Pesticides and the fertility of the Grey Partridge Perdix perdix.

Abstract

The recent decline in average annual productivity of the  grey partridge, Perdix perdix, on farmland is described. It is primarily attributed to an increase in the mortality rate of the chicks aged I to 15 days although increased nest losses have also occurred.

The chick survival rates were mapped in the Sussex study area and were found to vary greatly from farm to farm, even within the same year, e.g. from 6% to 84% in 1971. Over the 4 years, 1968-1971, such variations were compared with other biological data which provided explanations for the causes of the mortality variation.

Although partridges in cereal growing areas are exposed to many pesticides during reproduction, no biologically significant and direct effects have been demonstrated in the field. The main ihdirect effect is that the traditional weed fauna has declined, so that the average availability of food to the chicks has decreased. This effect, together with changing farming systems (especially the polarization of grass and cereal cropping), has resulted in malnutrition which has exaggerated other causes of mortality such as predation and disease rather than caused 'simple starvation'. Natural variations in the availability of food, in some years and areas, mean that the decrease of certain insects on modern farms does not materially affect the chick food supply and no adverse effects occur. In some cases, predation and disease can cause heavy mortality even where chick food is plentiful.

The adverse effects of pesticides can, therefore, only be established in such a complex situation if the natural causes of mortality are already understood. Since this is rarely, if ever, the case, a much greater importance should be attached to broadly based ecological studies. In future, more of the adverse effects of pesticides must be anticipated, otherwise the methods of environmentally conscious ecologists will increasingly resemble those of historians.