Common As Dirt
Our sub-terrestrial allies are nature's most efficient recyclers
by Stuart B. Hill
One of the articles in Exploring Our Interconnectedness (IC#34)
Winter 1993, Page 16
Copyright (c)1993, 1996 by Context Institute
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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.
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