A well-defined species, until 1984 known as Collema fluviatile (Huds.) Steud.
The species has been recently reclassified in the genus Lathagrium as Lathagrium dichotomum (With.) Otálora, P.M. Jørg. & Wedin (Otálora et al. 2014).

Species has suffered severe decline, or has always been very rare, in most of its range. This is less so in the British Isles (especially Scotland), but here its conservation status requires to be enhanced. Within the UK it is correctly evaluated as VU, but a higher ranking is deserved in a global context.

Assessment: the documentation of the species in the United Kingdom is the result of an intense, targeted survey effort that has not been exactly replicated in any other country. It is not certain that the absence of the species in large areas reflects actual absence.

Western and Central Europe, with a single report from Turkey (Kınalıoğlu 2009), and two 19th Century records from Russian Fennoscandia. Apparently extinct from France, Germany, Switzerland, Czech Republic, Slovakia and Romania. Now very rare (perhaps as only single occurrences) in Austria and Poland. Recently reported from single localities in Belgium (Diederich et. al. 2009) and southern Portugal (Raven et al. 2009). Dubiously reported from North America, being apparently based on an unlocalized and undated specimen collected by Thomas Drummond, who collected in North America in the 1830s.

Belgium: one recent record

Asian Russia: recently reported as new to Yakutia by Vershinina et al. (2012)

UNITED KINGDOM: detailed survey and monitoring information are available for sites throughout United Kingdom.

Outside of the British Isles, Collema dichotomum has suffered severe decline, and is now extinct in several countries. In countries where it has been seen in recent years, it is present in only one or two localities.

POLAND: “As good as I know only localities of this species from the Beskid Sądecki Mts could be checked by Lucyna Sliwa at the beginning of 90th (just after 20 years from the first findings by Olech (1973), but Lucyna qualified this species to the group of extinct lichens in that mountain region. Other localities form the Beskid Wyspowy Mts and Beskid Zywiecki Mts by Nowak (1998 - work based on studies made in 1960th) had not been checked since the comparative studies had not been repeated there. The same situation refers to the work of Kozik (1977) made in the Carpathian Foothills.” (Pawel Czarnota, pers. comm., e-mail to T. Spribille 27 Jan 2015)

BELGIUM (Diederich et al. 2009): “This species was considered as extinct in the area of study (Sérusiaux et al. 2004) as the latest localities of that species were explored in 2001 in vain. The most recent herbarium specimen was collected by J. Lambinon in 1967 near Nisramont, in the Ourthe valley. Quite surprizingly, healthy populations of the species were observed during this summer (August 2009) in the same localities. Obviously rather low level of the river allowed detection of the species which was probably overlooked in 2001.
Collema dichotomum is a rare species throughout Europe (Degelius 1954) and was included as ‘endangered’ in the Lichens Red List for Europe by Sérusiaux (1989), an option confirmed by Church et al. (1996) in a similar analysis for the lichens in Britain. In England as well as in Scotland, surveys to assess the status of the species and conservation projects have been conducted with the funding of official Agencies (see http://www. thebls.org.uk/content/survey.html) ; further a species action plan has been adopted (http://
http://www.ukbap.org.uk/UKPlans.aspx?ID=234). A single recent locality is mentioned by Wirth (1995) for SW Germany, and no recent locality
is known in France (C. Roux, pers. comm., 1 Sept. 2009). Collema dichotomum grows in freshwater rivers and is almost permanently inundated;
it is very sensitive to eutrophication and any pollution or mechanical damage to the river bed. It is thus very interesting that a completely
unprotected section of the Ourthe river in Belgium still hosts a nice population of that lichen. At the European scale, C. dichotomum is the most interesting macrolichen occuring in our area of study and deserves special monitoring and effective protection of its habitat.”

