Impacts of invasive birds: assessing the incidence and extent of hybridization between invasive Mallard Ducks (Anas platyrhynchos) and native Yellow-billed Ducks (Anas undulata) in South Africa

Stephens, Kirstin (2019-04)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Hybridization with invasive species is a major impact on native congeners, often leading to introgression and the loss of unique genotypes or co-adapted gene complexes. Therefore, hybridization needs to be managed through the removal of invasive species and their hybrids. Such management is often hampered, due to difficulties in identifying hybrids without genetic information, as hybrids are often morphologically indistinguishable from parental species. Consequently, genetic monitoring has become a useful tool in the detection and management of hybridization. Public opposition is often an additional barrier to the control of charismatic invasive species. In the case of the invasive Mallard Duck (Anas platyrhynchos) in South Africa, there is strong public opposition to their removal, founded in part, upon a lack of awareness of the potential threat that Mallard Ducks pose to the native Yellow-billed Duck (A. undulata) through hybridization and subsequent introgression. Despite this threat, hybridization between the two species is still based on observational evidence with no scientific evidence to support the occurrence and extent of hybridization between these two species. With this thesis I aim to provide baseline genetic data for the Yellow-billed Duck in South Africa, and use population genetic analyses to determine if there is population genetic structure and differences in genetic diversity measures between widespread native populations. I also aim to determine if hybridization and introgression has occurred and whether sex-biased mating between Mallard and Yellow-billed Ducks occurs in South Africa. I used nuclear microsatellite DNA markers to determine genetic diversity measures and structure of Yellow-billed Ducks across three populations and historical museum samples from South Africa. The current populations represent a panmictic population with sufficient migration between them to prevent the formation of a strong population genetic structure. This has two implications: firstly, resources do not need to be focused on the conservation of genetically unique populations in South Africa and secondly, that there is the potential for hybridization to spread through long-distance migration between populations. Genetic diversity and structure have not changed significantly over time suggesting that there have been no major changes in the population numbers of Yellow-billed Ducks and their genetic diversity, up until now, is not negatively affected by anthropogenic threats such as urbanisation. To determine whether hybridization between Mallard and Yellow-billed Ducks, and introgression, have occurred in South Africa I used microsatellite genotype data to identify hybrid individuals and assign them to hybrid genotype classes. There is genetic evidence of hybridization and introgression between Mallard and Yellow-billed Ducks but most backcrossing and introgression is occurring into the Mallard Duck population. This means that the Yellow-billed Duck population is currently largely unaffected by introgression, but that it may become more extensive in the future. I also used DNA sequencing of a mitochondrial gene region to determine if there was sex-biased mating occurring between these two duck species. This approach revealed that most mating is occurring between Mallard Duck hens and Yellowbilled Duck drakes. These findings indicate that Mallard Duck hens should be prioritised for removal and that it is advisable to remove Mallard Ducks while there is still a chance of protecting the genetic integrity of the Yellow-billed Duck. These findings have implications for managing hybridization between the two species such as the prioritisation of the removal of Mallard Duck hens and that removal of Mallard Ducks has a good chance of protecting the genetic integrity of the Yellow-billed Duck. Moreover, if these findings are communicated to the public, it may convince them in favour of the removal of invasive Mallard Ducks.

