Beneath our feet lies one of the most important and at the same time least understood ecosystems on Earth. In a single handful of soil live billions of microorganisms, fungi, and invertebrate animals—so-called soil-dependent species that spend a large part of their lives in the soil or in the litter layer. They drive key processes such as decomposition, nutrient cycling, and soil formation, thereby supporting both agricultural production and climate stability.
Despite this importance, we know surprisingly little about biodiversity in soil. Although it is considered the most species-rich habitat on the planet and is estimated to harbor around 60% of all species, many of these organisms have hardly been studied or have not even been scientifically described. This knowledge gap becomes clear in the IUCN Red List: of the world’s estimated seven million terrestrial arthropods, only about 0.18% have so far been assessed for extinction risk, and for fungi the figure is only around 0.04%.
This imbalance has consequences: while clearly visible soil species—such as rabbits, moles, or prairie dogs—have been studied comparatively extensively, the condition of the actual key organisms (including fungi, bacteria, and invertebrates) in soil remains largely unknown. Important groups of soil biodiversity, such as mycorrhizal fungi, are still absent from the Red List.
Accordingly, very little is often known about the extinction risk of individual soil-dependent species. Yet without knowledge of their conservation status, it is not possible to assess which protective measures are appropriate.
Against this background, researchers from the IUCN and Conservation International have for the first time systematically recorded how many soil-dependent species are listed in the Red List. In addition, more than 500 soil species were assessed for extinction risk for the first time.
The results of the current study show not only how large the knowledge gaps are, but also that species loss in soil is already under way.
Image: Andy Murray, CC BY-SA 2.0, via Wikimedia Commons
What are soil-dependent species?
To record soil-dwelling organisms in the Red List, a clear definition first had to be developed. Although almost every species is related to soil in some way (many animals burrow, plants root in the ground), the study considered only those for which soil plays not merely a secondary role, but is a central part of their life.
Accordingly, soil-dependent organisms are those that spend a significant part of their life cycle within the soil or live predominantly at the interface between soil and litter layer. What matters, then, is not just any contact with soil, but a close ecological dependence on this habitat.
This definition includes very different groups of organisms: from larger soil inhabitants such as burrowing vertebrates to the so-called macrofauna and mesofauna—for example insects, mites, or worms—all the way to microorganisms and fungi. Microfauna and microflora are also included. What all these groups have in common is that they govern key processes in soil, such as the breakdown of organic matter, nutrient availability, or the stability of soil structures.
By contrast, organisms for which soil does not play a central role are not included. Plants, too, despite their importance for soil formation, are not counted as soil-dependent species within the meaning of the Red List.
Moles spend most of their lives underground and therefore count among soil-dependent vertebrates. However, they are only one visible part of a far larger, mostly invisible biodiversity in soil.
Image: Christoph Moning, CC BY 4.0, via Wikimedia Commons
Result: More than 8,600 soil-dependent species assessed
The analysis of the IUCN Red List shows for the first time how many soil-dependent species have been recorded at all so far. In total, the researchers identify 8,653 species, including 503 invertebrates and fungi that were assessed for the first time in the study (a further 38 species were updated).
The composition of this group is striking:
– 5,010 terrestrial vertebrates,
– 3,133 invertebrates (predominantly arthropods and molluscs)
– and 510 fungal species.
This means that the largest share of recorded species consists of vertebrates—in other words, those groups that are comparatively well studied. The actual key organisms of soil, especially invertebrates and fungi, are clearly underrepresented.
The newly assessed taxa include, among others, numerous dung beetle species from southern Africa, selected North American fungal species, and more than 100 earthworm species from Brazil. One of the species recorded for the first time is the giant Brazilian earthworm (Rhinodrilus fafner), which is classified as Data Deficient (DD). So far, the species is known only from the more than two-meter-long holotype discovered in 1912.
The analysis shows that a substantial share of these species is already threatened: 1,758 species (20.3%) are assigned to the categories Vulnerable (VU), Endangered (EN), or Critically Endangered (CR). Another 35 species are already considered Extinct in the Wild (EW) or fully Extinct (EX), including the Lake Pedder earthworm († after 1972).
At the same time, data are insufficient for many species. 1,722 species (19.9%) are listed as Data Deficient (DD)—for them, basic information is lacking to assess extinction risk at all. This high share illustrates how large the knowledge gaps are for soil-dependent species. It is therefore likely that the actual share of threatened species is significantly higher.
The majority of recorded species are currently considered not threatened: 4,530 species (52.4%) are classified as globally Least Concern (LC), and a further 608 species (7%) as Near Threatened (NT). This classification, too, should be interpreted with caution, as it depends strongly on the data currently available.
