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Impacts of Non-Native Rainbow Trout on Stream Food Webs in the Cape Floristic Region, South Africa: Integrating Evidence from Surveys and Experiments

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Impacts of Non-Native Rainbow Trout on Stream Food Webs in the Cape Floristic Region, South Africa: Integrating Evidence from Surveys and Experiments Impacts of non-native rainbow trout on stream food webs in the Cape Floristic Region, South Africa: integrating evidence from surveys and experiments by Jeremy Mark Shelton Town Cape of University Thesis presented for the degree of DOCTOR OF PHILOSOPHY in the Department of Biological Sciences Faculty of Science UNIVERSITY OF CAPE TOWN July 2013 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University Declaration This thesis reports original research carried out in the Department of Zoology, University of Cape Town, between 2009 and 2013. It has not been submitted in whole or in part for a degree at any other university. All data presented are original. Any assistance received is fully acknowledged. Jeremy Mark Shelton Town Cape Date of University Town This thesis is dedicated to my mother, LynetteCape Hampton – your love, friendship and support have inspired me beyond words of University Acknowledgements Acknowledgements This project would not have been possible without the financial support I received from the CIB (DST-NRF Centre of excellence for Invasion Biology), DAAD (German Academic Exchange Service), UCT (University of Cape Town) and the Cape Tercentenary Foundation. Thanks first and foremost to my supervisors Jenny Day and Michael Samways. Jenny, your encouragement and enthusiasm have inspired in me a deep passion for freshwater conservation biology. Michael, your unquestioning belief in my abilities and rational, thoughtful approach to research have contributed greatly to my growth as a scientist. Without constructive and creative discussions with Jackie King, Jenny Day, Dean Impson, Sean Marr and Steve Lowe in the developmental stages, thisTown project would never have gotten off the ground. Sean – although not an official supervisor, your creative insights and extensive knowledge of aquatic systems that you have shared with me over the years have been invaluable. Cape Long, challenging hours in the field were ofrequired to collect the data presented in this thesis. For assistance with the field experiments, my thanks go to Kirsten Retief, Klaus Menck, Darragh Woodford, Matthew Bird and Kirsten Tilanus. For clambering up into countless isolated kloofs while carrying backpacks bulging with heavy sampling equipment, my sincerest thanks go to Matthew Bird and Kirsten Tilanus. Bird’o – you have a unique way of blending fun and Universityefficiency in the field, and thanks for the insightful and inspiring chats around the evening braai. Kirsten – your dedication, good attitude and companionship in the field was pivotal in collecting the data required for this project, and thank you for believing in me when times were tough. For hours and hours of committed laboratory work I am very grateful to Liesl Phigeland, Kristen Barnes, Sean Marr and Kirsten Tilanus. Thanks also to Denise Schael, Michael Samways, Mike Picker, Matthew Bird and Vere Ross-Gillespie for lending a hand when my invertebrate identification skills were inadequate. i Acknowledgements Coleen Moloney, Henning Winker, Colin Attwood, Deena Pillay and Sean Marr – I feel privileged to have received analytical advice from a group of such generous, logical and patient scientists. The writing presented in this thesis was much improved by constructive comments and criticisms provided by Sean Marr, Ian Hampton, Jenny Day and Michael Samways. To the members of the Freshwater Research Unit – Sean Marr, Alistair Fyfe, Heather Malan, Justine Ewart-Smith, Matthew Bird, Helen Dallas, Dean Ollis, Cecile Reed, Vere Ross- Gillespie and Carla-Louise Ramjukadh - I could not have hoped for a more interesting and friendly group with which to share a lab. Thank you for the cricket matches, and also for the thoughtful chats concerning topics ranging from freshwater biology to film reviews. I have thoroughly enjoyed each and every aspect of my PhD research, and I am certain that without the love and support of my friends and family that thisTown would not have been the case. Last, but certainly not least, Jordan-Laine – the love and inspiration you have breathed into my life over the past year has brought out the best in me and made the final stages of my PhD a thoroughly enjoyable and exciting experience.Cape Thanks also for the magnificent sketches on the first and last pages of this thesis.of University ii Table of contents Table of contents Acknowledgements i Abstract iv Chapter 1 - General introduction 1 Chapter 2 - Predatory