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Ecological replacement as a restoration tool: Disentangling the impacts of Aldabra giant tortoises (Aldbrachelys gigantea) using DNA metabarcoding

Moorhouse-Gann, Rosemary 2017. Ecological replacement as a restoration tool: Disentangling the impacts of Aldabra giant tortoises (Aldbrachelys gigantea) using DNA metabarcoding. PhD Thesis, Cardiff University.
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Species extinctions on islands are commonplace throughout history. Such extinctions can lead to dysfunctional ecosystems, especially when keystone species are lost. When the target species is extinct, an analogue species can be introduced to restore ecosystem function (known as ecological replacement). In Mauritius, exotic giant tortoises (Aldabrachelys gigantea) have been introduced to restore ecosystem function after the loss of their endemic counterparts, which were thought to be keystone grazers. Dietary analysis is essential to understand the impact that tortoises have on the ecological network. Metabarcoding of plant DNA from faecal samples is an invaluable tool to recover detailed dietary information. Such dietary analysis is often inhibited by the absence of comprehensive DNA barcode libraries. The aim of this PhD research was to use DNA metabarcoding to assess the impacts and interactions of introduced Aldabra giant tortoises on Mauritian islands undergoing restoration. Here, the direct effect of tortoise dietary preferences on the plant community and the knock-on effects on two vertebrate species endemic to Mauritius, the Telfair’s skink (Leiolopisma telfairii) and Pink Pigeon (Nesoenas mayeri), were investigated. A comprehensive DNA barcode library of the plants present on Ile aux Aigrettes and Round Island was created using the second internal transcribed spacer (ITS2) in order to maximize taxonomic discrimination at the species level in the dietary analyses. Ninety-nine percent of the Islands’ angiosperms were successfully sequenced. This is the first time island plant communities have been so comprehensively DNA barcoded in order to carry out dietary analyses and the library lays the foundations for the construction of more comprehensive food webs to further current understanding of ecological restoration. Universal short-amplicon plant DNA metabarcoding primers for the ITS2 region, capable of amplifying the degraded plant DNA found in faecal samples were designed. To increase the breadth of the application of these primers, they were tested on both Mauritian and UK plant species to prove that they can be successfully applied in both tropical and temperate systems. In silico testing suggested that 88% of 1,111 UK and 9 Mauritian plants were a good match with the novel primers. In practice, 99% of 202 UK and Mauritian plants amplified successfully. The diets of introduced Aldabra giant tortoises, Telfair’s skinks and Pink Pigeons were analysed by metabarcoding the plant DNA found in faecal samples, using novel primers. Giant tortoise grazing alters the plant community structure by cropping both exotic and native vegetation. This engineering of the vegetation structure indicates that tortoises create and maintain tortoise lawns in open areas, which together with the established forested areas constitutes a vegetation mosaic that may be beneficial for biodiversity. The giant tortoises also play a role in controlling the invasive weed species on Ile aux Aigrettes, by reducing plant biomass through grazing. However, Telfair’s skinks and Pink Pigeons exhibit dietary preferences for some exotic plant species. Thus, it is important to increase the availability and variation of native plant species that these endemic vertebrates prefer to consume in order to buffer the effect of reducing the availability of preferred exotic plants.

Item Type: Thesis (PhD)
Date Type: Submission
Status: Unpublished
Schools: Biosciences
Uncontrolled Keywords: 'Ecological replacement' rewilding, conservation, tortoise, bird, lizard, reptile, 'trophic interactions', metabarcoding, 'DNA barcoding', 'next generation sequencing'
Funders: Natural Environment Research Council (NERC), Durrell Wildlife Conservation Trust
Date of First Compliant Deposit: 14 May 2018
Date of Acceptance: 23 April 2018
Last Modified: 16 Apr 2021 13:37

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