Report from the Sub-comittee on the environment and health

Indirect effects on mammals
and birds
There are thus numerous examples of pesticides affecting <strong>the</strong> density of
prey, so that <strong>the</strong> predators lack food. If, on <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> predators
are <strong>the</strong> most sensitive to a given pesticide, <strong>the</strong> effect can be <strong>the</strong> opposite.
Treatment can <strong>the</strong>reby reduce <strong>the</strong> density of predators, leading to an
increased number of prey (Croft 1990). If <strong>the</strong> prey is primarily pests,
<strong>the</strong>re could thus be an unintended increase in <strong>the</strong> number of pests.
In several countries, including Denmark, a richer flora has been found in
organically cultivated fields (Moreby et al. 1994; Hald, Reddersen 1990)
and this – toge<strong>the</strong>r with o<strong>the</strong>r factors that differentiate <strong>the</strong> two cultivation
systems – has been followed by considerably richer insect fauna, with
respect to both species and individuals, in organically cultivated fields
(Reddersen 1998).
Several studies have shown a negative indirect effect on <strong>the</strong> insect fauna
<strong>from</strong> herbicide spraying in addition to <strong>the</strong> effect on herbivorous insects.
In particular, it is well documented that various general predators, such
as ground and rove beetles, are more numerous in cereals with ground
cover of wild plants (Speight, Lawton 1976; Powell et al. 1985;
Chiverton, So<strong>the</strong>rton 1991; Reddersen et al. 1998). Here, it is presumably
<strong>the</strong> ground cover, which provides concealment and changes <strong>the</strong>
microclimate in a favourable direction, that is <strong>the</strong> deciding factor.
However, it does not appear that spiders are affected by <strong>the</strong> ground cover
(Chiverton, So<strong>the</strong>rton 1991; Reddersen et al. 1998). It has also been
shown that a large number of o<strong>the</strong>r insect groups, including many
beetles, flies and midges, are indirectly affected by herbicides via <strong>the</strong>
quantity of weed. Serious and long-term indirect effects of herbicide
treatment on <strong>the</strong> entire insect community in cereal fields have been
observed, with greatly reduced total densities (20-85%) and also a
consistently lower species diversity (Reddersen et al. 1998).
Indirect effects of <strong>the</strong> use of pesticides have also been found one step
higher up <strong>the</strong> food chain. Common species of bird in farmland, such as
partridge, pheasant, yellowhammer and skylark, breed less successfully
in fields treated with pesticides than in unsprayed or organically
cultivated fields, even though <strong>the</strong> substances used are not directly
poisonous to birds in <strong>the</strong> dosages used (Potts 1986; Hill 1985; Petersen et
al. 1995; Odderskær et al. 1997). Attention has <strong>the</strong>refore been directed
towards effects on <strong>the</strong> birds’ sources of food. In <strong>the</strong> breeding season,
birds and, particularly, <strong>the</strong>ir young eat mainly insects <strong>from</strong> all trophic
levels, e.g. herbivores, fungivores and insectivores.
The relationship between source of food and <strong>the</strong> birds’ breeding success
has been closely studied in Denmark in <strong>the</strong> case of <strong>the</strong> skylark
(Odderskær et al. 1997). This study is one of <strong>the</strong> most detailed and
statistically best designed studies with respect to clarifying <strong>the</strong>
relationship between pesticides, source of food and <strong>the</strong> birds’ breeding
success.
The study showed that treatment with herbicides and insecticides impairs
<strong>the</strong> skylarks’ source of food, but <strong>the</strong> relationships are complex since
o<strong>the</strong>r factors than use of pesticides affect <strong>the</strong> system. For example, <strong>the</strong>
wea<strong>the</strong>r affects <strong>the</strong> amount of food and also affects <strong>the</strong> need for food and
<strong>the</strong> time available to <strong>the</strong> parent birds to search for food, since <strong>the</strong> young
cannot tolerate lying alone and unprotected in <strong>the</strong> nest in bad wea<strong>the</strong>r.
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