Geoscience Reference
In-Depth Information
occur under alternating bare ground and cropped soil. Direct comparisons have shown
that permanent grassland supports about twice as many species as arable land.
Earthworms show the effects of the change from grassland to arable cultivation
very clearly. Populations of worms are generally three times as high in grassland, and
have four times the weight due to the larger proportion of big species, for example,
the lob worm Lumbricus terrestris. When a field is ploughed, there is an initial loss
of worms caused by direct physical damage and also due to predation by birds. Gulls
are often seen following the plough in autumn, swooping down to pick up an exposed
worm before it can bury itself again, like gulls behind a trawler picking up scraps of
fish. There may also be a more gradual toll of worms because of increased drying out
of surface layers of the soil and penetration by winter frosts. Probably, however, the
most important factor is simply the gradual loss of organic matter as a result of con-
tinuous arable cropping, which is a relatively recent departure from traditional farm-
ing. About 50 per cent of the biomass of cereals remains behind in their roots, but this
is small compared to the organic matter built up in the soil under grassland. Crops,
like sugar-beet or potatoes, leave even less organic matter behind after harvest, and so
reduce the earthworm population still further.
Direct drilling of cereals gives an interesting new twist to the story, since by
eliminating cultivation there is a partial reversal of the trend. Eight years of continu-
ous cereal cropping with and without cultivation allows one to compare these altern-
ative systems of cropping. Differences in the size and composition of earthworm pop-
ulations are fairly consistent and often quite striking. Generally speaking, the total
worm populations decrease under a system of continuous long-term cereal cropping
irrespective of whether the soil is cultivated before sowing or not. This suggests that
there is a gradual depletion of the worms' food resources. However, fields that are
direct drilled support larger populations on average than those that are ploughed every
year. A less drastic form of cultivation such as chisel ploughing, which breaks up the
soil surface but does not invert it, has an intermediate effect on the worms.
Disruption is greatest for those species of worms that produce deep, semiper-
manent burrows. Thus, after eight years of regular ploughing, the long worm Apor-
rectodea longa and the lob worm Lumbricus terrestris are almost eliminated from
some soils. Smaller, surface-living forms, such as the green worm, grey worm and
rosy worm Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea
rosea are less affected; they decline in numbers progressively and almost equally un-
der all forms of continuous cereal growing.
The burning of straw destroys large numbers of surface-living creatures but the
effects on populations are not obvious since in any case they often experience large
seasonal fluctuations. The animals are usually very mobile and able to recolonize a
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