I’ve written before about how valuable hedgerows are for wildlife. But in places where trees and shrubs find it harder to become established, linear boundary walls can offer similar benefits. Not only do these walls, especially dry stone ones with lots of points of entry, provide shelter for a range of organisms, they are also growing surfaces for plants, breeding spaces for a range of creatures and also way markers for bats and birds. In the first of a two-part post on wall wildlife, I’ll show you why walls are so useful for wildlife and introduce you to some of the lichens and plants that call walls home.
Walls Far and Wide
Walls are not uniform structures, of course. How a boundary wall is built and what it is made of obviously affect the species using it, as well as the ways it is used. Different sections of a wall will have a distinctive character too, with each producing a different microclimate. Depending on its aspect, one side will usually be warmer and drier, with the other wetter and colder. And the base is naturally more sheltered than the exposed top. Some of the best walls for wildlife are dry stone walls, those built without any mortar between the stones. Used as field boundaries in Britain and Ireland for millennia, as well as acting as receptacles for the rocks cleared from stony ground, they are commonest in upland areas. Here, hedges are harder to grow, and bedrock is nearer to the surface and easier to access. The Yorkshire Dales is one of the best places to see them. It is estimated that over 5,000 miles of walls cross the Dales landscape.
Different regions developed their own unique styles of dry stone wall, largely down to the type of rock available locally. Where only harder rocks such as granite were present, walls would consist of large boulders. Where rock was easier to work and break up, such as the limestone of the Aran Islands in Ireland, people used much smaller, regular pieces to build walls. Whatever the style, however, the many holes provided by a lack of binding agent act as shelter for resting or hibernating creatures from insects to toads, and birds to hedgehogs. They also provide a growing surface for a range of mosses and lichens. Rock type determines which species will colonise where. Thin soils can even form over time in the various nooks and crannies of a drystone wall. This in turn allows plants such as ferns to grow.
Walls and Wildlife
All walls, dry stone or not, provide some measure of habitat for wildlife, though. Miles of boundary wall once enclosed many of Britain’s large country estates. Large sections of these are still standing today, even where estates have been broken up or have disappeared completely. Those of us with garden walls, rather than hedges or fences, mirror these in miniature. And many of our railways and canals have sections bounded by walls. Of course, there won’t naturally be as many cavities in these types of walls as their drystone cousins. But over time, mortar and stones naturally crumble, forming holes for creatures and roots to enter. And even before this happens, they can still offer useful perches for hunting raptors or displaying songbirds, more of which in part two. Let’s meet some of the plants and lichens you are likely to see on a wall near you, though.
Lichens are actually a partnership between a fungus and an alga (and sometimes also a bacteria). They are masters of finding a foothold on hard surfaces, including walls, with many adapted to life on exposed rocks. Individual species have particular preferences regarding the type of rock and how much moisture or sun they get. The bright orange sunburst lichen, for example, can cope with exposed conditions due to the sunscreen chemical that produces its colouration. In shadier habitats, it is often greener in colour. The species often grows on coastal walls, but also inland. Sea ivory, however, is more exclusively coastal and is salt-tolerant. Its pale green fronds produce thick sprouting tufts on seaside walls and rocks and in Shetland, sheep even eat it.
Lichens that might otherwise only grow in the uplands can sometimes be found at lower altitudes where walls act as a replacement for rocky outcrops. Different species will have varying levels of tolerance for pollution. This means that some happily colonise urban walls while others only grow on rural ones. Crucially, patches of lichen on a wall become a habitat in their own right. Tiny invertebrates live amongst the fronds and leaf-like structures of fruticose and foliose lichens. Fruticose lichens in particular can form dense mats on the tops of walls, able to hide many small invertebrates. Larger creatures then feed on these invertebrates, while slugs and snails often eat the lichens themselves.
Mosses and Liverworts
Unlike lichens, mosses and liverworts are plants. Because they don’t have roots, however, they can still easily attach to hard surfaces like walls. Instead of reaching into the earth to get nutrients and water, they get them from the air and rainfall or fog. Both mosses and liverworts tend to prefer shadier, moister areas of a wall, although mosses in particular are able to go into suspended animation for decades in dry conditions, only reviving after rain returns. In less exposed areas, they can form luxurious cushioned wall top covers.
