Theory suggests that species perform best at intermediate densities, where density-dependent facilitation and antagonism are balanced, but empirical evidence is scarce, particularly in plants. In a self-incompatible perennial herb (Saussurea nigrescens), whose recruitment heavily relies on seed output, we test whether both intraspecific facilitation and antagonism significantly affect seed production, resulting in highest seed yield at an intermediate capitulum density.Methods
Plots with different S. nigrescens densities were sampled in an Eastern Tibetan meadow during the growing season of 2012 to investigate the relationships between capitulum density and pollinator visitation rate, seed set ratio, parasite ratio, seed damage ratio, and capitulum size. Both simple linear and quadratic models were employed to determine the shape of relationships.Important Findings
In line with general theory, hump-shaped relationships of capitulum density versus seed set ratio and number of florets per capitulum indicate intraspecific facilitation in sparse populations, which can be attributed to positive density-dependent pollinator visitation and the amelioration of detrimental physical factors. However, the proportion of seeds damaged by pre-dispersal predators increased monotonically with capitulum density, which may have—in combination with increased intraspecific competition for light and soil nutrients—resulted in density-dependent antagonism. Both positive and negative density-dependent agents acted simultaneously throughout the density range investigated and led to the highest seed yield at intermediate density levels in the Tibetan lotus. More efforts concurrently exploring the two effects are needed to facilitate understanding species abundance and community structure.