Reproductive Hierarchies in Erodium: Effects of Variation in Plant Density and Rainfall Distribution

Reproductive hierarchies in even—aged plant monocultures often reflect the dynamics of intraspecific competition and, by their influence on effective population number, can alter significantly the relative importance of genetic drift in the evolution of plant populations. In experimental populations of Erosium boytrs and E. brachycarpum, I examined the interactive effects of intraspecific competition and rainfall variability on the development of reproductive hierarchies in these annual species. Reproductive inequality within monocultures of both species increased with increasing plant density and plant productivity, as measured by seed production, was determined by rainfall distribution. The magnitude of reproductive inequality was dependent on the interaction of sowing density, reproductive inequality was greatest in populations that experienced early autumn precipitation and lowest in populations exposed to drought during flowering. In contrast, at the lowest sowing density there was no significant effect or rainfall pattern on reproductive inequality. Effective population number was relatively insensitive to increases in population density because of increased inequality in reproduction at higher population densities. Reproductive variance could be a major contributor to drift variance in dense populations of both Erodium species.