The potential for birds to disperse the seeds of Acacia cyclops, an invasive alien plant in South Africa

摘要

Rooikrans Acacia cyclops is an aggressive invasive tree that threatens natural resources in South Africa. The seeds of A.cyclops have a prominent aril which attracts birds that ingest the seeds and disperse them endozoochorously. Two biological control agents, a Seed Weevil Melanterius servulus and a Flower-galling Midge Dasineura dielsi, were released on A.cyclops in 1991 and 2002, respectively. Together these agents have substantially reduced seed production and generally far lower numbers of seeds are now available to birds. A consequence of this transition from historically bounteous quantities of seeds to scanty seed availability is that birds may no longer associate with the trees and seed dispersal may be disproportionately reduced. To assess whether this has happened, seed attrition was measured by comparing the amount of seeds that disappeared from two groups of branches, one available to birds and the other enclosed in bird netting. Other types of granivores (mainly field mice) were excluded from both groups of branches with a plastic funnel placed around the stems. Mature seeds were also harvested and fed to caged bird species to determine gut retention times and germination rates of ingested seeds. Attrition rates of seeds showed that birds continue to remove seeds but that only a proportion of the crop is taken. Only two frugivorous species (Knysna Turaco Tauraco corythaix and Red-winged Starling Onychognathus morio) and two granivorous species (Red-eyed Dove Streptopelia semitorquata and Laughing Dove Streptopelia sengalensis) ingested A.cyclops seeds during feeding trials. Ingestion by birds enhanced seed germination except for those ingested by Laughing Doves. There were no apparent effects of length of gut passage time and avian body size on seed germination rates. Despite the diminished seed resource due to biological control agents, birds continue to disperse A.cyclops seeds.