Fleshy-fruited invasive alien plants and frugivores in South Africa.
South Africa is one of the world's most biologically invaded countries and has spent billions of rands on efforts to eradicate alien invasive plants. Chemical and mechanical control methods have varied in success and the need for integrated management strategies has been realised. This requires a better understanding of all aspects of the invasion process. Some of the most invasive plant species rely on vertebrate dispersers which facilitate long-distance seed dispersal. Frugivory is based on a mutualism in which the frugivores gain a resource and the plants benefit from seed dispersal away from the parent plant. Seed germination itself may either be enhanced, reduced or not affected at all after gut passage. The first aim of this study was to determine if generalist avian frugivores and a fruit bat species (Epomophorus wahlbergi) enhance or decrease seed germination of invasive alien plants in South Africa, by either pulp removal or seed coat abrasion, or if they serve as dispersers only. The second aim was to determine if avian frugivores are able to meet their energetic demands by feeding on a specific alien fruit diet. Finally, we also quantified the nutritional content and morphological characteristics of fleshy fruits of various invasive alien and exotic plant species. Avian frugivores: Red-winged Starlings (Onychognathus morio), Speckled Mousebirds (Colius striatus), and Dark-capped Bulbuls (Pycnonotus tricolor), varied in their effects on the germination success of seeds of four invasive alien species, namely: Lantana camara, Solanum mauritianum, Cinnamomum camphora, and Psidium guajava. However, this was not associated with differences in seed retention times. Similar germination success was observed for avian ingested and de-pulped seeds. This was also observed for fruit bat spat and depulped seeds of Psidium guajava, Melia azedarach, Eriobotrya japonica, and Morus alba. Therefore seed coat abrasion was not important for the germination of these fleshy-fruited invasive alien plants. Pulp removal resulted in significantly earlier seed germination as well as higher seed germination percentages than in the case of whole fruit controls for some of these invasive species. Gut passage is thus important for long-distance dispersal, and in some cases, for enhanced germination of seeds. The invasive Solanum mauritianum and indigenous congener S. giganteum showed similar germination responses, with both ingested and depulped seeds germinating profusely. However, S. giganteum benefited from pulp removal as seeds from whole fruits had less germination. Avian frugivores varied significantly in most energetic parameters calculated when given diets of invasive alien fruit. Speckled Mousebirds and Dark-capped Bulbuls were able to maintain body mass and efficiently process fruits of all four alien invasive plants, while Red-winged Starlings were only able to do so on lipid-rich C. camphora and sugar-rich S. mauritianum. Furthermore, frugivores also adjusted their feeding behavior by eating more nutritionally poor fruit and less energetically rewarding fruit. Fruit bats consumed more fruit per gram body mass than avian frugivores did. They therefore process proportionately more seeds than avian dispersers and thus their role in invasive seed dispersal, which has previously been underestimated particularly in South Africa, is highlighted. Fruits of invasive plant species were similar in morphology, but greater in nutritional content, than fruits of indigenous species. These fruits also contained small, light seeds with approximately only 30% having more than 10 seeds per fruit. The ability of frugivores to efficiently process these fruits and the greater nutritional rewards offered by these provide new insights into why these invasive fruits are preferred by frugivores. In addition, invasive alien plants may have a competitive edge over indigenous species because of their larger reproductive outputs and not necessarily because of greater germination success.
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