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  1. null (Ed.)
    Over the past five decades, many studies have examined the Janzen-Connell hypothesis, which posits that host-specific natural enemies, such as insect herbivores and fungal pathogens, promote plant species coexistence by providing a recruitment advantage to rare plant species. Recently, researchers have been exploring new and exciting angles on plant-enemy interactions that have yielded novel insights into this long-standing hypothesis. Here, we highlight some empirical advances in our understanding of plant-enemy interactions in tropical forests, including improved understanding of variation in plant species’ susceptibility to enemy effects, as well as insect and pathogen host ranges. We then review recent advances in related ecological theory. These theoretical studies have confirmed that specialist natural enemies can promote tree diversity. However, they have also shown that the impact of natural enemies may be weakened, or that natural enemies could even cause species exclusion, depending on enemy host range, the spatial extent of enemy effects, and variation among plant species in seed dispersal or enemy susceptibility. Finally, we end by discussing how human impacts on tropical forests, such as fragmentation, hunting, and climate change, may alter the plant-enemy interactions that contribute to tropical forest diversity. 
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  2. Microbes are thought to maintain diversity in plant communities by specializing on particular species, but it is not known whether microbes that specialize within species (i.e., on genotypes) affect diversity or dynamics in plant communities. Here we show that soil microbes can specialize at the within-population level in a wild plant species, and that such specialization could promote species diversity and seed dispersal in plant communities. In a shadehouse experiment in Panama, we found that seedlings of the native tree species,Virola surinamensis(Myristicaceae), had reduced performance in the soil microbial community of their maternal tree compared with in the soil microbial community of a nonmaternal tree from the same population. Performance differences were unrelated to soil nutrients or to colonization by mycorrhizal fungi, suggesting that highly specialized pathogens were the mechanism reducing seedling performance in maternal soils. We then constructed a simulation model to explore the ecological and evolutionary consequences of genotype-specific pathogens in multispecies plant communities. Model results indicated that genotype-specific pathogens promote plant species coexistence—albeit less strongly than species-specific pathogens—and are most effective at maintaining species richness when genetic diversity is relatively low. Simulations also revealed that genotype-specific pathogens select for increased seed dispersal relative to species-specific pathogens, potentially helping to create seed dispersal landscapes that allow pathogens to more effectively promote diversity. Combined, our results reveal that soil microbes can specialize within wild plant populations, affecting seedling performance near conspecific adults and influencing plant community dynamics on ecological and evolutionary time scales.

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