Friendly Neighbours? Investigating Ecological Facilitation Between Thunbergia Atriplicifolia (Acanthaceae) and Exochaenium Grande (Gentianaceae)
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Friendly neighbours? Investigating ecological facilitation between Thunbergia atriplicifolia (Acanthaceae) and Exochaenium grande (Gentianaceae) Simangele M. Msweli Submitted in fulfilment of the academic requirements for the degree of Master of Science In the discipline of Biological Sciences School of Life Science College of Agriculture, Engineering and Science University of KwaZulu-Natal Pietermaritzburg South Africa Supervisor: Prof. Steven D. Johnson [email protected] Co-supervisor: Dr Adam Shuttleworth [email protected] PREFACE The research contained in this dissertation was completed in the School of Life Sciences, of the College of Agriculture, Engineering and Science, University of KwaZulu-Natal (Pietermaritzburg), South Africa. It was conducted from January 2015 to December 2017, under the supervision of Professor Steven D. Johnson and Dr Adam Shuttleworth. The work presented in this dissertation represents the original work of the author and has not been submitted in any form to another university and, except where the work of others is acknowledged in the text, the results reported are due to investigations by the candidate. Signed: ________________________ Simangele Msweli (Candidate) ________________________ Prof. Steven D. Johnson (Supervisor) _______________________ Dr Adam Shuttleworth (co-supervisor) i DECLARATION 1: PLAGIARISM I, Simangele M. Msweli, declare that: (I) the research reported in this dissertation, except where otherwise indicated or acknowledged, is my original work (II) this dissertation has not been submitted in full or in part for any degree or examination to any other university (III) this dissertation does not contain other persons’ data, pictures, graphs or other information, unless specifically acknowledged as being sourced from other persons (IV) this dissertation does not contain other persons’ writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then: a) their words have been re-written but the general information attributed to them has been referenced b) where their exact words have been used, their writing has been placed inside quotation marks, and referenced; (V) this dissertation does not contain text, graphics or tables copied and pasted from the Internet, unless specifically acknowledged, and the source being detailed in the dissertation and in the References sections. _______________________ Signed: Simangele M. Msweli Date: June 2018 ii ABSTRACT Wildflowers that co-exist, share similar floral signals and flower simultaneously, may interact with the same pool of pollinators, therefore sharing pollinator species and individuals. Floral similarity could be a result of a common ancestor, convergent floral syndrome or mimicry. Pollinator sharing may have consequences for the pollination success and fecundity of one or more of the interacting wildflowers. This study was conducted in grassland vegetation of southern KwaZulu-Natal to investigate an observed floral similarity between Thunbergia atriplicifolia and Exochaenium grande, document pollinator sharing between the two species and investigate the consequences of pollinator sharing on (1) interspecific pollen transfer between the two species and (2) seed set of individuals of E. grande that grow within sparse and dense populations of T. atriplicifolia. It was hypothesized that E. grande is a mimic of T. atriplicifolia. Investigated aspects of floral similarity include: flowering phenology, flower colour, flower size and nectar production. Weekly surveys indicated that both species flower simultaneously but E. grande occurs at much lower density and peaks in flowering later than does T. atrplicifolia. Flowers of the two species have similar spectral reflectance and in terms of a bee vision model can be considered indistinguishable to bees. There is also overlap in flower size of the two species. They provide similar amounts of nectar but nectar of E. grande has a higher sugar concentration. Both species are dependent on pollinators for seed production and are pollinated by generalist solitary bees belonging to the Apideae and Hactilidae family, with some species shared by both plant species. Stigmas of T. atriplicifolia and E. grande carried heterospecific pollen in varying degrees. Estimates of pollen production indicated that E. grande produces more pollen than T. atriplicifolia. Although interspecific pollen transfer was confirmed, it is not yet clear whether this improper pollen transfer affects seedset. Plant density of E. grande and T. atriplicifolia was not a significant predictor of seed set in E. grande. However, there were very high levels of seed predation in