THE TROUBLE WITH WORMS
By Dennis Burton, Director of Land Restoration, at The Schuylkill Center for Environmental Education
"I doubt that there are many other
animals which have played so important
a part in the history of the world."
--Charles Darwin on earthworms
Until the 1980's, and the observations of some forest ecologists, earthworms rightly had the reputation of being the best thing for soil since the plow, especially for farmers and gardeners. Worms condition the soil for various crops, rapidly decompose organic matter and quickly return nutrients to the soil. In addition, their physical presence aerates the soil, allowing oxygen and water to filter into the plant roots, and create channels for roots to grow. One type of worm (endogeic) burrows deep into the soil, pulling organic matter as well as beneficial fungi and bacteria down with it. Another type (epigeic) lives close to the surface, churning the soil and organic matter into a nutrient rich frass of casts (worm droppings) that naturally fertilize crops. Without these worm related activities, farmers and gardeners would have to expend more physical, chemical and hydrologic energy to produce a high yield.
So what's the trouble with worms?
Enter the forest ecologists.
In the 1980's ecologists at the Institute for Ecosystem Studies (IES) in Millbrook, NY, created a 100-mile urban-to-rural gradient to study and measure the ecological health of forests from New York City to rural Connecticut. Along the gradient they discovered some obvious differences in the condition of the forests, such as more pollutants and heavy metals in the urban soils, and healthier, more diverse ecosystems in the rural soils. But they also made one unexpected discovery that applies as much to New York as to Pennís Woods and all eastern deciduous forests.
One IES researcher, Dr. Richard Pouyat, USDA Forest Service, now with the Baltimore Long Term Ecological Project, noticed that the forest floor of the urban and some suburban ecosystems along the New York urban-to-rural gradient had very thin, and in some areas, virtually no leaf litter on the ground. This phenomenon puzzled Dr. Pouyat until he finally dug a little deeper into the matter. What he found, unexpectedly, was that the soil in these areas literally writhed with earthworms. Though that may be cornucopia to farmers and gardeners, it turns out to be anathema for urban/suburban forests. And not just New York forests. These worms also writhe in many urban/suburban green spaces in Pennsylvania. The problem is that if these earthworms move into Pennsylvania's rural forests, the soils could be severely affected, as is the case with some wooded areas in rural Minnesota where earthworms are threatening the stability of that state's maple/beech ecosystem.
Healthy, undisturbed forests are typically stable ecosystems anchored in a very complex soil structure that teems with macro and microscopic life. Over the last fifteen thousand years the forests north of the glacial moraine evolved without earthworms. The glacier scoured the land down to bedrock, forcing all life forms to move south. The only native earthworms surviving the glacier in Pennsylvania are south of the glacial moraine and permafrost boundary near West Virginia and Maryland. The earthworms north of the glacial moraine were introduced by explorers and settlers after 1492. These European and Asian worms arrived in agricultural and horticultural material. The Japanese pachysandra and the European beech in your front yard probably came with these alien worms. Similarly, the fishing industry has fostered the spread of alien earthworms. The night crawlers, sold by bait shops and often dumped at the edge of wooded lakes and streams, originated in Europe.
Because Pennsylvania's forests evolved without earthworms these ecosystems depend upon fungi and invertebrate detritivores to slowly break down organic matter and gradually release nutrients back to the plants. The key to health in the Keystone state's forests resides in a fungal-based soil that slowly decomposes its organic matter. A healthy layer of leaf litter in a northern forest is woven together with filaments of fungi that also binds the litter to the soil. If you pick up a handful of leaves in an undisturbed forest you will notice lacy white threads weaving through it. That is the fungal hyphae. The slow decomposition by the fungi and other native detritivores keeps nutrients locked up in the leaf litter and only slowly releases it back to the plants: An oak leaf in this system typically takes three years to decompose.
The leaf litter in a forest is comparable to the skin on an animal. It retains moisture, protects the organs (roots), breathes, prevents erosion, deters pathogens (non-native plants), and promotes seed germination. A nutrient balance has evolved in this stable system between the vegetation above ground and the enormous biosphere in and below the leaf litter. When that system looses its leaf litter it is like puncturing your skin. Erosion follows and nutrients bleed quickly from the soil. Such disturbance soon exposes the soil, making it available to non-native, invasive plant species, compaction, and run-off of rainwater, which would normally keep the soil moist. The disturbed soil and invasive plants soon alter the stable system, causing a decrease in the diversity of plant/animal relationships, which in turn begins a cycle of non-native invasions that tend toward monocultures.
Earthworms are notorious disturbers of soil. Their disturbance creates healthy conditions for farms and gardens, but unhealthy conditions for forests. Earthworms have voracious appetites. In one acre of land, earthworms can turn over five tons of organic matter a year. When a forest system becomes heavily infested with earthworms they consume most of the leaf litter in one season. By summerís end the "skin" of the forest is depleted and the vulnerable soil becomes exposed to weathering and compaction. This rapid consumption and digestion of leaf litter also releases the stable nutrients, which in effect is like fertilizing the forest. Again, fertilizing farm fields and gardens, which are essentially monocultures, benefits those systems. On the other hand, fertilizing a forest would be like planting weeds in your garden.
