Bacidia incompta (Borrer ex Hook.) Anzi

Lichens are remarkable organisms; they are stable combinations of an alga and/ or a cyanobacteria with a fungus, living together in a symbiotic association (3). The fungus causes the alga to release sugars, which allow the fungus to grow, reproduce and generally survive. The fungus provides protection for the alga, and enables it to live in environments in which it could not survive without the fungal partner (3). A general rule is that the fungal component of a lichen is unable to live independently, but the alga may live without the fungus as a distinct species (5). Many lichens are known to be very sensitive to environmental pollution, and they have been used as 'indicators' of pollution (5).

A UK Biodiversity Action Plan Priority Species, Bacidia incompta has a Species Action Plan, which is lead by the wild plant charity Plantlife; this plan aims to maintain the existing populations and to encourage the species to spread by providing suitable habitat (2). In addition, Plantlife has included this lichen in its Back from the Brink programme (6).

The lichen Bacidia incompta does not have a common name. The Generic part of the scientific name, Bacidia means 'rod-shaped' and refers to the shape of the spores, called 'ascospores', which develop inside a tiny bag-like structure known as the 'ascus' (3). The specific part of the name, incompta is derived from the Latin word 'incomptus', which means untidy or unkempt, and refers to the scrappy outline of this lichen (4). This lichen grows as a whitish-fawn (5) or greyish-green crust with brown or blackish fruits, which look like dots (6).

Grows on the trunks of mature trees, mainly on elm in Britain, although it may also occur less commonly on ash, holly, beech, sycamore and hornbeam (2). It often grows in rain-seepage tracks on the bark, which are rich in nutrients, or underneath wounds in the bark (2). Main habitats supporting this species are old parkland and wayside trees in open conditions (2).

This lichen was formerly widespread in Britain, reaching as far north as Aberdeenshire, and was once particularly common in the south-east of England, but rare in more oceanic areas (5). However, it has suffered a severe decline, and has been completely lost from many of its former sites (5). It persists in the New Forest, and a few sites in Dorset, Devon and Cornwall (7), one site in Wales, and in some areas of Scotland (2). Outside of Britain, this species occurs in continental Europe, North America and Madeira (2).

Classified as vulnerable in Great Britain (2).

Prior to 1960, the decline of this lichen was largely due to air pollution and the felling of wayside or parkland trees (5). After 1960, the effects of Dutch elm disease took a huge toll on the species, as the disease destroyed many populations of the main host tree of this lichen (5).