Guest blog: targeted eradication of sour fig (Carpobrotus edulis) to protect localised populations of the Glanville fritillary butterfly on Guernsey

Guernsey is a small island of 65 km², situated in the English Channel, some 48km from France. It is at the southern limit of many species of plants and animals and at the northern limit of Mediterranean and southern species. With its mild climate and a variety of different natural habitats including wooded valleys, sandy beaches, sand dunes, wetlands, rock pools, and cliffs, it is home to a wide range of insects, birds, marine animals and plants.

The department for Agriculture, Countryside and Land Management Services are busy developing an invasive non-native species (INNS) action plan for the island. The plan is based on EU and UK guidance which has been adapted for Guernsey’s unique location and status. The islands INNS action plan is interesting in that it raises awareness of the threats to biodiversity from well documented invasive species that could arrive on its shores, whilst at the same time addressing the potential threat from non-indigenous species that are widespread across Europe. Some animals such as the Guernsey vole are unique to the island whilst others that are common throughout the British Isles are absent. For example, Guernsey has no moles, squirrels, foxes or badgers.

The INNS action plan proposes a series of objectives and key actions to deliver a co-ordinated approach for addressing the threats posed by alien species. These objectives will underpin action in three key areas:

  • Phase 1: Prevention measures (biosecurity and horizon scanning),
  • Phase 2: Early detection and rapid response, and
  • Phase 3: Management of established INNS
Whilst this plan recognises that greater effort and resources should be placed on preventing INNS establishing in the island, urgent action is now required to tackle some of the most damaging species already established. By far the most visible and pernicious is the sour fig.

Found across the Channel Islands, it was believed to have been introduced to Guernsey from South Africa sometime before 1886. Once established it forms dense, impenetrable mats where a single plant can dominate an area up to 50m carpeting coastal and dune grassland to the exclusion of all other species. Some research suggests it may also modify the soil conditions by reducing the pH and increase nitrogen and carbon levels. Historically, its spread was limited by heavy frosts, however as these frosts have become scarce it is now able to cover vast areas very quickly.
The large-scale removal of sour fig meets the following broad aims of the Biodiversity Strategy for Guernsey:

  • To conserve and enhance key local, regional and internationally important species, habitats and sites;
  • To increase public awareness and encourage communities and individuals to be involved in the conservation of local biodiversity
One of the animal species most at risk from sour fig encroachment is the Glanville fritillary, a stunning but vulnerable butterfly that is scarce on a national and local level. Removal by simply pulling up the fig and its root system has shown to be effective in the areas where this approach has been applied, allowing the butterfly’s food plant Ribwort plantain to survive. In these targeted areas the butterfly numbers are holding up well year on year.

This method involves physically removing the fig and pulling out as much root system as possible. The remaining root system and layers of fig debris which has built up over years is then be raked and removed - exposing clear topsoil beneath.

If the debris and root system is not removed, the grassland will not recover as quickly, if at all, and the change of regrowth rate of the fig will be greater. Even when the fig is thoroughly removed, the sites need to be revisited regularly to remove any small shoots of fig which regrow. Once pulled, the fig is loaded onto tarpaulins or into one-ton bags and dragged to skips ready to disposed of in a bio secure and sustainable way by the local waste disposal company.

Rocquaine Bay. Image: States of Guernsey

We propose that we continue to resource volunteer labour as well as community and corporate groups. This is a fine example of the collaboration between the States of Guernsey government and the local population, working together to protect Guernsey’s vulnerable wildlife and maintaining the value of our natural assets and the services they provide.

Using historic aerial photography, we were able to measure the spread of sour fig at Rocquaine Bay. From a small patch of only 6m², in 1990, the fig had spread to over 1,200m² in 2016. Without management, the fig would quickly cover the entire coastal verge at the expense of all other wildlife.

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Thin strip of image show tree trunk and bark