The arbuscular mycorrhizal (AM) fungi are a globally distributed group of soil organisms that play critical roles in ecosystem function. However, the ecological niches of individual AM fungal taxa are poorly understood.

Island biogeography theory is one of the most influential paradigms in ecology. That island characteristics, including remoteness, can profoundly modulate biological diversity has been borne out by studies of animals and plants. By contrast, the processes influencing microbial diversity in island systems remain largely undetermined. We sequenced arbuscular mycorrhizal (AM) fungal DNA from plant roots collected on 13 islands worldwide and compared AM fungal diversity on islands with existing data from mainland sites. AM fungal communities on islands (even those >6000 km from the closest mainland) comprised few endemic taxa and were as diverse as mainland communities. Thus, in contrast to patterns recorded among macro-organisms, efficient dispersal appears to outweigh the effects of taxogenesis and extinction in regulating AM fungal diversity on islands. Nonetheless, AM fungal communities on more distant islands comprised a higher proportion of previously cultured and large-spored taxa, indicating that dispersal may be human-mediated or require tolerance of significant environmental stress, such as exposure to sunlight or high salinity. The processes driving large-scale patterns of microbial diversity are a key consideration for attempts to conserve and restore functioning ecosystems in this era of rapid global change.

Questions: What is the overall restoration potential of the persistent soil seed bank of abandoned alvar grassland sites? To what degree does the share of characteristic alvar grassland species in the soil seed bank change during secondary succession from traditionally managed pasture towards stages overgrown by Corylus avellana and Juniperus communis?Location: Northern Estonia.Methods: The species composition of above-ground vegetation and the persistent soil seed bank were investigated in successional alvar grasslands. Particular attention was given to typical grassland species in the persistent soil seed bank. Soil seed bank density was also recorded. General linear models and non-metric multidimensional scaling were used to assess successional trends and the restoration potential of the soil seed bank.Results: The cessation of traditional management has led to a considerable decline in above-ground plant species richness. The composition of aboveground vegetation in different successional stages was considerably more variable than that of the seed bank. We detected large and species-rich persistent soil seed banks in all our study sites. About 70% of species in the seed banks of overgrown sites could be classified as typical grassland species. The frequency of typical alvar grassland species, such as Arabis hirsuta, Arenaria serpyllifolia, Cerastium fontanum, Helianthemum nummularium, Poa angustifolia, Potentilla crantzii, Sedum acre and Veronica spicata in the seed bank of long-term abandoned (25 and 50 yr) sites was unexpectedly high.Conclusions: Abandoned semi-natural grassland sites in northern Estonia still contain a relatively large grassland community species pool, including a large and effective soil seed bank. The soil seed bank could thus play a more important role in the restoration management of abandoned dry grassland communities than could be concluded from previous studies.

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Question There are contrasting opinions about how communities assemble along a productivity gradient, particularly in relation to competitive interactions. One view is that functionally similar, and closely related species, are less likely to co‐exist (limiting similarity). Alternatively, competitive exclusion may act on dissimilar species because species bearing traits associated with low competitive ability are excluded (weaker competitor exclusion). We ask if patterns of functional and phylogenetic diversity are related to changes in species diversity in response to fertility manipulations. Location Species‐rich mesophytic grassland in Estonia. Methods The grassland has been manipulated from 2002 to 2011 to increase (fertilizer addition) and decrease productivity (sucrose addition) in small‐scale (50 cm × 50 cm) plots. We linked annual increases and decreases in species diversity to changes in functional and phylogenetic diversity. We used abundance‐weighted mean pair‐wise functional or phylogenetic distance of all possible species pairs. Results We found convergence in four traits (plant height, leaf distribution, lateral spread, type of reproduction) and a decrease in mean functional and phylogenetic diversity, in support of weaker competitor exclusion or habitat filtering. There was less support for limiting similarity, with divergence found for two traits associated with decreasing species diversity (leaf distribution in the sucrose treatment and lateral spread in the fertilizer treatment). Conclusions Our results support the view that competition can lead to the exclusion of weaker competitors, rather than increasing functional and phylogenetic diversity, as expected from the principle of limiting similarity. However, multiple assembly processes, which are generally seen as mutually exclusive, are operating simultaneously, albeit on different traits and at different stages of community assembly.