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The Pevensey Levels Population
The Pevensey Levels comprise one of the largest continuous lowland wet grazing systems in south-east England. Although over 500ha have been converted to arable, 3500ha of low-lying grazing meadow remain, intersected by a complex network of ditches. The aquatic communities supported by the ditches include 68% of British aquatic plant species and many nationally rare invertebrates. The national and international importance of the area is reflected in its designation as an SSSI and RAMSAR site. Some of the best habitat (181.6 ha) is protected as a National Nature Reserve (NNR), 130.2 ha of which is managed by the Sussex Wildlife Trust (SWT). Although the Pevensey habitat is very different from that at Redgrave and Lopham Fen they share characteristics common to western European D. plantarius sites. Both are lowland wetlands fed by neutral to alkaline, base-rich water: the structure of the vegetation is more important than its species composition (Smith 2000). The requirement for stiff-leaved, emergent vegetation to support the nursery webs (Biology), which is met primarily by Cladium mariscus at Redgrave and Lopham Fen, is met by a range of emergent marginal species, such as Carex pseudocyperus, as well as by the stiff leaves of the floating rosettes of Stratiotes aloides on the Pevensey Levels (Figs. 1 and 2).
|Fig. 1 Ideal habitat on the Pevensey Levels - field with summer-wet scrapes and ditches with Stratiotes aloides.||Fig. 2 The wider drains (sewers) on the Levels are also suitable for D. plantarius. Cows are always best on the other side of the ditch!||Fig. 3 Pevensey Levels ditch colonised by the alien Hydrocotyle ranunculoides|
Following the discovery of D. plantarius in 1988, English Nature commissioned a large-scale survey of its range and abundance in 1990 (Jones 1992). This survey, of ca 90km of ditches throughout the SSSI and in adjacent areas, revealed a very extensive population. In the areas of best habitat, primarily on gravity-drained marshes, densities were often very high: nursery web densities averaged one per 2m of bank. Away from this core area, however, the pump-drained marshes supported a much lower population density and the population appeared to be very fragmented. On the basis of nursery web counts, the total adult female population was estimated to be in the order of 3000 at a time when the Redgrave and Lopham Fen population was thought to have, at best, only tens of adult females.
Although the Pevensey population was much larger than that at Redgrave and Lopham Fen in 1990, it is widely considered that habitat quality on the Levels has declined during the last fifty years. Drainage schemes, implemented in the 1960s and 1970s substantially reduced water levels and allowed further conversion of pasture to arable. Water quality has also declined and some of the main channels have become eutrophic. A further threat to the rich species assemblages of the ditches has come from dramatic increases in the populations of alien water plants, including Azolla filiculoides, Crassula helmsii and, most recently, Hydrocotyle ranunculoides (Fig. 3).
Since the early 1990s, efforts by the Environment Agency, Natural England, the Sussex Wildlife Trust and local landowners, have resulted in measures to address these problems. These have resulted in major changes in water level and land management on the Levels. A Wildlife Enhancement Scheme (WES) on the SSSI has provided landowners with tiered financial incentives for environmentally sensitive practices, including water level and ditch management, reduced stocking rates and restrictions on agrochemical use. More recently, the introduction of the Environmental Stewardship Schemes have provided further opportunities for landowners throughout the area to receive financial incentives for management practices that benefit wetland wildlife. Improved water level management has been made possible largely by the modification of existing sluices and construction new ones, together with better management of abstraction for public water supply.
To assess the response of D. plantarius to the changes in management on the Pevensey Levels, and to monitor changes in the extent and status of the population, Natural England are developing a long-term population monitoring programme for the area. In the medium term, sea level rise must be seen as a potenital threat to the Pevensey population, the greater part of which is on the lowest-lying Levels that were reclaimed from salt marsh in the Middle Ages.