Stochastic and Deterministic Processes Influence Saprotrophic Fungal Diversity.

A long-standing focus of community ecology has been to quantify patterns of diversity in order to understand processes (i.e., species-sorting, dispersal limitation) that influence community structure.  This study utilized molecular techniques to quantify α- and β-diversity for saprotrophic fungal communities located on senesced leaves in the forest floor of upland, riparian, and vernal pool habitats.  Utilizing clone library data, we found a significant taxa-area relationship for sugar maple leaves, but not beech leaves, consistent with Wright’s species-energy theory.  This suggests that energy availability associated with plant biochemistry is a key factor regulating the scaling relationships of fungal α-diversity.  We also compared taxa rank abundance distributions to mathematical models associated with niche or neutral theories of community assembly.  The zero-sum model derived from neutral theory showed the best fit to our data.  We collected leaves in a manner that allowed us to quantify spatial distance between communities and quantified community composition for 180 communities using TRFLP.  At five out of the six sites sampled, we found a distance-decay relationship where community distance was significantly correlated with spatial distance.  However, we found that depth also influenced community composition, providing support for species-sorting through the influence of moisture and other factors that vary by depth.  Depth was embedded within the spatial distance matrix, thus providing consistent support for both processes influencing fungal communities.  Collectors curve analysis yielded different patterns when all leaves at a site were randomly sampled or when leaves were stratified and randomly sampled by leaf pack depth.  Stratification indicated that the influence of environmental heterogeneity was most pronounced at vernal pool and riparian sites, and least pronounced at upland sites.   Support for neutral theory influencing α-diversity shows that stochastic processes are important regarding α-diversity, but β-diversity is influenced by a combination of stochastic and deterministic processes.