The Ecological Value of Metalliferous Mining Sites

Cornwall has more derelict land than any other county in England (4,888 ha), this being 12% of the national total. 3,899 ha of this is made up of metalliferous spoil heaps. Mining land presents special problems for plants and wildlife, but also unique opportunities. Some of the factors special to these sites include:

• A mosaic of hummocks and hollows
• Compacted and often contaminated soils, leading to long-standing bare ground, affected by wind and water erosion and prone to extremes of temperature
• Lack of topsoil, soil structure, nutrients and micro-organisms
• Steep slopes
• Toxicity due to mineral content – for example residual, fine particulate copper, zinc, lead and arsenic
• Unstable substrates
• Derelict buildings and building rubble
• Abandoned tanks and ponds, as well as poorly drained areas
• Shafts, adits and openworks.

The vegetation which develops on such sites is generally typical of the surrounding area, and may thus be composed of gorse scrub, willow, acid grassland, heathland, bare ground, ruderal habitats in disturbed ground, wetlands or open water. In Cornwall, gorse and willow scrub typically take the place of woodland. The precise composition of habitats on any site will be related to particular conditions, and may give rise to rare and unusual communities – for example, some mosses and lichens are associated with concentrations of heavy metals, particularly copper.

As a general rule, the longer such sites have been abandoned, the more diverse the vegetation development and the greater the nature conservation interest, though mere vegetation cover is not a guarantee of importance, since species requiring ‘pioneer’ conditions are a key feature of such sites. Variation in abandonment of different areas of a site can also play its part in ensuring biodiversity. Many of the most important sites in Cornwall and West Devon have been long abandoned and have been relatively isolated from the effects of recent disturbance by mineral reprocessing, land reclamation, tipping or motorcycle scrambling.

These sites now represent some of the last remaining semi-natural habitat in Cornwall, often forming islands of wildlife habitat within urban and industrial areas, or within areas of improved farmland. As such, they may well be important reserves for the wider community. Many also contain small areas of heathland, a declining resource both in Cornwall and nationally. The areas of bare ground they often include are also increasingly rare in the region.

Plants

Compacted, contaminated soils on such sites may provide suitable growing conditions for a wide range of species which may not be able to grow in more fertile areas where they are out-competed. Typical of these are grasses and other plants adapted to contaminated soils such as Common Bent (Agrostis capillaris), some varieties of Thrift and Heathers. The calcareous pointing on derelict walls may provide niches for plants unsuitable for acid Cornish soils. Rushes may grow in metal contaminated water and silts, whilst stream beds may be coated with the metal tolerant algae Microthamnion species.

Bryophytes, mosses and lichens

Soils rich in copper are globally scarce and over 25% of the total number of mosses and liverworts found in Cornwall have been recorded on abandoned metalliferous mine sites. Several of these species are nationally rare and many are restricted to metal-contaminated conditions. The rarest include Cornish Path Moss (Ditrichum cornubicum) which is unique to Cornwall, the moss Scopelophila which is found on only three or four sites in Britain, one of these being in Cornwall and the liverwort Cephaloziella nicholsonii, which is unique to Britain, over 90% of the population being found in Cornwall. Cornish mining sites also support nine ‘Nationally Rare’ species, including all of the national population of Cephaloziella integerrima and most of Cephaloziella massalongi and Scopelophilia cataractae. An SSSI including seven discrete sites have now been designated for its bryophyte importance. A further 138 SSSI’s protect sites of mineralogical or biological interest, as does one SAC designated for the importance of the flora of an area of former mining land.

Lichens are the born survivors of extreme environments, and are often important colonisers, occurring on spoil heaps, mine buildings and adit walls. It has recently been established that mineralisation type is a key factor in determining the composition of assemblages on mine spoil, some species favouring low pH iron-rich substrates, others high pH copper-rich sites. A visit to Cornwall by the British Lichen Society in 1987 identified a range of metallophyte lichens and metal-rich eco-types near Redruth and Liskeard. These are rare in Britain, and are best developed at some Cornish mine sites.

Animals

Disused shafts and ruined buildings often provide important nest and hibernation sites for a wide range of birds such as Raven and Peregrine, whilst bats, including the rare Greater Horseshoe Bat, use open mine workings as roost sites. Dry areas may be colonised by common lizards and slow-worms, whilst wetter areas provide habitats for frogs and toads.

Terrestrial invertebrates

Areas of heathland provide important autumn nectar sources for flying insects; heather is also an important food source for several invertebrates. Bare ground habitats are important for a range of insects which use open areas for nesting, chasing after prey and basking. Of note are the Mottled grasshoper (Myrmeleotettix maculatus), the Grayling butterfly (Hipparchia semele), the Silver-studded blue, tiger-beetle wasp and western bee-fly, several mining bees and other nationally rare species.

Freshwater invertebrates

Freshwater contaminated by metalliferous run-off is usually low in invertebrate diversity, the low pH and lack of available calcium making molluscs and crustaceans rare in such conditions. Some species thrive here, however, including Polycentropodid caseless caddisflies, Nemourid stonefly larvae, non-biting Chironomid midge larvae and the larvae of Dragonflies, including the Nationally Scarce Ischnura pumilio. The small red damselfly Ceriagrion tenellum has been found on several former mining sites near Camborne. In addition, a range of beetles and bugs live in the less contaminated upper areas of these water bodies.

The following species have been identified as priority species within the Cornwall Biodiversity Initiative Action Plan short and middle lists for metalliferous mine sites: