Aspects of avian thermal physiology and frugivory of indigenous and invasive fruits in South Africa.

Abstract

Indigenous and invasive plants have long been recognised as an important food source for avian frugivores worldwide. It has been suggested that seed germination success can vary amongst avian and plant species and that ingestion by avian frugivores can enhance, reduce or have no effect on seed germination. Few studies have looked at the role of both invasive and indigenous avian species on the germination success of invasive and indigenous plants in South Africa. Therefore the first aim of this study was to determine the effects of invasive rose-ringed parakeets (Psittacula krameri) and indigenous Knysna (Tauraco corythaix) and purple-crested (Gallirex porphyreolophus) turacos on seed germination of invasive alien plant species (Solanum mauritianum, Cinnamomum camphora, Psidium guajava, and Morus alba). The second aim was to determine the effect of invasive rose-ringed parakeets on seed viability and germination of indigenous Ficus species (i.e. F. lutea, F. burkei, F. sur and F. natalensis). Previous studies have shown that many avian frugivores prefer invasive alien fruits suggesting that they may be able to efficiently assimilate energy from these fruits. However few studies have focused on the consumption and digestive efficiency of invasive fleshly fruits by indigenous and invasive birds simultaneously in one study. Consequently this study also investigated the digestive efficiency of invasive and indigenous avian frugivores feeding on invasive fleshy fruits. Finally, this study assessed the seasonal effects on the thermoregulation of invasive rose-ringed parakeets. As invasive bird species are spreading in South Africa, understanding the physiological responses that equip them to tolerate a wide range of climatic conditions would be useful for modelling potential distributions and effectively managing them. Seed ingestion by Knysna and purple-crested turacos resulted in significantly higher germination success than those from whole fruits for all invasive alien plant species in this study. Germination success of manually pulp-removed seeds did not differ from that of turacos ingested seeds, suggesting that seed coat abrasion was not important for germination of these invasive alien plants. Seed passage through the digestive tract of rose-ringed parakeets resulted in significantly reduced germination success and viability of all ingested plant species, suggestion that seed ingestion by this species is disadvantageous to these plant species. These results suggest that Knysna and purple-crested turacos are legitimate seed dispersers of the four fleshy-fruited invasive plants, while rose-ringed parakeets are mainly seed predators. Results obtained from this study also suggest that rose-ringed parakeets are mainly seed predators of Ficus fruit, as none of the ingested seeds managed to germinate. In addition, all three avian species investigated managed to meet their energetic demands by feeding on invasive alien fruit only, suggesting that these plant species may sustain avian frugivores especially during periods of food scarcity. Furthermore, the results obtained also suggest that rose-ringed parakeets show seasonal thermoregulatory responses representing energy conservation, as expected. This suggests that rose-ringed parakeets are physiologically equipped to cope with a range of environmental conditions and this partly explains their global success as an invasive species.