impact of non-native rainbow trout on native fish 20 populations in headwater streams of the upper Breede River catchment, South Africa Town Chapter 3 - Influence of non-native rainbow trout on benthic community 67 structure in headwater streams of the upper Breede River catchment, South Africa Cape of Chapter 4 - Characterising and contrasting the trophic niches of native Breede 120 River redfin and non-native rainbow trout in headwater streams of the upper Breede River catchment, South Africa Chapter 5 - Relative top-down effect of native benthic-feeding redfin vs. 181 non-native drift-feedingUniversity trout on benthic community structure in a Cape Floristic Region headwater stream Chapter 6 - General discussion 240 References 261 Appendices 289 iii Abstract Abstract Impacts of invasive predators may be influenced by whether or not native predators which function in the same way as the invasive predator exist in the recipient system. Impacts are expected to be strong in isolated systems lacking functionally similar predators because native species will be naïve to the foraging behaviour of the introduced predator, and because the invasion is likely to change the role which the native predator assemblage performs. In this thesis I studied how the introduction of a functionally novel predatory fish, rainbow trout Oncorhynchus mykiss, has affected native fish, and how changes in the functioning of the predator assemblage have influenced lower trophic levels, in headwater streams in a catchment area within the Cape Floristic Region, South Africa. Fish populations, benthic invertebrate assemblages, benthic algaeTown and particulate organic matter were surveyed in each of 24 minimally-disturbed headwater streams in the upper Breede River catchment, and relevant environmental variables in each stream measured, over one summer. Twelve of the streams containedCape trout and 12 did not. It was hypothesized that native predatory fish wouldof be vulnerable to predation by trout because they evolved in the absence of a functionally similar predator. The mean density and biomass of the native fish Breede River redfin Pseudobarbus sp., Cape kurper Sandelia capensis and Cape galaxias Galaxias zebratus, was 5-40 times higher in streams without trout than in streams with them, and although present at all 12 sites without trout, native fish were only recordedUniversity at five of the 12 sites with trout present. Multivariate analysis of variance revealed no consistent difference in environmental conditions between sites with and without trout, and distance-based linear models identified trout density as the best predictor of Breede river redfin and Cape kurper density. Cape galaxias density, on the other hand, was best predicted by other environmental variables including mean substrate size, site slope and riparian vegetation cover, but analyses for this species may have been compromised by the low frequency of Cape galaxias occurrence. A predation experiment conducted in mesocosms in one of the survey streams revealed that large trout selectively consumed small Breede River redfin (the dominant member of the native fish assemblage), iv Abstract indicating that size-specific predation by trout reduces native fish abundance, and is also likely to change the size structure of native fish populations. The taxonomic and functional composition of the invertebrate assemblage, as well as the biomass of benthic algae, in streams with trout differed consistently from that in streams without trout, implying that trout impacts extend beyond native fish to lower trophic levels. Herbivorous invertebrates were more abundant, and algal biomass lower, in streams with trout than in streams without them. Distance-based linear models identified trout presence as the best predictor of these patterns. It was concluded that by reducing native fish abundance, trout indirectly reduce the predation pressure on herbivorous invertebrates, resulting in increased grazing pressure on benthic algae. These results indicate that trout do not functionally compensate for the native fish that they have replaced, being weaker regulators of herbivorous invertebrates than are native fish. Town Differences in the functional role performed by trout and redfin were examined by characterizing and contrasting their trophic niches in a subset of the survey streams (trout: n = 3, redfin: n = 3). Behavioural observations showedCape that while redfin fed mostly from the stream bed, trout fed primarily from the drift. Gut contents and stable isotope analysis revealed that herbivorous aquatic invertebratesof contributed more to the diet of redfin than to that of trout, and that trout augment their diet of aquatic invertebrate prey by consuming terrestrial prey more than do redfin. Collectively
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