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Abstract. Habitat fragmentation is well known to affect plant and animal diversity as a result of reduced habitat area and connectivity, but its effects on microorganisms are poorly understood. Using high-throughput sequencing of two regions of the rRNA gene, we studied the effects of forest area and connectivity on the diversity and composition of fungi associated with the roots of the dominant tree, Metrosideros polymorpha, in a lava-fragmented landscape on the Island of Hawaii. We found that local fungal diversity increased with forest area, whereas fungal species composition was correlated with fragment connectivity. Fragment size and connectivity were significant predictors even when we included environmental covariates, which were also associated with fungal diversity and composition. Fungal species composition was more similar among highly connected fragments than among poorly connected ones. We also identified individual taxa that varied in abundance with connectivity. Taken together, our results show that habitat fragmentation can alter microbial diversity and composition via differential response among fungal phyla and individual taxa to habitat connectivity.

Both top-down (grazing) and bottom-up (resource availability) forces can determine the strength of priority effects, or the effects of species arrival history on the structure and function of ecological communities, but their combined influences remain unresolved. To test for such influences, we assembled experimental communities of wood-decomposing fungi using a factorial manipulation of fungivore (Folsomia candida) presence, nitrogen availability, and fungal assembly history. We found interactive effects of all three factors on fungal species composition and wood decomposition 1 year after the fungi were introduced. The strength of priority effects on community structure was affected primarily by nitrogen availability, whereas the strength of priority effects on decomposition rate was interactively regulated by nitrogen and fungivores. These results demonstrate that top-down and bottom-up forces jointly determine how strongly assembly history affects community structure and function.

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Aim Priority effects, in which the order and timing of species arrival influence community assembly, are thought to be stronger in less stressful environments, reflecting increased competition, but this hypothesis has rarely been examined over evolutionary time. Here, we test the hypothesis using an island community of alpine plants. Location Murchison Mountains, South Island, New Zealand. Methods Lineage age data for 14 monophyletic New Zealand alpine plant genera, with stem ages ranging from 2 to 17 Ma, were paired with species presence and abundance data in 261 plots, covering nearly 400 km2. The relationships between lineage age and the relative abundance and richness of focal genera across elevation (800–1620 m) and precipitation (2500–5000 mm yr−1) gradients were investigated using linear models in a Bayesian framework. Results The relative abundance of focal genera increased with lineage age, but this effect was weaker at higher elevation and precipitation, where plants are likely to have experienced more stressful conditions. The relative richness of focal genera increased at a consistent rate despite significant changes in overall richness of focal genera across both elevation and precipitation gradients. Main conclusions Our results suggest that priority effects, on a time‐scale of millions of years and involving significant evolutionary change, influence community assembly, leading to increased dominance of older lineages, but that the strength of these effects may vary with environmental conditions. Environmental gradients correlated with abiotic stress, such as elevation in alpine systems, may be particularly important for predicting the strength of both ecological and evolutionary priority effects.