October is a month of great change. While the beginning of the month is still dominated by the green of the forests, insects singing, and wildlife busily preparing for winter, by the end of the month, our North American temperate forests will have experienced their first killing frosts. In many ways, the high winds and rapidly changing weather brought on by October are much like those of March. Truly, these two equinox months bridge the gap between the annual season extremes.
As we walk through the woods, the colors of the fall foliage dominate our views. The reduced sunlight has limited the production of the photosynthetic pigment chlorophyll, enabling other yellow, orange and red pigments to dominate leaf coloration. The plant community is ending it’s growing season and sending the energy-rich sugary sap down to it’s roots for the next season’s fuel. It is a beautiful, if not ambivalent, end of the summer growing season.
Not all activity in the forest is in decline, however. Under our feet lies a world of activity known only to a few hikers. In the leaf litter lies a vast community of small invertebrate animals (arthropods, nematodes and earthworms), protozoa, fungi and bacteria. These are the decomposers, and this is a very busy time for this community, second only to spring in energy consumption.
Arthropods range in size from microscopic to several inches in length. They include insects, such as springtails, beetles, and ants; crustaceans such as sowbugs; arachnids such as spiders and mites; myriapods, such as centipedes and millipedes; and scorpions.
Nematodes are microscopic non-segmented worms. Nematodes can be divided into bacterial-feeders, fungal-feeders, predaceous and omnivorous feeders.
Protozoa are single-celled primitive animals, including such things as amoebas and disease forming giardia and malaria. Protozoa feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are several times larger than bacteria.
Bacteria, a major source of decomposition, convert energy in soil organic matter into forms useful to the rest of the organisms in the soil food web.
To give you an ideal of the subterranean populations, here is a recipe for a typical woodland soil. In each square yard of soil, add
100 - 500 earthworms
500 – 200,000 arthropods (including 500 to 5,000 springtails)
20 million nematodes (roundworms)
And, to each teaspoon, add:
1,000 – 1 million protozoa
100 million – 1 billion bacteria
60,000 yards of fungal hyphae
It is with numbers like these that we appreciate that this subterranean food web is so very important to the health of our forest ecosystems. This is not so surprising when one realizes that every living thing above the ground ultimately dies and must be consumed by the decomposers before their nutrients can be recycled for the next generation of life forms.
These soil microorganisms have evolved to take advantage of every possible niche. With
few exceptions, each of these decomposers include species that have become detrivores,
herbivores, fungivores, bacterivores, carnivores and omnivores.
The interaction of these inhabitants is a model of nature’s diverse web of life. As John Muir put it, “When we try to pick out anything by itself, we find it hitched to everything in the universe.”
Take many of the arthropods, for instance. As arthropods graze on bacteria and fungi, they stimulate the growth of mycorrhizae and other fungi, and the decomposition of organic matter. If grazer populations get too dense the opposite effect can occur – populations of bacteria and fungi will decline. Predatory arthropods are important to keep grazer populations under control and to prevent them from over-grazing microbes.
The largest number of arthropods is in natural plant communities with few earthworms (such as conifer forests). Natural communities with numerous earthworms (such as grassland soils) have the fewest arthropods. Apparently, earthworms out-compete arthropods, perhaps by excessively reworking their habitat or eating them incidentally.
The lives of earthworms and microbes are closely intertwined. Earthworms derive their nutrition from fungi, bacteria, and possibly protozoa and nematodes, and they promote the activity of these organisms by shredding and increasing the surface area of organic matter and making it more available to small organisms.
A major role of protozoa is in controlling bacteria populations. When they graze on bacteria, protozoa stimulate growth of the bacterial population (and, in turn, decomposition rates and soil aggregation.) Exactly why this happens is under some debate, but grazing can be thought of like pruning a tree – a small amount enhances growth, too much reduces growth or will modify the mix of species in the bacterial community.
Protozoa and bacterial-feeding nematodes compete for their common food resource: bacteria. Some soils have high numbers of either nematodes or protozoa, but not both.
The numbers, biomass, activity and community structure of the organisms which comprise the soil foodweb can be used as indicators of ecosystem health because these organisms perform critical processes and functions. Soil decomposers are responsible for nutrient retention in soil. If nutrients are not retained within an ecosystem, future productivity of the ecosystem will be reduced as well as cause problems for systems into which those nutrients move, especially aquatic portions of the landscape. An example of this is a comparison of two large clearcuts in the Pacific Northwest coniferous forests. Two sides of a valley showed distinctly different regrowth characteristics, with one appearing healthy, while the other supported only a paucity of weak scrub growth. On inspection, it was found that the north-facing slope of healthy growth contained a diverse community of soil microbes, while the hotter, drier, south-facing slope had lost virtually all of it’s soil life forms.
It is equally noteworthy that in healthy ecosystems, while nutrient productivity and cycling increases, nutrient loss decreases. What makes this possible is the increasing complexity of the soil foodweb. As total ecosystem productivity increases, biodiversity below ground, i.e., the structure and function of the soil foodweb, also increases.
The similarity between the communities above ground and under ground is based upon the same tenets of nature. Whether the ecosytem is terrestrial, aquatic, or subterranean, an undisturbed natural community is a wonderfully diverse and interactive web of life.