Small farms are key to bio d – literature consensus

Farm size has become a key variable of interest in discussions surrounding food security, development, and the environment1. Most of the world’s farms are small—of the 570 million farms in the world, 84% are <2 ha in size2. Smallholders are facing growing pressure on their livelihoods from low prices in global markets and climate change-induced production losses3. Accordingly, smallholders have been the target of global development policies such as the Sustainable Development Goal (SDG) target 2.3, which seeks to support smallholders by increasing their productivity, incomes, and access to land. Many countries’ Intended Nationally Determined Contributions (INDCs) of the UN Conference of Parties on Climate Change (COP21) also aim to bolster smallholders’ adaptive capacity. Numerous scholars argue that smaller farms perform better than larger farms in terms of production, environmental, and socioeconomic outcomes4. On the basis of these arguments, scholars, policy makers, and social movements argue in favour of land reforms to redistribute farmland5,6. Although 84% of the world’s farms are <2 ha in size, they only constitute 12% of farmland—increasing the proportion of farmland in smaller farms will arguably increase its benefits. At the same time, consumers have increased their willingness to pay for products with labels associated with smaller farms7,8. Thus, there is a growing call for support for small farms. While this support is important, the performance of small farms in terms of productivity, resource efficiency, biodiversity, and greenhouse gas (GHG) emissions has itself remained highly contested9–14. Here, we synthesize the relationship between farm size and six socioeconomic and environmental outcomes, leveraging the past 50 yr of empirical evidence that directly assessed crop production, environmental performance, and economic outcomes as they relate to farm size. Our systematic assessment of the multidimensional outcomes related to farm size builds on past reviews that focused on single outcomes (for example, yield, economic performance, or biodiversity metrics for specific species)15–18, non-systematic reviews15,16, studies based on indirect measurements of farm size and the outcome variables of interest19–21, and studies with specific regional foci15,17. We present evidence from 118 studies (318 observations) from 51 countries on the relationship between relative farm size (along a continuum) and: (1) yields as value of crop output per area (value ha–1) or total crop production per area (kg ha–1), (2) crop diversity at species and varietal levels, (3) non-crop biodiversity at field and landscape levels, (4) resource-use efficiency as measured in terms of technical efficiency22, (5) GHG emissions per unit output, and (6) profit per unit area. Results Our analysis finds that smaller farms have higher yields and harbour greater crop diversity and higher levels of non-crop biodiversity at the field and landscape scales than larger farms (Table 1). We find no conclusive evidence for a relationship between farm size and resource-use efficiency, GHG emissions, or profit. In the remainder of this article, we will address each of these key findings in turn and discuss their implications for policy initiatives and consumer sup-port for small farms globally. Smaller farms have higher yields. Our synthesis shows that, in the literature, when primary studies assess yield across farm sizes, 79% (95% confidence interval, CI = 58–100%) of them report that smaller farms have higher yields (in either weight ha–1 and value ha–1terms) (Fig. 1). We also find that yields typically decrease by 5% for each hectare increase in farm size (−5% mean effect; 95% CI =−9 to −1%; Fig. 2a and Fig. 3), within the range of studied observations (mean = 7.5 ha; s.d. =22.7 ha). While the distribution of effects includes deviant cases (Fig. 3 and Supplementary Fig. 1), these new findings show that, on average, the available evidence supports the idea—which originated in the 1920s and has been studied extensively since the 1960s—that smaller farms are higher yielding than larger farms14,23,24. Moreover, we find that controlling for labour removed the effect—our model not controlling for labour has stronger effect size (although not statistically different from zero at the 95% confidence level) than the one that does. This result is in line with Sen’s 1964 prediction24 and subsequent literature25, that suggests that labour markets are an important reason for the inverse farm size–productivity relationship (Fig. 2b). Smaller farms have greater crop diversity. While many field studies have explored in situ crop diversity on small farms26–28, few directly measured the relationship between farm size and crop diversity. In our review, four studies show higher crop diversity on smaller farms, while three found the opposite; much too small a sample for statistical inference. But we previously conducted an in-depth quantitative analysis on the relationship between crop diversity and farm size across 55 countries and 154 crops using a newly harmonized dataset of nationality representative farmer surveys and agricultural censuses29. We found that, except for an unexplained dip in the 2–5 ha size range, there is a strong inverse relationship globally between farm size and the number of crop species found across the landscape—with higher species diversity within smaller farms than larger farms when controlling for area (Supplementary Fig. 2). Crop diversity on small farms is selected by farmers for a range of reasons such as improved nutrition30, market diversification31 and mitigation of drought risk32. Smaller farms harbour greater non-crop biodiversity. There are three key pathways by which smaller farms could be beneficial for non-crop biodiversity covered in the literature. The first is through ecological management practices, such as limited insecticide use and use of organic management practices. The second is through increased field edges (increased margin-to-field area ratio); increased field edges can lead to larger available breeding habitats for arthropods33,34, provide refuge for arthropods and smaller species to colonize after escaping recently disturbed fields35,36, increase the number of pollinators and beneficial predators within fields4,34 and act as conservation corridors for arthropods and small mammals37,38. The third is through landscape composition, with small-farm-dominated landscapes harbouring diverse land cover types such as forests and wetlands, fields of different crops or fields in different phenological stages of production39,40. In the studies we reviewed, there is evidence for all of these effects. When combined, 77% of studies (95% CI = 61–99%) reported that smaller farms and fields have greater biodiversity at the farm and