Individual study: Arable field reversion to grassland reduces grey partridge Perdix perdix densities in farmland on the South Downs, Sussex, England
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. Gibier Faune Sauvage, Game Wildlife, 15, 293-308
In Britain in the 1950s, a sharp decline in grey partridge Perdix perdix numbers followed agricultural intensification. By the mid-1980s the partridge population was estimated to have declined to less than 20% of that in the 1950s. In the early 1990s there were estimated to be only 145,000 pairs. Primarily herbicide application used to control weeds (many of which are food plants of insects upon which young partridge chicks feed) but also insecticide application, loss of hedgerows (nesting cover), and increased predation (due to a reduction in gamekeeping), are all considered contributory factors.
A long-term study of grey partridge has been made on farmland in southern England; since 1970 partridge numbers have fallen by 72%. However, numbers have remained constant on the only farm that has retained a traditional mixed farming system with rotational grass sown under spring crops. On the other four farms within the area there is no longer any rotational grassland and winter cereals have replaced spring-sown cereals. Since the study began, the South Downs was declared an Environmentally Sensitive Area (ESA) in 1987/92. Through incentives, the ESA scheme encouraged farmers to undertake ‘arable reversion’ i.e. a switch from cereals to grass, the aim being to increase the area of grassland (non-rotational or permanent) to benefit wildlife. Whether the ESA arable reversion scheme has benefited grey partridges was investigated.
Study area: The study area consisted of 28 km² of farmland on the South Downs (a series of hills) in Sussex, southern England.
Comparison of arable reversion and non-reversion fields: Fields were assigned to one of two categories, ESA arable reversion fields and those that were not. This portioning changed slightly from year to year after 1987 as further fields entered into the scheme. Autumn partridge flock (covey) densities (coveys/km²) were compared between areas in the pre-ESA (1970-1986) and ESA (1987-1994) periods. The relative quality of the two areas was also quantified on an annual basis and the change in relative quality over time was compared between pre-ESA and ESA periods.
Grey partridge surveys Partridges were counted after the autumn harvest at the end of August or beginning of September, by driving across fields in a four-wheel-drive vehicle at dawn and dusk and mapping the position of each observation.
Partridge densities: Prior to introduction of the ESA arable reversion scheme, the average annual autumn partridge flock density on fields destined for arable reversion was 6.5 (± 1.0). This was higher than the density (4.9 ± 0.6) for the rest of the study area. Subsequent to the start of the ESA scheme, average density on the reversion fields dropped to 1.1 (± 0.3). This was less than half the average density, 2.5 (± 0.1), in those areas outside the scheme.
When comparing the relative quality of reversion fields to the rest of the area, the effect of the long-term change (i.e. a decline) in partridge numbers in much of the study area was largely removed. Based on the annual autumn covey densities, there was little change in relative quality between 1970 to 1986, with only a sight downward trend of -1% per year, but from 1987 to 1994 a decline of -23% per year. This steep decline suggested that arable reversion was having a negative impact on autumn covey densities.
There were also similar trends in total autumn density (-2% to -28%) and for spring pair density (-2% to -25%).
Conclusions: The emphasis of the ESA scheme in Sussex was to encourage reversion of arable land to grass. In this study, the large grassland areas (resulting from the ESA arable reversion scheme) became increasingly unfavourable for grey partridges and also another declining farmland bird in the UK, the corn bunting Milaria calandra. Therefore to benefit such species, a low-input (in terms of fertilisers and biocides) mixed arable with grass under-sowing ESA option is suggested as an alternative to arable reversion.
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