Between a rock and a hard place
Rocky habitats are found wherever the underlying rock reaches the surface and are predominantly a feature of the uplands, where scouring by glaciers and weathering by the elements have exposed the underlying bedrock. The geology of the UK is surprisingly diverse for a small country and there are examples of rocks from almost all geological eras, dating back 3,000 million years. A useful way to categorise them in terms of the vegetation they can support is according to whether they are acid or lime-rich. Siliceous (acidic) rocks are those with a high silica content. They predominate in the uplands and include granite, gneiss, sandstone and quartzite. Basic rocks are those rich in calcium such as limestone (including chalk), some calcareous schists and some igneous (volcanic) rocks such as serpentine and basalt. Their distribution is much more limited in the uplands, but they generally support a richer array of plants and include a number of sites long celebrated by botanists for their outstanding flora.
Rocky habitats include cliffs, crags, rocky outcrops fell-field (areas of shattered rock and gravel on mountain summits left by retreating glaciers), scree and boulder fields, and limestone pavement (a unique habitat with a special flora). Features common to all of these habitats are the lack of soil (resulting in droughty, nutrient-poor conditions) and a harsh environment where the flora and fauna are exposed to the wind and plenty of rain. Plants are usually confined to ledges and crevices and are specially adapted to cope with a challenging environment – for example many have a cushion form with small, in-rolled leaves and strong root systems, offering protection and security on exposed ledges. These species are generally unable to cope with competition from more robust vegetation elsewhere and so are confined to rocky habitats. Examples are cyphel, moss campion, purple saxifrage, snow pearlwort, alpine speedwell, highland cudweed, mountain sorrel, and rarities such as alpine woodsia, alpine gentian and saxifrages.
On wider cliff ledges, out of the reach of grazing animals, a distinctive tall-herb flora may develop. On lime-rich rock, this includes globeflower, meadowsweet, angelica, lady’s-mantles, roseroot, wood crane’s-bill and marsh hawk’s beard. Acid ledges support great wood-rush, bilberry and several ferns. This vegetation is particularly intriguing as it was probably once much more widespread but is now severely constrained by grazing animals including livestock. At lower altitudes, such ledges can also support trees such as rowan and aspen that are prevented from regenerating by grazing elsewhere.
Scree is generally found below cliffs and crags, and is formed of shattered rocks and boulders that have fallen from the cliffs above. Colonised first by lichens then grasses, heath bedstraw and fir clubmoss (an ancient species), they can eventually support a range of ferns (particularly on lime-rich sites) including limestone fern, parsley fern, holly fern, oak fern and mountain male fern. However, the succession can be reset at any time by fresh rock falls and movements of the scree downhill. Again, lime-rich scree supports a wider range of plants than that formed by acidic rock, but is more local in distribution.
What to look for
Ring ouzel and wheatear nest in crevices and golden eagle, raven and peregrine nest on cliffs. Very keen botanists may find some of the UK’s rarest plants on lime-rich rocks (such as alpine woodsia, yellow oxytropis, alpine bartsia and alpine gentian), but these require determination and a good deal of know-how to find. However, other alpine species are more widespread and equally attractive and interesting – look in gullies, under overhangs, and anywhere with seeping water, particularly where the aspect is north-facing
Conservation and threats
Rocky habitats are some of our most natural. Remote, hard to access and unsuitable for agriculture, they have not been shaped by human activities and are not dependent upon human intervention, unlike many other UK habitats. However, grazing animals are prevalent in the uplands and it is generally only the most inaccessible ledges and rockiest screes that remain unaffected by grazing livestock (including feral goats). A reduction in grazing pressure would allow some species to spread and increase the resilience of the population to other adverse factors. The vegetation is often very sparse, so populations are particularly vulnerable. For example, recreational climbing and bouldering is also an issue at some sites, and climbers are encouraged not to “clean” vegetation away and to avoid vulnerable areas. Climate change is likely to alter the artic-alpine component of the flora, which can only move higher up to avoid the effects of a warmer climate, and once at the summit, will have nowhere else to go.
How you can help
You can help at home by not buying rocks for your garden that have been quarried from limestone pavements, and when visiting rocky habitats in spring and summer, take care not to disturb nesting birds such as ring ouzel.
Wildlife Trusts are actively involved with the conservation and restoration of rocky habitats – you can support the work of your local Wildlife Trust by becoming a member.