Conserving change: occurrence and fertility in a critically endangered species of a dynamic riparian habitat

Abstract

Many species of river riparia are threatened by habitat loss due to altered flood and sediment regime, and associated shifts in vegetation structure. However, their ecological niche is often obscure, especially in inconspicuous organisms such as lichens, hindering their conservation and use as indicator species in river restoration. We studied if variation in sediment size distribution, gravel bank elevation and vegetation structure drive presence-absence and fertility (fruit body production) in the endangered, soil-dwelling lichen Stereocaulon incrustatum along two Swiss braided rivers, using binomial generalized linear mixed effect models in a Bayesian framework. Data was sampled on 811 plots randomly placed along 41 transects perpendicular to the main channels. Presence probability was highest on the most elevated plots, at 30% vascular plant cover in the herb layer, and 30–40% cobble cover, and increased with moss cover. Fruit body production probability was highest under closed canopies of woody plants > 3m. We show that in braided rivers, S. incrustatum is most likely found on elevated, coarse-grained sediments with increased moss but moderate vascular plant cover. This indicates a niche comprising relatively stable riparian environments, where a dry, cryptogam-dominated vegetation establishes on raw soils and competition with vascular plants is moderate. Fertile thalli are mostly found under closed canopies and high densities, suggesting a shift to sexual reproduction with increasing habitat age. While rare but strong disturbances are therefore necessary for habitat creation, older, densely populated habitat patches may harbor important source populations for colonization, thereby representing focal areas for conservation.