Identification

The White-faced Darter is a small dark libellulid, which as the name suggests has a pale creamy white frons. The male has a narrow black abdomen, which is marked with red and orange on the dorsal and lateral segments. The thorax is also black with faint red antehumal stripes and lateral markings¹ (Fig. 1). These markings often darken with age and in flight the male can appear almost completely black². Consequently, the white-faced darter can be confused with the Black Darter. However, the flight periods of these two species usually only overlap for a very short period, making identification easier. Furthermore, the white frons is very distinctive, but specimens should be examined in the hand to confirm a positive identification³. Females and teneral males are also predominantly black with a white frons, but have pale yellow thoracic and abdominal markings (Fig. 2).

Larvae are typically libellulid. They are easily distinguishable from other Darter larvae, by the presence of dark stripes on the underside of the abdomen².

Status

In Britain the White-faced Darter is a rare dragonfly with a disjunct distribution¹. It has declined, notably in England, in the last 35 years and it is now present at only half of the localities at which it occurred in the mid 20th Century⁴. Currently there are only five breeding sites in England¹ and fears for the future of this species have led to its inclusion in the Biodiversity Action Plans of Cheshire and Cumbria⁴. It is also afforded protection under the Wildlife and Countryside Act, 1981.

Distribution

L. dubia is found throughout northern Europe and east to Siberia. In Britain it is essentially a northern species and at the southerly limits of its range it is restricted to mountain areas such as the Pyrenees and Alps¹.

In Britain this scarce species is found at isolated sites from the Midlands to north Scotland. Historically it occurs as far south as Surrey, but this population now appears to be extinct. Consequently, Chartley Moss is the most southerly distribution in the UK⁵. Major strongholds for the species occur in the highlands of Scotland. The populations in both Inverness-shire and Ross-shire are particularly important, as the species appear to have declined in Perthshire and been lost from Argyllshire.

Distribution map from the National Biodiversity Network Gateway website.

Ecology and Habitat Requirements

The White Faced Darter is a species of lowland peatbogs. It requires relatively deep, oligotrophic, acidic bog pools with considerable rafts of Sphagnum at the edges^{1, 3, 4} in which to breed (fig. 3). Larvae also occur among waterlogged Sphagnum in depressions devoid of standing water³. The larvae live within the matrix of submerged and floating sphagnum which offers them support and shelter⁶ and are confined to waters without fish^{1, 7}. Despite the historic belief that the larvae of the White-faced Darter require acidic water in order to survive, research indicates that acidity is not as important as the presence of fish. Fish are unable to survive in acidic waters and thus bog habitats do not support them thereby removing the predation pressure upon the larvae^7,8. This is important because the larvae actively hunt during the day and are very vulnerable to predation.

L. dubia is a territorial species, particularly near water, and the male holds a relatively small territory^{1, 3}. Females are seldom found near water, preferring to bask on open ground or perch amongst the surrounding vegetation. Adults of both sexes roost in trees and bushes that can be up to 50m away from the nearest pool⁹ thus avoiding the low nocturnal temperatures⁶. The White-faced Darter is adapted to the cold climate of the bog by its dark colouration, which enables it to achieve rapid warming^6,10. On leaving these roosting sites individuals prefer to settle on a pale background, basking on birch logs and bare ground, which speeds up the warming of the body through extra radiation¹¹. In these situations the males are less territorial and tolerate each other probably because all sexual behaviour takes place near water. Copulation is a relatively short affair, taking place amongst low bushes or heather¹. The females then oviposit alone, flying low over suitable pools and dropping the eggs onto the waterlogged Sphagnum moss¹ or among the stems of cotton grass^3,5,9. For egg laying to be elicited the Sphagnum needs to be covered by standing water¹² and studies also indicate that the bright reflections from the exposed water around the floating moss within pools appear to be important in selecting suitable breeding sites³.

The larvae generally take 2 years to develop, although some individuals complete their life cycle in less than a year⁸. Emergence usually begins early in May, and takes place between early and mid morning. The larvae use a variety of supports such as the stems of cotton grass, heathers and rushes and a number of exuviae are often found on the same stem (Fig. 4).

Larvae leaving bog pools in Britain emerge on the northwest margin where they are warmed by the direct rays of early morning sun⁹. Emergence is successful under a variety of weather conditions and can occur on overcast days with low air temperatures⁹ and light rain¹¹. However, calm weather seems to be a prerequisite as "larva appear to be reluctant to leave the water on windy days"⁸. The minimum night temperature seems to affect emergence as much as sunshine and individuals can arrest metamorphosis temporarily if adverse conditions occur, such as low temperatures⁸. Observations indicate that tenerals tend to fly to the nearest suitable scrub or woodland³.

Summary of Habitat Requirements

The White Faced Darter requires oligotrophic bog pools with a considerable, semi-submerged Sphagnum raft. Emergent vegetation, such as cotton grass or rushes provides emergence supports and resting sites. Away from its aquatic habitat it also requires scrub or woodland, which provides important roosting and feeding sites.

