Post-dispersal seed removal of Carduus nutans and C. acanthoides by insects and small mammals

The journey of wind-dispersed seeds does not necessarily end when they land. Secondary dispersal and/or predation can positively or negatively affect the spread of invasive plants. Here we studied post-dispersal seed removal of nodding and plumeless thistle (Carduus nutans and C. acanthoides) in part of their invaded range in Pennsylvania, USA. The relative impact of small mammals and insects was determined using exclusion treatments in the field. In cages that allowed insect access, 88 % of the seeds were removed after 1 day, and 99.9 % were removed after the 6-day trial. When insects were excluded, the removal rate was significantly lower (18 % after 1 day, 40 % after 6 days). The seed removal rates provide an upper limit to the seed predation rate, with the understanding that it is also possible for seed removal to be an important secondary dispersal mechanism. We discuss a combined empirical-theoretical approach to evaluate the impact of these alternative seed fates on the spread and management of these thistles.