The Changing Status of Butterflies On Portsdown Hill

The following article was requested by the Hampshire branch of 'Butterfly Conservation' for publication in their 2005 issue of the "Hampshire and Isle of Wight Butterfly and Moth report 2005."

Portsdown today

Portsdown Hill is a 12 km east-west aligned chalk ridge that lies immediately to the north of Portsmouth. Despite the advance of urbanisation, the south-facing scarp slope retains a substantial area of unimproved chalk grassland and mixed scrub. It provides many hectares of accessible, relatively butterfly-rich habitat. Beginning in the early 1990's, in an effort to protect and restore chalk grassland, management has been applied to most of the area. This discussion concentrates on the SSSI which occupies 50 ha in the centre of the Hill and other nearby sites

The majority of the Hill is given over to intensive agriculture which dominates the northern dip slope. The non-agricultural land associated with covered reservoirs, fortifications and research establishments that occupy a substantial part of the ridge of the hill provides a potentially very good butterfly habitat. This potential is currently not well recognised

Portsdown's history

Written descriptions, paintings, maps and photographs show that open chalk grassland was a prominent feature of the south face of the hill, until the abandonment of grazing in the 1950s. The livestock of passing drovers and local farmers had maintained a constant grazing pressure and kept the hill open.

Portsdown has been subjected to disturbance over the centuries; therefore soil depth and topography vary considerably. Archaeological evidence indicates that Portsdown has been occupied since Neolithic times. However, the most pronounced disturbance occurred more recently, with the construction of the Victorian hill forts, defensive work during WWII and chalk quarrying.

There remains a diverse, if fragmented, flora with many species that are known to be slow to colonise, such as Bastard Toadflax and Horseshoe Vetch. The widespread presence of such species suggests a longstanding chalk grassland habitat.

History of Portsdown's butterflies

An indication of the richness of Portsdown's habitat in the past, is portrayed by Fassnidge, (1923) who describes as common, species that today are absent. Adonis Blue, Grayling and Silver Studded Blue all disappeared in the twenty years after WWII. Like many chalk grasslands there has been a steep decline in its butterflies. The abandonment of grazing and loss of its rabbits led to scrub invasion and coarsening of the vegetation. The predictable disturbance and damage that accompanies Portsdown's open access status has added to the problems. The decline of the hill's butterfly population was noted in the mid 1970s, (Dickson, 1975).

An absence of management continued into the 1990s. By then the site was 70% scrub and most of the grassland was a coarse, Upright Brome dominated thatch, with incipient scrub. The few remaining patches of rich grassland were restricted to isolated fragments on the thinner soils of old trackways and past earthworks.

Three butterfly transects and numerous observations from the last 15 years record the consequences for butterflies of no management. The aim of the management is to bring about a recovery of the butterfly population and other chalk grassland wildlife. This information, especially that which relates to downland dependant species, should provide a baseline to judge the effectiveness of the downland restoration.

During the 1990s, transects and sightings record a general decline in most of the 28 species found on Portsdown, especially downland butterflies. On one transect route, Portsdown West, the Small Blue fell from 55 individuals in 1993, to 3 between 1996-1998. Portsdown was one of the best Hampshire sites for this species in the 1970s. A similar decline was seen with the Wall Brown, Small Heath, Dark Green Fritillary and Dingy Skipper. They had annual indices comfortably in double figures in the early 1990s. A decade later, ones or twos were seen all season, if at all. Not all downland species crashed, a small Chalkhill Blue colony held steady through the 1990's, only to fall by 80% in 2001. Common Blue numbers remained little changed and so did several other species, Skippers (Small, Essex and Large), Green Hairstreak, Orange Tip, Small Copper, but at low numbers. Gatekeeper and Marbled White fell noticeably, but were found in hundreds over a season

Over the same time period, the Portsdown East transect recorded the presence of Small Blue, Wall Brown, Small Heath, Dark Green Fritillary and Dingy Skipper, in low numbers and not necessarily every year. In the previous years they were noted as quite common but they no doubt declined in response to the loss of grassland habitat.

Another notable trend was that Speckled Wood numbers doubled on both transects during the 1990s, probably because of the increasingly scrubby nature of the site.

Current management of the SSSI and its effect on butterflies

Scrub clearance and livestock fencing began in 1994 on the SSSI. Grazing started soon after and has gradually expanded as the fenced compartments were completed. The site now has winter grazing of sufficient intensity to remove the annual growth of grass from most of the site. A marked increase in rabbit numbers augments the grazing pressure provided by livestock so that rank grass has been largely replaced by short turf. Without the thatch of dead Upright Brome there were no grass fires and the consequential devastating affect on invertebrates. The fires were especially common on the grassland occupied by the largest Chalkhill Blue colonies.

The majority of the scrub clearance has occurred in the last 5 years. Since then grassland connectivity has increased. Small pockets of grassland are no longer surrounded by an encircling mass of bushes. Scrub cover has fallen to approximately 35%. The clearance has also produced 18 ha of new habitat, which can only benefit the dwindling butterfly population. When cleared of scrub the ground is re-colonised by scrub regrowth and ruderal plants. The extra nectar sources provided by the flowers of early colonists, such as ox-tongues, are utilised by butterflies. There are also chalk grassland species that are relics from the seed bank. It is hoped that larval food plant species such as Kidney Vetch and Birds Foot Trefoil will continue to establish themselves. There are encouraging signs that diverse grassland will re-establish on cleared scrub with a good representation of desirable plants. However, a high level of management is needed, mowing and grazing, to achieve this.

In 2005 a new butterfly transect was established by the South East Area Group to monitor the effects of large scale scrub clearance at the western end of the SSSI. Whilst not directly comparable with the Portsdown west transect there is a partial overlap and it gives a useful comparison. A notable effect was the increased growth of Mignonette in one cleared area which produced impressive numbers of Small White. The Small White outnumbered Meadow Browns and Gatekeepers, its larvae presumably feeding on the super abundant Mignonette.

Other findings were that Dark Green Fritillary, Wall Brown and Small Blue and Chalkhill Blues were still to be found at the western end of the hill but Small Heath and Dingy Skipper were not. An interesting additional species was the Brown Argus as several were seen near to a Rockrose patch.

A transect carried out between 2000-2002 in an area in which grazing had been established in 1996 showed that these six butterflies were present, although not every year and not in high numbers. A new species was recorded, the Ringlet. Presumably the site has developed sufficiently lush vegetation on its lowers slopes to host it.

The impression created by field observations is that the Chalkhill Blue is recovering well as is the Wall Brown. The Dark Green Fritillary, Dingy Skipper and Small Heath are very near to extinction but the Brown Argus is to be found in more than one part of the hill. The most unusual finding was a Silver Spotted Skipper in August 2005

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Conclusions

The restoration of Portsdown Hill's chalk grassland started too late for at least three butterfly species, but this may not be the case for other downland-associated species that seemed to have entered a final decline in the 1990s. The remaining presence of typical downland species and the recent appearance of two previously unrecorded ones suggests that a more complete and historically representative butterfly fauna may be achievable. Whether they are able to expand from dangerously small internal colonies or re-establish from external colonies remains to be seen.

It may be possible to host sustainable populations of butterflies that were common to our ancestors. This will certainly require the continuation of suitable management on the SSSI and probably the adoption of more wildlife-sympathetic management on other areas of Portsdown.

Bibliography

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