Fungus-growing termites live in obligate mutualistic symbiosis with species of the basidiomycete genus Termitomyces, which are cultivated on a substrate of dead plant material. When the termite colony dies, or when nest material is incubated without termites in the laboratory, fruiting bodies of the ascomycete genus Xylaria appear and rapidly cover the fungus garden. This raises the question whether certain Xylaria species are specialised in occupying termite nests or whether they are just occasional visitors. We tested Xylaria specificity at four levels: (1) fungus-growing termites, (2) termite genera, (3) termite species, and (4) colonies. In South Africa, 108 colonies of eight termite species from three termite genera were sampled for Xylaria. Xylaria was isolated from 69% of the sampled nests and from 57% of the incubated fungus comb samples, confirming high prevalence. Phylogenetic analysis of the ITS region revealed 16 operational taxonomic units of Xylaria, indicating high levels of Xylaria species richness. Not much of this variation was explained by termite genus, species, or colony; thus, at level 2-4 the specificity is low. Analysis of the large subunit rDNA region, showed that all termite-associated Xylaria belong to a single clade, together with only three of the 26 non-termite-associated strains. Termite-associated Xylaria thus show specificity for fungus-growing termites (level 1). We did not find evidence for geographic or temporal structuring in these Xylaria phylogenies. Based on our results, we conclude that termite-associated Xylaria are specific for fungus-growing termites, without having specificity for lower taxonomic levels.

Ecosystem engineers typically exert positive feedback on their environment, which enhances their performance. Such positive feedback is lacking in the establishment phase, when densities are too low and/or patches are too small. There is a strong need to unravel the mechanisms for overcoming the resulting establishment thresholds, both for ecological restoration purposes and to be able to use their services. In the present study, we question whether providing a transient substratum can be used as tool to overcome establishment thresholds, by creating a window of opportunity for initial settlement, using mussels (Mytilus edulis) as a model system. Combining field and flume experiments, we study how biogenic substratum enrichment in the form of a shell layer on a soft mudflat affects the critical dislodgement thresholds and, thus, the chances of mussel establishment at different mussel densities and aggregation states. Flume results showed that the presence of a shell layer reduced dislodgement of mussel patches in low‐energy environments but was conditional for establishment in high‐energy environments. That is, in high‐energy environments with shells, aggregation into clumps enhanced dislodgement, while dislodgement was reduced with increasing overall mussel biomass and overall mussel patch weight. Without shells, dislodgement was always 100%. These findings agreed with our field studies, which showed that coarse shell material reduced mussel losses (by a factor of 3), reduced aggregation (by a factor of 2.4), and increased attachment strength (by a factor of 2.4). Overall, our results show that the local presence of biogenic substratum increases the chance of mussel establishment by enhancing the critical hydrodynamic dislodgement threshold. Thus, the local addition of a biogenic substratum may create a window of opportunity to initiate settlement in more dynamic environments, to shift at a local scale from a bare mudflat state into an established biogenic reef state. Our findings have clear implications for how to approach restoration and management of ecosystem engineers dominated systems. For instance, when positive feedback of ecosystem engineers is lacking, (1) the transient offering of suitable settling substratum may be a necessary step to overcome establishment thresholds, and (2) this becomes increasingly important with increasing abiotic stress.

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