Size asymmetry plays an important role in structuring and maintaining diversity of plant communities. Such asymmetries can be generated by a range of mechanisms, take a variety of distributional forms (e.g., bimodal, uniformly distributed across all size classes, Gaussian), and have implications for infectious disease dynamics. We use models to examine the role of size asymmetry in hosts of infectious disease, motivated by an empirical system consisting of a fungal pathogen (Bipolaris) and its annual grass host, Microstegium vimineum. Fungal infection reduces individual Microstegium growth rates. Two modes of disease transmission are feasible in this system: litter-based (pathogens in grass litter produced at the end of one growing season transmit to germinating individuals the following season), and plant-based (pathogens move from plant to plant over the growing season via, e.g., wind). The relative importance of these modes is not yet known. We incorporated within-host pathogen growth and transmission into physiologically structured annual plant models to explore the seasonal patterning of infection from each mode. Because infection reduces individuals’ grow rates, different transmission routes imply different distributions of end-of-season biomass. These processes in turn could alter the seasonal unfolding of intraspecific competitive interactions.
Results/Conclusions
When Bipolaris is mainly transmitted through litter, and assuming that germination of all seeds occurs at roughly the same time, the entire cohort is exposed to infection around the same time, resulting in a bimodal (infected versus uninfected) end-of-season Microstegium biomass distribution. In contrast, when we assume that Bipolaris transmits mainly plant to plant over the growing season, and that some host individuals are infected at the start of the season (e.g., from litter or perennial plants), we find that rates of infection accelerate over the growing season, before declining late in the season once few uninfected plants remain. This results in a broad distribution of infection times, with the greater proportion clustered near the middle of the growing season. The resulting biomass distribution has a distinctly different pattern than in the litter-based transmission case. We extend these results to examine how asymmetries in size might alter intraspecific competition, and when such competition might dampen, versus exaggerate, size asymmetries. By comparing biomass distribution patterns to data from Microstegium-Bipolaris empirical systems, we can infer which of the two transmission modes (litter-based versus plant-based) seems most likely to be prevalent.