This past year at the University of Tennessee I took a class entitled Writing for Science and Medicine. One of our reading assignments in the Best of Science Writing booklet was a piece by David Quammen on earthworms…It was, appropriately, called “Thinking About Earthworms”. Quammen confronts society’s tendency to be mentally absorbed in our current ways-of-the-world and suggests we all take more Darwinian approach to life. In a sense, he dares we step outside the music-blasting party and venture off into the backyard woods. His point: break free from a designated place, physically or mentally. Be like Darwin, the famous man with controversial ideas, who spent hours of his life thinking about something so forgotten and understudied as earthworms. Venturing into the unknown opens us up to new discoveries and fascinations. Certainly with Darwin this was the case. He wrote a book which nowadays stands in the shadows of On the Origin of Species but was, according to Quammen, a great success initially. It’s called The Formation of Vegetable Mould, Through the Action of Worms. Now doesn’t that sound exciting? Actually, it turns out that earthworms (Oligochaeta) do play a critical (and almost shocking) role in shaping the lands we live on, and are quite interesting little guys themselves…
#1: There are more than 7,000 species of earthworms. The Oregon giant earthworm (Driloleirus macelfreshi) can grow 3 feet long…and what’s crazier, is that it apparently gives off the smell of lily flowers when handled! Whaaaat?!
#2: Earthworms are hermaphrodites: organisms possessing both female and male sexual organs. An earthworm, unlike a chicken, relies on intercourse to reproduce (did I shock anybody there? Because I certainly didn’t know chickens laid eggs on a cyclic basis, no fertilization required). So as earthworms are doin’ the dirty, sperm from one earthworm is deposited into the other worm, and vice versa; but the fertilization happens later…The worm excretes a mucous ring from its clitellum (ever noticed that funky thicker ring around one end of a worm?) that travels to the other end. Along the way the ring passes first the eggs, and then the sperm. When the ring passes over the egg receptacles, eggs stick to it, and slide along until they reach the sperm receptacles, where they are deposited. Egg + sperm = fertilization. The fertilized ring slips off the worm’s head, self-seals into a cocoon, and waits on the ground for baby worms to develop and break free. Voila! Weird.
#3: A long, long time ago my sister, two friends and I were down the hill playing in the sandbox. And our playmate found a worm; and he tore it in half. My sister and I were horrified. I’m pretty sure that the worm did NOT evolve into two separate worms, like Ryan suggested. But he was on to something. Earthworms DO have the power of regeneration, but this power is not all-encompassing. Worms cannot grow new heads, but they can grow new tails. A torn/cut/broken worm may only survive if the area of severance is located roughly 10 segments behind the clitellum (that thick ring). An earthworm generally has about 100-150 segments. Along these segments are bristles, which the worm uses to maneuver through the ground and hold on tight to the walls of its burrows if a bird tries pulling it out.
#4: Earthworms are decomposers; they can digest around 36 tons of soil each year! Their excrement is deposited in the form of castings, which are rich in nutrients and help create the soil that grows our crops and trees (microbial activity in the castings ensures the nutrients are readily available for plants.) Worms constantly borrow through the layers of the soil and castings are partially deposited on the soil’s surface, specifically at night and during rain when the worm’s skin can remain moist. In this way, earthworms can turn over the top six inches of soil in 10-20 years. The fact that worms turn the world “inside out” was the central theme of Quammen’s article.
#5: Earthworms enhance soil porosity, which refers to how much water the soil can hold. When the soil is already packed full of moisture, rainwater has nowhere to go, thus leading to the washing-away of soil layers. The burrows of earthworms serve as passages for water drainage, helping curb erosion (the passages also provide space for root growth, allowing plant and trees to grow tall and strong.) Surface runoff decreases with increased soil porosity. Agricultural runoff, high in nitrate and phosphate from fertilizer residue, stimulates the growth of algal blooms, which deprive bodies of water of oxygen and lead to aquatic ecosystem deterioration.
It may be helpful to know that there are 3 major ecological groups of earthworms: epigeic species (found in compost piles), endogeic species (live in the upper soil strata) and anecic species (deep-burrowing). The latter make burrows that can extend several meters into the soil; therefore, they are important for increasing soil porosity!
#6: An acre of land can contain millions of worms (each square yard may have 50-500 worms depending on soil type: cropland is typically less nutrient-rich than, say, grassland or woodlands, so you’d expect to find more worms in the latter. There are 4840 square yards in one acre…So, upwards of 2.4 million worms per acre!)
But there’s a catch to all this. Everything exists in balance. And various ecosystems exhibit differentiating physical and chemical characteristics; species adapt to these specific ecosystem requirements and play specific roles in particular environments. Earthworms illustrate the concept of order perfectly: an earthworm in your garden is a friend, but an earthworm in the forest is a pest.
Earthworms are non-native in areas stretching from central-Illinois to Canada; those that exist in the temperate forests are considered invaders. They have opposite, negative impacts on the ecosystem: they increase soil erosion and decrease soil porosity. The forest is covered by a rich layer of leafy, organic debris; earthworms gobble up this layer, thereby exposing the ground and making it susceptible to erosion and soil compaction.
Many years after Darwin, the earthworm remains an understudied topic of interest; and concern. In 2007, researchers Nick Mikash, Kaloyan Ivanov, and Shimshon Balanson submitted a proposal to survey the earthworms and terrestrial isopods present around Acadia National Park. Surveys not only give us a picture of what communities of these creatures look like right now, but also provide “baseline data” that can be used as the foundation for future research projects. Future data (collected, say, in a few decades or even a century from now) can be measured against this baseline data to assess ecological changes over time. Are earthworms more abundant now than they were then? Has the distribution of different types of earthworms shifted over time? How have these earthworms altered the forest ecosystem? Some scientists suggest that the worm is altering the ability of the forest to act as a natural carbon sink because of its impacts on nutrient cycling. Continued research on the earthworm is needed to evaluate the risks it poses to forest ecosystems and global cycles. That’s a lot of impact from such a little guy!