, for example, land-use history (Lunt & Spooner 2005). Furthermore, the environmental impacts of farming may occur at scales beyond the farm, for example, through nitrogen leaching and greenhouse gas emissions (Ewers et al. 2009), and similarly, the biodiversity benefits of organic practice may affect neighbouring farms (Gabriel et al. 2010). Additionally, our yield figures do not include the full life cycle economic and environmental costs of inputs, such as fertilizer and pesticides. Hence, we may underestimate the negative impacts of (conventional) farming on local (and indeed global) biodiversity.
Second, apart from birds, diversity was measured at the field scale, and the observed patterns in diversity may change at larger scales. For example, as organic fields have more diverse crop rotations with a smaller proportion of wheat, higher alpha diversity in fields of a different crop and higher beta diversity between fields and farms may lead to higher species numbers at coarse scales (Gabriel et al. 2006).
Third, the pairing of farms and landscapes improves statistical comparability, but it usually narrows down the selection and reduces contrasts. The most intensive conventional farms and the most productive arable landscapes are not selected because of the scarcity of organic farms in the most productive landscapes (Rundlof & Smith 2006; Gabriel et al. 2009). Hence, the conventional farms in our study may perform better in terms of biodiversity and worse in terms of yield than more typical conventional farms. However, the comparison of diversity at similar yields should not be influenced by this selection.
Finally, our study examines only a single crop type, winter cereals (predominantly winter wheat). While this is the most important arable crop in Europe, it is far from being the only one. Absolute yields and differences between organic and conventional yields vary between different crops, between crops and animal products and between different landscapes and regions (de Ponti, Rijk & van Ittersum 2012; Seufert, Ramankutty & Foley 2012). Seufert, Ramankutty & Foley (2012) report organic yields to be 34% lower when farming systems are most comparable. Our figures show more pronounced differences than this (54% lower), although we used farms that were matched for enterprize type, soil and location and thus accounted for potential biases due to the farm and landscape types that organic farms occur in other studies (Gabriel et al. 2009; Norton et al. 2009). However, farm management decisions are generally made on the basis of profitability rather than yield per se, and the higher market price of organic produce and lower input costs may compensate farmers for the lower yields, allowing enhanced biodiversity while maintaining profitability (Sutherland et al. 2012). Alternatively, organic yields might be even smaller if we accounted for non-food crops, that is, green manure crops, within the crop rotation. Therefore, it is difficult to predict how much more land is needed to produce the same amount of food with organic agriculture, but it seems clear that it requires substantially more land (Goklany 2002; Trewavas 2004). Whole-farm approaches or indeed whole economy approaches are required, where biodiversity and yields at larger spatial and temporal scales should be compared.