Stone walls are central to the rich cultural heritage of human history. Some of the oldest stone walls in the world still stand in ancient Mediterranean lands, and also provide the foundations for Incan architecture (think Macchu Picchu) and the castles and ramparts of feudal Japan. In the UK, Ireland and Europe, stone walls are key elements of pastoral landscapes from a thousand postcards, and numerous regional specialities maintain their own unique cultural and ecological foundations. This colonial heritage is also preserved in the new world, particularly North America’s New England region and Australia’s southern states.
Stone walling is more than simply stacking rocks. A harmonious balance of art and science are needed to keep the wall standing. Each stone is fitted into the negative space around its neighbouring stones, like a jigsaw, so that the final wall holds itself against the pull of gravity.
The resulting structures are works of art, but they also have function. They mark field boundaries, keep livestock from straying, and give shelter to people and farm animals. Across ancient Britain and Europe, these walls also sheltered honeybee hives, an essential part of rural life until sugar and paraffin wax came along. Before the invention of the modern wooden bee hive, hollows called boles were built into south-facing walls to shelter straw bee skeps. (IBRA has a register of bee boles in the UK if you want to find them.) The classic Cornish hedge is another stone wall specialty – two faces of rock wall filled with earth so that trees can be planted in the middle.
National park managers often contract stonemasons to create paths and bridges that fit the amenity of the landscape, creating aesthetically functional structures to please visitors keen to experience the ‘natural’ environment. The added benefit is that the structures are made of real stones, in most cases from the land they stand on. No cement, no imported products, no synthetic materials, no footprint.
Despite increased interest in dry stone structures, in both town and country, the value of them to wildlife is often overlooked. Yet the gaps and crevices left in stone walls by omitting mortar or synthetic sealants create natural interstices for plants to grow and animals to shelter in…a fact that should be included in conservation goals.
A study just published in the journal Biodiversity & Conservation found that dry stone walls in the northern Appennines in Italy created more heterogeneous habitat for salamanders and snails than concrete and other types of retaining walls. The dry stone walls also supported more lichen, little ecological wonders that are essential to ecosystem function and important indicator species.
Structural diversity in ecosystems creates more ecological niches, which means there are more spaces for different plants and animals to hold their ground. But structural diversity doesn’t just come from fallen dead wood, complex forests and leaf litter. Man-made stone structures can also contribute, directly and indirectly, to landscape heterogeneity and biodiversity conservation.
Only a handful of other studies have directly considered the biodiversity value of dry stone walls, showing that the walls are important habitat for small reptiles, insects, mammals and birds. In the 1970s, the Codys found that most of the wrens they saw on the Shetland Islands preferred foraging in dry stone walls rather than their usual habitat, the flat sea cliff faces. The authors concluded that the increased surface area of the walls, compared to the flat cliff face, provide a more rewarding foraging habitat for the birds.
The walls are constructed sufficiently loosely that the birds forage within them with much freedom; a Wren might remain out of sight in the wall interior for some minutes, to emerge perhaps a hundred feet further on.
And it’s not just lichen, birds and reptiles that benefit from these structures. Dry stone walls can benefit solitary and social bees. The walls provide cavities for bees to nest in, but they also create permanent, undisturbed habitat for perennial herbs to grow alongside them, indirectly providing diverse floral resources for bees to forage on.
Stonework is an art, a tradition, a challenging skill that we, as mammals, should be mighty proud of. As far as we know, primates and other dextrous animals can use stone tools, and many creatures nest in stone, but no other mammals are known to build structures with stone. Species that are known to build stone nests or ramparts to protect eggs include small arid-zone birds (e.g. Black Wheatear, Rock Wren and the Desert Lark) and several fish in the minnow family. But few of these structures stand for centuries, providing shelter for a myriad creatures and a heritage of skills and songs to pass down the ages.
Replacing plans for cement walls with ones of stonework and preserving the stone walls we already have in our landscapes creates heritage, history and biodiversity, not just art.
Sacred, blood-stained walls, your peaceful front
Sheltered the fateful fires of Lexington;
Builded to fence green fields and keep the herds at pasture,
Ye became the frowning breastworks of stern battle;
Lowly boundaries of the freeman’s farm,
Ye grew the rampart of a land at war;
And still ye cross the centuries
Between the ages of monarchs and the age
When farmers in their fields are kings.
~ ‘The Song of the Stone Wall’ Helen Keller
© Manu Saunders 2014
Great post! Walls have fascinated me for a long time, not specifically dry stone walls, but rather urban stone (and brick) walls. They are certainly poorly studied and have significant nature conservation potential, I agree! I have an old post that may be of interest: http://jeffollerton.wordpress.com/2012/03/25/the-walls-of-the-garden/
Thanks Jeff, and thanks for the link to your post! I wish I had found it before writing this one – I was surprised I didn’t find more studies about bees nesting in dry stone walls, it seems so obvious, especially for solitary bees and wasps. I only did a quick search, but the thesis I linked to above is the only one I found about solitary bees, although I did find a couple about honeybees. An interesting topic for future research! Especially for urban conservation!
Yes, it is. One of my PhD students, Muzafar Hussain, has been researching urban bees and looking at nesting sites in walls so we hope to publish that soon. I also have a data set of nectar and pollen plants that grow on urban walls in Britain that I want to write up at some point.
Nice article Manu. Definitely worth looking into these structures an what they provide in the landscape
Thanks Lisa. I agree! huge research potential there 🙂
Meant to send you a link to this which might be of interest: http://researchrepository.ucd.ie/handle/10197/3949
Thank you Jeff! I hadn’t seen that one. Interesting to think of stonewalls as ‘novel’ ecosystems – perhaps that would depend on the age of each individual wall relative to the ecological communities using it. Lots of food for thought!
Yes I’m not convinced by the argument that they are novel ecosystems, but it’s an interesting idea.