Northern Brown Argus

Durham Priority Species
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Magnesian Limestone Grassland Action Plan

Priority Habitats and Species: Magnesian Limestone GrasslandCG8 GrasslandChalk Carpet moth, Least Minor moth, Cistus Forester moth, Northern Brown Argus

Priorities
1.     Protect and maintain the current extent of Magnesian Limestone Grassland and re-create it where opportunities allow.
2.     Restore degraded sites and ensure appropriate management
3.     Establish sustainable populations of all priority species supported by Magnesian Limestone Grassland
4.     Raise public awareness of the importance and special characteristics of Magnesian Limestone Grassland.

Magnesian Limestone Grassland Targets
Vision Statement: To increase the extent of magnesian limestone grassland in good condition, and to expand the area of species rich calcareous grassland.

Target Type Unit Value
1. To maintain the extent of magnesian limestone grassland in the Durham BAP area maintain ha tbc
2. To expand the extent of magnesian limestone grassland in the Durham BAP area expand ha  75
3. To achieve good condition of magnesian limestone grassland within SSSIs in the Durham BAP area. achieve condition ha  tbc
4. To achieve good condition of magnesian limestone grassland outside SSSIs in the Durham BAP area. achieve condition ha  tbc

CG8 Grassland Targets
Vision Statement: To maintain all CG8 grassland in good condition, and to maintain accurate information on its status

Target Type Unit Value
1. To maintain the extent of CG8 grassland in the Durham BAP area. maintain ha  tbc
2. To achieve good condition of CG8 grassland within SSSIs in the Durham BAP area. achieve condition ha  tbc
3. To achieve good condition of CG8 grassland outside of SSSIs in the Durham BAP area. achieve condition ha  tbc

Chalk Carpet Moth Targets
Vision Statement: To have a better understanding of the species’ distribution in the Durham BAP area

Target Type Unit Value
1. To maintain the range of the chalk carpet in the Durham BAP area. maintain  tbc

Least Minor (moth) Targets
Vision Statement: To have a better understanding of the species’ distribution in the Durham BAP area

Target Type Unit Value
1. To maintain the range of the Least Minor in the Durham BAP area. maintain occupied tetrads tbc

Cistus Forester (moth) Targets
Vision Statement: To have a better understanding of the species’ distribution in the Durham BAP area

Target Type Unit Value
1. To maintain the range of cistus forester in the Durham BAP area. maintain  tbc

Northern Brown Argus Targets
Vision Statement: To maintain its current distribution of sites and to restore it to further suitable sites.

Target Type Unit Value
1. To restore the historic range of the northern brown argus in the Durham BAP. restore number of sites 1 colony >100 individuals

Magnesian Limestone grassland is a type of calcareous grassland that occurs on outcrops of Magnesian limestone, laid down in the Permian period (about 225 million years ago) when shallow seas covered the UK.  Like other unimproved calcareous grassland occurring on other types of limestone and chalk it is species-rich and particularly important for its botanical and invertebrate interest.

The Magnesian Limestone lies on a climatic divide intermediate between the chalks of southern England and the northern carboniferous limestones, occurring in a narrow band stretching from Nottinghamshire to Tyneside.  Consequently the magnesian limestone grasslands hold an unusual assemblage of plants, some at the limits of their southern or their northern ranges.  Eighty-four nationally scarce invertebrates and 13 nationally scarce plants are recorded from this habitat.

Like all species rich grasslands, magnesian limestone grassland has declined markedly since the second world war, largely through agricultural intensification.  The remaining grasslands of this type are often restricted to steeper slopes, which have proved more difficult to plough or ‘improve’.

Primary grasslands, which have a long history of extensive management, are particularly rare and secondary grasslands, which have developed on disused quarries or on road verges, now make up a significant proportion of the resource.

