Specialized pathogens are thought to maintain plant community diversity by producing a stabilizing mechanism (often called the Janzen-Connell hypothesis, plant-soil feedback, or conspecific negative density-dependence). However, recent studies suggest that this mechanism is less effective if species differ in susceptibility to their specialist pathogens. Many studies have shown that species differ in their susceptibility to pathogens, although the mechanism for this difference is usually unknown. A recent hypothesis proposed that differences in susceptibility occur because populations differ in their diversity of pathogen resistance genes (R-genes), with genetically diverse populations being more resistant. Here, we use theoretical models to test whether R-genes alter how pathogens affect tree species coexistence. In our model, seedlings under a parent or non-parent adults of their same species are exposed to pathogens; however, such seedlings can be protected if their R-genes differ from those of the adult. We analyze both a deterministic model (to study selection on genes and populations) and a stochastic simulation model (to analyze the impact of drift and immigration).
Results/Conclusions
We find that seedlings from populations with low R-gene diversity are more susceptible to their pathogens, and thus suffer higher seedling mortality. High seedling mortality puts populations at a competitive disadvantage, causing them to become rarer and potentially excluded. This effect is particularly strong when seed dispersal is low, because seeds of rare species are more likely to encounter pathogens (i.e. as they cannot disperse far enough to escape them). Additionally, R-gene diversity may be reduced during population bottlenecks. We show that this reduces the ability of populations to recover from rarity, and may cause a vicious cycle: bottlenecked populations lose R-gene diversity, making them more susceptible to pathogens, which in turn makes them rarer, leading to further loses in R-gene diversity, and so on. We show that high seed dispersal, the immigration of new R-genes, and species-specific pathogens that are not affected by R-genes can counteract these effects. Together, our results suggest that while species-specific pathogens generally promote plant diversity, genotype-specific pathogens may undermine it, especially when seed dispersal is low.