What drives variation in conspecific negative density-dependence? A demographic perspective

Conspecific negative density dependence (CNDD), the reduced survival of juvenile trees at high conspecific density, is considered a key force maintaining plant diversity. Variation in CNDD across species and environments is typically attributed to variation in sensitivities to natural enemies (e.g., pathogen transmission/virulence) or sensitivity to intraspecific competition. We show that the density-independent component of vital rates -- baseline mortality and growth -- substantially alters the cumulative consequences of conspecific density on survival, independently of any change in instantaneous sensitivity to conspecific crowding. This reflects genuine demographic effects on opportunities for crowding to shape survival, and on population characteristics shaping the negative influence of conspecifics, such as infection prevalence and size structure. It is not an artifact of how CNDD is measured. Our results, derived from a pathogen model and a size-structured seedling model, provide a demographic framework for assessing potential drivers of CNDD variation among species and across abiotic gradients.