Soil Invertebrates and Abiotic Factors
Introduction
Over the next two weeks, we will be sampling soil organisms in different
microclimates on campus. In addition to enumerating the number and types of invertebrates
in these areas, we will also be obtaining measures of physical factors that might influence
the distribution and abundance of these organisms.
Invertebrates living in the soil and the organic litter above it are difficult to
sample. Most are small and many live in the interstices of soils, decomposing wood, and
leaves. Because of this, hand sorting of soil and litter samples under a dissecting microscope
is a very tedious and inefficient method of obtaining counts of soil and invertebrates. An
alternative would be to use a technique that forces the invertebrates to leave the litter-soil
matrix on their own. We will be using just such a technique (known as the Tullgren funnel
method) to obtain counts of soil invertebrates in lab over the next two weeks. The Tullgren
funnel method employs heat and desiccation to drive organisms downward from the sample
through a funnel apparatus and into a vial of preservative.
Sample Collection and Processing
The soil arthropod sampling procedure outlined in this exercise involves three main
steps: (1) collection of samples, (2) Tullgren funnel extraction, and (3) identification and
counting of the extracted organisms.
This will be a ‘descriptive’ type study. We will focus on distribution of various types
of soil invertebrates across two microhabitats. Before going out to sample, the class will be
given a opportunity to provide input in the design of the study (i.e. choose the two areas to
be compared).
Collection of Samples
The procedure for collecting samples should be designed to allow standardization of
the surface area and depth of the soil or leaf litter samples collected. For general
ecological work, this requirement can usually be satisfied through the use of some type of
cylindrical metal coring device. For sampling in porous soils or sands, a tin can with the top
and bottom removed may be satisfactory. We will be using a tin can with the top and
bottom removed. To obtain the sample, force the edges of the can into the soil or leaf litter
to a depth of 2 centimeters. The sample should be removed from the corer with as little
disturbance as possible and placed in the containers provided.
Extraction Procedure
The Tullgren funnel employs electric lights to heat and dry the
samples. The soil or leaf litter material to be extracted should be
wrapped in one to two layers of cheesecloth and placed on the hardwarecloth screen above the funnel. Extraction is usually most efficient when
the sample is kept intact and placed upside down in the sample chamber.
The cheesecloth functions to reduce the amount of soil sifting down into
the specimen vial as the sample dries. For most purposes, 80% ethanol is a satisfactory
preservative. Ethanol acts as a repellent for some organisms, however, so for careful
studies of some groups other preservatives may be desirable The time required for
extraction varies with the size of the sample. For samples of normal size, extraction should
be nearly complete after 36-48 hours.
Identification and Counting
Animals may be identified and counted by placing the sample in a flat dish and
examining it with a dissecting microscope. Although most animals can be identified to order
or family with the aid of standard reference books, we will be using a "natural taxonomy"
approach. For most soil arthropod groups, however, identification beyond this level is very
difficult. The large number of species and small body size of many groups mean that
careful anatomical study is required for complete identification. Furthermore, many of the
individuals encountered in samples are immature forms for which descriptions are generally
unavailable. We will identify and count the invertebrates during the second week of this
lab.
Abiotic Factors
We will determine soil temperatures using soil thermometers. Take at least two
measurements per site. We will also determine soil moisture using a drying methodology.
The Ecology of Soil Invertebrates
The soil is a radically different environment for life than the ones on and above the
ground; yet the essential requirements do not differ. Like organisms that live outside the
soil, life in the soil requires living space, oxygen, food, and water. Without the presence and
intense activity of living organisms, soil development could not proceed. Soil inhabitants
from bacteria and fungi to earthworms convert inert mineral matter into a living system.
Soil possesses several outstanding characteristics as a medium for life. It is stable
structurally and chemically. The atmosphere remains saturated or nearly so, until soil
moisture drops below a critical point. Soil affords a refuge from extremes in temperature,
wind, light, and dryness. These conditions allow soil fauna to make easy adjustments to
unfavorable conditions. On the other hand, soil hampers the movement of animals. Except
to such channeling species as earth worms, soil pore space is important. It determines the
living space, humidity, and gaseous conditions of the soil environment.
Only a part of the upper soil layer is available to most soil animals as living space.
Spaces within the surface litter, cavities walled off by soil aggregates, pore spaces between
individual soil particles, root channels, and fissures all are potential habitats. Most soil
animals are limited to pore spaces and cavities larger than themselves.