FRANCE: four of seven sites are considered historical (C. Roux, pers. comm., Jan. 2015). See also attached graphic. Roux wrote (pers. comm., e-mail to T. Spribille dated 27 Jan 2015): “Cette espèce a été récemment trouvée dans les Pyrénées par Clother Coste en 2011 :
1) in Roux et al. 2011 : 67 ; Languedoc-Roussillon, Pyrénées-Orientales, Nohèdes, à proximité de la réserve naturelle de Nohèdes, bac des Torrelles, ruisseau des Torrelles, latitude : 42,64707° N, longitude : 2,26032° E, sur une surface de granite, horizontale, dans l’eau, orient. gén. SE, orient. loc. SE, pente 0°. Alt. 1485 m. Date : 2009/07/29. herbier C. Coste.
2) in Coste 2011 : 102 ; département des Pyrénées-Atlantiques (64) et des Hautes-Pyrénées (65).
Je n’ai aucune information précise sur ces deux dernières mentions.
Toutes les autres mentions sont anciennes ou d’après des spécimens anciens. Extrait du Catalogue des lichens de France :
Lathagrium dichotomum (With.) Otálora, P.M. Jørg. et Wedin — Syn. Collema dichotomum (With.) Coppins et J. R. Laundon, Collema fluviatile (Huds.) Steud., Leptogium cataclystum (Körb.) Harm. — Lichénisé, non lichénicole — Lorraine, Massif central (Corrèze et monts de l’Espinouse), Pyrénées. Très rare. Patrimonial d’intérêt international. En danger critique d’extinction [cr] — 34a, 57a, 64r, 65r, 66!, 87a, 88a — Saxicole, sur rochers et blocs de roches silicatées basiques ou modérément acides, calcifuge, moyennement acidophile ou subneutrophile, très hydrophile (longtemps ou constamment inondé), plutôt héliophile, non nitrophile. Étages collinéen et montagnard. Ombroclimats humide et hyperhumide — Clauzade et Roux 1985 : 342-343; Degelius 1954 : 361-366 ; Ozenda et Clauzade 1970 : 310 ; Companyo 1864 : 814 ; Coste 2011 : 102 ; Crozals 1912 : 268 ; Harmand 1905 : 106-107 ; Lamy 1880 : 341 ; Roux et al. 2011 (Pyrénées-Orientales) : 67 — Rem. Voir sous Leptogium rivulare.”

SPAIN AND PORTUGAL: no population and trend data were obtained.

It grows, often permanently inundated, in little polluted, freshwater rivers (rarely inland lakes) on ± basic siliceous rocks, especially where the latter form slabs, forming a stable riverbed. It requires some water movement, but is absent from rapids and areas of fast flow, and can tolerate some degree of silt deposition. It can tolerate some dappled shade from overhanging trees, but does not occur where there is permanent shading by trees. It mainly occurs in the middle reaches of rivers. Higher upstream conditions are too unstable, whilst lower downstream the riverbed is too heavily silted. At some of the lower sites on a river course, the species becomes confined to ribs and the edge of rocks facing upstream, where these niches are kept free of silt by the current.

The major threat to this species is from small or medium-scale hydro-schemes in the relentless and often reckless persuit of ‘green energy’. Currently, the probable largest known population of the species, on the River Isla in eastern Scotland, is under threat from a hydro-scheme proposal, even though both the lichen and its site (a Site of Special Scientific Interest) supposedly have the highest protection. Other threats are numerous, and include anything that increases the silt load in rivers, reduces water quality, exposes the lichen to longer than normal periods of drought (e.g. extraction of water for agricultural, domestic or commercial use during periods of low rainfall), or increases amount of shade from overhanging trees.

In the UK, Collema dichotomum is red-listed as VU, and is listed on Schedule 8 of the Wildlife and Countryside Act. For other countries for which a red-list exists, it is evaluated as either EX or CR.

Most knowledge of this species’ biology and ecology is based on observations and a small amount of monitoring. Much more research is needed into its population dynamics, quantification of its ecological and physiological tolerances (e.g. relating to silt deposition, drought, eutophication) and reproductive biology so as to advise future conservation efforts. Standardized recording and monitoring protocols need to be developed, given the difficulties involved with an underwater organism.

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