The Sussex Study

The Sussex study started out as the Partridge Survival Project, with the intended aim of investigating exactly why the chick survival rate of the grey partridge had declined.  Dick Potts soon established that one of the causes was a shortage of chick-food insects in cereals, which led to poor survival of the partridge chicks.  Consequently, from 1970 onwards, he and his colleagues have monitored cereal invertebrates annually on the South Downs study area in June. 

At the same time, details of crop type, disease and flora are recorded on a field-by-field basis.  The monitoring of partridges and other game has been continued as well, with the whole area being counted every autumn after harvest. Data on pesticide use (herbicides, fungicides and insecticides) was collected through on-site interview with farmers and their agronomists. 

The Sussex study has expanded beyond just monitoring grey partridges and can now be considered to monitor the cereal ecosystem across the 62km² of the South Downs between Arundel in the west and Worthing in the east.  Throughout the 38 years that the study has run there have been many changes in land-use (Figure 1) and pesticide use, the associated cereal flora and invertebrates and finally the grey partridges, which depend on the cereal ecosystem. 

Changes in land-use

The changes in land-use that have occurred are particularly striking when displayed in map form (Figure 1).  In 1970, the extent of light blue illustrates that spring cereals, mainly barley, were the dominant crop type.  Winter wheat (red) was unevenly distributed. Rotational grass (undersown grass leys - light green) was widespread, with four of the five large farm units managed using traditional rotations that included undersowing of cereal crops to establish grass.  Non-rotational grass (dark green) was restricted to the slopes of the Downs.  Winter barley/oats (dark blue) and miscellaneous or break crops (orange) were mainly to be found in the one farm that had moved from traditional rotations to one that included mixed arable and grass (without undersowing and using break crops such as oilseed rape and linseed).  

By 1980, the preponderance of spring barley had been replaced with winter cereals, with three of the five large farms now abandoning traditional rotations in favour of rotations that featured arable and grass not necessarily rotating with no undersowing. By 1990 only one of the farm units retained traditional rotations, with another having moved to continuous winter wheat. 

The South Downs Environmentall Sensitive Area Scheme (ESA) encouraged farmers to revert arable land to permanent grass, with a resultant increase in the amount of non-rotational grass.  By the turn of the century, the ESA together with the introduction of set-aside had resulted in large changes in land-use at this time.  The following decade has seen the introduction of Environmental Stewardship, through both the Entry and Higher Level Schemes, and the loss of set-aside.  By 2008, fields had been divided where they had been combined over the past 30 years and there was a return to the planting of spring cereals with a patchwork pattern to the cropping. 

Figure 1

Changes in partridge numbers

The changes in partridge numbers that have accompanied the alteration in land-use are equally striking and illuminating (Figure 2).  In 1970, the density of spring grey partridge pairs averaged 11.3 pairs per 100 hectares or 250 acres.  By 2000 this had dropped to 2 pairs per 100 hectares, with only one pair per 100 hectares in 2004.  Since then things have improved and in 2007 there were 2.6 pairs for every 100 hectares. 

Figure 2.  The changing distribution of grey partridge coveys through the 38 years of the Sussex study.  Each blue dot is the location of a partrige covey in Autumn.

Changes in cereal flora

Although the use of herbicides was wide-spread at the beginning of the monitoring in Sussex (1970), there has been changes in their efficacy and intensity of use (number of treatments per crop).  This has not resulted in sterile crops with a number of weed species, particularly perennial dicotyledon weeds, increasing throughout the monitoring. (Floral diversity in cereals).   A few species have declined and many of these are once common annual dicotyledons which are important as both food plants for chick-food insects and whose seeds are important to seed-eating farmland birds (for example, Chickweed - Stellaria media - Figure 3 below).

Figure 3.  The red dots are locations where Chickweed was identified in cereal samples in June.  The occurrence of Chickweed has declined throughout the study area.

Changes in cereal invertebrates

Five groups of cereal invertebrates are particulary important in the diet of farmland bird chicks:

• Araneae & Opiliones (spiders and harvestman)
• Carabidae & Elateridae (ground and click beetles)
• Symphyta & Lepidoptera (sawflies, butterflies and moths)
• Chrysomelidae & Curculionidae (leaf beetles and weevils)
• Non-aphid Hemiptera (plant bugs/hoppers, excluding aphids)
• Aphididae (aphids)

The abundance of spiders, harvestmen, ground and click beetles declined through time in Sussex, whilst the abundance of caterpillars declined until the mid-1980s and then increased (Figure 4). There was no change in the abundance of leaf beetles & weevils, plant bugs/hoppers and aphids.  These changes have been shown to be related to the use of insecticides. 

Figure 4