E. grande, and that hinders our ability to rule out the any effects of pollinator sharing in seed set. These findings reveal a striking similarity and pollinator sharing between the two species, but despite this, there appears to be no negative or positive effects of this on seed set. Seed set only accounts for half of fitness, however, and the effects of pollinator sharing may therefore be evident on other measures of fitness. The role of trait similarity for pollinator attraction still requires further experimental investigation before the hypothesis of ecological facilitation (or mimicry) can be accepted. iii ACKNOWLEDGMENTS This work was made possible by the contribution of the following organisations, and people whom I extend my gratitude to: a) The National Research Foundation for funding in the first two years of the degree b) The Mandela Rhodes Foundation (MRF) for financial assistance in the third year and for leadership workshops that developed me on a personal level c) Professor Steven D. Johnson for supervising the project, offering guidance where needed and monitoring my progress d) Dr Adam shuttleworth for co-supervising the project and providing technical assistance where needed e) Dr Karl Duffy, Dr Dara Stanley and Dr Timo Van der Niet for proof reading my drafts. As a non-native English speaker, I relied on their assistance in checking all language related mistakes f) Several postgraduate students, especially Zwelibanzi Mdlalose, Mthokozisi K. Zuma and Sachin Doorsamy for driving me to the study sites and assisting with data collection g) My friends (Naledi Zama, Rerani Ramaano, Abongile Mbande, Sithembile Ndlovu, Lutendo Mugwedi and Hloniphani Mthunzi) for their generous support h) And lastly but not least, my family. As a first person to do an MSc degree at home, they had less understanding of what exactly I was doing, but still supported me wholeheartedly. Thanks to my mom; Nombuso T. Msweli, my aunt; Hlengiwe T. Msweli, my brothers; Sibusiso Nxumalo, Ntuthuko Nxumalo and Siboniso Msweli and my sister; Samukelisiwe Sukazi. iv Table of Contents Preface ………………………………………………………………………………… i Declaration of plagiarism……………………………………………………………... ii Abstract…………………………..................................................................................iii Acknowledgements……………………………………………………………………iv Chapter 1: Introductions………………………………………………………………. 1 Chapter 2: Sharing of floral signals and bee pollinators in two grassland flowers: syndrome, mimicry or coincidence?............................................................................. 18 Chapter 3: Implications of pollinator sharing by two grassland wildflowers for interspecific pollen transfer and fecundity…………………………………………….49 Chapter 4: General discussion and conclusion………………………………………. 65 v CHAPTER 1: LITERATURE REVIEW Pollinator diversity, flower diversity and pollinator sharing Approximately 87.5% of all flowering plants rely on animal vectors for pollination but a minority rely on abiotic vectors such as wind and water (Ollerton et al. 2011). Animals are good pollinators since they are mobile and can carry pollen to distant conspecific flowers, therefore promoting outcrossing and ensuring increased seed set and fruit set. Insects are the dominant group of animal pollinators (Ollerton et al. 2011). Animal pollinators are a key driver in diversification of flowering plants (Grant and Grant 1965; Stebbins 1970; Ollerton 1999). This is because flowering plants have evolved a plethora of strategies to attract pollinators which are often geographically localized, thus creating ecological gradients over which diversification takes place (Johnson 2006). To attract and maintain pollinators, plants provide rewards and exhibit floral traits that are attractive to pollinators. Examples of floral traits include colour, shape, orientation, olfactory and tactile cues that are used by pollinators to either locate food or brood sites. Particular combinations of these traits are attractive to locally available and effective pollinators (Dyer et al. 2012). Plants often occur in communities where they co-exist with other flowers sharing similar floral traits. This creates potential for pollinator sharing. As a result, a pollinator may have interactions with various co-occurring flowers. These interactions then make up a plant- pollinator network of specialized and generalized pollinators (and flowers). Pollinator sharing may be detrimental (i.e. competition), beneficial (i.e. facilitation and mimicry) or have no effects on fitness of the plant species concerned. Since animal vectors are major pollinators, their foraging behavior is very important in understanding plant-plant interactions. In this study, I investigate the interaction between Exochaenium grande (Gentianaceae) and Thunbergia atriplicifolia (Acanthaceae) which