Project: Ecology of drystone walls

Ecology of drystone walls

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Dry stone walls are structures that provide habitats for many animals and plants.  As man-made objects of the cultural landscape, which at the same time provide habitats for many endangered species, dry stone walls play an important role in the conservation of biodiversity. As linear landscape elements, they make an important contribution to ecological connectivity. The benefits of dry stone walls for biodiversity are particularly high when they interact with other landscape elements that are important for biodiversity (hedges, rough pastures, water bodies). When restoring dry stone walls, special attention should be paid to the conservation of other landscape elements.

The ecological value of dry stone walls can be described from two perspectives:

  • Seen from close up the focus is on the ecological value of the individual wall as a habitat for various animals and plants.
  • Seen from afar the focus is on the ecological value of the network of different walls in the landscape. The drystone walls form ecological stepping stones in the landscape among other structures (hedges, rough pastures, water structures, etc.).

The ecological value of dry stone walls is high in both perspectives.

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"Close-up view", ecological value of the individual walls as habitats for various animals and plants

The colonisation of a wall consisting of newly broken, 'sterile' stones takes place from the neighbouring habitats. The radius of action of the animals, how fast they can move and what demands they make on the habitat are decisive here. This makes it clear how important it is for newly constructed walls to have a diverse environment and old walls that are already colonised. Therefore, renovations should be carried out in stages so that there are always old sections of wall in the vicinity and the recolonisation of the renovated sections of wall can take place without any problems. If intact sections of the old wall are left standing, mobile species find replacement habitat and recolonisation of the new wall can take place more quickly. Normally, the colonisation of dry stone walls by plants and animals proceeds without human intervention. Structural facilitation for safe migration of animals from the surrounding area can promote colonisation. It is also important that the passage through the crevice system of the wall to the soil is not interrupted. Furthermore, it may be possible to try to make the wall more attractive for animals and plants as a breeding and hibernation site by installing caves and passages. For example, the installation of nesting aids or caves in the dry stone wall can promote the settlement of cavity-nesting bird species. However, this should be approved by the competent cantonal authorities (Art. 19 NHG). Whether and how a settlement serves biodiversity must be clarified in each individual case.

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Mammals that use dry stone walls as habitats depend on sufficiently large cavities. Too small a back wall should be avoided, otherwise there will be too few crevices and cavities. Mammals that inhabit dry stone walls include mice, weasels, hedgehogs, dormice and bats.

Dry stone walls are colonised by mammals independently. However, the installation of caves can create an offer for colonisation. Renovations must take into account that in winter some mammals use dry-stone walls as winter quarters and cannot escape.

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Dormouse, found during wall demolition in Verdabbio in autumn 2005

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Dry stone walls are valuable and extremely important habitat elements in the cultural landscape for amphibians, but especially for the heat-loving reptiles. Many species use the masonry as an annual habitat: they live continuously on or in the wall throughout the year. Other species need the masonry at least temporarily, for example as frost-proof winter quarters. The masonry also serves as a place for sunbathing and hiding, as a hunting ground and as an egg-laying site. Dry stone walls are also important as functional connecting corridors between two partial habitats.

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All types of dry stone masonry are usable for amphibians and reptiles. The exception is combined natural stone-concrete masonry, which usually cannot meet the requirements of the two species groups. Particularly valuable are single-sided constructions, e.g. EH2S, EH3S, EH1S, EHBlock, EHKorb, which are additionally backwalled or backfilled with suitable material. This should largely consist of a heterogeneous mixture of stones with a diameter of at least 20 - 40 cm, so that there are sufficient usable cavities and passage systems. It is important for amphibians and reptiles to have sufficient joint width, which should be at least 20 - 30 mm in places, and 40 - 50 mm if large species are present. Specific access points should be created at the base of the wall. If possible, backfilling with finer material should be avoided. The backfill must be accessible to amphibians and reptiles; the use of filter fleeces should be avoided whenever possible. Particularly valuable are frost-free areas at a depth(h) of more than 100 cm that serve as winter quarters.

Dry stone walls are colonised independently by amphibians and reptiles; settlements should be avoided. Renovations must take into account that amphibians and reptiles use dry stone walls as winter quarters and cannot escape during hibernation. The dismantling of walls with machines is more harmful than dismantling by hand. If renovation is carried out in stages, the animals can escape to old sections of the wall or be relocated.

In dry stone wall projects (renovation and new construction), it must be clarified whether there is a known population of amphibians or reptiles in the project area. This information can be obtained from the karch (www.karch.ch) and the cantonal nature conservation offices. In consultation with these offices, the best course of action can be determined, or additional measures in the area can be planned (piles of branches, stone lenses, etc.).

