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The UK translocation programme
Translocation programme summary
Between 2010 and 2015, around 6,000 hand-reared, three-month old fen raft spiderlings and 56 adult females with their nurseries containing hundreds of week-old spiderlings, were released at new sites in the Norfolk and Suffolk Broads. These nurseries contributed in the regional of 30,000 spiderlings although their survival chances were much lower than those of the older, hand reared spiders.
The timetable of the releases, and a summary of the numbers of nurseries recorded in the new populations by 2015 is given in the following table - the rationale for the work and details of the translocation programme are given below.
|Site||Owner/manager1||Introduced in:||2015 range2||
2015 minimum estimate
|Castle Marshes||SWT||2010-'11||>3km ditch||800|
|Carlton Marshes||SWT||2011-'12||ca 1km ditch||(140 in 2014, no equiv. est. in 2015)|
|Mid-Yare Marshes||RSPB||2012-'13||ca 1.1km ditch||480|
|Ludham Marshes||NE||2014-'15||0.35km||introductions only|
1SWT - Suffolk Wildlife Trust, RSPB - Royal Society for the protection of Birds, NE - Natural England
2 current length of ditch occupied; the spiders were introduced to a maximum of 0.5km of ditch at each site
D. plantarius is intrinsically very vulnerable to extinction because it is restricted to only three sites, at one of which its popoulation is extremely small. Its Species Action Plan, first produced in 1999 and revised in 2005 and 2008, seeks to address this issue with the following ambitious targets for its future status:
- an increase the overall range of D. plantarius at Redgrave & Lopham Fen to 13 Ha of habitat occupied 3 years in 5 by 2010 and to 65 Ha by 2020
- the number of sites with sustainable populations of D. plantarius should be increased by six by 2010. By 2020 the total number of sites with sustainable populations should be increased to 12.
Despite an increase in range at Redgrave and Lopham Fen since 2006 it become clear that both of these targets would have to be met by deliberate translocation to new sites - both within the Redgrave and Lopham Fen nature reserve complex and more widely in both East Anglia and Southern England. Research evidence (i) that D. plantarius has a low propensity to disperse (Pearson 2008) and (ii) that there had been a significant decline in genetic diversity in the Redgrave and Lopham Fen population since 1992, suggested that establishment of new populations was likely to be essential to ensure the long-term future of this species in the UK. Natural England, the lead organisation for the Action Plan for this species, in consultation with the project Steering Group (Suffolk and Sussex Wildlife Trusts, the Broads Authority, RSPB, the British Arachnological Society, Buglife and Nottingham University), committed to a translocation programme if it could be established that it could satisfactorily meet the criteria for translocation laid down in guidelines produced by the IUCN, JNCC and Invertebrate Link.
Preparation for the translocation programme
During 2008 and 2009 British Arachnological Society experts surveyed potentially suitable habitat to ensure that our current understanding of the UK distribution of D. plantarius was correct and that major populations were unlikely to have been overlooked or misidentified. Their work was augmented by appeals for Dolomedes sightings through articles in specialist magazines, a Broads Authority identification card issued to those working in the wetlands of the Norfolk and Suffolk Broads, and through this website. New populations of Dolomedes fimbriatus but not of D. plantarius were discovered.
Over the same period, techniques for rearing newly-hatched D. plantarius in captivity were tested at the John Innes Centre Insectory, Norwich. Because the D. plantarius population at Redgrave and Lopham fen was so small, removal of spiders for translocation was not considered to be a viable option. However, this work, and subsequent captive rearing trials, showed that capture of adults females with egg sacs, and rearing of their progeny for the first few months of life, yeilds survival rates often in excess of 90% over a period when survival in the wild is almost certainly very low. This meant that the source population could be reinforced at the same time as a new population is established, by the release of both a proportion of the reared spiderlings and of the captive adult females when they produce second broods. Successful production of second broods in the wild is uncommon in the Redgrave and Lopham Fen population but much commoner amongst captive females. Return of the females to the wild just after their brood emerged from the egg sac decreased the probability of both mother and egg sac succumbing to predation.
