There are 57 billion nematodes per human. Boreal forests pack 7× more per square meter than cropland. The numbers, the mechanisms, and why pesticides might make things worse.

This is part 2 of a five-part sequence on welfare ecology. Part 1 introduces the ethical premises. Part 3 covers interventions. Part 4 explores a model of invertebrate suffering. Part 5 covers AI.

If you decided in part 1 that you care – even a little – about the welfare of invertebrates, the next question is: Where are they, how many are there, and what drives their population sizes?

The Numbers

Terrestrial Soil Fauna

The foundational data comes from two landmark studies:

Van den Hoogen et al. (2019) estimated 4.4 × 10²⁰ nematodes in the top 15 cm of soil globally. Stefan Geisen, the second author, clarified that this accounts for roughly 90% of all soil nematodes, putting the total at about 4.89 × 10²⁰ (Grilo 2025a).
Rosenberg et al. (2023) estimated 10¹⁹ soil arthropods, of which roughly 95% are mites and springtails, with the remainder being ants, termites, and other groups.

For context: There are 8 billion humans and roughly 80 billion farmed land animals at any given time. Soil nematodes alone outnumber all farmed animals by a factor of 6 billion.

Marine Fauna

From Bar-On, Phillips, and Milo (2018):

Approximately 10²¹ marine nematodes – about 2× the number of soil nematodes.
Approximately 10²⁰ marine arthropods (copepods, krill, amphipods, etc.) – 10× the number of soil arthropods.

Marine nematodes are overwhelmingly benthic (living in seafloor sediment). Marine arthropods are split between benthic and pelagic (water column) habitats.

Density by Biome: The Terrestrial Landscape

Not all land is created equal. Rosenberg et al. (2023) provided mean densities of soil arthropods by biome. Grilo (2025a) combined these with van den Hoogen et al.’s nematode data to estimate total soil animal density. The results:

Several things jump out:

Forests are packed. Boreal and temperate forests have 7–8× the soil animal density of cropland. The mechanism is well understood: forests produce continuous, diverse leaf litter that feeds a complex soil food web. They have deep root networks supporting fungal communities that mites and springtails feed on. The soil is structured with pores and channels that provide habitat at every scale.

Cropland is relatively sparse. Tillage destroys soil structure. Monocultures reduce ...

Welfare Biology and AI