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Die kleinen Bewohner der Trockensteinmauern (Spinnen, Insekten, Schnecken) werden wenig wahrgenommen, stellen aber mit Abstand die grösste Gruppe an Tieren dar, die Trockensteinmauern als Lebensraum benutzen. Die Mobilität der verschiedenen Gattungen und Arten ist unterschiedlich. Ameisen, Wanzen und Spinnen sind sehr mobil und können schnell grössere Gebiete besiedeln. Gliedertiere (Hundertfüssler, Tausendfüssler und Käfer) sowie Schnecken sind nicht so wanderfreudig. Sie benötigen mehr Zeit zur Besiedlung. Erst in älteren Trockenmauern sind Tiere mit sehr geringem Aktionsradius, oder sehr speziellen Ansprüchen anzutreffen, Schnecken sind beispielsweise auf Feuchtigkeit und Humus angewiesen und bewegen sich nur sehr langsam fort. Auch Asseln sind auf ausreichende Feuchtigkeit und Humus angewiesen und die Raupen einiger Schmetterlinge, welche auf an Mauern vorkommende Nahrungspflanzen (z.B. Flechten, Sedum) spezialisiert sind, besiedeln die Mauer erst, wenn die entsprechende Pflanze vorhanden ist.

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Trockensteinmauern werden von Spinnen, Insekten und Schnecken selbständig besiedelt. Der Einbau von Hohlräumen kann ev. die Ansiedlung von Hummeln, Wespen und Hornissen begünstigen

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Some bird species (e.g. cavity-nesting birds such as hoopoes, tits, wheatears, little owls and dippers) like to use suitable cavities in dry stone walls for breeding. Too small a back wall should be avoided, as otherwise there will be too few crevices and cavities. The installation of suitable cavities can support settlement at suitable sites. Some cavity-nesting bird species (e.g. hoopoe, wryneck, various tit species, wheatear, little owl and dippers) like to use suitable cavities in dry stone walls for breeding. While titmouse species already use small cavities as nest sites, larger species such as the hoopoe or the little owl need niches with a larger volume. The dipper is a special case, as it is bound to sites along watercourses. The installation of cavities or specific nesting aids at suitable sites can promote the settlement of cavity-nesting bird species (cf. SVS/Bird Life Switzerland leaflet "Construction of hawk nesting aids"). Too small a back wall should be avoided, otherwise there will be too few crevices and cavities. When installing cavities, make sure that no surface water or seepage water gets into the nest, even during heavy rainfall.

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The vegetation of dry stone walls is specialised for this habitat. The development of a wall vegetation can take decades and begins with the colonisation of the stone surfaces by so-called cryptogams, which include algae, fungi, mosses and lichens. The latter are organisms in which fungi and algae grow in symbiosis. Cryptogams are alternately moist. They can therefore dry out without dying. Later, when humus has formed in the joints, ferns and flowering plants can colonise the walls. Colonisation usually occurs through the entry of seeds by wind, insects and birds. Typical plant species are those that can survive longer dry periods, such as striped fern, wall pepper, houseleek and cinnamon weed. Depending on the orientation, rock type and age of the walls, different plant species and plant communities occur.

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Plants do not need support during colonisation, but if possible, stones overgrown with lichen or moss are reinstalled in the new wall (same exposure). If there is agricultural use in the immediate vicinity, care should be taken to promote an extensively used herbaceous margin at the base of the wall. Late mowing would be an option to prevent the growth of woody plants. An unfertilised and non-pesticide-treated buffer strip of 3 to 5 metres along the wall is indispensable for ecologically valuable objects in order not to endanger the inhabitants of the walls and their biodiversity.

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Distant view, "macro" perspective, walls as a network in the landscape

The dry stone walls form a networking landscape element alongside other landscape structures (hedges, rough pastures, water bodies, etc.).

Criteria for recording and evaluating entire object areas in the overall local ecological context:

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  • Structural diversity of the object area

Composition of habitat types according to "Delarze + Gonseth, Lebensräume der Schweiz" -> www.infoflora.ch/lebensraeume as well as www.lebensraeume.unr.ch, see Anthropogenic stone corridors (Gives hints)

  • Ecological significance of the dry stone wall based on the surrounding wall landscape (stand-alone/only dry stone wall in the area, part of a wall complex of several closely spaced individual walls or retaining walls in a large-scale terraced landscape) (low, medium, high, very high)
  • Networking function of the dry stone wall (low, medium, high, very high)
  • Quality of connectivity between the individual biotopes and small structures of the project area (low, medium, high, very high)
  • Quality of the biodiversity of the object area (low, medium, high, very high)
  • Type of zone (spatial planning)
  • Ecologically relevant economic implications (e.g. agriculture, transport, special uses)

Inventory inputs and assessments (protective measures).