Report from the Sub-comittee on the environment and health

Assumptions and
uncertainties
Results of <strong>the</strong> calculations
of bird populations
196
54 large Danish farms, where information was available on crop and
biotope conditions, toge<strong>the</strong>r with data on all pesticide treatment. The
distribution of <strong>the</strong> different species in relation to biotope conditions,
crops and treatment frequency indices could <strong>the</strong>n be calculated and tested
by means of covariance analyses. These data have been reanalysed, so
<strong>the</strong> logarithm for a given species' population density in each crop is
expressed as a linear function of <strong>the</strong> treatment frequency index, while <strong>the</strong>
effects of <strong>the</strong> o<strong>the</strong>r biotope conditions are kept constant. The treatment
frequency indices for <strong>the</strong> pesticides that have shown statistically
significant effects and <strong>the</strong> areas used for <strong>the</strong> crops have been varied in
<strong>the</strong> analyses.
It is assumed that <strong>the</strong> average field size remains unchanged, that <strong>the</strong>re are
no general changes in <strong>the</strong> amount of hedgerow and o<strong>the</strong>r hedge
vegetation, that <strong>the</strong> number of small biotopes does not change and that
<strong>the</strong> natural content of <strong>the</strong> farmland generally remains unchanged. It is
also assumed that <strong>the</strong> population density of each species can be
calculated independently of o<strong>the</strong>r species. Lastly, it is assumed that if
herbicides and insecticides act simultaneously, <strong>the</strong>n <strong>the</strong> total effect will
be <strong>the</strong> product of <strong>the</strong>ir effects, i.e. a mutually intensifying effect. Lastly,
it is assumed that <strong>the</strong> calculated population densities can be extrapolated
to national level without taking account of <strong>the</strong> localities' bearing
capacity. The calculated population increases can <strong>the</strong>refore be interpreted
as an upper limit for <strong>the</strong> changes that could be expected.
The results of <strong>the</strong> calculations are shown in figure 10.6. It will be seen
that <strong>the</strong> populations of partridge, whitethroat and yellowhammer increase
in all scenarios compared with present production and that all <strong>the</strong>
scenarios show significantly increased population density for <strong>the</strong>se
species. This applies to <strong>the</strong> 0-scenario, <strong>the</strong> +scenario and <strong>the</strong> ++scenario.
For <strong>the</strong> o<strong>the</strong>r species, <strong>the</strong> index would be unaffected by <strong>the</strong> use of
pesticides, compared with present production. It is noteworthy that <strong>the</strong>
countrywide population density of <strong>the</strong> skylark is not affected by <strong>the</strong> use
of pesticides. In studies described in section 5.1, it has been found that
<strong>the</strong> use of pesticides reduces <strong>the</strong> production of young as a consequence of
effects on food resources. However, <strong>the</strong> relationship between <strong>the</strong> annual
production of young and <strong>the</strong> population trend is not known (Elmegaard,
Axelsen 1999; Petersen, Jensen 1998). Similarly, we do not know
precisely how increased reproduction at field level is reflected at farm
level because <strong>the</strong> redistribution of young birds in <strong>the</strong> following season
depends on <strong>the</strong> density of older birds, <strong>the</strong> crop rotation, etc., and only to a
minor extent on <strong>the</strong> fields' ownership conditions (farm characteristics).
The partridge is adversely affected by herbicides, while <strong>the</strong> population
density of whitethroat and yellow bunting is affected by both herbicides
and insecticides.