Earthworms as drivers of global food production

By Steven Fonte, Colorado State University

Earthworms serve an important role in processing and restructuring of soils and are considered as important indicators of soil health. They are also known to benefit plant growth by feeding on decaying plant material and incorporating surface residues (plant material left on the field after harvest) in the soil. In this way, earthworms help to release nutrients (mainly nitrogen) making them available for uptake by growing plants. Earthworms also form extensive networks of tunnels belowground and their castings (excrement) turn into highly stable soil aggregates, making them key soil ecosystem engineers. These engineering activities can drastically improve water infiltration, thus helping soils to capture and store rainfall for use by plants and protecting soils against erosion. Along with their effects on plant growth and soil structure, earthworms interact with other soil organisms in complex ways, and in many cases can help plants defend against pathogens. Despite the range of ways that earthworms impact soils and plant growth, the contribution of earthworms to agricultural production remains poorly understood. In our recent study, we aimed to fill this knowledge gap by quantifying the global impact earthworms have on agricultural production.  

To gauge their impact, we leveraged findings from a previous meta-analysis quantifying the effects of earthworms on plant growth across a range of experiments conducted in varying soil types, management regimes, crop types, and with a gradient of densities of earthworms. We then combined global data layers (i.e., maps) of soils, fertilizer management, and crop yields, along with a recent map of earthworm abundance and diversity to estimate the impact of earthworms on global agricultural production. We focused on common grains (corn, rice, wheat, barely) and legumes (e.g., dry beans, garbanzos), as these were the crops most commonly considered in the meta-analysis.

Our study estimated that earthworms contribute to roughly 6.5% of global grain production and 2.3% of global legume production. This is equivalent to 140 million metric tons of food each year, which puts earthworms on par with Russia or Brazil in terms of their contribution to the global grain supply! Earthworm contributions were especially notable in places that have reported large earthworm populations, such as Europe and East Asia, indicating that more worms translate into a greater impact on crop yield. Additionally, we found their contributions to be quite important in regions with more acidic soils and lower fertilizer inputs rate, such as Sub-Saharan Africa, where earthworms were estimated to contribute as much as 10% of annual grain production. And we suspect these numbers might be an underestimate since the maps of earthworm abundance for much of the Global South were based on limited data and may not accurately reflect earthworm populations in these regions. This is the first study to our knowledge that has sought to estimate the contribution of a beneficial soil organism to global agricultural production. In summary, our research highlights the need to better understand the role of soil biodiversity in agricultural systems and then apply this knowledge toward the improved management of soils for achieving multiple sustainability goals.

In highlighting the potential value of soil organisms to global agricultural production, we hope that our findings motivate farmers and policy makers to further invest in soils and gain a true appreciation for the biodiversity beneath our feet. And the next time you are working in your garden or on your farm and come across an earthworm, you might think twice about rototilling your soils, and perhaps leave some mulch or manure for them to feed on.