Parasitism by fungi and insects plays a crucial role in shaping plant community, yet its ecological functions remain underexplored. In urban areas, many environmental factors such as habitat fragmentation, the heat island effect, low humidity, and nitrogen deposition may affect parasitism in wild plants. The urban stress hypothesis suggests that parasitism increases in urban areas due to stressful environmental conditions. On the other hand, the habitat fragmentation hypothesis suggests that habitat fragmentation suppresses parasitism in urban areas.
This study tested these hypotheses by investigating foliar fungal diseases that are easily observable with eyes (e.g., powdery mildew-like and rust-like diseases) and insect herbivory (e.g., leaf-eating insects) in herbaceous plant communities across urban, suburban, and rural landscape gradients in Japan and China. Epidemic triangle (i.e. three factors as pathogen occurrence, environment, and host susceptibility, as determinants of outbreak intensity) was formulated by zero-inflated binomial regression, and we estimated environmental effects on parasitism-occurrence and outbreak-intensity by Bayesian method.
Many parasitism were found in spring and autumn, and seasonal changes were generally the same to cultivated crops. Results indicate that urbanization increased parasitism occurrence of powdery mildew-like disease. Powdery mildew-like disease might decrease susceptible plants in urban area.
Wind along urban roads might transport powdery mildew spores, and connect their habitats (i.e. host plants). Dry air humidity and rich nitrogen are well known environmental conditions causing powdery mildew on cultivated crops. These environments may enhance powdery mildew in urban regions. Various local environmental factors at study plots, which were examined in a previous study but not included in this one, might mask differences in outbreak intensity among regional factors.
Habitat fragmentation hypothesis suggesting less parasitism in urban region was rejected in this research at least on powdery mildew-like disease. Our result was similar to urban stress hypothesis. However, the epidemic triangle does not consider changes in susceptibility due to the environment, but rather divides “changes in susceptibility due to the environment” into “inherent susceptibility” and “environmental effects.” The epidemic triangle approach is suitable for ecological studies, whereas the stress mechanism approach may contribute to a physiological understanding.