If we take the time to see the world from a mite’s point of view, we get a glimpse at how interdependent we are with so many of the Earth’s creatures. Stuart B. Hill is associate professor of entomology at McGill University and director of the Macdonald College’s Ecological Agriculture Projects in Quebec. His research focuses on soil life (he’s particularly fond of mites) and safe methods of pest control, but his interests range from science policy and ethics to human behavior and sustainable development. This article first appeared in Seasons, a publication of the Federation of Ontario Naturalists.
In most parts of the world there is more life beneath the surface of the soil than above it. In a few shovelfuls of forest soil there may be more than 1,000 different types of organisms and as many as 100 different types of mites. In a square meter there can be more than 10 million nematodes and protozoa, 1 million mites and springtails, and thousands of other invertebrates, including several hundred earthworms. Even in a teaspoon of soil there can be as many as 10 million bacteria and two kilometers of fungal filaments.
The soil is indeed a living, breathing firmament in which the primary action taking place is the breakdown of organic matter and the release of plant nutrients.
The soil is the decomposer part of the natural cycle that flows from plant and animal production, to consumption and use, to waste decomposition and recycling, and then back to plant production again. If there were no organisms in the soil we would soon be up to our ears in accumulated dead animals and other forms of organic matter.
Increasingly, however, these organisms are having a harder and harder time getting their work done. Nearly everything that humans do to the soil kills them – directly or indirectly. They are poisoned by pesticides and some fertilizers, and injured or exposed to the damaging rays of the sun or to predators by cultivation and bare-soil agricultural practices.
They’re starved of a balanced diet by our failure to return our wastes to the soil and by our tendency to grow the same crop year after year, providing the soil population with a monotonous diet. In addition, desiccation, flooding, fire, compaction from heavy machinery, and contamination with a vast range of pollutants add to the hazards of life in the soil.
Yet it is upon these very organisms and this free decomposition service that the productivity of our farms and forests, and ultimately, we ourselves rely.
IT’S A MITE’S WORLD
The key to working with (rather than against) our allies in the soil is to try to see the world from their point of view.
From the perspective of a soil organism, about half of the average soil consists of solid material, mostly mineral particles, and half consists of spaces between the particles. Half of these spaces are filled with water that occurs as a film around the particles.
This situation has resulted in the evolution of three primary strategies for living in soil. Protozoa, nematodes, and some other small organisms swim or creep around in the water film, feeding primarily on bacteria and on one another.
Mites, springtails, and other small arthropods wander around in the air spaces (up to their knees in water) browsing on fungi, nematodes, and one another.
Earthworms, some insects and other larger arthropods, slugs, and snails burrow through the soil, independent of the size of the air and water spaces, feeding primarily on the dead organic matter and on the microorganisms that colonize it.
VIBRANT SOIL HABITATS
The key soil management question is: what can we do to provide these organisms with optimal food and space conditions so they can get on with their jobs?
Clearly, the key is to permit the return of a suitable mix of uncontaminated organic wastes to the soil and to avoid stressing the system physically, chemically, and biologically.
In a stressed and starved soil, the earthworm population can fall to nearly zero, whereas in a well-managed soil, their activity may result in the production of large quantities of castes (their very fertile waste) in the soil’s upper layers. In addition, their burrows help to aerate the soil and provide channels for root growth and water drainage.
As well as these more obvious beneficial roles, most soil organisms have many more subtle positive effects. For example, the breakdown of leaves requires the action of a succession of different species of fungi. The problem is that because fungi have limited dispersal powers they soon run out of suitable substrate. They also pollute the environment around them with antibiotics, making it difficult for other fungi to grow.
The animals, especially the mites and springtails, provide part of the solution to this problem. Most of the fungal-feeding mites prefer certain species of fungi. The mites generally digest only the filaments, leaving the spores to pass through their guts undamaged and get deposited in little packages of their waste throughout the soil. Because this waste is fertile, like potting soil, these spores will germinate and form new fungal colonies in new locations.
In this way the mites help the process of organic matter decomposition, while at the same time essentially practicing a primitive form of farming. Actually, it is not so primitive because their waste is deposited in a piece of cast-off gut lining that can contain substances that prevent other fungi from growing on their package of waste. Hence, mites practice a fairly sophisticated method of "weed" control. Furthermore, the hairs on their bodies may disperse the spores of the fungal species on which they prefer to feed. It is humbling to think that this sort of farming has been going on in soil for more than 400 million years.
These kinds of relationships operate by means of very delicate balances. Thus, if the mite population is decimated by, for example, the application of a highly toxic pesticide, microbial activity may initially increase in the absence of cropping and then slow down as antibiotics accumulate.
It is this regulatory function carried out by these animals that is so important for the maintenance of soil fertility. This has not been widely recognized because the effects of these organisms are catalytic and indirect, and also because of the belief that the regular use of chemical fertilizers can keep the soil productive. In fact, as is the case with many analogous processes in the human body, the artificial provision of a naturally produced substance will inhibit its production and create dependence on repeated applications.
It is important to understand that there are many functions performed by the vast array of creatures in the soil, many more than we are fully aware of. Whenever we eliminate one of these organisms we inherit its job – a job at which it is an expert and we are, at best, novices.
The message is clear: we must support our allies in the soil and help them to support us and the other organisms with which we share this planet.