Common wall-growing moss species include the aptly named wall screw-moss which produces tiny red chilli-like fruiting bodies on the end of long stems. Another common species, grey-cushioned grimmia forms compact cushions with silvery leaf tips and tucks its fruiting capsules back into the cushion. The beautiful silky wall feather-moss, meanwhile, grows in distinctive mats with runners creeping away from the central section.
Like mosses, liverworts are bryophytes, a group of plants that lack some of the features of the so-called ‘higher’, or vascular plants such as ferns and flowering plants. Like other plants, bryophytes do photosynthesise, converting sunlight, water and CO2 into energy. However, they don’t have proper roots or proper vascular systems so are unable to transport water and nutrients around their structures. A number of liverwort species can be found on the bases of damper walls, including great scented liverwort. Crescent-cup liverwort, meanwhile, is often found on walls close to human habitation. Mosses and lichens both help to create small amounts of soil on the tops of walls, and on ledges and flatter sections. They do this by trapping wind-blown soil particles from elsewhere and by breaking down the stone itself into soil.
That thin soil then allows various species of vascular plant to colonise, including a number of fern species. Although classed as higher plants, ferns evolved before their flowering relatives and use spores rather than seeds to reproduce. These spores are contained in structures called sporangia which are then arranged in groups called sori, usually on the underside of the leaf. Different species have different arrangements of sori, which can help with identification. Wall-growing ferns would naturally grow on rocky outcrops and substrates, so make do with walls where these aren’t available, even in urban environments. They tend to prefer shadier areas of wall, out of direct sunlight.
Maidenhair spleenwort is one of our prettiest and most distinctive wall species. Often growing on garden walls, it has long, narrow fronds made up of lots of individual leaflets. Wall-rue is an even smaller species that often grows near to maidenhair spleenwort but looks quite different. It’s small, club-shaped leaflets are a waxy green colour, and the overall appearance is not very fernlike. As its name suggests, the underside of the rusty-back fern’s leaflets appear covered in rust due to its brown scales. The much larger hart’s tongue fern is easily identified as it is our only fern with single, undivided leaf fronds. Being evergreen, all these species provide valuable shade and shelter for invertebrates all year round.
Although mosses and lichens do help create soil on a wall, amounts are still minimal. A number of flowering plants can cope with the fact there is little or no soil to root into or provide water and nutrients, however. These include drought-resistant stonecrops such as wall pennywort. Also known as navelwort due to the belly button like depression in the middle of the fleshy, pale green leaves, it grows on shady, damp walls. It produces tall flower spikes in summer. Other flowering plants commonly found growing in the cracks of walls or at their bases include herb robert, dandelion and the non-native, but well-established, ivy-leaved toadflax. Where boundary walls are made up of a combination of earth bank and stone wall, such as the Cornish hedges of Cornwall, an even wider variety of plants can flourish due to access to more soil and therefore more water and nutrients.
One of the most important flowering plants found on walls is the common ivy subspecies Hedera helix helix. (The subspecies Hedera helix hibernica doesn’t climb but spreads at ground level.) Although its root system grows in nearby soil, this climbing ivy uses special hairs to stick to walls and trees as it climbs. The leaves have between three and five lobes on immature plants but become oval without the lobes when mature. As an evergreen it provides shelter and shade for a huge number of organisms. Invertebrates from earwigs to snails and moths to spiders, as well as birds and small mammals, all benefit from the cover ivy gives to an otherwise exposed wall. It is also late to flower and fruit, which means that it provides sustenance for pollinators and birds long after other food sources have disappeared for the winter.
The relationship between plants and walls isn’t always a happy one. Sometimes trees can force themselves through a wall as they grow, destroying them, for example. But a substantial number of lichens and plants take advantage of the benefits walls offer, many more than can be covered here. Whether it’s the mosses and small ferns of an urban garden boundary, or the luxurious lichen of a rural dry stone one, walls are effectively linear wildlife refuges, able to support a huge range of species. And these plants and lichens in turn enable a large number of birds and animals to inhabit or utilise walls. Look out for part two where we’ll meet some of them.