Doctoral Degrees (Botany and Zoology)

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Browsing Doctoral Degrees (Botany and Zoology) by browse.metadata.advisor "Altwegg, Res"

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    Ecology of the Black-faced sheathbill on Marion Island
    (Stellenbosch : Stellenbosch University, 2013-12) McClelland, Gregory T. W.; Chown, Steven L.; Altwegg, Res; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology, Centre for Invasion Biology.
    ENGLISH ABSTRACT: As the pace of climate change has begun to accelerate so too has it become clear that the direct impacts thereof are likely to have profound consequences for many island systems. Moreover, it has also been suggested that climate change will exacerbate the effects of many invasive species, so further impacting both diversity and ecosystem functioning. Forecasts for such interactions have been most pronounced for the Southern Ocean islands, which are home to a wide variety of endemic species. This thesis is about such interactions and their specific impacts on a key endemic, the black-faced sheathbill (Chionis minor) on the Prince Edward Islands. Of increasing concern is how invasive rodent populations in the Southern Ocean may be responding to global climate change, as ameliorating conditions on these islands are forecast to decrease thermal and resource restrictions on rodents. However, firm evidence for changing rodent populations in response to climate change, and demonstrations of associated impacts on the terrestrial environment, are entirely absent for the region. In Chapter 2 of this thesis, these relationships are explored for invasive house mice (Mus musculus) on Marion Island. Using spatially explicit capture-recapture modeling, it is determined that mouse populations across a range of habitats have increased over time. Owing to an extended breeding season, made possible by ameliorating conditions brought on by climate change, the total number of mice on the island at annual peak density more than doubled over the past decade. It is also demonstrated that mice directly reduce invertebrate densities, with biomass losses up to two orders of magnitude in some habitats. Because of the importance of invertebrates to nutrient cycling on the island, such changes are likely to have significant ecosystem-level impacts. In Chapter 3 the focus expands to examine how increasing mouse impacts and other outcomes of climate change are affecting the ecology of the black-faced sheathbill. It has been established that invasive house mice are capable of suppressing the populations of several seabird species in the Southern Ocean. However, mouse impacts on the region’s few island endemic land-birds remain largely unexplored. Further, a significant effect of climate change may be realized by altering interspecific interactions, specifically food webs. A significant portion of sheathbill diets is derived from rockhopper penguins, a species currently under a climate-change-driven decline, which may have significant effects on sheathbills. The study found that terrestrial invertebrates are no longer a significant prey resource for sheathbills on Marion Island, and that sheathbills have effectively been displaced from a formerly important winter food resource by mice. In response, the number of sheathbills foraging in king penguin colonies increased. Moreover, a reduced rockhopper penguin population lead to significant declines in both the number and proportion of sheathbills foraging in rockhopper penguin colonies. The sum result was a significant decline in the body condition of female sheathbills. Rather than decrease reproductive output, sheathbills responded by decreasing clutch size and producing significantly fewer male nestlings. While population estimates did not detect a reduction in the number of sheathbills, population projections suggest that the population is in decline, with the reproductive population declining faster than the absolute population. There is need for greater study of island species, as for even relatively well-studied taxa such as birds many aspects of ecology remain significantly less studied when compared to species occurring on continents. For example, basal metabolic rate (BMR) is a fundamental characteristic of all endotherms, yet only a handful of island birds have had their BMR measured, and fewer still to a level that allows intraspecific analysis. In Chapter 4 the BMR of black-faced sheathbills on Marion Island was measured to determine whether the unique phylogenetic position and ecology of sheathbills equate to a unique BMR when allometrically compared to other birds. It was found that the BMR of sheathbills is typical for a bird of its size. However, significant intraspecific variation was found to occur, with differences in habitat quality a likely driver. The results of the study show that the combined effects of climate change and invasive species can have significant consequences for terrestrial endemics in the Southern Ocean. Further, the long-term changes observed in sheathbills make clear the need for improved documentation and study of island species in general, as many of the responses observed in this study are significant but subtle and would not have been evident without detailed knowledge of species ecology and vital rates. Giving greater focus to insular biota is imperative to understanding their current status and ecology as well as establishing a barometer against which further global change can be measured and mitigation measures evaluated. Specific conservation responses for the black-faced sheathbill on Marion Island include the provision of nest boxes at king penguin colonies, and eradication of house mice. The latter would have long-term benefits for the species, invertebrates, ecosystem functioning generally, and likely also for important seabirds such as several species of albatrosses whose chicks are being increasingly preyed on by mice. Eradication would, however, be difficult and expensive, and with substantial potential non-target effects, including on sheathbills, that would have to be carefully managed. In the absence of local mouse eradication, and with ongoing climate change, specific management of the sheathbill population through the provision of supplementary nesting sites seems the most appropriate conservation action. It should therefore be examined in small-scale trials to ascertain the likelihood of unintended consequences. Importantly, the maintenance of Prince Edward Island as largely free of invasive species is key to the conservation of the local black-faced sheathbill subspecies, Chionis minor marionensis, endemic to the Prince Edward Island group.