Population dynamics of the invasive, annual species, Carrichtera annua, in Australia

Carrichtera annua (L.) DC. (Brassicaceae) is a widespread weed of the southern rangelands of Australia and there is currently no effective control strategy for this weed outside agricultural systems. Field and glasshouse experiments were used to target various stages of the life cycle of C. annua that, from initial field observations and a review of the literature, appeared to be important or were poorly understood in Australia. Seed production was found to be prodigious with up to 30000 seed m-2 recorded in the field and extensive collection of dry-dispersed seed by ants was documented, similar to that in the native range of C. annua. Two seedbanks, an aerial pod seedbank and a soil seedbank, are key features contributing to the success of this invasive species as the seedbanks are subject to, and protected from, contrasting pressures. The aerial seedbank, usually the larger of the two, protects seed from collection by ants but is susceptible to vertebrate grazing and fire, while the soil seedbank is depleted by ants and seed decay although the adhesive nature of wetted seeds helps stabilise this seedbank. The population can be replenished by either seedbank in one generation, hence both seedbanks need to be targeted to allow successful control. Inhibition of germination by high temperatures in unfavourable conditions and the potential to reach maturation and fruit production very quickly also contribute to the high seed production of this species.