Dispersal and predation of alien Acacia seeds: effects of season and invading stand density

Acacia saligna and A. cyclops are invasive alien shrubs in lowland ecosystems of the fynbos biome, South Africa. Both species typically form dense thickets and accumulate large soil-stored seed banks. Seed removal rates, relative to seed availability in the litter layer of Acacia-infested vegetation, were studied to determine the importance of indigenous ants and vertebrates as dispersors and predators, respectively. There was a relative loss to the annual seed-fall of 50% and 80–95% in dense A. saligna and A. cyclops stands, respectively. Ants and rodents removed significant quantities of seeds, but a large proportion may have rotted. In a dense A. saligna stand, ants usually removed all seeds presented to them after 7 days presentation, compared to only 36% of seeds in a dense A. cyclops stand. In dense stands of both species, rodents were relatively slow to take seeds, removing 31% within 7 days. Removal from trays was least during and following seed-fall (January-March) and greatest prior to seed-fall (September-November) and decreased as A. cyclops canopy cover increased. In a low-density A. cyclops stand, ants and rodents competed for seeds. The results indicate that indigenous ants have played a critical role in maintaining and accumulating Acacia seed banks and thus may have facilitated the development of dense Acacia stands. Acacia saligna en A. cyclops is uitheemse indringerstruike wat in die laagland-ekosisteme van die fynbos-bioom, Suid-Afrika, voorkom. Dit is kenmerkend van albei spesies dat hulle digte ruigtes vorm en dat groot saadvoorrade in die grond opgegaar word. Die tempo van saadverwydering in verhouding tot beskikbaarheid in die afvallaag van plantegroei waarin Acacia vervuil is, is bestudeer. Daar was ’n relatiewe verlies van onderskeidelik 50% en 80–95% in die jaarlikse saadval in die digte Stande van A. saligna en A. cyclops. Miere en knaagdiere het betekenisvolle hoeveelhede saad verwyder, maar ’n groot deel mag moontlik verrot het. In ’n ruie stand van A. saligna het miere gewoonlik al die saad wat beskikbaar was binne 7 dae verwyder, teenoor slegs 36% van die saad in ’n ruie stand van A. cyclops. Knaagdiere was betreklik traag om saad in ruie Stande van albei spesies te verwyder, en slegs 31 % is binne 7 dae verwyder. Verwydering uit bakke was die laagste gedurende en na saadval (Januarie–Maart) en die hoogste voor saadval (September-November), en het verminder soos A. cyclops se blaredak digter geword het. In ’n minder-digte stand van A. cyclops was miere en knaagdiere in kompetisie om die saad te bekom. Die resultate toon dat inheemse miere ’n kritieke rol in die handhawing en opberging van Acacia-saadvoorrade gespeel het en sodoende moontlik die ontwikkeling van ruie Acacia-Stande aangehelp het.