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Environmental Change
Network
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ECN SITE LINES: PORTON DOWNEstablishment History Porton Down has been part of the Ministry of Defence estate since the First World War. It was at this time that chemical weapons were first used on the battlefield against British troops and the requirement for experimental chemical warfare and protection testing areas resulted in the setting up of the first restricted area at Porton. By 1918 the experimental Range had been aquired, encompassing approximately the same area as now owned by the Chemical and Biological Defence (CBD) sector of the Government's Defence Evaluation and Research Agency (DERA), the sponsors of the ECN on this site.
Porton is one of the most important lowland sites left in Britain for Juniper The UK gave up its offensive capability in the 1950s and now carries out research and development on defensive measures only. The continued need for the Range for experimental testing has prevented the advent of modern agricultural development seen elsewhere in southern England, resulting in the core of the area remaining as a unique snap-shot of the ancient Wessex downland as it might have appeared centuries ago. The integrity and size of the Range (approximately 1600 ha of which 1277 ha are designated 5551), and diversity of habitats appear to provide optimum conditions for a large number of species which are uncommon or are found in lower numbers over the rest of southern Britain. Even among the wonderfully rich wildlife sites owned by the MOD, Porton Down is considered the 'Jewel in the Crown'.
The ECN Site The ECN site is the whole of the SSSI, of which approximately 1000 ha is semi-natural chalk grassland with successional scrub. The area lies on the southern edge of Salisbury Plain. It is underlain by Upper (Cretaceous) Chalk with a few patches of clay-with-flints on high ground. The soils are therefore almost entirely free-draining and the site is considered to be a dry one for ECN purposes. The vegetation has only recently been well documented, with two detailed surveys, one in the early 1970s and one in the early 1990s. Most of the grassland can be assigned to the National Vegetation Classification calcareous grassland CG types with CG2 (Festuca ovina-Avenula pratensis), CG3 (Bromus erectus), CG6 (Avenula pubescens) and CG7 (F.ovina-Hieracium pilosellaThymus spp) the most common. Some very rare subtypes of CG7 are present, only elsewhere found in the East Anglian Brecklands. This is due to the intensive disturbance by the high rabbit population which is effectively allowed to manage the grassland through close grazing.
A Porton vista with rabbit grazed grassland in the foreground and open Juniper scrub in the background Elsewhere, there are areas of calcareous heath, scrub and secondary and ancient woodland. The sampling site (SS) is fairly typical of the grassland habitat, comprising mostly open or lightly scrubbed (with Juniper Juniper communis) cropped grassland, but with some small patches of Dogwood, Hawthorn and Buckthorn scrub. As with the rest of the Range, ant hills of the Yellow Meadow-Ant Lasius flavus, are abundant here. The Target Sampling Site (TSS) is situated in the slightly more scrubby part of the area.
ECN Measurements Most of the ECN monitoring is carried out on and around the TSS in an area called Roche Court Down. Although this area has a history of being cultivated during the past few centuries (as was much of the rest of the Range at times), the proximity of large areas of untouched ancient grassland enabled this area to quickly re-establish its biodiversity. The wide range of abundant food plants, for example, results in the largest count of butterfly species over the entire ECN with some national rarities such as Silver-spotted Skipper commonly recorded. In 1995, thirty-three butterfly species were recorded, the second commonest being Dark Green Fritiliary!
Coarse-grain vegetation survey work on semi-natural grassland The bird monitoring is interesting more for the unusual species than abundant ones. The Stone Curlew has one of its strongest outposts at Porton where between ten and twelve per cent of the total UK population regularly breed. The Common Birds Census (CBC) plot, covering 153 ha of grassland and scrub, held two pairs in 1996, while Yellowhammer, Skylark and Green Woodpecker are present in good numbers in their natural habitat. Most CBC plots with similar species to Porton are farmland or other marginal habitats for these species. This is also one of few CBC plots which regularly has the nationally declining Tree Pipit breeding. Other biological monitoring protocols which have raised much interest include the Carabid beetles. As with other groups, the Porton beetle fauna is unique in that species found among similar habitats elsewhere are relatively uncommon, but one in particular, the orange-spotted Panageus bipustulatus sometimes abundant. This discovery caused great stirrings among our carabidophile friends at Rothamsted and elsewhere. It is the rabbit monitoring protocol which has highlighted Porton among the other ECN sites. Rabbits are super-abundant on the chalk grasslands here and monitoring has shown an increase of over 200 per cent since 1994 according to spring counts. This is the period following the last major outbreak of Myxamotosis at Porton. However, it is the sheer number of indicators (faecal pellets) which are present which is astonishing. The autumn 1996 count was almost 23,000 faecal pellets over the two transects, with probably ten times that number having to be cleared in the first place. There must be a market for ECN Environmentally Friendly Fertiliser! With the prevailing wind coming from the sea and rural south-west, it would be expected that the Porton site would not experience elevated pollution levels. Indeed, nitrogen dioxide levels appear to be in midrange compared to the other ECN sites, and it remains to be seen whether other factors, such as phosphate deposition, are relatively important here. The Porton ECN site is very exposed and experiences a wide range of climatic vagaries from severe winter frosts to searing summer heat. It is this type of stress which may prove to be a more important factor in determining biological species abundance and community composition than constant but moderate pollution levels.
Research at Porton CBD have recognised the importance of associated research projects alongside the ECN at Porton. The availability of such a large out-door semi-natural laboratory is an unusual resource in the lowlands of Britain these days, particularly one where natural processes are allowed to run unchecked to a large degree. Two doctoral research projects are currently in progress, one looking at the relationship between butterfly abundance and habitat availabilty, the other looking at Orchid mycorrhizae as a factor determining their abundance. Both projects will hopefully have an important input into future practical management planning where optimisation of populations is the target. Other research, particularly relating to Porton's most important groups (such as Stone Curlews and rabbits) are likely to be implemented in the near future.
Clive Bealey, |
