Patterns of granivory in Acacia cyclops stands under biological control at Langebaanweg, South Africa

Abstract

Low seed abundance associated with effect of biological control agents in an invasive shrub Acacia cyclops may limit its seed banks and further spread of the remaining seed crop in the South-western Cape, South Africa. However, there is a limited knowledge on how a reduced seed abundance, and vegetation cover which is positively correlated to seed bank size, affects patterns of granivory in A. cyclops stands. To fill this knowledge gap, granivory rates were measured using seed exclosure cages located both in closed and open A. cyclops tree canopy covers at Langebaanweg. Fresh seeds of A. cyclops were presented in tens per cage, and monitored in four-hour intervals of the day during the seeding season (December–July, 2013). Seed removal by rodents (74%) was not affected by vegetation cover suggesting an increased demand of the scarce seeds of A. cyclops. Conversely, seed removal by invertebrates (16%) was lowest among treatments, and was restricted in low tree canopy cover possibly due to competition for seeds under shady canopy. About 10% of the remaining seeds were consumed by vertebrates during the afternoon times associated with limited dispersal chances. In combination, biological control agents and rodents’ seed predation may effectively reduce seed banks of A. cyclops and invasion of this species in South Africa.