Population dynamics of an annual legume, Kummerowia stipulacea, were followed in permanent plots in a mown field for 4 years. There was a large reduction in the number of plants within seasons.
Laboratory studies showed that biotic factors were responsible for seed death in the soil. Using soil from the field plot location, we isolated fungi pathogenic to K. stipulacea seeds and seedlings.
Two pathogenic fungi (Rhizoctonia solani and Pythium irregulare) were used in a glasshouse study of the effects of pathogens and intraspecific competition on plant mortality and growth. Our null hypothesis was that death of individual plants due to disease would be balanced by increased growth of survivors. A factorial experiment was established with five soil treatments (three controls, P. irregulare and R. solani) and seven seed densities.
Inoculation by both R. Solani and P. irregulare led to a reduction in final numbers of plants, with the reduction greatest for populations inoculated with R. solani. Average biomass per plant was reduced with increased sowing density in all treatments except the R. solani inoculation, where surviving plants in high-density pots may be larger because of the reduced density resulting from pathogen-induced death. No treatment differences in size hierarchies were evident.
Surviving plants in both inoculation treatments could not take full advantage of the reduced density, and thus total biomass and number of trifoliate leaves per population (a surrogate for seed production) were reduced compared to controls, leading us to reject the null hypothesis. The reduction was more severe at high sowing densities. These results suggest that disease has the potential to reduce seed production and thus plant population numbers, if populations are seed limited. Shapes of yield-sowing density relationships were asymptotic for all treatments; thus, disease-induced ‘overcompensation’ leading to fluctuating population dynamics was not suggested.