Geoscience Reference
In-Depth Information
rich in nitrogen and sugars: hawthorn, apple, ash, alder, birch, sycamore, nettle and
dog's mercury are more palatable than oak, beech, pine, spruce or larch which have
distasteful tannins or polyphenols.
It is a common sight in autumn to see the ends of leaves protruding out of the
soil on a path or lawn; and it was Darwin who showed unequivocally, through a large
number of experiments, that a leaf was most often seized by the narrow tip, which
would offer least resistance when drawn into its burrow. The same applied to elong-
ated triangles of paper smeared with fat. This showed that the worm was guided in
some way by shape not texture. The aspiring researcher would do well to read this ac-
count as a model of thorough, step-by-step discovery. Darwin did not have the benefit
of statistics to help him reach 'significant' conclusions. Nevertheless, he realized the
need for repeating experiments many times to be sure that the results could not be due
to chance.
On bare, stony ground, such as a gravel path or weathered quarry floor, the
mouth of the burrow is often marked by a small cairn of stones (
Fig. 35
). Some of
these stones may have been pushed up from below to make more space. Others have
certainly been dragged there. A colleague, Christine Brown, once witnessed, by the
light of a dim street lamp, a large
Lumbricus
feeling the surface of an angular pebble
with its lips. It was apparently selecting the best corner to get hold of for, after a few
seconds, it fastened its mouth over this, and jerked the stone a centimetre or two to-
wards its burrow.
The less palatable kinds of leaves need a period of weathering before they be-
come acceptable, but nevertheless, earthworms can consume more oak and beech lit-
ter than all other soil invertebrates put together. This has been shown by burying leaf
discs in nylon bags with various mesh sizes so as to exclude different groups of in-
remove 90 per cent of the fallen leaves during the winter months, equivalent to 1.2
tonnes per hectare dry weight. Thus the cycle of nutrients from plant to soil is set in
motion again.
What would happen if there were no worms? Frank Raw looked at an orchard
from which all worms had been eliminated by residues from copper-based sprays over
several years, and found that a layer of peat-like organic matter had developed in the
top 1-4 cm. Dutch investigators found the same thing, and they also noticed that the
underlying soil structure was itself starting to deteriorate. Where there had been a
stable, subangular, blocky condition with high porosity, the soil was becoming more
cloddy, less stable and less porous. The grass was more shallowly rooted as well and
was more easily damaged by mowing.
