The impact of grazing management on plant and invertebrate populations in chalk grassland in South England.

Abstract

Many centuries of grazing by sheep on chalk hills (South Downs) in Southern England has produced a species-rich chalk flora (downland turf) on steep, nutrient-poor slopes. However, an increase in arable production and use of artificial fertilizers on the grassland has made downland turf an endangered habitat type. In 1987, The Ministry of Agriculture, Fisheries and Food (MAFF) introduced an Environmentally Sensitive Area (ESA), scheme to encourage the conservation of downland turf, paying farmers to stop arable production and to sow permanent grassland (arable reversion) which must be managed largely without agricultural chemicals.
Surveys of plants and invertebrates were made on chalk grassland at Plumpton (Lewes, Sussex) in June and July 1995. The plant survey was carried using the Braun-Blanquet approach. 21 phytosociological records were investigating the effects of grazing on (a) seven areas of species-rich downland turf, (b) eight fields of arable reversion grass that was maintained below 8 cm tall, by grazing with sheep (short sward fields) and (c) six fields of arable reversion grass that was maintained above 20 cm by less intensive grazing (tall sward fields). Invertebrates were sampled in four short sward arable reversion grass fields and four tall arable reversion grass fields with pit-fall traps.
The plant survey has showed that nine years after the end of arable production, arable reversion fields supported less than half the number of plant species as downland turf. The downland turf supported sustainable chalk associations of Festuco-Brometea class, with many rare and vulnerable species such as Dactylorhiza fuchsii, Anacamptis pyramidalis, Gymnadenia conopsea, Listera ovata, Polygala calcarea, Phyteuma orbiculare, Hippocrepis comosa etc. However, these associations were absent from the arable reversion fields, which were dominated by sown agricultural species of the Molinio-Arrhenatheretea class, such as Lolium perene, Phleum pratense, Poa pratensis, Trifolium repens (species usually absent in the downland turf). This indicates that establishment of species-rich grassland on land which has been taken out of arable production is a slow process, especially if competitive agricultural species such as Lolium perenne and Trifolium repens are sown at the time of reversion. There is no significant difference in the number of plant species between the short sward and tall sward arable reversion grass fields.
However, the tall sward arable reversion grass fields supported approximately twice as many invertebrates as the short sward arable reversion grass fields and approximately 50% more species of invertebrates than the short sward arable reversion fields. The tall sward fields therefore provided more nesting cover and invertebrate food items for ground-nesting birds such as skylarks Alauda arvensis.