School of BioSciences - Theses
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ItemNatural history and the effects of artificial light at night in an urban-exploiting orb-weaving spider( 2024-01)Spiders are the most speciose and abundant lineage of terrestrial predators on the planet. They play vital ecological roles as major consumers of insects across habitat types, and many spider species are now urban exploiters, taking advantage of the resources afforded by anthropogenic change. However, the degree to which the stressors present in these environments, including artificial light at night (ALAN), affect their behaviour, development and physiology remains largely untested. In this thesis, I explore how the natural history of spiders can help us understand urban exploitation, and examine some of the effects of ALAN on one of these important urban predators, the nocturnal Australian garden orb-weaving spider (Hortophora biapicata). My thesis begins with a general introduction (Chapter 1), outlining the rationale for my project. The core of the thesis is then divided as follows. In Section I (Chapters 2 and 3), I explore the current knowledge of urban exploitation. I start with a review of the ways in which a broad range of animals are able to exploit anthropogenic change despite the multitude of stressors present in these environments, how this exploitation shifts species interactions, and the likely ecological consequences (Chapter 2). Animals exploit anthropogenic resources such as abundant food sources and novel habitats, and the resulting shifts in species interactions can further benefit some species. However, this has costs such as decreased biodiversity, poorer food security, and increased spread of disease. Following this, I present a review of the urban ecology of spiders broadly (Chapter 3), highlighting which spiders are successful in cities, which urban habitats they occupy, and the many traits that can facilitate their success. This review establishes an important question at the centre of this thesis: why are some spiders more successful than others at exploiting urban habitats? Section II (Chapters 4 – 7) presents a case study of a particular urban-exploiter, H. biapicata. This section starts with a description of the natural history of H. biapicata, including their morphology, development, foraging, and mating (Chapter 4). This chapter highlights important traits for their urban exploitation, including generalist habitat and dietary preferences, and an ability to change colour between moults to camouflage on new surfaces. Supporting this, I then explore the flexibility they exhibit in foraging and mating behaviours, and discuss how an orb-weaver performing these tasks without an orb-web provides insight into the evolution of web loss (Chapter 5). The next two chapters move on from natural history to explore the effects of ALAN on these spiders. First, I test the impacts of ALAN exposure on development and climbing performance, highlighting the importance of the timing and duration of ALAN exposure: ALAN accelerates juvenile development but prolongs subadult development, and the resulting morphological shifts may impact climbing speed and other behaviours (Chapter 6). Second, I test the effects of ALAN on neuroanatomy in garden orb-weavers, finding that even short-term ALAN exposure negatively impacts brain volumes (Chapter 7). In particular, I find a negative effect of ALAN on the volume of the primary eye visual pathway, which is associated with the synthesis of melatonin. Melatonin is an important hormone in the brain, the suppression of which by ALAN is thought to be a major driver of the impacts of ALAN in general, potentially providing insight into both the mechanisms and consequences of these effects. Finally, I conclude my thesis with a general discussion that links natural history and responses to ALAN to urban exploitation in H. biapicata, explores how these findings can inform our understanding of the success of other animals in cities, and highlights important areas for future research (Chapter 8). Australian garden orb-weavers are charismatic and ecologically important predators across Australia, yet they are symbolic of the general lack of research for spiders in urban areas. The findings in this thesis convey the importance of natural history research, the value of both behavioural experiments and recent technological advancements for understanding biological processes, and the need to bridge the gap in knowledge about these vital but underappreciated predators. This not only improves our knowledge about spiders in cities, but because urban ecology is often taxonomically limited, will also help to develop more robust theory and predictions for the future of animals in a changing world.