In spring the project Steering Group reviewed the case for translocation and concluded that translocations of D. plantarius should go ahead in England subject to establishing the most appropriate provenance of stock for new populations. As yet, there is no translocation programme in Wales, where NRW (then CCW) wish to undertake more survey work before making any decision on whether translocation is necessary or appropriate.
An experimental approach was taken to determining the most appropriate provenance of D. plantarius for use at new sites - from one or other of the two English populations, or from both sites. Although combining these two long-isolated populations to increase genetic diversity by might be expected to result in offspring better able to adapt to their new environment and to future environmental change (hybrid vigour), there are circumstances in which such hybrids can be less successful (outbreeding depression). To ensure that populations established by deliberate translocation would stand the best possible chance of long-term success, an experiment was undertaken in 2010 to compare courtship and mating behaviour, fecundity, and survival and growth of progeny in crosses between and within the two English D. plantarius populations. Sub-adult spiders collected under license from the Sussex Wildlife Trust's nature reserve on the Pevensey Levels and the Suffolk Wildlife Trust's Redgrave & Lopham Fen nature reserve were matured in captivity. Once adult, within and between population matings were set up in an artificial arena in which all aspects of courtship behaviour could be observed and recorded. Fecundity, and the growth and survival of the resulting progeny over a 15 week period, were measured. No mating barriers between the populations were detected and subsequent rearing of the resulting offspring - from 18 successful parings - showed that the hybrids were as successful as the within-populations crosses.
Translocations in 2010
Because of this positive result, all of the spiderlings produced by this experiment were used to establish the UK's first new populations of this species, in October 2010. The Steering Group for the project approved release of both the between population crosses, and the spiderlings of Pevensey Levels and Redgrave & Lopham Fen origin, at the Suffolk Wildlife Trust's Castle Marshes reserve. This internationally important area of grazing marshes is on the River Waveney, downstream from Redgrave & Lopham Fen at the source of the river. It is highly likely that this species occurred in this area in the past but was lost when the quality of the ditches was degraded by conversion of the marshes to arable agriculture. The ditches have now been restored and support a rich assemblage of invertebrates and plants, very similar to those on the Pevensey Levels, the spider's stronghold on the south coast of England. The area is, in many respects, very much more similar to the Pevensey Levels than to Redgrave and Lopham Fen.
In addition to releasing around 2800 spiderlings at Castle Marshes, over 1900 spiderlings of single-site provenance were returned to their natal populations on the the Pevensey Levels and at Redgrave and Lopham Fen. At the latter site they were used not only to boost numbers in the two small areas to which they had become confined, but also to create a new, third focus for the population within the nature reserve. Although habitat restoration by the Suffolk Wildlife Trust at this site over the last decade has been very successful, the spiders have so far failed to recolonise ideal new areas. Translocating them within the reserve, in addition to creating chains of new turf ponds and scrapes linking the sub-populations, was expected to aid recolonisation of extensive areas of the reserve. In future years, the possibility of boosting depleted genetic diversity at Redgrave and Lopham Fen by introducing spiders from the Pevensey Levels will be evaluated.
Spider-rearing in a Suffolk kitchen....
UK media attention was briefly diverted from the gloom of the UK Government's Comprehensive Spending Review in October 2010 by the story of the spiderling releases at Castle Marshes and Redgrave and Lopham Fen. National as well as local radio channels and newspapers picked up this good news story in the International Year of Biodiversity. The fact that the spiderlings parents had been mated, and most of the spiderlings hatched and reared in a domestic kitchen added human interest to the story! Although some of the spiderlings were reared in a conventional laboratory setting at the Insectory at the John Innes Centre in Norwich most lived in my kitchen where they were reared them individual test tubes (to avoid cannibalism) and fed them on an abundant supply of fruitflies from the garden compost bins and lesser dung flies, netted from fresh pony dung!
