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

Abstract

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.

Biotic Interactions as Mediators of Biological Invasions: Insights from South Africa

“…Indeed, native frugivorous species are often responsible for the increased invasiveness of many alien plants in South Africa (Jordaan et al 2011;Wilson and Downs 2012;Thabethe et al 2015;Mokotjomela et al 2016). For example, for the highly invasive Acacia cyclops, germination is greatly enhanced following ingestion of its seed by two native frugivorous birds, the Knysna Turaco, Tauraco corythaix, and the Red-winged Starling (Mokotjomela et al 2015(Mokotjomela et al , 2016. Similarly, Thabethe et al (2015) reported enhanced seed germination for S. mauritianum, C. camphora, P. guajava, and M. alba as a result of ingestion by two native Tauraco species.…”

“…Few studies have reported on the complete seed dispersal cycle of both native and alien plant species in the same environment (Wang and Smith 2002). Nonetheless, the increasing number of invasive fleshyfruited plants in South Africa indicates that their seeds are effectively dispersed, and establishment success is high (Mokotjomela et al 2015). Because of the commonly smaller seed size of invasive alien fruits (Gosper and Vivian-Smith 2010;Mokotjomela et al 2013a), more seeds can be dispersed by vertebrates than those of native species, implying that each dispersal event will likely carry more seeds of alien than native species.…”

Ingestion by an invasive parakeet species reduces germination success of invasive alien plants relative to ingestion by indigenous turaco species in South Africa

“…Avian frugivores play a key role in seed dispersal of many plant species worldwide (Cain et al 2000;Renne et al 2000;Vivian-Smith and Gosper 2010;Jordaan et al 2011a;Mokotjomela et al 2013bMokotjomela et al , 2015. The process of seed dispersal by avian frugivores usually involves the consumption of pulp and regurgitation or excretion of intact seeds (D'Avila et al 2010;Fedriani et al 2011;Czarnecka et al 2012).…”

Interspecific competition in germination of bird-dispersed seeds in a habitat with sparse tree vegetation in South Africa

“…The germination of ingested seeds represents the quality component, which is regarded as more in uential in plant recruitment than the quantity component (Mokotjomela et al, 2016;Schupp et al, 2010;Schupp, 1993). Avian frugivores occasionally modify the rate of germination and enhance the chances of seeds' survival (Kleyheeg, 2018;Mokotjomela et al, 2016Mokotjomela et al, & 2015Thabethe et al, 2015;Fricke et al, 2013;Mokotjomela, 2012; Barnea et al, 1991). For example, Traveset et al (2001) tested the germination characteristics of the seeds from different plants (Rhubus ulmifolius, Osyris alba, Rubia peregrine, Asparagus acutifolius, and Phillyrea spp) that passed through birds' guts in the western Mediterranean shrubland, and found that different germination speeds/rates were promoted by the treatment in the birds' gut.…”