Long Term Arable Ecosystem Studies (Sussex Study)

August 2007
38 YEARS OF CHANGE IN FARMLAND WILDLIFE

The Sussex Downs study area in 1970. Blue spots show grey partridge coveys (groups of birds - usually single broods with some old birds) counted in autumn.

The distribution of grey partridge coveys counted in autumn on the Sussex Downs study area in 2007.

Our study on the Sussex Downs is one of the longest-running projects on arable farmland in the world.  It started in 1968 with an  investigation into the causes of the decline in numbers of the grey partridge. Dick Potts, the project officer, began counting partridges on 62 square kilometres of the downs.  He found that the main reason for the decline was a reduction of chick-food insects in cereal crops caused by the disappearance of arable weeds. This followed the first use of herbicides in the late 1950s, and led to the starvation of partridge chicks¹.   

Since 1970 we have sampled insects and weed abundance on this study area annually in June, recorded the cropping on each field, and counted partridges each autumn.  Since we started insect numbers in cereals have dropped by about a half².  Caterpillar numbers (sawfly and Lepidoptera larvae), in particular, have dropped on all farms except two where traditional mixed farming with ley rotations is still practised³. 

Although the Sussex Study began a decade after the introduction of herbicides, it started before the widespread use of foliar fungicides and of insecticides.  Herbicide use has increased thoughout the study and a cocktail of herbicide chemicals are now applied against a much wider range of plants⁴.  The number of fungicide and insecticide treatments has also increased, with consequent effects on insects and birds. 

Populations of slow-reproducing insects like sawflies (as well as other invertebrates such as spiders) can take up to four years to recover from a single summer application of an insecticide like dimethoate^{5, 6}.

Partridge chick survival on areas that are regularly treated with summer insecticides is now a third lower than it is on other areas where insecticides are little used.  Results from the Sussex study contributed to the now well-accepted theory that grey partridge declines are due, at least in part, to the indirect effect of pesticides⁷.

1. POTTS, G. R. (1970) Recent changes in the farmland fauna with special reference to the decline of the grey partridge (Perdix perdix). Bird Study, 17, 145-166.

2. AEBISCHER, N. J. (1991) Twenty years of monitoring invertebrates and weeds in cereal fields in Sussex. In: The Ecology of Temperate Cereal Fields. Eds L. G. Firbank, N. Carter, J. F. Darbyshire, G. R. Potts, Blackwell Scientific Publications, Oxford. 305-331.

3. AEBISCHER, N. J. & POTTS, G. R. (1998) Spatial changes in Grey Partridge (Perdix perdix) distribution in relation to 25 years of changing agriculture in Sussex, U.K. In: Perdix VII: International Symposium on Partridges, Quails, and Pheasants. Eds. M. Birkan, L. M. Smith, N. J. Aebischer, F. J. Purroy, P. A. Robertson, Office National de la Chasse, Paris. 293-308.

4. EWALD, J. A. & AEBISCHER, N. J.(2000) Trends in pesticide use and efficacy during 26 years of changing agriculture in southern England. Environmental Monitoring and Assessment, 64, 493-529.

5. AEBISCHER, N. J. (1990) Assessing pesticide effects on non-target invertebrates using long-term monitoring and time-series modelling. Journal of Functional Ecology, 4, 369-373.

6. EWALD, J. A. & AEBISCHER, N. J. (1999) Pesticide use, avian food resources and bird densities in Sussex. Joint Nature Conservation Committee, Report No 296, Peterborough.

7. CAMPBELL, L. H., AVERY, M. I., DONALD, P. F., EVANS, A. D., GREEN, R. E. & WILSON, J. D. (1997) A Review of the Indirect Effects of Pesticides on Birds. Joint Nature Conservation Committee Report, No. 227, Peterborough.