Healthy forests also depend on a balance of soil structure and chemistry. For instance, most plants are very sensitive to the pH of soil. The pH scale, from 0 to14, measures the acidity or alkalinity of a substance. Calcium has a pH around 10, and vinegar has a pH of 3. Agricultural fields and most gardens do best with a neutral pH of 7. Most forests, on the other hand, require lower pHs between 4 and 6. An oak/hickory forest, and its abundance of ericaceous shrubs, in the Ridge and Valley section of Pennsylvania, will have a pH closer to 4, because oaks and ericaceous plants such as blueberries, Rhododendrons, and mountain laurel, the state flower, thrive in acidic soils.
Earthworms can't tolerate acidic soil, which is another indication that our forests evolved without them. However, many worms have calciferous glands near their gizzards. These glands neutralize acidic organic matter by producing calcium carbonate, a substance used to manufacture lime, which is used to neutralize (sweeten) acidic soil. Thus, these earthworms have their own strategy for overcoming nature's defense against them.
And as if that wasn't enough, earthworms affect the important nitrogen cycle in the forest soil. As with the other nutrients, forests contain a stable form of nitrogen known as ammonium nitrogen. This nitrogen remains locked up in the vegetation and is released through the gradual decomposition of organic matter. Because only a small amount of nitrogen is available, weedy vegetation, which generally requires large amounts of nitrogen, is deterred.
However, earthworms quickly release nitrogen into the soil by rapidly consuming the organic materials containing it. The stable ammonium nitrogen changes into unstable forms such as nitrates. Nitrate nitrogen then becomes available to any and all plants, including the alien invasives that might be languishing at the edge of the system. The excess nitrates then leach into the ground and eventually wash out into nearby streams and lakes causing even more problems.
Perhaps the most disturbing aspect of earthworms in the forest is the disturbance itself. Acting as mini-rototillers, the worms churn the soil into nutrient rich casts, often several inches deep. This action affects the root systems of many native perennial woodland wildflowers. Some studies have shown a net loss of spring ephemerals and other herbaceous species in the forest as the worms advance. The overall affect of this, again, is loss of diversity in the food chain: Along with a reduction of the herbaceous layer there is a reduction of insect pollinators and herbivorous insects. With fewer insects there are fewer insectivores, most notably forest bird species. Also, the niche opened by the loss of herbaceous plants soon becomes filled by non-native herbaceous species. Studies have shown that native insects do not favor many of these alien plants, which further reduces the diversity in the food chain.
The sky is not falling, itís changing
It is easy to become disheartened by our environmental woes. Pennsylvania's forests have enough stress from over a century of logging, an over-abundance of deer browsing, acid rain, invasive vegetation, and now invasive earthworms. But the picture is not as dire as it seems: Logging practices have improved to sustainable levels; recent deer management strategies have great potential for reducing local herds; acid rain has long been on the decline; and weed-warrior groups such as the Exotic Pest Plant Councils (EPPC) around the country, Audubon, local conservancies, and scores of concerned volunteers are successfully, if slowly, battling invasive vegetation.
Likewise, strategies for managing invasive earthworms are being tested at the Schuylkill Center in Philadelphia. Several substances, including tobacco, coffee, black walnuts,and others, have been tried by researchers there. Their studies have shown promise and, as with the other concerns in Pennís Woods, there is hope.
Invasive worms at the Schuylkill Center are an Asian species in the Amynthas genus. Unable to survive Philadelphia's winters, the adults die off after depositing thousands of cocoons with eggs. In April, when the soil warms, the eggs hatch and young worms begin the cycle all over again. In this sense these worms are annuals, similar to crabgrass or ragweed, dependent upon the previous year's seed to survive.
YOU CAN HELP: Invasive worms enter the forest primarily through horticulture, agriculture, and recreation. If you fish DO NOT dump your extra worms out on the ground. Throw them entirely in the water or take them home. If you plant trees and shrubs on your property examine the root balls for worms and destroy any you find. If you plant non-native trees and shrubs, which you shouldn't, DO NOT plant them near woodlots or forest edges.
A Dutch study demonstrated that worms migrate at 10 meters per year (about 30 feet). If native worms began migrating north from Baltimore after the last glacier they would just about reach Philadelphia now, if they could get across I95.
-Worms can live up to ten years
-The largest worm is the Grippsland Giant. Grows to one meter (about 3 feet) and stretches to two meters
-Worms can jump up to 30 centimeters (about a foot)
-There are 2700 known species of earthworms worldwide
-Worms breathe through their skin and can survive short periods under water
-Worms have a variety of colors: orange, yellow, green, and blue
-Ancient South Pacific societies roasted worms and mixed them with coconut oil to cure small pox.
-Middle Easterners mixed worms with oil to cure baldness
-Worms produce their own weight in casts in 24 hours