Browsing by Subject "science"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Open Access
    Assessing the effect of feather wear on carbon and nitrogen stable isotope ratios, and the use of stable isotopes to determine predator diets in the Namibian Islands marine protected area
    (2019) Johnson, Laurie; Ryan, Peter; Connan, Maëlle
    The stable isotope (SI) approach is widely used in ecological research to tackle problems such as delineating food web structure or tracing the migratory origins of various organisms. This thesis first tested the widely accepted assumption that SI ratios are fixed in an inert tissue, and then used the SI approach to infer the food web structure, from a marine top predator point of view, of a profoundly impacted marine ecosystem off southern Namibia. In bird research, it is assumed that SI ratios are fixed in feathers once they have completed their growth during moult. This assumption is crucial in determining where birds moult, and has been used to infer changes in the environment over time, as well as changes in the trophic levels of individuals. Recent comparisons of feathers collected from several penguin species during their annual moult have shown systematic differences between newly moulted and old feather SI ratios. I thus tested whether a change in SI ratios occurs as feathers age by comparing the carbon and nitrogen SI ratios of black and white feathers collected from captive, individually known African (Spheniscus demersus) and northern rockhopper (Eudyptes moseleyi) penguins at three occasions over a year. I found a clear trend for the rockhopper penguin feathers with new and old black feathers differing in their δ13C and δ15N values; this trend was not as clear for the African penguins. I then tested factors related to feather wear as a possible mechanism for differences in SI ratios between new and old feathers; these factors were feather reflectance and microstructure. In both penguin species, old black feathers reflected more light, and had a larger proportion of their barbs without barbules near their tips compared to new feathers. Feather wear may result in melanin leakage, which may explain the observed trends in the SI ratios between new and old pigmented penguin feathers. Differences in SI values were observed between species and may be a result of facility at which the penguins were housed, where one facility was exposed to more sunlight than the other, rather than the differences being a result of species. Although the differences observed were subtle, the state of feather wear (i.e. timing of feather collection within the moult cycle) should be considered in order to make accurate ecological inferences based on their SI ratios. Further research is needed to fully understand the phenomenon and to test whether the same process affects pigmented feathers of flying birds. I then used SI ratios to update our knowledge of resource partitioning among a marine top predator community in southern Africa, and to infer the marine food web structure in a Marine Protected Area off the southern Namibian coast. The Namibian Islands Marine Protected Area supports the most important breeding population of bank cormorants (Phalacrocorax neglectus; Endangered), and historically was important for two other Endangered seabirds: African penguins and Cape gannets (Morus capensis). Non-threatened marine top predators studied in the system were: greater crested terns (Thalasseus bergii) and Cape fur seals (Arctocephalus pusillus). In the last 50 years, shelf waters off southern Namibia have been overfished, resulting in an altered marine ecosystem; shoaling fish have been replaced by less nutrient-rich species such as hake (Merluccius spp.), jellyfish, and salps. Previous studies of the region’s food webs were based on traditional dietary analyses such as stomach content and scat analysis which provide short-term diet information. Here, I measured carbon and nitrogen SI ratios in several tissues of the marine top predators, and those of their potential prey species, to infer the marine food web for this region, and to complement short-term data obtained from traditional dietary analyses. Among the predator guild, Cape gannets had the lowest δ13C and δ15N values, indicating that they fed the farthest offshore and at the lowest trophic level both during the breeding season (from whole blood) and moulting period (from feathers). As expected, bank cormorant δ13C values indicated that they fed more benthically than the other predators, and Cape fur seals fed at the highest trophic level. African penguin tissue SI ratios were intermediate between those of Cape gannets and Cape fur seals. Greater crested terns exhibited the highest δ13C values, suggesting that they fed closest to shore. Bayesian mixing models used with species-specific discrimination factors (when available) revealed some resource partitioning among the marine top predators in this region but many made use of similar resources; sardine (Sardinops sagax), squid (Loligo reynaudii), and rock lobster (Jasus lalandii). This work highlights the most likely prey items used by marine top predators outside of the breeding period, and provides new insights into the food web of this region. Prior to the start of industrial fishing, marine top predators in this region mainly ate sardine and anchovy. Despite the collapse of these species’ populations in the 1970s, marine top predators currently still make use of these prey resources, which concurs with the results found from stomach content and scat analyses. In this thesis, I have shown experimentally that SI ratios vary slightly as feathers age in penguins. I have also used the SI technique in an ecological context to add to the knowledge on the diet of marine top predators of an overfished ecosystem. Overall I have shown how the SI approach can add to our understanding of trophic ecology, and also how the method is dependent on accurate SI inputs in order to make accurate dietary inferences.
  • Thumbnail Image
    Item
    Open Access
    Radio monitoring of the hard state jets in the 2011 outburst of MAXI J1836−194
    (2015) Russell, T D; Miller-Jones, J C A; Curran, P A; Soria, R; Altamirano, D; Corbel, S; Coriat, M; Moin, A; Russell, D M; Sivakoff, G R; Slaven-Blair, T J; Belloni, T M; Fender, R P; Heinz, S; Jonker, P G; Krimm, H A; Körding, E G; Maitra, D; Markoff, S; Middleton, M; Migliari, S; Remillard, R A; Rupen, M P; Sarazin, C L; Tetarenko, A J; Torres, M A P; Tudose, V; Tzioumis, A K
    MAXI J1836-194 is a Galactic black hole candidate X-ray binary that was discovered in 2011 when it went into outburst. In this paper, we present the full radio monitoring of this system during its 'failed' outburst, in which the source did not complete a full set of state changes, only transitioning as far as the hard intermediate state. Observations with the Karl G. Jansky Very Large Array (VLA) and Australia Telescope Compact Array (ATCA) show that the jet properties changed significantly during the outburst. The VLA observations detected linearly polarised emission at a level of ~1% early in the outburst, increasing to ~3% as the outburst peaked. High-resolution images with the Very Long Baseline Array (VLBA) show a ~15 mas jet along the position angle -21 ± 2o, in agreement with the electric vector position angle found from our polarisation results (-21 ± 4o), implying that the magnetic field is perpendicular to the jet. Astrometric observations suggest that the system required an asymmetric natal kick to explain its observed space velocity. Comparing quasi-simultaneous X-ray monitoring with the 5 GHz VLA observations from the 2011 outburst shows an unusually steep hard-state radio/X-ray correlation of LR ∝ LX1.8±0.2, where LR and LX denote the radio and X-ray luminosities, respectively. With ATCA and Swift monitoring of the source during a period of re-brightening in 2012, we show that the system lay on the same steep correlation. Due to the low inclination of this system, we then investigate the possibility that the observed correlation may have been steepened by variable Doppler boosting.
  • No Thumbnail Available
    Item
    Open Access
    Seeing our world through science
    (2013) Sithaldeen, Riashna; Jarrett, Thomas
    We live in a time of exciting discoveries and developments in the field of science, allowing us to 'see' more than ever before, including 'seeing' into the past, the present and the future. For anyone who wants to learn more about seeing our world through science.
  • Thumbnail Image
    Item
    Open Access
    Wishful thinking and the unconscious: A reply to Gouws
    (2005) Galgut, E
    This paper argues against the view that the Freudian unconscious can be understood as an extension of ordinary belief-desire psychology. The paper argues that Freud’s picture of the mind challenges the paradigm of folk psychology, as it is understood by much contemporary philosophy of psychology and cognitive science. The dynamic unconscious postulated by psychoanalysis operates according to rules and principles that are distinct in kind from those rules that organise rational and conscious thought. Psychoanalysis offers us a radical reconception of our ordinary way of thinking about our own minds.