What, where and how do soil animals eat? And why do we need to know?
Dr. Anton M. Potapov1,2 and Dr. Olaf Schmidt3
1Postdoctoral Researcher, University of Göttingen, Germany 2Researcher, Russian Academy of Sciences, Moscow, Russia; 3Professor, University College Dublin, Ireland
Shots of soil animals feeding on different resources, from micro-to macro-scales. Microarthropods like springtails (A) and mites (B) feed mainly on fungi and leaf litter. Tardigrades (‘water bears’) don't mind having single-celled soil algae in their diet (C). Microscopic protists, extremely diverse in forms and feeding habits, also feed on each other – in this case a testate amoeba catches a ciliate (D). In comparison to these organisms, earthworms are so large that they swallow many small animals and microbes together with soil organic matter (E). Photograph credits: A, B – Andy Murray (https://www.chaosofdelight.org); C – Adam Trautwig; D – Vincent Jassey; E – Frank Ashwood (https://www.frankashwood.com). Size scale bars are approximate.
We all know the early bird catches the worm, but what does the worm catch? What animals that live in soil eat down there is hard to observe but important to know. Over the past two decades or so, ecologists interested in this underground world have done lots of research to find out what exactly different soil animals feed on, how they are connected by these feeding (trophic) interactions, and how these interactions in turn support the many functions that we expect soils to deliver. These widely scattered studies have now been compiled and summarized by an international team of experts in a comprehensive review published in the journal Biological Reviews.
Soil is an incredibly densely populated habitat, literally teeming with various organisms that interact with each other, with plants and the soil environment in many complex ways. From microscopic single-celled protists to arthropods and earthworms, the so-called soil animal ‘decomposers’ consume soil bacteria, fungi, decaying plant roots and dead organic matter (Figure 1). Diverse and dangerous predators are waiting for you in the dark soil labyrinths, whether you are of microscopic size or a predator yourself, and even ‘giants’ like earthworms can fall prey to vertebrate animals (Figure 2).
Predation in soil. It is estimated that more than a half of all vertebrate predators on land may feed on soil invertebrates. Here is an early song thrush catching an earthworm (A). A mite that feeds on an ant – this photo illustrates ‘scavenging’ a poorly explored but likely common strategy in soil invertebrates (B). A money spider that has caught a springtail represents one of the most common predator-prey interactions in soil food webs (C). Many soil-associated mites are parasitic, like this one, clinging on its bigger relative – a harvestman (D). Photograph credits: A – Vladimir Avdeev; B – Tim Jonas (https://www.jonasweb.co.uk); C – Andy Murray (https://www.chaosofdelight.org); D – Frank Ashwood (https://www.frankashwood.com). Size scale bars are approximate.
In most cases, what a particular soil animal feeds on is difficult to study by direct observation. In spite of this, soil zoologists have made great progress in this field by applying various methods and analytical techniques. For example, stable isotope ratios in the animal body can tell us if this animal feeds on plants, microbes or other animals. The analysis of ‘biomarkers’ in fat tissues can tell us if bacteria or fungi were on the menu. And molecular gut content analysis can identify down to the species level what prey a predatory animal, as small as a mite, has ingested. The published review has summarized this new knowledge for the first time for the full spectrum of animals that are associated with soils, including unicellular protists, nematodes, segmented worms, arthropods, molluscs, and vertebrates – from moles and burrowing rodents to ground-foraging birds. The literature review also revealed that our understanding of soil food webs should be revised and suggested a new generalizable classification for all groups of soil animals.
Figure 3. Feeding of soil animals supports vital soil functions. Feeding on dead organic matter (detritivory, brown) regulates decomposition, carbon sequestration and supports soil structure. Feeding on bacteria and fungi (microbivory, dark yellow) indirectly regulates nutrient cycling in soil and plant growth. Feeding on living plants and algae (herbivory, green) directly affects plant biomass and contributes to dispersal of lower plants (mosses, algae). Feeding on other animals and protists (predation, red) regulates prey populations and thus supports biodiversity and stability of soil communities. All these functions are jointly supported by a plethora of litter- and soil-dwelling organisms, from micrometres to centimetres in body size. The illustration was prepared by Svenja Meyer.
Why do we need to know? Understanding what the many animals in the soil do is fundamental to describing, modelling and managing soil biological processes and related functions (Figure 3). For instance, if we understand soil food webs better, we can predict how they will respond to changes in land use, soil management and climate. In the wider picture, this will help us maintain soil functions and health, including fertility, the physical structure of soil and the amount of carbon it locks up, or emits to the atmosphere. And not to forget, soil biodiversity itself, it deserves to be protected for its own value just like the other biodiversity above-ground that is much more familiar to most people. If there are no worms, the bird can be as early as it wants, it will go hungry.
The review article was produced as a collaboration of 26 experts from Germany, Russia, the Czech Republic, Ireland, Spain, Canada, USA, and the Netherlands.
Publication (open access):
Potapov, A. M., Beaulieu, F., Birkhofer, K., Bluhm, S.L., Bryndova, M., Degtyarev, M.I., Devetter, M., Goncharov, A.A., Gongalsky, K.B., Klarner, B., Korobushkin, D.I., Liebke, D., Maraun, M., McDonnell, R.J., Pollierer, M.M., Schaefer, I., Schrubovich, J., Semenyuk, I.I., Sendra, A., Tuma, J., Vassilieva, A., Chen, T.-W., Geisen, S., Schmidt, O., Tiunov, A.V., Scheu, S., 2022. Feeding habits and multifunctional classification of soil-associated consumers from protists to vertebrates. Biological Reviews. https://doi.org/10.1111/brv.12832