Limnetica 36

One of the major challenges in microbial ecology is to unravel the mechanisms that operate at the spatial-temporal scales structuring microbial communities and how these communities respond to environmental change. It has been argued that the biogeography of microorganisms solely reflects the influence of contemporary environmental variation and “everything is everywhere”. Since the discovery of the key role that decomposer fungi, in particular aquatic hyphomycetes, play in leaf-litter decomposition in streams, mycologists were interested in deciphering several ecological aspects, including the factors that shape the distribution of these communities in streams. At local and regional scales, community structure is affected by several abiotic factors such as pH, temperature, conductivity and nutrients in the stream water, but at a global scale, community similarity also decreases as a function of the geographic distance. However, very few attempts have been conducted to assess the relative influence of environmental versus geographic factors in structuring decomposer fungi in streams. In the current study, we attempted to fulfil this lacuna by quantifying the relative contributions of environmental factors versus spatial factors to community composition, by using data of species composition and environmental (temperature, pH, conductivity and nutrient concentrations) and spatial variables (latitude, longitude and altitude), collected during a recent extensive literature search. Alfa diversity was negatively affected by pH, conductivity, nitrates and phosphorus concentrations in the stream water. On the other hand, a unimodal type relationship was found between species richness and temperature, latitude or altitude. Beta-diversity was also strongly influenced by environmental factors, in particular conductivity, nitrates and phosphorus concentrations, and pH in stream water, but also by the geographic location (latitude and longitude) of the study sites. However, when the effects were disentangled a significant effect was found only for environmental variables. This suggests that stream-dwelling decomposer fungi may exhibit biogeographic patterns, but as proposed by Baas-Becking “the environment selects” and is in part responsible for the spatial variation of stream-dwelling decomposer fungi. However, our conclusions should be taken cautiously because our data were extracted from the literature and reflect a geographical bias in the collection effort. Thus, future investigations should involve broad-scale coordinated surveys, incorporating a larger number of streams in different latitudinal bands and along large environmental gradients, to better assess the relative importance of environmental and geographical factors in structuring fungal communities in streams.