Abstract. An essential ecosystem service is the dilution effect of biodiversity on disease severity, yet we do not fully understand how this relationship might change with continued climate warming and ecosystem degradation. We designed removal experiments in natural assemblages of Tibetan alpine meadow vegetation by manipulating plot-level plant diversity to investigate the relationship between different plant biodiversity indices and foliar fungal pathogen infection, and how artificial fertilization and warming affect this relationship. Although pathogen group diversity increased with host species richness, disease severity decreased as host diversity rose (dilution effect). The dilution effect of phylogenetic diversity on disease held across different levels of host species richness (and equal abundances), meaning that the effect arises mainly in association with enhanced diversity itself rather than from shifting abundances. However, the dilution effect was weakened by fertilization. Among indices, phylogenetic diversity was the most parsimonious predictor of infection severity. Experimental warming and fertilization shifted species richness to the most supported predictor. Compared to planting experiments where artificial communities are constructed from scratch, our removal experiment in natural communities more realistically demonstrate that increasing perturbation adjusts natural community resistance to disease severity.

Acinetobacter baumannii is an opportunistic pathogen that is a cause of clinically significant nosocomial infections. Increasingly, clinical isolates of A. baumannii are extensively resistant to numerous antibiotics, and the use of polymyxin antibiotics against these infections is often the final treatment option. Historically, the polymyxins have been thought to kill bacteria through membrane lysis. Here, we present an alternative mechanism based on data demonstrating that polymyxins induce rapid cell death through hydroxyl radical production. Supporting this notion, we found that inhibition of radical production delays the ability of polymyxins to kill A. baumannii. Notably, we demonstrate that this mechanism of killing occurs in multidrug-resistant clinical isolates of A. baumannii and that this response is not induced in a polymyxin-resistant isolate. This study is the first to demonstrate that polymyxins induce rapid killing of A. baumannii and other Gram-negatives through hydroxyl radical production. This significantly augments our understanding of the mechanism of polymyxin action, which is critical knowledge toward the development of adjunctive therapies, particularly given the increasing necessity for treatment with these antibiotics in the clinical setting.

The artificial fertilization of soils can alter the structure of natural plant communities and exacerbate pathogen emergence and transmission. Although the direct effects of fertilization on disease resistance in plants have received some research attention, its indirect effects of altered community structure on the severity of fungal disease infection remain largely uninvestigated. We designed manipulation experiments in natural assemblages of Tibetan alpine meadow vegetation along a nitrogen-fertilization gradient over 5 years to compare the relative importance of direct and indirect effects of fertilization on foliar fungal infections at the community level. We found that species with lower proneness to pathogens were more likely to be extirpated following fertilization, such that community-level competence of disease, and thus community pathogen load, increased with the intensity of fertilization. The amount of nitrogen added (direct effect) and community disease competence (indirect effect) provided the most parsimonious combination of parameters explaining the variation in disease severity. Our experiment provides a mechanistic explanation for the dilution effect in fertilized, natural assemblages in a highly specific pathogen-host system, and thus insights into the consequences of human ecosystem modifications on the dynamics of infectious diseases.

A number of theoretical and empirical studies have demonstrated the effects of perturbations on ecosystem stability. Compensatory dynamics among taxonomic units have been proposed as a major mechanism regulating the temporal stability of biomass production (hereafter ‘temporal stability’). However, most studies have focused on the effects of species asynchrony on temporal stability in response to perturbations, and few studies examined how compensatory changes among functional groups affected temporal stability. Here, we conducted a 4‐year functional group removal experiment and a 4‐year experimental warming and nitrogen addition experiment in an alpine meadow of Qinghai‐Tibetan Plateau to investigate the effects of perturbations (functional group removal, experimental warming and nitrogen addition) on temporal stability and the potential mechanisms. In both experiments, temporal stability was positively related to both species and functional group asynchrony. However, species asynchrony and functional group asynchrony responded differently to different types of perturbations. In the removal experiment, although asynchrony among both species and functional groups decreased as more functional groups were removed, structural equation modelling showed that removal of different functional groups could affect temporal stability through altering either species or functional group asynchrony. Warming suppressed temporal stability through decreasing asynchrony among species, while nitrogen addition reduced temporal stability mainly through decreasing functional group asynchrony. Synthesis. Our findings demonstrate the importance of considering compensatory dynamics at different taxonomic levels for predicting temporal stability under anthropogenic perturbations in alpine meadows, and throw light on the importance of protecting both species and functional group richness to maintain temporal stability in the context of global change.