What threatens soil-dependent organisms
The current analysis makes clear that the threat to soil-dependent species follows familiar patterns. Classification is based on the criteria of the IUCN Red List, which capture different risk factors:
Image: Thorectes hispanus (Reitter, 1892) Observed in Spain by Juan José Areso, CC BY-NC 4.0, via GBIF)
– 1,136 species: small and still shrinking range
– 335 species: extremely small or severely restricted populations
– 245 species: strong and rapid population decline
– 130 species: very low number of mature individuals
– 2 species: assessment based on quantitative analyses
In addition, 109 species meet several of these criteria simultaneously, indicating complex threat situations.
The researchers identify the main threat factors for soil-dependent organisms:
- intensive agriculture and land-use change, for example through soil compaction, drainage, and the use of fertilizers and pesticides
- settlement and infrastructure development, which destroys or fragments habitats
- logging and forestry, which alter soil structure and microhabitats
- invasive species, which displace or alter existing soil ecosystems
- climate change and extreme weather events, such as droughts or heavy rainfall
- fire and altered fire management
A central driver of this development is intensive agriculture. A current study (2026) shows that pesticide residues can be detected in around 70% of the soils examined in Europe, and not only on arable land, but also in forests and meadows. These substances demonstrably influence the composition of soil communities and harm key organisms such as fungi, bacteria, and invertebrates.
In practice, these factors rarely act in isolation. They often overlap and reinforce their effects on one another. Soil organisms are especially sensitive to changes in the chemical and physical properties of their habitat. Even comparatively minor interventions can durably alter nutrient cycles, soil structure, and microbial communities.
The threats are therefore not new, but their effects on soil biodiversity are often underestimated. The study shows that the same drivers that also put aboveground ecosystems under pressure play a central role in soil—usually largely unnoticed.
The consequences of losing soil biodiversity
The study authors make clear that the importance of soil-dependent organisms for the functioning of ecosystems can hardly be overstated. They govern a large share of key processes in soil.
Image: GLJIVARSKO DRUSTVO NIS from Serbia, CC BY 2.0, via Wikimedia Commons
Accordingly, the consequences of their loss would be severe. A decline in soil biodiversity affects not only individual species, but endangers entire ecosystems. Nutrient cycles fall out of balance, soils lose fertility, and their ability to store water and bind carbon declines. This has direct effects on agricultural productivity, food security, and the global climate system.
What is particularly critical is that many of these changes are gradual and remain unnoticed for a long time. While aboveground species losses are often visible, changes in soil usually become apparent only once key functions are already disturbed.
To counter these developments, land use plays a key role. Agriculture in particular offers major potential to stabilize or restore soil ecosystems. Organizations such as Conservation International therefore promote cultivation methods that work more closely with natural processes.
One example is agroforestry systems, in which crops are combined with trees and nitrogen-fixing plants. Such approaches improve soil structure, promote biodiversity, reduce erosion, and increase resilience to climatic changes. At the same time, they can contribute to the regeneration of degraded land and secure stable yields in the long term.
Improve the protection of soil-dwelling species
The study shows that soil-dependent organisms are still clearly underrepresented in conservation. A central point of leverage is the IUCN Red List, which serves as a global benchmark for the extinction risk of species. To obtain a more realistic picture, invertebrates and fungi in particular must be included much more strongly in assessments.
Progress is slowed, however, by limited financial resources and a lack of taxonomic expertise. At the same time, the existing assessment system reaches its limits: a large part of soil biodiversity consists of microorganisms that cannot be captured using the current criteria. Unclear species definitions, hard-to-detect distributions, and missing data make their assessment even more difficult. New methods such as genetic analyses offer better possibilities here, but are still only at the beginning of broad application in conservation.
To close these gaps, the authors propose concrete measures: building more closely networked expert panels, closer cooperation with existing research and conservation initiatives, and better funding for corresponding programs. At the same time, a stronger communication of the importance of soil biodiversity is needed—both in politics and among land users and in the public sphere.
Approaches such as sustainable agriculture or agroforestry systems show that use and protection of soils can be combined. They can help stabilize soil ecosystems and preserve their functions in the long term.
Sources
- Cox, N. A., Westrip, J. R. S., Bowles, P., Hilton-Taylor, C., et al. (2026). Global extinction risk assessment of soil-dependent species: Recent progress and recommendations. Oryx, 1–8. https://doi.org/10.1017/S003060532510272X
- McCoy, M. K. (2026, 16 April). Study: One in five soil species face extinction. Conservation International. https://www.conservation.org/news/study-one-in-five-soil-species-face-extinction
About the author: Doreen Fräßdorf
Doreen Fräßdorf is the author and publisher of artensterben.de. She researches and writes about extinct and endangered species in the modern era, with a focus on red lists, scientific studies, historical sources, and current conservation efforts. The goal is a clear, evidence-based overview of biodiversity loss and species protection.
She is also the author of a non-fiction book about extinct modern-era mammals.
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