Current Threats

• Habitat Destruction and Fragmentation: - approximately 95% of the original lowland raised bogs have been destroyed in Britain, limiting the availability of habitat for the White-faced Darter. Industrial scale peat extraction, agriculture reclamation, drainage, and afforestation have significantly reduced the number of breeding sites in the last 40 years⁴.
  
  
• Removal of Sphagnum Moss: - In the past breeding sites have been threatened by the commercial collection of moss from bog pools for the horticultural trade. Not only does this practice make the habitat untenable, but also removes the larvae, which live within the Sphagnum moss³. Protection under the Wildlife and Countryside Act helps to guard against this.
  
  
• Succession: - The transient nature of wetlands and bog pools means that suitable habitats are threatened and lost through encroachment of scrub and trees. Seral changes are also intrinsically linked with drying of the habitat.
  
  
• Altered Hydrology: - Changes in hydrology, which lead to lowering of the water table and drying of the habitat, have serious implications for the White-faced Darter, destroying and further fragmenting the habitat. Over abstraction of water may pose a serious threat, particularly as the species has a two year development period. Any drying of the habitat may exterminate the population¹.
  
  
• Pollution: - Changes in water chemistry and pH are likely to be detrimental to the White-faced Darter. Historically it was thought that acidity was a prerequisite for the species, but research now suggests that it is not as important as predation by fish⁷. In the 1980's liming of Lake Gårdsjön elevated the pH from 4.7 to 7-8 and despite the change in water chemistry L. dubia were still recorded 5 years after liming⁷, thereby supporting the hypothesis. Furthermore, most bog pools are too acidic to support fish. Changes in pH, are likely to have an indirect impact on the White-faced Darter by altering the habitat. At Black Lake, a Cheshire Wildlife Trust Nature Reserve, a Limestone path was constructed around the lake and it appears that leachate has neutralised the water. This has killed the Sphagnum moss removing the White-faced Darters habitat¹³.
  
  
• Eutrophication: - The runoff from agriculture is another area of concern. Nutrient enrichment caused by leaching of agricultural fertilisers may be deleterious to this species, causing changes to the vegetation structure depending on the extent of the problem.
  
  
• Climate Change: - As a holarctic species, the White-faced Darter is threatened by the prospect of climate change. The species may already be retreating northwards in response to a warming climate and this may partially explain the demise of the White-faced Darter at Thursely Common in Surrey.
  
  
• Predation: - research suggests that predation by fish may be a factor limiting the suitability of habitats for the White-faced Darter⁷. L. dubia is more active during daylight periods and has a lesser tendency to hide in the bottom debris compared to other dragonfly larvae^{7, 14}. This behaviour makes nymphs conspicuous and susceptible to fish predation. In addition, research suggests that L. dubia does not have an anti-predator reaction, generally trying to escape fish attacks rather than feigning death as witnessed in other libellulid species. Consequently, the presence of fish seems likely to be one of the factors limiting the distribution of the White-faced Darter.
  
  
• Impact of Visitor Pressure: - At some sites, damage to the habitat through visitor pressure is thought to be a particular problem that may need to be addressed¹³.

Management

General Management Guidelines

• Maintenance of the Lowland Peatland Habitat: - As a result of the rarity of lowland Peatlands in Britain, the maintenance and protection of this habitat for a variety of different species is very important, including a number of dragonfly species. Without careful management, further habitat loss and fragmentation could seriously threaten the future of the White-faced Darter.
  
  
• Control of Scrub Encroachment: - Although woods provide roosting and feeding areas for the White-faced Darter, active management is required to ensure that scrub encroachment is controlled, invasion of birch and conifers is prevented, and the open peatland habitat is maintained at existing and potential breeding sites.
  
  
• Maintenance of Water Quality - Water quality should be maintained to ensure that pollution and eutrophication do not threaten the White-faced Darter.
  
  
• Maintaining Water Levels: - The control of scrub encroachment will help prevent drying of the habitat, but any activity that results in reduced water levels throughout lowland Peatland habitats should also be avoided. These diverse wetlands are reliant on their water supply. Improved knowledge regarding the hydrology of inhabited sites might enable improved management at adjacent sites.

Best Management Practice

• Control of Scrub Encroachment Scrub and trees should be cut back from areas where it is encroaching on to open peatland. Once cut the timber should be removed to prevent damage or eutrophication of the fragile habitat.
  
  Birch scrub is more difficult to remove simply by cutting as tree can quickly regenerate from remaining stumps. As a consequence it may be necessary to use an herbicide, such as glyphosate. This must be undertaken with care, using the relevant guidelines.
  
  
• Management of the Aquatic Vegetation: - Management of the aquatic vegetation is important, as bog pools will ultimately be lost through successional processes.
  
