گفتمان خاكشناسی - earth worms _ part 2

گفتمان خاكشناسی

به امید روزی كه قدر خاك را هم بدانیم


[edit] Rainstorms

Earthworms are seen on the surface after heavy rain storms flood the soil because, despite needing a moist environment to allow the diffusion of gases across their skin membrane, the soil becomes saturated and they begin to drown. Earthworms also come to the surface during rain to mate. To protect themselves they escape to the surface, but if the ground is un-naturally hard they may later become stranded and die from exposure. This is why they are seen in places like driveways after a storm. However, certain earthworm species can survive immersion for several days in oxygenated water.


[edit] Rainstorms

Earthworms are seen on the surface after heavy rain storms flood the soil because, despite needing a moist environment to allow the diffusion of gases across their skin membrane, the soil becomes saturated and they begin to drown. Earthworms also come to the surface during rain to mate. To protect themselves they escape to the surface, but if the ground is un-naturally hard they may later become stranded and die from exposure. This is why they are seen in places like driveways after a storm. However, certain earthworm species can survive immersion for several days in oxygenated water.

An earthworm being eaten by an American Robin.

An alternative hypothesis concerning this behavior is that as some species (notably Lumbricus terrestris) come to the surface to mate they may become stranded. However, as this behavior is limited to only a few species and L. terrestris is rarely, if ever, one of those found stranded on impermeable surfaces, this hypothesis does not seem a very likely explanation.

Another hypothesis is that the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly. Since the relative humidity is higher during and after rain, they do not become dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content, and are more vulnerable to predators such as birds.

A further hypothesis is that, as there are many other organisms in the ground as well, and their respiration increases carbon dioxide, this gas may dissolve into the rainwater to form carbonic acid. As the soil becomes too acidic for the worms, they seek a more neutral environment on the surface.

[edit] Locomotion and importance to soil

Close up of an earthworm in garden soil

Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. (In all the body segments except the first, last and clitellum, there is a ring of S- shaped setae, embedded in the epidermal pit of each segment,perichaetine) The whole burrowing process is aided by the secretion of lubricating mucus. Worms can make gurgling noises underground when disturbed as a result of the worm moving through its lubricated tunnels. They also work as biological "pistons" forcing air through the tunnels as they move. Thus earthworm activity aerates and mixes the soil, and is constructive to mineralization and nutrient uptake by vegetation. Certain species of earthworm come to the surface and graze on the higher concentrations of organic matter present there, mixing it with the mineral soil. Because a high level of organic matter mixing is associated with soil fertility, an abundance of earthworms is beneficial to the organic gardener. In fact as long ago as 1881 Charles Darwin wrote: It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures [1]

[edit] Benefits

The major benefits of earthworm activities to soil fertility can be summarized as:

  • Biological. In many soils, earthworms play a major role in converting large pieces of organic matter (e.g. dead leaves) into rich humus, and thus improving soil fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the dried dirt, such as leaf fall or manure, either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 9" of soil in which the worm is living.
Faeces in form of casts
  • Chemical. As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough—including stones up to 1/20 of an inch (1.25mm) across—into its gizzard wherein minute fragments of grit grind everything into a fine paste which is then digested in the intestine. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, minerals and plant nutrients are made available in an accessible form. Investigations in the US show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, in itself an indicator of why it pays the gardener or farmer to keep worm populations high.
  • Physical. By its burrowing actions, the earthworm is of great value in keeping the soil structure open, creating a multitude of channels which allow the processes of both aeration and drainage to occur. Permaculture co-founder Bill Mollison points out that by sliding in their tunnels, earthworms "act as an innumerable army of pistons pumping air in and out of the soils on a 24 hour cycle (more rapidly at night)" [2]. Thus the earthworm not only creates passages for air and water to traverse, but is itself a vital component in the living biosystem that is healthy soil. Earthworms continue to move through the soil due to the excretion of mucus into the soil that acts as a lubricant for easier movement of the worm.

See Bioturbation. The earthworm's existence cannot be taken for granted. Dr. W. E. Shewell Cooper observed "tremendous numerical differences between adjacent gardens" (Soil, Humus And Health), and worm populations are affected by a host of environmental factors, many of which can be influenced by good management practices on the part of the gardener or farmer.

Darwin estimated that arable land contains up to 53,000 worms per acre (13/m²), but more recent research from Rothamsted Experimental Station has produced figures suggesting that even poor soil may support 250,000/acre (62/m²), whilst rich fertile farmland may have up to 1,750,000/acre (432/m²), meaning that the weight of earthworms beneath the farmer's soil could be greater than that of his livestock upon its surface. One thing is certain however: rich, fertile soil that is cared for organically and well-fed and husbanded by its steward will reap its reward in a healthy worm population, whilst denuded, overworked, and eroded land will almost certainly contain fewer, scrawny, undernourished specimens.

[edit] Earthworms as invasive species

From a total of around 6,000 species, only about 120 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.[3]

[edit] North America

A total of approximately 182 earthworm taxa in 12 families are reported from America north of Mexico, i.e., USA & Canada, of which 60 (ca. 33%) are exotic/introduced. Only two genera of Lumbricid earthworms are indigenous to North America while introduced genera have spread to areas where earthworms did not formerly exist, especially in the north where forest development relies on a large amount of undecayed leaf matter. When worms decompose that leaf layer, the ecology may shift making the habitat unsurvivable for certain species of trees, ferns and wildflowers. Another possible ecologic impact of greater earthworm numbers: larger earthworms (e.g. the night crawler, Lumbricus terrestris, and the Alabama jumper, Amynthas agrestis) can be eaten by adult salamanders, and when the salamanders do consume the earthworms they are more successful at reproduction. However, those earthworms are too large for juvenile salamanders to consume, which leads to a net loss in salamander population.[4]

Currently there is no economically feasible method for controlling invasive earthworms in forests. Earthworms normally spread slowly, but can be quickly introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings from other areas. [5]

[edit] Australia

Australia has 650 known species of native earthworm that survive in both rich and in nutrient-poor conditions where they may be sensitive to changes in the environment. Introduced species are commonly found in agricultural environments along with persistent natives. Most of the 75 or so exotics have been accidentally introduced into Australia. The total species numbers are predicted to exceed 2,000. [6]

[edit] Special habitats

While, as the name earthworm suggests, the main habitat of earthworms is in soil, the situation is more complicated than that. The brandling worm Eisenia fetida lives in decaying plant matter and manure. Arctiostrotus vancouverensis from Vancouver Island and the Olympic Peninsula is generally found in decaying conifer logs or in extremely acidic humus. Aporrectodea limicola and Sparganophilus and several others are found in mud in streams. Some species are arboreal. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own.

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