Understanding mechanisms maintaining forest biodiversity and relative abundance patterns remains an elusive problem in community ecology. The role of pathogens and other enemies has been explored, and shown to maintain biodiversity through negative density dependent processes involving specialist pathogens, but these mechanisms have not maintained relative abundance patterns. Recent work on the enemy susceptibility hypothesis has suggested that generalist pathogens may be able to maintain biodiversity levels through differential impact on survival of species, but a thorough testing of the hypothesis has not yet occurred. This work utilizes simulation-based modeling of disease and plant dynamics in a spatially explicit context to answer the question: can generalist pathogens maintain both biodiversity and relative abundance patterns in a simulated forest? The work first explores Janzen-Connell style density-dependent interactions in a dispersal limited environment, then considers a single generalist pathogen. It then moves into a model with two quasi-generalist pathogens, each infecting a group of species that can be viewed as representing a deep phylogenetic clade. Finally, a mixed model of generalist and specialist pathogens is considered. The impact of seed and fungal dispersal distances are explored for each model scenario.
Results/Conclusions
Janzen-Connell processes produced expected results - species were maintained in the community, but in a fluctuating pattern that did not maintain relative abundance patterns throughout the duration. A single generalist pathogen ultimately led to extinction of rare species. However, when two quasi-generalist pathogens were introduced, representing pathogens that are able to infect major clades of plants, but not all species, biodiversity and relative abundance patterns were maintained stably throughout the duration of the simulations. Spatial analysis suggests that rarer species that were more susceptible to the pathogen were nonetheless able to persist in spatial refugia that were pathogen-free for extended periods of time because of clumping and dispersal limitation of the pathogen. The mixed model produced more varied results, depending upon the relative virulence of the generalist vs. specialist pathogens. Under some scenarios, long-term maintenance of both biodiversity and relative abundance patterns occurs, but simulations suggest that it is extremely sensitive to parameterization of the model.