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Redgrave and Lopham Fen
Eric Duffey's first record of D. plantarius at Redgrave and Lopham Fen was from an area of mixed fen vegetation including Schoenus nigricans (Black Bog-rush) and Cladium mariscus (Great Fen-sedge), as well as some Phragmites australis (Common Reed) around the many turf ponds (Duffey 1958). The spiders have subsequently been found to be largely restricted to areas of pools with tussock-forming emergent and marginal vegetation, in open, unshaded areas. They are rarely found in dense stands of P. australis. Much of the fen surface was pitted with, often deep, turf ponds created by the digging of peat for domestic fuel - a practice that probably continued until the 1930s. The dominant vegetation over much of the fen was Cladium mariscus which was traditionally cut on a 4-5 year rotation to provide material for thatching roof ridges.
Four years after the discovery of D. plantarius, an artesian borehole was sunk close to the edge of the fen. Removal of 3,500 tonnes of water a day from the underlying chalk aquifer over the following four decades, exacerbated by droughts in the 1970s, '80s and '90s, progressively dried out and degraded the site. The original copious supply of base-rich, nutrient-poor artesian groundwater arising from marginal springs and seepage lines was cut-off. It was replaced by a hydrological regime controlled by rainfall patterns and inflow of eutrophic water from a catchment dominated by intensive pig and poultry farming. Profound changes in the habitat resulted from the desiccation of the fen, most conspicuously the invasion by scrub of many formerly open, wet areas. Internationally rare plant assemblages were degraded or lost, together with many rare species of plants and invertebrates. D. plantarius is one the fen's nationally rare species to survive this period.
As the fen dried out, and its habitats changed, D. plantarius became progressively more restricted in range. Although no systematic survey of the extent of the population was carried out, information on the decline can be derived from collation of casual records and comparison with the area of the fen that must formerly have supported suitable habitat. These records show that, by the mid 1990s, the population had been lost from 30% of the area from which it had formerly been recorded. Comparison with the area over which it is likely to have occurred in the more distant past suggests an 85% decline. By the late 1980s, the population had become confined to two small areas of the fen, separated by a distance of ca 0.4km and a wide belt of intervening woodland. Both of these areas were dominated by C. mariscus, growing around the margins of deep turf ponds. The depth of these turf ponds, enabling them to hold water even in dry summers, must have been a major factor in the survival of the spiders as the fen suring this period.
Artesian abstraction finally ended in July 1999, at the end of a 5-year EU-funded programme to restore the fen. During this period large areas of scrub were removed, oxidized and enriched surface peat was stripped off the fen surface and an extensive grazing regime introduced. Peat removal was mostly to a depth of 50cm, at which nutrient levels remained low enough to support typical fen vegetation. Relocation of the bore-hole at the end of the restoration programme co-incided with a series of wet seasons and resulted in rapid hydrological recovery.
Recovery of the fen's biodiversity has been a much slower process. Common reed Phragmites australis initially became dominant in and around the new scrapes, with C. mariscus only starting to reappear in many of these areas ca eight years after the hydrological restoration. This was probably because high nutrient levels were maintained for a protracted period by water flushing through the remaining oxidised and degraded surface peat.
As the fen became conspicuously drier during the 1970s, concern grew about the fate of the D. plantarius population. Additional, deep ponds were hand-dug in the late 1960s and more were excavated mechanically during the late 1970s (Duffey 1977) and 1980s (Duffey 1991) to ensure a summer water supply for the spiders. Despite this, the droughts of the late 1980s left very little standing water on the fen and D. plantarius became confined to two, small, isolated areas of deep turf ponds, on Middle Fen and Little Fen. In the drought of 1976 Eric Duffey failed to find any spiders and feared that the population had been lost. He re-found them again in small numbers in the following year and increased the political pressure for action to save them.
In 1991, English Nature (now Natural England) initiated a Species Recovery Programme project which aimed to prevent extinction of the residual population; significant expansion was not a realistic objective while the fen remained dry. Systematic monitoring and positive habitat management measures were established. The latter included re-instating cutting of C. mariscus on a traditional rotation, removal of encroaching scrub and the excavation of additional turf ponds and deepening of existing ones. However the most radical measure, and probably the most important factor ensuring the survival of the population during this period (Smith 2000), was the introduction of an irrigation supply, piped from the borehole to the core of the spider population.
When artesian abstraction ended in 1999, much more ambitious targets were set for the recovery programme. The very high potential fecundity of the spiders led to an expectation that the population might recover very rapidly.
Systematic monitoring of the D. plantarius population, undertaken between 1991 and 2013 within and just beyond the two core areas for the spiders, showed no evidence of any sustained or significant recovery in either sub-population since the hydrological recovery. Population trends over this 22-year period showed significant variation between years and between the two sub-populations, but no evidence of any long-term trend (Fig. 1).
Fig. 1 Annual population indices for the two residual sub-populations of D. plantariuson Redgrave and Lopham Fen NNR, in July 1991-2013. Data generated by a log-linear Poisson regression model plotted on a linear scale. 2SEs shown as positive bars for the sub-population on Middle Fen and negative bars for that on Little Fen.
