Feeding dynamics of invasive (Tarebia granifera) and native (Melanoides tuberculata) freshwater gastropods.

Abstract

Tarebia granifera has invaded several South African estuaries, outnumbering native gastropods such as Melanoides tuberculata. Comparative feeding dynamics can aid in estimating and understanding invasion impacts. This study aimed to explore the feeding dynamics of these gastropods with the use of two different approaches. Stable Isotope Analyses (SIAs) were used to investigate the dietary preferences and niche overlaps in the St Lucia Estuary, while Functional Response Experiments measured the relationship between the consumption rate of a food resource and its availability. These experiments were run both with and without the presence of heterospecific chemical cues (secondary metabolites secreted by either species) to determine whether chemical cues influence the functional responses of T. granifera and M. tuberculata. Mixed species experiments were also conducted to observe individual behaviour (active, inactive and feeding) over time and measure resource consumption when both species are placed together under high and low food availability. The SIA showed that both gastropods exhibited generalist diets, however, due to niche partitioning, there was no significant difference in their dietary niche overlap. In the FR experiments, both species exhibited Type II FRs and had similar feeding rates. However, in the presence of heterospecific chemical cues, M. tuberculata exhibited a higher feeding rate at high food availabilities, whereas T. granifera showed efficient feeding dynamics under limited food resources. In the mixed species experiments, T. granifera spent more time feeding under low food availability while M. tuberculata spent more time feeding under high food availability. This study showed that in certain conditions (e.g. high resource availability) the alien species did not always have a feeding advantage. A multiple method approach is recommended when assessing ecological impacts of invasive species. iv ACKNOWLEDGEMENTS First and foremost, I would like to express my sincere gratitude to my supervisor Dr Nicola K.Carrasco and co-supervisor Dr Nelson A.F. Miranda, for giving me this opportunity. I would like to thank my supervisors for always having faith in me and for always providing me with invaluable guidance throughout my project. It was a great privilege to work and study under two proficient supervisors. I would like to extend my gratitude to Professor André Vosloo, for his valuable input towards my project.A special thank you to Caroline Fox from Ezemvelo KZN Wildlife for logistical support and for providing us with the helpful information in the field. Dr Grant Hall of the Stable Isotope Laboratory at the Mammal Research Institute (University of Pretoria) is also acknowledged for processing and analysing the stable isotope samples.A sincere thank you to Jamila Janna for providing valuable data for this study. I would also like to thank my research colleagues, Merusha Govender, Merrisa Naidoo, Njabulo Mdluli, Natleen Govender and Mandy Jones, for their assistance and continuous support in the fi...