Rare and phylogenetically distinct plant species exhibit less diverse root-associated pathogen communities

One of the central questions in ecology is why some species are abundant while others are rare. In plant communities, some studies show that rare plant species are rare because they suffer stronger negative density-dependent effects from pathogens compared to abundant plant species. Moreover, such pathogen effects are also suggested to be related to phylogenetic distance among plant species. However, the importance of pathogens has been commonly inferred by treating the entire pathogen community as a "black box" without explicitly characterizing pathogen richness and community composition. Here, we test two predictions. First, if rare plant species are more susceptible to pathogens, we predict that rare plant species are associated with a higher richness of specialists (i.e., pathogens that attack only a single plant species) and/or the total pathogen community. If phylogenetically distinct plant species are less susceptible to pathogens, we predict that plant species with higher phylogenetic distinctiveness (i.e., a measure of how phylogenetically distant a species is from other co-occurring species) are associated with a lower richness of phylogenetic specialists (i.e., pathogens that attack closely related plant species) and/or the total pathogen community. We conducted a survey of the root-associated pathogen communities from 45 plant species in a subtropical forest. We showed that approximately 40% and 25% of the pathogens were specialists and phylogenetic specialists respectively. In contrast to our first prediction, the richness of the total pathogen community but not the richness of the specialists was found to be positively related to plant species abundance, indicating that rare plant species suffer less from pathogens. Consistent with our second prediction, both the richness of the phylogenetic specialists and the total pathogen community were found to be negatively related to plant species phylogenetic distinctiveness. Furthermore, these correlations were stronger at the earlier plant life stages examined. Synthesis. We found that the root-associated pathogen communities were less diverse in rare plant species and plant species with few close relatives. These associations varied across multiple plant life stages, suggesting that the strength of the above-ground-below-ground interactions change dynamically across plant life span.