Water in the pore spaces is essential. The majority of soil organisms are active only
in water. Soil water is usually present as a thin film coating the surfaces of soil particles.
This film contains, among other things, bacteria, unicellular algae, protozoa, rotifers, and
nematodes. The thickness and shape of the water film restrict the movement of most of
these soil organisms. Many small species and immature stages of larger centipedes and
millipedes are immobilized by a film of water and are unable to overcome the surface
tension imprisoning them. Some soil animals, such as millipedes and centipedes, are highly
susceptible to desiccation and avoid it by burrowing deeper.
When water fills pore spaces after heavy rains, conditions are disastrous for some
soil inhabitants. If earthworms cannot evade flooding by digging deeper, they come to the
surface, where they often die from ultraviolet radiation and desiccation or are eaten.
A diversity of life occupies these habitats. The number of species of bacteria,
fungi, protists, and representatives of nearly every invertebrate phylum found in the soil is
enormous. A soil zoologist found 110 species of beetles, 229 species of mites, and 46
species of snails and slugs in the soil of an Austrian deciduous beech forest.
Dominant among the soil organisms are bacteria, fungi, protozoans, and nematodes.
Flagellated protozoans range from 100,000 to 1,000,000, amoebas from 50,000 to 500,000,
and ciliates up to 1000 per gram of soil. Nematodes, an unsegmented worm, occur in the
millions per square meter of soil. These organisms obtain their nourishment from the roots
of living plants and from dead organic matter. Some protozoans and free-living nematodes
feed selectively on bacteria and fungi.
Living within the pore spaces of the soil are the most abundant and widely
distributed of all forest soil animals, the mites (Acarina) and springtails (Collembola).
Together they make up over 80 percent of the animals in the soil. Flattened, they wiggle,
squeeze, and digest their way through tiny caverns in the soil. They feed on fungi or search
for prey in the dark interstices and pores of the organic mass.
The more numerous of the two, both in species and numbers, are the mites, tiny
eight-legged arthropods from 0.1 to 2.0 mm in size. The most common mites in the soil and
litter are the Orbatei. They live mostly on fungal hyphae that attack dead vegetation as
well as on the sugars produced by the digestion of cellulose found in conifer needles.
Collembolae are the most widely distributed of all insects. Their common name,
springtail, describes the remarkable springing organ at the posterior end, which enables
them to leap great distances for their size. The springtails are small, from 0.3 to 1.0 mm in
size. They consume decomposing plant material, largely for the fungal hyphae they contain.
Prominent among the larger soil fauna are the earthworms (Lumbricidae). Burrowing
through the soil, they ingest soil and fresh litter and egest both mixed with intestinal
secretions. Earthworms defecate aggregated castings on or near the surface of the soil or
as a semiliquid in intersoil spaces along the burrow. These aggregates produce a more open
structure in heavy soil and bind light soil together. In this manner earthworms improve the
soil environment for other organisms.
Feeding on the surface litter are Myriapod millipedes. They eat leaves, particularly
those somewhat decomposed by fungi. Lacking the enzymes necessary for the breakdown of
cellulose, millipedes live on the fungi contained within the litter. The millipedes' chief
contribution is the mechanical breakdown of litter, making it more vulnerable to microbial
attack, especially by saprophytic fungi. Accompanying the millipedes are snails and slugs.
Among the soil invertebrates they possess the widest range of enzymes to hydrolyze
cellulose and other plant polysaccharides, possibly even the highly indigestible lignins.
Not to be ignored are termites (Isoptera), white wingless, social insects. Except
for some dipteran and beetle larvae, termites are the only larger soil inhabitants that can
break down the cellulose of wood. They do so with the aid of symbiotic protozoans living in
their gut. Termites dominate the tropical soil fauna. In the tropics termites are
responsible for the rapid removal of wood, dry grass, and other materials from the soil
surface. In constructing their huge and complex mounds, termites move considerable
amounts of soil. Detrital-feeding organisms support predators. Small arthropods are the
principal prey of spiders, beetles, pseudoscorpions, predaceous mites, and centipedes.
Protozoans, rotifers, myxobacteria, and nematodes feed on bacteria and algae. Various
predaceous fungi live on bacteria-feeders and algae-feeders.
Cox, G. W. 1996. Laboratory Manual of General Ecology, 7 th edition. McGraw-Hill Higher
Education, Boston, MA
Smith, Robert Leo and Thomas M. Smith. 1998. Elements of Ecology. Addison Wesley
Longman, Inc. Menlo Park, CA