AFRIKAANSE OPSOMMING: Hibridisering met indringerspesies is 'n belangrike impak op nabyverwante inheemse spesies, wat dikwels lei tot introgressie en die verlies van unieke genotipes of mede-aangepaste geenkomplekse. Daarom moet hibridisering bestuur word deur die verwydering van indringerspesies en hul basters. Sulke bestuur word dikwels bemoeilik weens probleme met die identifisering van basters sonder genetiese inligting, aangesien basters dikwels nie morfologies van ouerspesies onderskei kan word nie. Gevolglik is genetiese monitering 'n nuttige hulpmiddel in die identifisering en bestuur van hibridisasie. Openbare opposisie is dikwels 'n addisionele versperring vir die beheer van charismatiese indringerspesies. In die geval van indringer Mallardse eende (Anas platyrhynchos) in Suid-Afrika, is daar sterk openbare teenkanting teen hul verwydering, wat gedeeltelik gebaseer is op, op grond van 'n gebrek aan bewusmaking van die potensiële bedreiging wat Mallard eende vir die inheemse Geelbekeend (A . undulata) inhou deur hibridisasie en daaropvolgende introgressie. Ten spyte van hierdie bedreiging, is hibridisering tussen die twee spesies steeds gebaseer op waarnemingsgetuienis sonder wetenskaplike bewyse om die voorkoms en omvang van hibridisasie tussen hierdie twee spesies te ondersteun. Met hierdie proefskrif poog ek om basiese genetiese data vir die geelbekeend in Suid-Afrika te verskaf om bevolkingsgenetiese ontledings te gebruik om te bepaal of daar 'n bevolkingsgenetiese struktuur en verskille in genetiese diversiteit tussen wydverspreide inheemse bevolkings is. Ek poog ook om vas te stel of hibridisasie en introgressie plaasgevind het, en of geslags-bevooroordeelde paring tussen Mallardse eende en geelbekeende in SuidAfrika voorkom. Ek het kernmikrosatelliet-DNA-merkers gebruik om genetiese diversiteit en struktuur van geelbekeende van drie populasies en historiese museum monsters in Suid-Afrika te bepaal. Die huidige bevolkings verteenwoordig 'n panmiktiese bevolking met voldoende migrasie tussen hulle om die vorming van 'n sterk bevolkingsgenetiese struktuur te voorkom. Dit het twee implikasies: eerstens hoef hulpbronne nie op die bewaring van geneties unieke bevolkings in Suid-Afrika gefokus te word nie. Tweedens, daar is die moontlikheid dat hybridisasie kan versprei deur middel van langafstand-migrasie tussen bevolkings. Genetiese diversiteit en struktuur het nie oor tyd beduidend verander nie, wat daarop dui dat daar geen groot populasiegetal veranderinge in geelbekeende was nie en dat hulle genetiese diversiteit, tot nou toe, nie negatief beïnvloed was deur menslike bedreigings soos verstedeliking nie. Om vas te stel of hibridisering tussen Mallardse eende en geelbekeende, en introgressie, in Suid-Afrika plaasgevind het, het ek mikrosatelliet genotipe data gebruik om hibried individue te identifiseer en hul aan baster genotipe klasse toe te ken. Daar is genetiese bewyse van hibridisering en introgressie tussen Mallardse eende en geelbekeende, maar die meeste terugkruising en introgressie vind plaas in die Mallardse eend bevolking. Dit beteken dat die geelbekeend bevolking tans grotendeels onaangeraak word deur introgressie, maar dat dit in die toekoms meer prominent kan word. Ek het ook DNA-volgordebepaling van 'n mitochondriale geen-streek gebruik om vas te stel of daar geslags-bevooroordeelde paring tussen die twee eendspesies voorkom. Dié benadering het aan die lig gebring dat die meeste paring tussen Mallardse eend wyfies en geelbekeend mannetjies plaasvind. Hierdie bevindings dui daarop dat wyfie Mallardse eende geprioritiseer moet word vir verwydering en dat dit raadsaam is om Mallardse eende te verwyder terwyl daar nog 'n kans is om die genetiese integriteit van die geelbekeend te beskerm. Hierdie bevindings het implikasies vir die bestuur van hibridisasie tussen die twee spesies, soos die prioritisering van die verwydering van wyfie Mallardse eende en dat die verwydering van Mallardse eende oor die algemeen 'n goeie kans bied om die genetiese integriteit van die geelbekeend te beskerm. Verder, as hierdie bevindinge aan die publiek oorgedra word, kan dit hulle oortuig ten gunste van die verwydering van indringer Mallardse eende

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