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ItemThe effects of artificial light at night on the behaviour and physiology of Drosophila melanogaster( 2019)Artificial light at night is one of the most pervasive and least recognised anthropogenic pollutants. Its extent and intensity is expanding globally, at an estimated rate of 2.2% per annum, such that many urban and peri-urban animals no longer experience natural darkness. The majority of animal species have evolved under bright days and comparatively dark nights, with their physiology and behaviour synchronised to this rhythm. Accordingly, disruption to this cycle is linked to a suite of behavioural and physiological shifts across taxa. Experimental and correlational evidence documenting phenotypic responses to the presence of artificial light at night are accumulating but the underlying mechanisms driving the observed negative impacts associated with artificial light at night are not resolved. A likely reason is that light at night disrupts circadian rhythms and contributes to perturbed oxidative status through its interaction with the indolamine, melatonin, a key driver of circadian rhythm and powerful antioxidant. A major and currently untested gap in the literature is whether light at night can act as a driver of evolutionary change. In this thesis, I investigated the short-term phenotypic impacts of ecologically relevant levels of artificial light at night on life history traits (throughout Chapters 2 to 5) and oxidative status (Chapters 2 and 4), using the model organism Drosophila melanogaster. I then used experimental evolution to explore whether artificial light at night can induce local adaptation in populations evolved over 15 to 25 generations (Chapter 4). Finally, using a melatonin dietary supplementation experiment, I attempted to investigate the relationship between artificial light at night and the melatonin pathway, thus hoping to demonstrate disruption to the melatonin pathway as a causal mechanism underpinning the observed phenotypic responses (Chapter 5). My results demonstrated experimentally, that individuals exposed to artificial light at night have short-term phenotypic changes, with disrupted mating behaviour, reduced fecundity and size, altered development patterns and reduced longevity in D. melanogaster (Chapters 2 to 5). Additionally, I present novel (and somewhat counter intuitive) evidence, that artificial light at night is associated with lower levels of reactive oxygen species in ovaries (Chapter 3) and, aligned with this, reduced oxidative DNA damage in female flies (Chapter 4). In contrast, I found no evidence for such effects in males (Chapters 3 and 4). Despite potential for evolutionary change, I found little evidence for adaptation in most fitness traits after evolution under artificial light at night (Chapter 4), with mating propensity the only life history trait to exhibit limited local adaptation. This suggests that the selective pressure of artificial light at night in the laboratory, may be weaker than anticipated. However, the fact that some level of adaptation was evidenced in relatively benign conditions, suggests that, in a more natural competitive environment, the selection pressure of light at night may be stronger. I was unable to conclude whether melatonin is a potential mechanism driving phenotypic variation following exposure to light at night (Chapter 5), as the melatonin doses and design used, despite replicating a previous study, resulted in negative fitness consequences (reduced longevity under dark nights and disrupted eclosion regardless of light treatment), suggesting the melatonin supplementation was potentially toxic. Nonetheless, these data potentially support the importance of circadian disruption under artificial light at night as a driver of the observed negative effects. I propose that further research is warranted to test different melatonin concentrations and dosage duration at different life stages and at different times relative to circadian rhythm. This variation in dosage regimens may be able to better define its role (and the relevant contributions of circadian disruption and oxidative status) in the effects of artificial light at night. This thesis demonstrates the negative impacts of artificial light at night in the model species, D. melanogaster and adds to the growing body of literature documenting taxa-wide damaging effects of artificial light at night on animal behaviour and physiology. As we continue to light our nights and reduce natural darkness globally, understanding the consequences and mechanisms behind the effects of artificial light at night is paramount to informed decision making around urban lighting strategies. Moreover, understanding the role of artificial light at night in driving evolutionary change, is of the utmost importance in maintaining wildlife stability in an increasingly urbanised world.
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ItemImpacts of streetlights on sleep in urban birds( 2019)Over the past century, artificial light has dramatically transformed our environment. Light at night is increasing globally, to the extent that in many places, true darkness no longer exists. As the timing of light can influence almost all aspects of biology, the alteration of natural light cycles could pose a severe threat to wildlife. One particularly harmful impact could be the disruption of sleep. In this dissertation, I investigate the impacts of artificial light at night on sleep. Despite the importance and prevalence of sleep across the animal kingdom, sleep is arguably underappreciated in studies of ecology and conservation. After providing a general introduction (Chapter 1), I begin by giving a broad perspective of sleep research, including current methods, opportunities, and the significance of sleep for issues such as artificial light at night (Chapter 2). I then provide a review of the evidence for impacts of artificial light at night, in both humans and wildlife (Chapter 3). Finally, I explore the effects of artificial light at night on two diurnal bird species: pigeons (Columba livia) and black swans (Cygnus atratus). I focus on the effects of one of the most common sources of outdoor lighting: streetlights. Light at night from LED streetlights caused pigeons to have less rapid eye movement (REM) sleep and non-REM sleep, have more fragmented sleep, and sleep less intensely than during darkness (Chapter 4). Some of these effects persisted for more than a day after exposure to light at night. In black swans, light at night in a naturalistic environment reduced night-time rest, which we demonstrate reflects reduced sleep (Chapter 5). This research provides the first direct evidence that exposure to environmentally-realistic artificial light at night can disrupt sleep in birds. One possible strategy for reducing disruption of sleep could be to alter the colour of lighting. To test this idea, I compare the effects of two different lighting colours: white (blue-rich) and amber (blue-reduced) light. Previous research has shown that blue wavelengths of light have the greatest effect on melatonin, a hormone important for sleep regulation. However, contrary to my predictions, amber and white light had very similar effects on sleep in both pigeons (Chapter 4) and swans (Chapter 5). Together, these findings will help councils and other land managers to make more informed decisions about lighting, particularly for areas that might offer important refuges for wildlife.