Magnesian limestone comprises four distinct plant communities.  They are known by their National Vegetation Classification (NVC) codes:

CG8 – Sesleria albicans – Scabiosa columbaria

CG2 – Festuca ovina – Avenula pratensis

CG3 –  Bromopsis erectus

CG6 – Helictotrichon pubescens

CG8 Grassland

Whereas most of these communities occur on other calcareous substrates within the UK, CG8 is entirely restricted to the magnesian limestone in the Durham area, and is probably the rarest lowland calcareous grassland community in the UK. Natural England estimates put the amount of CG8 grassland at less than 65ha in all.

CG8 is most characteristic of the drift-free slopes along the Durham escarpment and East Durham Plateau, and its finest example can be found at Thrislington National Nature Reserve.  Because it is unique to the Durham Magnesian Limestone Natural Area and because of its rarity, this community is considered as worthy of a target in its own right.

Other communities

CG2 generally has a more southerly distribution in the UK, and in this area, many of the typical southern components are absent.  It is commonly found on south facing cliff slopes of the Durham coast, where it includes more maritime species such as sea plantain (Plantago maritima).

CG3 is also more typical of southern limestones, while CG6 is characteristic of damper, more mesotrophic soils. Both communities are sparsely distributed within the Natural Area.

Apart from Castle Eden Dene, there has been very little work on the invertebrate fauna of magnesian limestone influenced sites.  Most information on other sites is restricted to butterflies and moths.

Invertebrates

Northern brown argus

There are 34 known sites for the northern brown argus in the Magnesian Limestone Natural Area (11 inland and 23 coastal).  Twelve sites have been lost since the 1982/3 survey.  The northern brown argus requires a varied sward with a large proportion of common rock-rose, the larval foodplant, in the medium (6-10cm) and tall (>10cm) categories; suitable habitat is best maintained by light grazing up to 0.2 Livestock Units/ha/year.

Chalk carpet

Chalk carpet moth has declined in the Durham BAP area to the point where there is only one recent record from Wingate Quarry. The chalk carpet breeds on several leguminous plants, especially common bird’s-foot-trefoil, and also requires a varied vegetation structure with a continuity of areas of short turf with patches of bare ground.

Cistus forester

There are only a handful of records for cistus forester moth on the Durham coast. Like northern brown argus the cistus forester breeds on common rock-rose but its precise habitat requirements are unknown.

Least minor

Least minor moth has declined in the Durham BAP area to the point where there are no recent records.  Historic records exist from the coastal magnesian limestone grasslands and from Cassop Vale. Least minor feeds on glaucous sedge, but again, little is known about its precise habitat requirements.

Threats

  • Withdrawal of funding/subsidies.  Managing magnesian limestone grassland is not in itself an economic proposition.  All sites require some kind of subsidy to maintain their value, whether this be in the form of agri-environment funding, or as part of the revenue budgets of local authorities and agencies.
  • Undergrazing threatens some sites that currently have no grazing infrastructure.  Scrub is a serious problem on a number of sites, in particular gorse.
  • Many types of cotoneaster have become established at some sites.   If left unchecked they can become dominant, leading to acidification of the soils.
  • Inappropriate grazing, either at the wrong time or with the wrong stock, or too heavily, will all lead to a deterioration in species diversity.
  • Some sites are very difficult to manage because of their position on slopes or with pitfalled ground, and require a labour intensive approach to management.
  • Agricultural practices such as ploughing and reseeding and fertiliser application still threaten some grasslands, especially undamaged sites as ownership changes hands, and despite EIA legislation which covers the larger sites.
  • Disturbance.  Some sites in urban areas are particularly prone to vandalism through arson and fly tipping and dog fouling, and to erosion of footpaths.
  • Coastal erosion threatens grassland which is sandwiched between the sea and built development, with no room to retreat.
  • Overzealous removal of scrub in some locations, can lead to habitat loss for invertebrates associated with limestone grassland and scrub.
  • Limited information on distribution and ecology of invertebrates of magnesian limestone may lead to inappropriate management on some sites.
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