Leptogium rivulare is a globally rare lichen with very restrictive habitat requirements (the periodically inundated substrate, bases of trees or rocks; Jørgensen 2007). Potential habitats are easily diminished or even eliminated by natural processes (e.g. interference with the extent or duration of spring flooding) or the deterioration of habitats may be caused by human activities.
This taxon has only a few rich subpopulations of stable condition worldwide (in Republics of Marii El and Komi, Russia, and in Ontario, Canada). In most documented locations it is represented by single or few thalli although suitable substrate is present and careful searching for the species has been performed. In six historical locations (Canada, Estonia, Finland and USA), this taxon is considered extinct.
Considering criterion A, information about population reduction is based on records of this species. Altogether 30 localities in Europe and North America are documented and of these six are considered extinct (single localities in Estonia and Finland, subpopulations in Illinois and Vermont in USA, and two subpopulations in Ontario, Canada). Population size reduction is estimated at 20% over the last three generations. This is less than 30% and thus, this species does not qualify for Vulnerable. However, future decline is projected in quality of habitat as the largest subpopulations in Canada (Ontario, Ottawa) are under threat of urban development and recreational activity. In Russia, for example in Republics of Marii El and Komi, the pollution of rivers is another potential threat for this species as only unpolluted water streams are considered suitable for it. This species is assessed as Near Threatened.
Information about population reduction is based on records of this species. Altogether 30 localities in Europe and North America are documented but of these, six localities are considered extinct or possibly extinct (single localities in Estonia and Finland, subpopulations in Illinois and Vermont in USA, and two further subpopulations in Ontario, Canada).
It seems that subpopulations are more threatened in the European Nordic countries (it is Extinct in Finland and Estonia, and Endangered in Sweden), while there are new records from Belarus and Lithuania (one locality in either country). Russian subpopulations, especially those from the Republic of Marii El, are evaluated as stable (Bogdanov and Urbanavichus 2008). In the Republic of Komi, there are 30 separate observations of which only two were evaluated as rich populations; all other records were of a few thalli (Pystina et al. 1999). In the Middle Urals, this species is found as the only large specimen and no other findings were made so far although suitable substrate is abundant in this area and despite thorough searches (Paukov and Teptina 2012, Paukov pers. comm.).
In Canada, Leptogium rivulare is currently known in Manitoba, and in three localities in Ontario, where the subpopulations are 15 to 35 km apart. In two of these localities, the lichen occurs almost exclusively on tree bases around seasonal ponds. It forms irregular growths on dozens to many hundreds of trees in these places. On some trees, it is barely present; on others, it forms large, encrusting patches about 50 cm across. The great numbers of small thalli often scattered over the bark indicate that successful reproduction is occuring, while the large patches show persistence over many years. Almost all individuals are fertile, and are capable of reproducing when only a few years old. The total amount of Leptogium rivulare in Ontario is c. 40 m2 (COSEWIC 2004).
On the other hand, the region where the species is now found has been well studied by lichenologists, and careful searching of the full range of habitat possibilities in more than 60 sites in the vicinity and region of the existing and historical subpopulations, has failed to turn up any sign of this species (COSEWIC 2004).
In summary, Leptogium rivulare is a globally rare lichen having only a few rich subpopulations of stable condition (in Republics of Marii El and Komi, Russia, and in Ontario, Canada). However, even in these countries it is at high extinction risk; it is threatened in Canada, category 1 (Endangered) in Komi and category 3 (Rare) in Marii El Republics. In most documented localities, it is represented by single or few thalli although suitable substrate is present and careful searching for the species has been performed. In six historical locations worldwide (Canada, Estonia, Finland and USA) the taxon is considered extinct; however, most of the historical localities have not been revisited because of too vague locality data.
Population Trend: decreasing