landscape levels compared to larger farms and fields (Fig. 1).The three remaining variables we tested—GHG emissions, resource-use efficiency, and profits—did not show conclusive relationships for the effect size magnitude and sign (Figs. 2, 4, and 5), even though the majority of studies concluded that larger farms had greater resource efficiency than smaller farms (Fig. 1). For example, while the evidence we reviewed shows that GHG emissions per unit output tend to be higher on smaller farms, suggesting that smaller farms might be less efficient per unit output, the confidence intervals around this effect crossed zero (−4% mean effect; 95% CI =−10–2%). We found no clear difference between small or large farms in technical efficiency (our proxy for resource efficiency per unit output with 0% mean effect; 95% CI =−1–2%), even after controlling for a variety of moderating factors, such as access to credit, extension services or cooperative membership (Fig. 2c). See the Supplementary Information for further discussion of these results. Similarly, while profitability per unit area also declined with increasing farm size, statistical confidence in the effect was also low (−2% mean effect; 95% CI =−10–5%) (Fig. 2a). Discussion Our evidence review and meta-analysis of the current evidence base for these six outcomes associated with farm size finds strong support for the inverse farm size–productivity and farm size–diversity relationship (empirical findings that both have strong theoretical support24,41–43). While a couple of emerging studies44,45, with limited coverage, suggest that the inverse size–productivity relationship might simply be a result of measurement errors and while we were unable to rule out this possibility, our synthesis did find the inverse farm size–yield effect was removed when controlling for labour, suggesting that smaller farms may be more productive due to the availability of family labour24. Similarly, we recognize that socio-political context is important and small farms may not always be more biodiverse46 but most of the evidence base across a broad geo-graphic range of agricultural systems is in support of this positive relationship. An important caveat to interpreting the past literature for each of these outcomes are regional biases and variation. Regional biases are most evident in the biodiversity literature that has predominantly focused on higher-income countries, mainly in North America and Europe (Supplementary Fig. 4). Different regions may contain species that prefer environments that larger farms foster, either through their larger fields or different farm management techniques (Table 1). While we could not test for these factors explicitly due to our sample size, a dominant theme in the literature for other indica-tors (yield, profitability, and resource efficiency) was that a farm’s political, socioeconomic, and geographic context (a farmer’s access to training, credit, machinery, insurance, inputs, markets, and/or subsidies) may explain the farm size to outcome relationships. For instance, the relationships between farm size, resource efficiency, and profit were the most spatially heterogeneous across all variables examined. For certain smallholder-dominant countries (for example, India and Ethiopia) we found that smaller farms were more profitable, whereas larger farms were more profitable in countries dominated by large farms, higher incomes, and better rural infra-structure (for example, the United States). This may suggest that smallholders have better access to markets, inputs, and technologies in a smallholder-dominant system that may affect their profitability and resource efficiency (see Supplementary Information for expanded discussion). Systematic evidence syntheses, such as meta-analyses, are an iterative process47. For some of the outcomes (such as GHG emissions, profits, and resource-use efficiency) we were unable to identify consistent or confident outcomes from the existing literature. This may be due to the small number of included studies, limited cross-country analyses, or that these relationships are too context-specific to be generalized. Future work should continue to assess these outcomes and build on this study to include other important outcomes, such as mental and physical health of workers and farmers, employment opportunities, pesticide or fertilizer use efficiency, and other key ecosystem services, such as pollination, in addition to examining literature in languages other than English. New primary work is also needed to further explore the functional form of the effects we present here, and to explore how different socioecological and political conditions and measurement methods may mediate positive or negative outcomes across farm size classes. This could further inform policies on land reform (such as redistribution or consolidation) that address market failures so that such policies can maximize the multidimensional benefits of farming systems to society48.To support sustainable transitions in farming practices across a range of farm sizes, more evidence-based synthesis is needed at broad regional scales. Until recently, the role of farm size in the global food system has been largely assessed by independent case studies. As international commitments (for example, SDGs and COP21 INDCs) begin to evolve into actionable funding plans and as countries continue to decide on land use policies that directly affect the size of farms, it is critical to identify how farm size affects different social, economic and environmental outcomes. Our study lends evidence to boost support for policies targeting smallholders. Most of the world’s farms (84%) are operated by smallholders2 and smallholders in lower-income countries are also among the poorest people on the planet49. Our study shows that smallholders are both productive and stewards of biodiversity. Rewarding smaller farms for their conservation benefits may be one policy pathway towards supporting smallholders. Biodiversity could be promoted on larger farms by promoting more ecologically friendly management practices and increasing biodiversity refuges such as buffer strips and increased natural perimeters. These findings come at a time where donor countries need to invest an estimated US$14 billion annually to achieve the goal of SDG 2.3 to double the incomes and productivity of smallholders49. Our review adds to the motivation for these investments. We found that, despite smallholders’ increased yields and role in provision of ecosystem services, there is not enough evidence for equivalent gains in smallholders’ profits. Thus, development support for smallholders is imperative from multiple viewpoints: the data not only show that investing in smallholders could lead to humanitarian benefits but also to increases in food production and benefits to biodiversity.