Exotic weevil invasion increases floral herbivore community density, function, and impact on a native plant

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

Whether plant populations are limited by seed or microsite availability is a long-standing debate. However, since both can be important, increasing emphasis is placed on disentangling their relative importance 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. Such data are also vital to understanding when biotic pressures, such as herbivory, influence plant abundance. In this study, we experimentally added seeds of a native thistle, Cirsium canescens, at four augmentation densities to plots at two long-term study sites and quantified densities of seedlings and reproductive adults over 9 years. Recruitment to both seedling and adult stages was strongly seed-limited at both sites; however, the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were important as well. Strong density-dependent mortality limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at these sites remains suitable for C. canescens 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, rather than shifting microsite conditions, are driving C. canescens population declines. Shifts in the importance of density-dependent recruitment limitation between sites highlights that alternate strategies may be necessary to recover plant populations at different locations.

Cumulative herbivory outpaces compensation for early floral damage on a monocarpic perennial thistle

Floral herbivory represents a major threat to plant reproductive success, driving the importance of plant tolerance mechanisms that minimize fitness costs. However, the cumulative insect herbivory plants experience under natural conditions complicates predictions about tolerance contributions to net fitness. Apical damage can lead to compensatory seed production from late season flowering that ameliorates early season fitness losses. Yet, the compensation realized depends on successful development and herbivore escape by later season flowers. Using monocarpic perennial Cirsium canescens, we quantified seed-reproductive fitness of plants with vs. without experimental damage to the early-developing large apical flower head, with and without a 30-40% herbivory reduction on subsequent flower heads, for two flowering cohorts. Plants with reduced herbivory clearly demonstrated the release of apical dominance and compensation, not overcompensation, for apical damage via greater seed maturation by later flower heads. In contrast, plants that experienced ambient herbivory levels on subsequent heads undercompensated for early apical damage. Individuals had lower total seed set when the apical head was damaged. Compensation was, therefore, possible through a small increase in total flower heads, caused by a higher rate of floral bud survival, and a higher seed maturation rate by subsequent heads, leading to more viable seeds per matured flower head. With ambient cumulative floral herbivory, compensation for apical damage was not sufficient to improve fitness. Variation in the intensity of biological interactions played a role in the success of plant tolerance as a mechanism to maximize individual fitness.