Nowell's limestone moss  (Zygodon gracilis)

Classified as Endangered in the UK, and protected under Schedule 8 of the Wildlife and Countryside Act. Classified as Vulnerable under the European Red Data Book 1995.

This rare moss is a brownish-green in colour and forms dense cushions. It is named after John Nowell, an amateur bryologist, who first discovered the species in 1860.

The moss is endemic to Europe, and is found in Britain, Austria, Germany, Switzerland, Italy and Poland. In the UK, it has been found only in the West Yorkshire Pennines.

Nowell’s limestone moss has been found only on old limestone walls and, more rarely, on a few rocky outcrops, all in dry but exposed situations.

Very little is known about the biology of this species, except for the fact that the last known occasion when it produced fruiting bodies was in 1866. It was believed that the reason for this was because the individual specimens were too far apart to cross-fertilise. However, in the autumn of 2002, Fred Rumsey, a researcher from the Natural History Museum in London, was one of a pair of bryologists who discovered nearly 500 patches of the moss on an old dry stone wall in West Yorkshire. Out of all these, only one patch had produced fruit capsules. Many species of moss fail to reproduce this way, but they can propagate by budding off pieces of themselves. However, to colonise new sites, they need to produce spores as these can travel much further than the moss can achieve by budding. It also improves the species' chances of survival by spreading the populations over a larger area.

With a species confined to just one relatively small area, the threats to its survival are great. In the case of this moss, the destruction or re-building of dry stone walls poses a serious danger unless this work is carried out sympathetically. The moss also seems to be threatened by the toxins associated with the zinc netting sometimes placed on top of walls to increase their stock-proofing.

Nowell’s limestone moss is listed as a priority species in the UK Biodiversity Action Plans (UK BAP), and is included in English Nature's Species Recovery Programme (SRP). Until the discovery of the fruiting moss in 2002, plans for conserving the species included establishing an ex-situ colony, in order to ensure the possibility of propagating specimens for a future re-introduction programme. However, in spite of this discovery, and because it is such an endangered species, ex-situ colonies will be maintained, and trial re-introductions will go ahead. It might even prove possible to carry out some careful 'match-making' on the plants in the wild. Fred Rumsey and his colleagues hope to aid the mosses' chances of reproducing by placing pieces of male and female moss next to one-another. If it works, it will avoid the need to re-create an artificial habitat in the lab on which to propagate the moss prior to cross-fertilisation.

• Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
• Ex-situ: measures to conserve a species or habitat that occur outside of the natural range of the species. E.g. in zoos or botanical gardens.
• Spores: microscopic particles involved in both dispersal and reproduction. They comprise a single or group of unspecialised cells and do not contain an embryo, as do seeds.