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Knowing Nature . . .

The Dirt on Worms

March 18, 2008
Jocie Ingram

Earthworms photo © Dave Ingram

At this very moment, thousands of pink earthworms are wriggling and writhing through my freshly dug garden soil. There is an even greater concentration in the compost bin, where the worms are happily devouring a stinking assortment of discarded grapefruit halves, tea leaves and veggie peelings, and converting them into rich organic soil. Worms are commonplace, and each acre of soil is home to about 50, 000.

Earthworms are invertebrates, animals with no backbone, and are classified as annelids due to their segmented body. Worms have 100 or more segments, seen as grooved rings along the worm’s length. Slippery mucous enables them to slide easily through soil, and the mucous binds with the soil so the walls of the tunnel don’t collapse.

Each segment is ringed with bristle-like hairs called setae that give the worm traction. Robins are often seen tugging at a worm that is gripping the soil with its hairs. The robin may tear the worm in two, which is OK for the worm, since it can regenerate new parts.

Worms have no eyes, teeth, ears or nose. They also have no lungs, and breathe through their skin, which must always be moist. Worms have a mouth, and eat by sucking food into a crop where it is ground up by a gizzard. They are voracious eaters of dead plant and animal matter, and in 24 hours can produce their own weight in castings (worm droppings). Above the mouth is a lobe called a prostomium that enables the worm to sense light and dark. Worms can also feel vibrations.

About a third of the way down the body is a saddle-like swelling called a clitellum. This is only visible in worms that are ready to reproduce. Earthworms are hermaphrodites, possessing both male and female parts. To mate, two worms line up head to tail and exchange sperm. After mating a slime-tube forms around the clitellum, and it slides forward over the worm’s head, picking up an egg and the stored sperm on the way, before being deposited in the soil. Worms often come to the surface to mate at night.

After a heavy rain, worms seem to be everywhere. When soils become saturated with water, worms can’t absorb as much oxygen through their skin, and are forced to come to the surface. This is hazardous, since they are easily preyed upon by birds, which enjoy this high protein (70%) snack. Worms can also die when exposed to the drying effects of daylight.

During unfavourable conditions, such as freezing or drought, worms burrow deep then curl up and go dormant. This enables worms to live in places as diverse as Greenland or the Sahara Desert.

Worms, called “nature’s ploughs” are good for gardens, and agriculture in general. Their burrows aerate the soil, allowing oxygen in. Burrows also provide paths for rainwater, which reach plant roots and prevent water run-off. Worms are important decomposers, breaking down organic matter and changing it into nutrients that are available to plants and other organisms.

Unfortunately, there is also a downside to worms. Almost all of the worms here are introduced, or “exotic”. The ice age, 10, 000 years ago killed off most of the native worm species in BC. Only 4 species survived, in spots along the coast that the glaciers missed. The other 19 species of our province were brought here by us. These invaders are spreading from gardens and fields into forests. They are changing soil ecology, and are having a negative impact on native plants.

Across Canada and the US, worms are increasingly turning up in wilderness areas. Once in the wild, they eat up the leaf mold and litter, called “duff”, which many native plants depend upon. Scientists, such as Dr Jan Addison, a retired professor from Royal Roads in Victoria, have studied the alarming effect exotic worms have in BC’s forests. In addition to eating up the duff layer, worm burrowing mixes deeper mineral soil with topsoil. Eventually, this changes the soil’s pH. Many of our forest trees and plants, such as Douglas-fir, require an acidic soil. According Addison, “the very rug of a forest is being pulled out from under the trees.”

There are even more serious implications, since worms are increasing with global warming, are able to overwinter and reproduce more rapidly. They are also contributing to the release of carbon into the atmosphere. When the duff layer is eaten, the carbons trapped in the decomposing matter are released. 80% of the carbon in Canada’s forests is stored in soil.

Once worms have become established in wild areas, there is no hope of eradicating them. It is best, therefore, to focus on prevention. Anglers need to pack out worm bait rather than leaving them along streams and rivers. Gardeners should avoid transporting worm-laden soils into natural areas, and wanton dumping of garden refuse.

In short, worms are good for gardens and farms, but bad for forests and wild areas. Keep the worms in the garden, where they belong!

Canadian scientists are asking for the public’s help in gathering more data about worms. For information, go to www.frogwatch.ca/english/wormwatch