Population status and ecology of the narrow endemic moss Thamnobryum cataractarum N.G.Hodgetts & Blockeel in England

Of the eight species of Thamnobryum (Hypnales: Neckeraceae) known to occur in Europe and Macaronesia (Hodgetts et al. 2020), four are narrow endemics, and of these, three, T. fernandesii Sérgio of Madeira and T. angustifolium (Holt) Nieuwl. and T. cataractarum N.G.Hodgetts & Blockeel of Britain, are all confined to extreme rheophilous habitats. Although these rheophilous species are similar morphologically, Olsson et al. (2009) concluded that this was probably the result of convergent evolution, and that each appears to have evolved independently from surrounding populations of T. alopecurum (Hedw.) Gangulee. Thamnobryum cataractarum (Figure 1) is one of 25 moss species that are listed on the world IUCN Red List as Critically Endangered, the highest category of global extinction risk (IUCN 2021). It is known from only a single site: Twistleton Glen, Mid-west Yorkshire (v.-c. 64), UK. Searches for other populations at potential sites within the wider region have been unsuccessful (Hodgetts 2005a). Within the riverine gorge of Twistleton Glen, the moss occupies a narrow niche, limited to rock substrate in areas of cascades and waterfalls. The species has been the subject of a national conservation action plan and in England is considered a species of principal importance for conservation under Section 41 of the Natural Environment and Rural Communities Act 2006. Despite the high degree of conservation concern, since the discovery of Thamnobryum cataractarum in September 1991 (Hodgetts and Blockeel 1992) there has been only a single survey of its population at Twistleton Glen, this having been done in September 2005 (Hodgetts 2005b), plus a brief and partial monitoring visit in May 2010 (Goodison 2010). The aim of the present study was to investigate the current population status and ecology of T. cataractarum. Taxonomy follows Blockeel et al. (2021). Twistleton Glen is a deep and narrow wooded gorge that has been carved by an existing river through interbedded sandstones and siltstones of the Ordovician Ingleton Group, located on the western edge of the Yorkshire Dales National Park in northern England. These rocks are base-deficient, but the river water flowing through the gorge is baseenriched because the rock of the wider catchment is mostly Carboniferous limestone (Garsdale Limestone Formation). The glen is located within the Thornton and Twistleton Glens Site of Special Scientific Interest and forms part of a walking route known as Ingleton Waterfalls Trail, managed as a tourist attraction by a private company. The climate is oceanic, with 187 rain days/year and average air temperatures of 12.7°C during the hottest month (July) and 1.2°C during the coldest month (February) for the period 1961–2002 (Met Office data supplied through the UK Climate Impact Programme). Geographical coordinates follow the Ordnance Survey National Grid reference system. Grid cells are referred to by the coordinates of the southwest corner. Fieldwork was carried out during September 2021. A detailed search was made for Thamnobryum cataractarum within accessible parts of the gorge, and locations of colonies were indicated by a temporary marker flag, ignoring further locations if < 1 m from an existing flag. Each flagged location was considered a separately occupied 1 m grid cell, and following Bergamini et al. (2019), each occupied 1 m grid cell was considered an ‘individual-equivalent’. Coordinates of all flagged locations were recorded with a hand-held GPS (Garmin GPSMAP 64s, Garmin Ltd, Olathe, USA), which reported an accuracy of≤ 10 m. Subsequent mapping was undertaken in Quantum GIS (QGIS.org 2022). Relevés were recorded to describe habitat conditions and community composition of locations occupied by Thamnobryum cataractarum, generally following the method of Bates (2011). Sample locations were selected to represent the range of conditions occupied by the moss. Relevés measured 50 × 25 cm, and within each, percentage cover of each species of bryophyte, vascular plants, lichen and