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

Abstract

Avian frugivores play a key role in seed dispersal of many plant species, including invasive alien plants. We assessed the effect of gut passage on the germination of selected invasive alien plant species in South Africa. Fruits of four fleshly-fruited invasive alien plant species: Solanum mauritianum, Cinnamomum camphora, Psidium guajava, and Morus alba, were fed to two species of indigenous turacos, Knysna (Tauraco corythaix) and purple-crested (Gallirex porphyreolophus) turacos, and to invasive roseringed parakeets (Psittacula krameri). Seed retention time was determined as this can influence both seed dispersal and germination success. Germination success of ingested seeds was compared with that of manually de-pulped seeds, as well as to seeds in whole fruit. The germination success of seeds of all the invasive plant species increased significantly after ingestion by both turaco species compared with seeds from whole fruits. Germination success of manually de-pulped seeds did not differ significantly from that of turaco ingested seeds. In contrast, seed passage through the digestive tract of rose-ringed parakeets significantly reduced germination success and viability of ingested invasive plant species. Our results suggest that Knysna and purple-crested turacos are legitimate seed dispersers of fleshy-fruited invasive plants, while rose-ringed parakeets are mainly seed predators. Although seed predation by rose-ringed parakeets negatively affects the reproductive success of these plants, it is unlikely that this will suppress the spread of these invasive alien plants in South Africa as they are already well established. Furthermore, they can facilitate dispersal by seed regurgitation and dropping uneaten fruits away from the parent plant. Similar trends could be expected for indigenous seeds that rose-ringed parakeets feed on and therefore these birds remain a negative influence within invaded ecosystems.

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

“…Causes trait/ phylogenetic over-dispersion. Fleshy fruits of alien trees/shrubs in fynbos are nutritionally more rewarding than the fruits of native species (Knight 1988;Jordaan et al 2011;Mokotjomela et al 2013a, b;Thabethe et al 2015) Darwin's Naturalisation Hypothesis Alien species with close native relatives in the recipient ecosystem have either a reduced or increased chance of successful establishment and invasion depending on the spatial scale and processes operating (notably competition vs. climatic fit). Using phylogenetic analyses, Bezeng et al (2015) found that invasive non-native trees and shrubs are less closely related to native species than their non-invasive non-native counterparts.…”

“…Hypothesis Based on interactions between alien and native species in the recipient community, invasion success is maximised when enemies in recipient community are specialists (unable to prey on alien species) and when native mutualists are generalists (facilitate invasion). Native South African birds are mostly generalist frugivores and readily consume and spread seeds of the fleshy fruits of alien trees/shrubs and thus facilitate invasion (Knight 1988;Jordaan et al 2011;Mokotjomela et al 2013a, b;Thabethe et al 2015).…”

“…Another factor promoting the preference of fruits of invasive species over those produced by native species by birds is that some invasive species offer higher nutritional rewards (e.g. Cinnamomum camphora, Lantana camara, Morus alba, Psidium guajava, Solanum mauritianum; Jordaan et al 2011;Mokotjomela et al 2013a;Thabethe et al 2015). The reproductive phenologies of some invasive plant species also ensure that their fruits or seeds are available for longer periods compared to many native species (Knight 1988;Mokotjomela 2012).…”

“…successful dispersal and germination (Mokotjomela et al 2016). 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.…”

The role of frugivorous birds in fruit removal and seed germination of the invasive alien Cotoneaster franchetii in central Argentina

“…; Thabethe et al . ). On the other hand, studies on alien species rarely consider the effect of dispersal agents on fruit removal and seed germination simultaneously ( but see Ferreras & Galetto ), which is fundamental to understand the importance of plant–animal interactions in the population dynamics and spread of invasive alien species.…”