  White-faced Darter nymphs appear to survive in pools at different successional stages, including those that are completely covered by submerged Sphagnum (Beynon, pers. com). However, pools with extensive floating rafts and some open water appear to be the favoured breeding habitat. Females seem to choose to oviposit into small areas of open water among the semi submerged Sphagnum moss. Therefore, the vegetation should be managed to ensure the pools remain attractive. Although some open water is required, the optimum amount is unknown and research is needed to ascertain the optimum percentage of vegetation to open water required by the White-faced Darter. However, in pools that are becoming overgrown some Sphagnum should be raked back by hand.
  
  Any Sphagnum removed must be treated with care as the larvae live within the vegetation. Where possible the Sphagnum should either be left within the pool or used to seed newly created habitats. If the Sphagnum must be removed it should be left on site to allow the larvae to return to the bog pool before being removed. By leaving the Sphagnum on site, but in areas where decomposition will not affect the surrounding vegetation, it can create habitats for other invertebrate species.
  
  
• Fish: - The introduction of fish into pools supporting the White-faced Darter should be avoided. White-faced Darters are not adapted to live within the same environment as fish. They are active during the day and do not have anti-predator reactions. Therefore, they are likely to be predated by fish. In acidic habitats fish cannot reproduce below pH 5.4^{7, 15, 16} and are unlikely to survive below pH 4.4¹⁷. Consequently the introduction of fish into acidic habitats would be unsuccessful.
  
  
• Improving Knowledge: - The White-faced Darter has been the subject of a number of studies. However, further research is required to determine more exactly the habitat requirements of this species to help us manage habitats more successfully and to inform and guide restoration and recreation of sites.

Habitat Creation and Restoration

This should be considered as an option where possible and the creation of new pools within existing sites are of considerable importance for the long-term survival of the species³. Peat cutting by hand in the traditional, low intensity, piecemeal way has maintained a steady supply of suitable habitat for this species and remains a good method of creating new habitat³. The position of newly created pools is important and the closer the proximity to inhabited pools the greater the chance of colonisation. The potential of the White-faced Darter to spread naturally to any suitable unoccupied areas has always seemed limited⁴ and recent research could provide an answer to this observation. Research suggests that the White-faced Darter is a homing species, returning to the pool from which it emerged to breed¹⁰. Therefore, adults are unlikely to colonise newly created pools quickly.

Where management of the bog pools is difficult, due to treacherous ground, a possible solution is to create new pools as older ponds become overgrown. However, there are important considerations that should be taken into account if this method of management is used.

• Homing and seeding - If research proves that the White-faced Darter is a homing species, newly created pools could benefit from the introduction of larvae.
  
  
• Successional stage of the vegetation - new pools should to be created a number of years before the old pools become untenable. This should allow healthy development of the Sphagnum raft, which appears to be particularly important for the larvae.

Translocation

If further research indicates that White-faced Darters are homing species, translocation of nymphs and seeding pools with Sphagnum from inhabited pools may be an important way of ensuring colonisation and should be considered.

Case Study

Chartley Moss National Nature Reserve

(Beynon. pers. comm)

Management work that has been undertaken by English Nature at Chartley Moss, to restore the bog habitat, and this has indirectly benefited L. dubia. Work undertaken on the Western Basin has included the felling and removal of Scot's pine, control of Birch scrub through cutting and application of glycophosphate and creation of pools during removal of Scot's pine stumps. In addition damming of the East - West Ditch on the Eastern Basin to restrict drainage and re-wet the peatland has provided good pool habitats, which have been colonised by L. dubia. Scrub control on the ditch edge has opened the habitat increasing its suitability for L. dubia.

Some specific management has also been undertaken to increase the amount of suitable habitat, thereby encouraging the L. dubia population. 2 large pools (fig 6.) and 7 small pools have been created specifically for this species. The smaller pools are situated near the pathway at the edge of the Sphagnum lawn and it is hoped that these will attract L. dubia enabling visitors to see the dragonflies without the need to take them onto the fragile raft. Felling of Scots Pine on the edge of habited pools has also removed shading.

Scaleby Moss SSSI¹⁸

English nature has undertaken work at Scaleby Moss, where "succession was leading to the decline of the White-faced Darter as well as a number of other local plants and invertebrates". Growth of vegetation in the bog pools and invasion of scrub onto the surrounding bog and heath were considered to be particular problems as they were changing the nature of the habitat. A programme of scrub control was undertaken to halt succession and new pools were created to provide suitable aquatic habitats. These were located in sufficiently wet peat in order to maintain water levels. Where pools were being lost through seral succession, a limited amount of Sphagnum was removed and transferred into the newly excavated pools.

The work was undertaken with care to ensure minimal damage to the existing bog habitats of greatest value. The authors consider that the newly created pools were a timely move to safeguard the L. dubia population.

References

Management file updated on 16 January 2003