It might appear that dry stone walls are a barren, hostile environment for wildlife, but this is not the case. Although the range of plants and animals which make use of walls is fairly limited, walls have considerable habitat value for those species, and often provide an important habitat in otherwise inhospitable upland areas. Stone faced banks are even more valuable, because the soil core increases the range of plants and animals which can be supported.
Wall flora
Walls of all types, including retaining walls and those with a soil core, are well-drained habitats. However, the particular community of plants which develops depends very much on the aspect of the wall face. The south face of a wall may present virtual desert conditions in summer, and support true xerophytes, which are plants that thrive in dry conditions. Meantime the north face will remain cool and relatively moist, colonised by species normally associated with the surrounding area.
Local sources of moisture play an important part in determining the point-by-point variation in wall flora. Slight changes in slope, small ledges or crumbling stone tend to retard runoff and allow plants a foothold. Unmortared copings which retain more moisture than mortared or flat copings tend to support more plant growth. Any damp spots or drip lines have more luxuriant plant growth than elsewhere on the same wall.
Stone walls of all types are particularly important habitats for lichens, mosses and ferns, and allow these plants to thrive in areas where there are few or no natural rock exposures. In many parts of Britain, walls extend or substitute for natural scree, cliff and open stone face habitats for many species of both lower and higher plants. For example, navelwort or wall pennywort (Umbilicus rupestris), which is common on non-limestone rocks and banks in western Britain, is found on walls as far east as Kent. Walls provide the main habitat for the rusty-back fern (Ceterach officinarum).
The type of stone is a major determinant of the type of plant community. Gritstone and other hard acid rocks support a completely different flora from limestone. The species of acid situations are fairly limited, especially in dry lowland districts, but are richer in wetter areas. Limestone walls tend to have a far richer flora, from the orange lichens (Caloplaca), through mosses, spleenworts and ferns to the higher plants. In the Mendips, two rarities, the brittle bladder fern (Cystopteris fragilis) and the limestone polypody (Thelypteris robertiana), thrive on derelict walls in the area, which resemble a scree.
Walls are rather like hedges in supporting the maximum amount of wildlife at the stage when the wall is mature or beginning to deteriorate, but before it becomes derelict. Rebuilding a wall, like laying a hedge, is initially destructive, but within a few years the wall starts to be recolonised. If a wall is left to become derelict through not wanting to disturb the wildlife it supports, there will eventually be little wall left. Walls decrease significantly in the species they can support when they drop below 2′ (600mm) in height. Although a pile of stones is a valuable habitat, it tends to eventually become grown over with grasses, brambles, ivy and other plants, and the stones themselves become buried in the ground. Such a mound in no way compares to the specialised habitats provided by a dry stone wall, of mature age, but in good condition.
Before restoring a wall, it may be worth arranging for an expert to check for any rarities, as it may be possible to avoid disturbing a particular section, or to transplant individual plants. When restoring a wall, destructive effects can be lessened by replacing stones ‘green side out’ if they are colonised with lichens or mosses, although this should not be done at the expense of good construction. A well-constructed wall should be neat and tight, with few nooks and crannies, and so may be slower to become colonised than a poorer quality repair with larger gaps and holes. However, the better-constructed wall is likely to remain in good repair for much longer, and provide the continuity of habitat which many slow growing species require.
In practice, there is such a huge length of deteriorating and derelict wall in all walling areas, that any repairs have a very minor effect on the total picture. What is important for wildlife is the mosaic of different habitats, and apart from the unlikely event of all walls being simultaneously rebuilt, there will always be a range of habitats from new walls to remnants.
The value of dry stone walls and earth banks for mosses and liverworts is also very dependent on the level of environmental pollution. Although the situation has greatly improved, industrial towns tend to be more polluted than elsewhere, and the wall flora is relatively impoverished. Roadside walls, especially on major routes with heavy traffic, tend to be so polluted with grime, salt and fumes that little survives. The side away from the traffic is usually better colonised with plants. The most luxuriant walls are those found in sheltered rural valleys and woodlands, where the stones may be completely covered with lichens, mosses, ferns and liverworts, as well as some higher plants.
A large proportion of the higher plants characteristic of walls are introductions, many of them from Mediterranean countries, which have escaped from gardens. They flourish in the well-drained, warm conditions on walls. Examples include ivy-leaved toadflax (Cymbalaria muralis), Oxford ragwort (Senecio squalidis) and mind-your-own-business (Soleirolia soleirolii), a close relative of the native pellitory-of-the-wall (Parietaria judaica). The species name muralis indicates that the plant is usually found on walls. Native species which favour wall habitats include many members of the stonecrop (Crassulaceae), saxifrage (Saxifragaceae) and cabbage (Cruciferae) families.
While some wall plants such as Oxford ragwort grow also along disturbed ground or on waste and barren places, quite a few are limited almost exclusively to walls and earth banks, and are at risk where walls and banks are disturbed or destroyed. In contrast to hedgerows, which although a good habitat, tend to comprise common plant species, walls and earth banks are an important refuge for less common plants.
Wall fauna
A variety of animals make use of walls and earth banks. Some are permanent wall-dwellers, while nocturnal animals use the crannies in dry stone walls as daytime resting places. Dry stone walls are especially valuable habitats for insects and spiders.
Limestone walls support many types of snails, and sometimes the larvae of the glow-worm (Lampyris noctiluca) which feed on them. Banks and walls are useful to reptiles, particularly adders, which use them for hibernation, spring feeding, autumn nesting and as migration routes between dry wintering sites and wetter feeding areas. Walls and banks provide virtually the only places in which a variety of animals can overwinter as adults. South-facing limestone walls in particularly sunny parts of the country such as the Gower peninsula and the Isle of Purbeck are valuable in this respect.
A number of bird species sometimes nest in walls, although they may normally choose other sites instead. Blue and great tits, pied and grey wagtails, house and tree sparrows, spotted flycatchers, nuthatches and wheatears all use walls, while a few species such as redstart actually favour walls for nesting. Our smallest seabird, the storm petrel, is famous for its use of wall nesting sites in addition to rocky storm beaches and creviced cliff faces. It prefers the walls of the ruined field systems and abandoned dwellings of small and now uninhabited islands; sizeable colonies nest in the great broch of Mousa in the Shetlands and in the monastic beehive cells of Great Skellig, County Kerry.
Small mammals which make good use of earth banks and hedgerows often also inhabit dry stone walls. Field voles, house mice, rats, rabbits and hares may all seek safety in walls, while red squirrels have been known to store nuts under the stones. Bank voles seem particularly to favour old ivy-covered walls for dwelling. Rabbits may burrow below very shallow foundations, and rabbits and rats wreak havoc by tunnelling into earth banks. Voles and squirrels do not dig below foundation level, and so cause little damage. Weasels and stoats hide and hunt among the stones, and polecats sometimes winter in walls. Foxes take advantage of whatever prey presents itself and may dig into a wall or bank for dinner. Larger mammals, including on occasion man himself, use walls for temporary shelter.
Walls and banks, like hedgerows, provide protected ‘corridors’ for small animals to move between areas of favourable habitat. This corridor effect is most beneficial when walls are bordered by strips and unmown and ungrazed grasses and herbs. This, plus the direct use of walls as habitat means that, while no animal species appears to be directly threatened by wall removal, many will find their habitat further restricted. Dereliction is less of a problem, as a low wide heap of stones still has some habitat value, although this decreases as the stones become overgrown, buried or dispersed.
