Peltigera gowardii

Lendemer & OBrien (2011) split Peltigera hydrothyria (Hydrothyria venosa). The name Peltigera hydrothyria was kept for eastern populations while western populations became Peltigera gowardii.

Peltigera gowardii is an aquatic foliose lichen occurring in cool, clear, free-flowing streams with low nitrogen and neutral pH in western North America, where it is widely distributed in mountain regions from Alaska and British Columbia through Washington and Montana south to Wyoming and California in the United States and Canada. There are an estimated 80 occurrences of this species, with the bulk of its distribution located in the Oregon Cascades region south through California. Climate change is the primary threat to this species, which requires a narrow band of climactic conditions, and does not tolerate increases to water temperature. Additional threats include grazing, timber harvest, fuels reduction, herbicide, flooding, recreation, garbage dumping, trail and road development, dam construction, water diversion, and succession. There are taxonomic issues in this group which have made it difficult to identify western lichens to species, which has stymied conservation efforts. Further research is also needed to clarify trends for Peltigera gowardii.

Peltigera gowardii occurs in western North America, where it is widely distributed in mountain regions from Alaska and British Columbia through Washington and Montana south to Wyoming and California in the United States and Canada (Lendemer and O'Brien 2011). A molecular study (Miadlikowska et al. 2014) found support for a split of P. gowardii into two species; P. gowardii, which extends from Washington state northward, and P. aquatica, which extends southward. However, the two species overlap in range, and are morphologically and chemically identical, making it impossible to identify this taxon to the species level without molecular analysis (McCune and Stone 2022). McCune and Stone (2022) proposed classifying all North American members of this group once again as P. hydrothyria with three varieties (vars. hydrothyria, gowardii, and aquatica). Due to the shifting taxonomy of the Peltigera hydrothyria complex, range extent and other rating factors included the following synonyms, following Dean (2023): Hydrothyria venosa, Peltigera gowardii, P. hydrothyria (of western distribution), and P. aquatica. Range extent was estimated using herbarium specimens, California Natural Diversity Database data, and NatureServe Network occurrence data collected between 1994 and 2025 (RARECAT 2024, CLH 2025, CNDDB 2025, GBIF 2025, NatureServe 2025).

By applying a 1 km separation distance to herbarium specimens, California Natural Diversity Database data, and NatureServe Network occurrence data collected between 1994 and 2025, it is estimated that there are 80 element occurrences (RARECAT 2024, CLH 2025, CNDDB 2025, GBIF 2025, NatureServe 2025).

Peltigera gowardii is dependent on cool, clear, mountain streams in old growth forests and is primarily threatened by any actions altering water quality, quantity, temperature, sediment load, chemistry, light regime, or stream bank stability (Poulsen and Carlberg 2007). Climate change is a major threat to this species, which has a very narrow climactic tolerance and is highly correlated with low minimum winter temperatures (Glavich 2009). Temperature increases of upwards of 2.5 °C are predicted for the Pacific Northwest region during this century (Mote et al. 2003), which could make some of the current occupied habitat of this lichen uninhabitable (Glavich 2009). In addition, climate change could increase the frequency and severity of storms and flooding, which may scour Peltigera gowardii from stream channels during high flows. Additional threats to this species include timber harvest, fuels reduction efforts, herbicide use, livestock-related impacts (such as sedimentation, erosion, disturbance, and pollution from cattle), natural erosion and flooding, camping, and other recreational activities, disturbance and sedimentation caused by off-highway vehicles, garbage dumping, trail and road development, dam construction, water diversion, and succession of trees into mountain meadow habitats (COSEWIC 2013, Dean 2023, CNDDB 2025, CNPS 2025).