Geoscience Reference
In-Depth Information
field very quickly. Also, many species have young stages which remain largely un-
scathed within the soil. Nevertheless, numbers of beetles, mites and springtails seem
to decrease with repeated episodes of straw burning, especially on the large scale that
has been practised in parts of East Anglia; compared with those on unburnt areas,
their numbers may remain low for several months after burning. Earthworms are af-
fected indirectly through the loss of a food resource and the rapid drying of the soil
surface. A single event of this kind shows little effect but repeated burning over sev-
eral years reduces worm populations.
P
ESTICIDES
During the last forty years or so, pesticides have been developed and used on an ever
increasing scale. They are powerful tools in the service of man for combating a wide
range of medical and agricultural pests. Ever since early agriculturalists were con-
fronted with tares and locusts as serious competitors for the resources they won from
the soil, means have been sought to limit their effects. Only with the growth of the
modern chemical armoury of plant protection chemicals have we fully realized the
extent of losses sustained, and the potential benefits of clean, healthy crops. As one
major problem after another has been overcome, so new ones have been uncovered.
The challenge from weeds, fungal diseases, eelworms, insects, mites, slugs and ro-
dents has been met by a battery of herbicides, fungicides, nematicides, insecticides,
acaricides, molluscicides and rodenticides. These chemicals permit the channelling of
energy and nutrients into chosen crops instead of allowing them to be shared, as in
Nature, among other forms of life in complex food webs.
Like an unpredictable genie, pesticides have proved to be a somewhat mixed
blessing, for their overall effects can seldom be fully predicted. There are few if any
pesticides that are completely specific to the target organisms; discrimination between
harmful and harmless organisms is rarely adequate. This was particularly the case in
the early days, when compounds like DNOC (dinitro-ortho-cresol) were equally ef-
fective against certain insects and some weeds, and when DDT was considered out-
standing precisely because of the wide range of insect pests that it controlled. Later,
this broad spectrum of activity was seen to have its drawbacks, especially after the
many ramifications of its toxicity and persistence were revealed through patient re-
search.
For instance, there was the famous case in the late 1950s of the death of robins
on the campus of Michigan State University (The American robin is more like our
thrush or blackbird). It appeared that the birds had absorbed large amounts of DDT
by feeding on earthworms. These, in turn, had accumulated the insecticide from elm
leaves months after the elm trees had been sprayed with DDT to kill the beetles that