The news story travelled around the world, not only via the BBC and national newspaper websites, but also reaching newspapers from Iceland, to the Netherlands and the USA. It's good to know that small spiders can make big news! Click on the links to read the Press releases issued by
Suffolk Wildlife Trust.
View coverage on the BBC website.
Read some of the newspaper coverage of the story:
The Daily Telegraph
The Daily Mail
The East Anglian Daily Times
The Eastern Daily Press
The Diss Mercury
The Diss Express
The Lowestoft Journal
and from Iceland...
Translocations in 2011
The D. plantarius translocation programme continued in East Anglia in 2011 with the establishment of a further new population at Suffolk Wildlife Trust's Carlton Marshes reserve on the lower Waveney and a second new focus of population at Redgrave & Lopham Fen. In addition, a planned, second introduction of spiderlings was made to the 2010 sites; the spiders at Redgrave & Lopham Fen appeared to have a two year life cycle and so this was needed to give the new populations a natural age structure. Mixed Redgrave & Lopham Fen and Pevensey Levels provenance was, once again, used at the lower Waveney sites while only local provenance was used within Redgrave & Lopham Fen.
In 2011 the spiderlings used for the releases were the progeny of gravid, wild caught females. Those from the small Redgrave & Lopham Fen population were once again reared in captivity for three months, ensuring high survival rates, before release in the autumn. Their mothers were retained in captivity until their second broods hatched and were then released back at their site of cature to augment their natal population. In contrast to 2010, however, the adult females from the much larger Pevensey Levels population were not returned to their natal areas but were released, together with their first nurseries, at the new sites in July and August. These females built their nursery webs in aquaria made from 5l plastic water bottles. Once the spiderlings were ca five days old - the age at which they would normally start to disperse from the nursery - the 'bottled' nurseries, together with their guarding mothers, were taken to their new homes. They were anchored in suitable vegetation and opened to allow the spiders to escape. Several of the mothers of these broods went on to produce successful second broods in their new homes.
Another new departure for 2011 was the very welcome help of four UK zoos, all members of the British & Irish Association of Zoos and Aquaria (BIAZA), in rearing the Redgrave & Lopham Fen test tubes spiderlings. Ian Hughes of Dudley Zoo, Karen Entwhistle of Chester Zoo, Keith Russell of Chessington World of Adventure and Caroline Howard of Lakeland Wildlife Oasis all became foster parents to many tiny spiderlings and suffered all longs hours, anxieties and rewards of this very unusual job! This new partnership with the BIAZA zoos brought huge benefits in terms, not only of the numbers of spiderlings that could be reared, but also in expertise in developing the captive rearing methods. It also brought opportunities to raise awareness of the plight of this species, and its wetland habitats, with a huge new audience.
Translocations in 2012
Another year and another new population founded, this time further afield in the Broads catchment, on the RSPB's extensive Mid-Yare reserves complex east of Norwich. At the same time, second introductions were made to the new 2011 sites at Carlton Marshes and Redgrave & Lopham Fen.
Translocation in 2013
This year the second introduction was made to the Mid Yare site.
Translocation in 2014 and 2015
A fourth new Broadland population was established in 2014 and 2015 - even further afield from Redgrave & Lopham Fen, on the Ludham-Potter Heigham National Nature Reserve on the Thurne grazing marshes. A major change in the methodology was made possible by the success of the first new population, at Castle Marshes. Densities in this population were so high that it was clear that small numbers of adult females with egg sacs could be harvested without impact. The spiders introduced to Ludham-Potter Heigham were Castle Marshes adults females with their 'bottled' nurseries. This change brought an end to the captive rearing programme and a considerable time and cost saving. It also ended the need to collect any spiders from the natural populations.
Monitoring the new populations
Monitoring is integral to the translocation process. Counts of nursery webs are used to assess the success of the new populations and to inform modification of the translocation methods - such as that implemented at Ludham-Potter Heigham in 2014. At present monitoring provides measures of the range and densities of the populations but moulted skins are also collected to allow eventual assessment of their changing genetic makeup, including the relative contributions of the two parental populations. This will provide a guide to the need to augment the new populations with additional stock.