From 2006 until 2010, despite the lack of increase in the population index, the range of the Middle Fen sub-population at Redgrave and Lopham Fen started to expand along a chain of turf ponds leading west from the core area. By 2010 the spiders had reached a series of ponds 300m away, on which they were last recorded in 1985. This spread is thought to have been facilitated by a combination of substantially improvement in the quality of vegetation in the area, together with a relatively long series of summers in which these pools have not dried out. However, in 2011 spiders were found on only one turf pond within this area and even by 2015, sightings were sporadic; the fate of this new new 'spur' of the Middle Fen population remains uncertain.
Evidence from this and other areas of the Fen that the population was starting to increase in extent, if not if density, led to a change in the monitoring method in 2014. Instead of counting spiders in a sub-set of turf ponds, regular counts were made of nursery webs along transects over a much wider area of the fen, throughout the breeding season.
This change in methods was also necessitated by the need to monitor two additional areas of the fen to which spiders spiderlings were introduced in 2010-12. Research evidence that D. plantarius has poor dispersal ability, together with the demonstration that genetic diversity at Redgrave & Lopham Fen has declined rapidly, suggested, first, that natural recolonisation of the fen would be an extremely slow process and, secondly, that the window of opportunity for this population is small. Because of this, and following a long process of evaluation, Natural England and the project Steering Group decided that deliberate introductions of the spiders should be made to areas of restored habitat within the fen complex. This work was carried out between 2010 and 2012 using captive-reared spiderlings from the Fen population. For more information on the translocation programme, click here.
The methodology for the new monitoring programme is still being developed - the gain in spatial information is inevitably accompanied by a loss in sensitivity to population changes between years. In 2015 it showed that the new population established by translocation on Great Fen was well established and that the spider's range on Little Fen was increasing.
Many factors may have contributed to the extremely slow progress of the Redgave and Lopham Fen spider population towards recovery since 1999. These include:
- A reduction in area of suitable habitat. Phragmites australis, which is avoided by the spiders, colonised both the margins of the flooded scrapes created by peat stripping and invaded areas of Cladium mariscus. However, the areas of apparently suitable habitat remained that were uncolonised had or had very low densities of spiders.
- Deep inundation of the core spider areas with stagnant water, often throughout the year, as a result of impeded drainage following the restoration operation.
- Eutrophication of the water supply to the fen. The relative contributions from the surrounding arable catchment and from nutrient release from the degraded surface peat are not known.
- Genetic problems resulting from genetic drift or from tight bottlenecks in the size of the spider population.
- A lack of either ability or propensity of the spiders to disperse.
- A food supply that is relatively poor in comparison with grazing marsh habitats, and results in a great average time to maturation.
- A low tendency to produce second broods relative to that in grazing marsh habitats.
The problems of the Redgrave and Lopham Fen population are being addressed through a combination of research on the spiders themselves, on the hydrology of the fen and on vegetation management. Studentships at the University of East Anglia investigated both the genetics of the spiders and aspects of their autecology. The genetics work is on-going and now based at the University of Nottingham. At the same time, work continues to understand and improve both control of the complex hydrology of the fen, and the management of vegetation to increase the area of internationally rare fenland associations. This includes the Cladium mariscus swamp favoured by the spiders.
A cause for optimism about the future of the D. plantarius population is the colonisation by Cladium mariscus of the fringes of many of the large water bodies created as part of the restoration operation. 15 years after restoration the fen's hydrology, C. mariscus is becoming increasingly dominant. At the same time Phragmites australis, the initial dominant, is becoming less vigorous. This progression will result in a substantial increase in the area of the fen that could potentially support D. plantarius.
One impediment to the spread of spiders from their two residual sub-populations is the availability of deep turf ponds that hold water throughout dry summers: protracted droughts remain a severe threa to this population. New chains of ponds, linking these areas with other areas of suitable habitat including the recently-created larger water bodies, is likely to be the only effective means of promoting natural increase in range and restoring gene flow between the sub-populations. Work on a rolling programme of pond creation began in 2009 and 2010 under the Suffolk Wildlife Trust's Higher Level Stewardship Scheme and still continues.
It seems increasingly likely that the Redgrave and Lopham Fen habitats can support a relatively low density population of this species; the very high densities seen in grazing marsh habitats are unlikely ever to be attained here. Nevertheless, as the area of suitable habitat on the fen increases, and if summer water levels and surface-wet linkages can be maintained, it shoud be possible to achieve a very substantial increase in the range of the spiders and to have a single, functional metapopulation rather than a series of isolated sub-populations.
To download summary reports on the status of Dolomedes plantarius at Redgrave and Lopham Fen in 2006 click here (764 KB), in 2007 click here (754 KB), in 2008 click here (1.1 KB), in 2009 click here (1.07 KB) and in 2010 click here (1.2 KB), in 2011 click here, in 2012 click here and in 2013 click here. Earlier reports on the status of D. plantarius at Redgrave and Lopham Fen NNR are published as English Nature Research Reports and can be obtained from Natural England (Enquiries, Natural England, 3rd Floor, Touthill Close, City Road, Peterborough PE1 1XN Tel: 0845 600 3078 or by visiting this link.