Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of native thistle

Authors: Rand, Tatyana; West, Natalie; RUSSELL, F LELAND - Wichita State University; LOUDA, SVATA - University Of Nebraska

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2020
Publication Date: 4/22/2020
Citation: Rand, T.A., West, N.M., Russell, F., Louda, S.M. 2020. Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of native thistle. Oecologia. 193(1):143-153. https://doi.org/10.1007/s00442-020-04656-2.
DOI: https://doi.org/10.1007/s00442-020-04656-2

Interpretive Summary: Determining the extent to which plants are limited by how much seed they produce or by other factors in the environment in which their seedlings come up, is critical to understanding the extent to which their populations can be controlled by the insects that feed on them. Answering this question has direct relevance to managing plant densities, for example recovering rare plant populations, or using biological controls to limit weed populations. In this study we show that populations of a native plant are highly seed limited, suggesting that the invasive biological control insect that destroys their seeds is a likely factor in the decline in their numbers. The work further suggests that the extent to which seed is limiting plant numbers varies across sites, suggesting that management aimed at manipulating insects to impact plant densities (e.g., either increase rare plants or decrease weeds) will need to be tailored to specific site conditions.

Technical Abstract: Whether plant populations are limited by seed or microsite availability is a long standing debate. However, both can be important, and increasing emphasis is placed on disentangling their relative strengths and how they vary through space and time. Although uncommon, seed addition studies that include multiple levels of seed augmentation, and follow plants through to the adult stage, are critical to achieving this goal. Additionally, such data are vital to understanding when biotic pressures, such as herbivory, influence plant abundance. We experimentally added seeds of a sparse native thistle, Cirsium canescens, at four densities to plots at two long-term study sites and quantified seedlings and reproductive adults over nine years. Recruitment to both seedling and adult stages was strongly seed limited at both sites, and the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal factors were additionally important. Strong density-dependence limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition clearly demonstrates that the post-dispersal environment at these sites remains suitable for C. canescens survival to reproduction, and that seed availability limits adult densities. Consequently, the results provide support for previous work suggesting seed losses due to the invasive weevil, Rhinocyllus conicus, rather than shifting microsite conditions, are driving population declines at these sites. Differences in the relative strength of post-dispersal limitations between sites, highlights that alternate strategies may be necessary to recover populations at each.