Movement and modelling of buried seed of the invasive perennial Chrysanthemoides monilifera in coastal dunes and biological control

Movement in the soil of seeds of the invasive shrub Chrysanthemoides monilifera was studied in relation to the success of proposed biological control treatments. Marked seeds were buried at depths from 0 to 6 cm and recovered after periods of 3–24 months. Survival declined with time but was higher for more deeply buried seeds. The average movement of seeds was 0.7 cm per year downwards which enhances their survival but also tends to remove them from the zone of highest likelihood of germination. A model was constructed to predict the size of the pool of potentially germinable seed under treatments including pre-dispersal predation and burning. It indicated that satisfactory control with a pre-dispersal seed predator will be achieved only if there is a < 95% reduction in seed-fall to the soil surface over the entire year. A control agent that is 100% effective for most of the year will probably be unsatisfactory if its rate of predation drops to > 90% for a part of the year (e.g. during winter when seed-fall is abundant). The effectiveness of the control could be enhanced by using intense fires to reduce the soil seed-pool and to kill many of the mature shrubs. However, the feasibility of this combination of treatments will depend on the reaction of the biological control agent to fire.