Hairstreak Butterfly Survey & Mitigation: Cambridgeshire

Background

BSG Ecology provided baseline survey and ecological impact assessment (EcIA) services for this large residential development in Peterborough that was granted planning permission in 2013.

During the scoping stage of the EcIA, the potential for two species of butterfly, the black hairstreak Satyrium pruni and the purple hairstreak Neozephyrus quercus to occur, was noted. Any populations found to be present were considered likely to be of regional and local importance respectively and the local planning authority requested that both species should be considered carefully as part of the EcIA as valued ecological assets.

Black hairstreakBlackHairstreak-Butterfly

In the UK, black hairstreak is restricted to woodlands in the East Midlands: the site is on the north-eastern limit of its range. It has declined in many areas since 1950, but its distribution appears to have been stable since the 1970s.  Most populations breed in dense, mature stands of blackthorn growing in sheltered, sunny positions, often on the edges of glades and rides, or along sheltered woodland edges.

The species is highly sedentary, and usually confined to small areas within woodland and as such can present a challenge to survey.

Purple hairstreak

Purple hairstreak butterfly is relatively common and widespread in suitable habitat (oak woodland and standard mature oaks) within the UK, but has a southerly distribution. The flight period is from early July to early September and the adults tend to fly high over the canopy of woodland, particularly around mature oak trees Quercus spp.  They mainly feed on the nectar-like secretions (honeydew) of aphids on the leaves of trees.

The eggs are laid at the base of oak leaf buds of and the caterpillars (larvae) feed solely on the leaves of the oak tree.  Colonies may be associated with a single oak tree, and the species is not highly mobile although occasional dispersal and colonisation have been observed.

BSG Ecology’s Role in the Project

BSG Ecology was commissioned to design and undertake a suite of butterfly surveys in suitable habitat within and adjacent to the proposed development to inform the EcIA and any mitigation or compensation measures that might be required.

An experienced lepidopterist within BSG Ecology undertook the work to ensure an appropriately detailed survey was completed as both butterflies can be hard to locate as adults, and to provide an experienced assessment of potential management and mitigation measures that could be undertaken. The main aims of the survey were to:

  • confirm the presence or likely absence of black and purple hairstreak butterflies;
  • identify known and potential habitat for these species; and,
  • propose suitable mitigation and habitat enhancement measures to maintain the favourable conservation status of theses butterflies as part of the development.

Results

The survey confirmed the presence of both hairstreak butterflies, which were associated with the woodlands adjacent to and within the site.  Black hairstreak was recorded in small patches of blackthorn on the edges of two woodlands (one individual). Other small patches of potentially suitable habitat were identified along the edge of two other woodlands, although the interior of these woods were considered to offer very limited suitable habitat due to the lack of any wide rides or clearings and the very sparse distribution of blackthorn.

The majority of hedgerows within the study area were assessed as unsuitable habitat for the species due to intensive management and the limited extent of blackthorn. However a strip of scrub and woodland along a stream running across the site was assessed as having some small patches of potentially suitable habitat.

Purple hairstreak butterflies were observed during the survey at three of the woodlands within the site.   Suitable habitat for the purple hairstreak was present within the majority of the semi-natural patches of woodland within the site particularly where mature oaks constituted part of the canopy.

Veteran Oak TreeRecommendations

The proposed urban extension presented the potential for populations of both butterflies to become isolated through habitat fragmentation, with the risk that the populations could eventually die out. To address this potential long term threat BSG made a series of recommendations based on research on the habitat requirement of black hairstreak in particular. These were aimed at making the population more robust by increasing the extent of potential habitat and improving connectivity.

Proposed measures  included:

  • Retention of existing stands of blackthorn, complemented by positive woodland edge and hedgerow management to encourage dense scrub 3-4m high. Flailing of hedges and woodland edges can destroy egg laying sites for black hairstreak in particular.
  • Planting of blackthorn to increase habitat availability in close proximity to areas of known occurrence, but also in woodland clearings, on woodland margins along hedgerows and as part of wider landscape design (improving connectivity)
  • Cutting of blackthorn to encourage regeneration, and in order to create areas of localised shelter within large stands

No specific habitat management was recommended for the purple hairstreak beyond the creation of linkages between woodland habitat patches through further woodland and hedgerow planting, with a high proportion of English oak Quercus robur, and the retention of existing habitat.

To ensure the objectives for the butterflies were embedded within the development proposals BSG liaised with the master planner in relation to the design of green infrastructure within the development layout. This not only meant consideration of the location of green space, but also its character and functionality. As such new native woodland with strong elements of oak and blackthorn were included in the master plan adjacent to existing woodland blocks. We also discussed the composition and structure of new hedgerows with the landscape architects to ensure commitments were made to high proportions of blackthorn within new hedgerow, scrub and woodland planting. Finally management recommendations were made to the land owner for the existing woodlands within the site.

Some elements of the green infrastructure strategy for butterflies have been implemented with new woodland and over 1.3 km of new hedgerow (with high proportions of blackthorn) being planted to  link two existing woodland blocks. Further elements of the strategy for butterflies will be implemented as development proceeds and we continue to discuss management options with the land owner. Monitoring will determine how successful the strategy has been at protecting and enhancing the populations of these two butterfly species.

Hairstreak Butterfly Survey & Mitigation: Cambridgeshire – Project Profile

Top Photograph: Black Hairstreak Butterfly

Bottom Photograph: Veteran Oak

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