Data inizio
27 Sep 2022

Biodiversity is a term used to refer to the number of species in one region or ecosystem, and for the first time, scientists have developed a new tool that assesses the value of converting farmland to organic when factoring its impact on both crop yield and biodiversity, published online in Ecology Letters.

“This tool can help identify the best strategy of land use intensity” said Dr Yi Zou, the leader of the study, and an ecology researcher at Xi’an Jiaotong-Liverpool University.

The study brought together a diverse team of scientists across China and Europe, and their results show the possibility for high biodiversity gain at the cost of little or no, yield loss, when converting from conventional to organic farming in non-cereal crops, indicating that organic farming can be a preferred option to maximize biodiversity in these farming systems.

More simply, their takeaway is that organic farming offers roughly a 25% gain in biodiversity with a similar cost in yield decline: these values can be higher for plants and lower for birds in cereal crops.

Because lower yields are often the case for organic in comparison to conventional, this sparks a debate that more land is required to produce the same amount of food, with an increase in acres under organic production resulting in more natural habitat needing to be converted, cancelling its benefits to biodiversity.

While natural habitats are home to higher biodiversity than farmland, in general, studies show that organic farming, which prohibits synthetic agrochemical use, hosts higher diversity than conventional farming and a lower output in yield.

Until now, it was an unknown whether a reduction in yield always implies a gain in biodiversity and vice-a-versa, or how this relationship varies between certain crops and organisms. Over the long run, yield gaps can close between organic and conventional as organic systems mature.

Creating a smart decision-making tool for switching to organic

To understand this, the authors proposed evaluating the relationship between biodiversity gain and yield loss to provide a good basis for deciding whether it is worthwhile to make the switch from conventional to organic farming.

To measure these tradeoffs, scientists performed a meta-analysis; an approach in which scientific studies are compiled for their specific attributes and a statistical analysis is performed to provide some type of consensus or bigger takeaway.

To perform the meta-analysis, researchers combed databases high and low for studies that compared gains or losses of biodiversity as a measure of the number of species in an area, or yield, measured as dry weight per plant by area for cereal and non-cereal crops in organic and conventional farming over the last 30 years.

After choosing carefully for those that matched their criteria, their search turned out 75 studies to analyze, and yielded 177 biodiversity comparisons and 175 yield comparisons. Organisms were classified into five groups: birds, mammals, invertebrates, microbes, and plants.

In quantifying these tradeoffs in their meta-analysis, the study showed that when comparing biodiversity gain and yield loss in moving from conventional to organic farming, the majority, 111 out of 205 comparisons, had a positive tradeoff with a gain in biodiversity and a lower yield in organic, 47 cases were win-win, and most of the remaining cases lose-lose.

But when looking across studies at the linear relationship between biodiversity gain and yield loss for switching from conventional to organic, there was no significant relationship. This finding provides evidence that a higher biodiversity gain does not always mean a loss in yield.

The finding is not surprising, as biodiveristy and crop yields are often influenced by a suite of environmental factors, such as climate, species, and so on, plus, biodiversity can enhance yield through provision of ecosystem services.

Researchers also collected data from the studies to create a couple of indices: a “compatibility index” to compare the relative gain in biodiversity or loss for organic and conventional farming; and a “substitution index” to infer whether a transfer from conventional to organic farming would create more advantages for biodiversity conservation.

“Higher values of the substitution index mean that organic farming is likelier to maximise biodiversity despite the extra land it takes up,” added Zou.

Supporting the organic movement in providing a win-win, for most noncereal crops, the compatibility index came out positive but close to zero, indicating that biodiversity gain is almost equivalent to yield loss in organic farming, and in cases where there is no yield loss, biodiversity gain is similar for both cereal and noncereal crops.

In their substitution index, cereal crops, birds, microbes, and invertebrates showed a low scoring, indicating that it is easier for natural areas to exceed the threshold, whereas plants show a high substitution index.

A path forward

On larger scales, the substitution index can be used to assess biodiversity conservation in farmland and natural habitat, and inform land use intensification debates in supporting policy makers in determining if a less intensive system is more beneficial in terms of yield production and conservation.

This tool should help local decision makers address whether a less intensive farming system is beneficial for yield production and biodiversity conservation. Although there is some noise in the output of their indices, this also speaks to the importance of context when using tools like this in decision-making.

Source: The Organic Center