Seed availability and post-dispersal limitations combine to drive plant population abundance. Understanding the relative contribution of each to plant recruitment will be vital to determining when biotic pressures, such as herbivory, influence population dynamics. However, few experimental studies follow the impact of seed addition through successive life stages to assess population-level consequences. Long term demography data suggest population densities of the native thistle, Cirsium canescens (Platte thistle) are in decline. Whether this decline might be attributed to decreased seed numbers due to the invasion of a novel herbivore, Rhinocyllus conicus, requires evaluating the importance of seed limitation relative to post-dispersal processes limiting recruitment and survival. We conducted a seed addition study to evaluate the relative importance of seed and microsite limitation (density-dependent and –independent processes) in describing patterns of plant recruitment at the seedling and adult stage. We experimentally added seeds at four augmentation densities to long-term demography plots at two study sites and quantified densities of seedlings and reproductive adults in control and addition plots over nine years.
Results/Conclusions
Though the relative strength of seed limitation decreased with plant age, recruitment to both seedling and adult stages was strongly seed-limited at both sites. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were also important but varied between sites. Strong density-dependent and -independent mortality limited recruitment at one site, while only density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at both sites remains suitable for C. canescens seedling establishment and survival to reproduction, and that seed availability limits adult densities. The results thus provide support for the hypothesis that seed losses due to the invasive weevil, Rhinocyllus conicus, even in the presence of shifting microsite conditions, are driving C. canescens population declines.