School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe influence of fire mosaics on insectivorous bats: From genes to communities( 2023-11)Australia is one of the most fire-prone countries in the world, with many ecosystems adapted to historical fire regimes. However, fire regimes are shifting, and significant changes to historical patterns of fire are recognised as a considerable threat to biodiversity. Inappropriate fire regimes have been linked to population declines of mammals across Australia. Despite this there are significant knowledge gaps in understanding how different components of the fire regime effect biodiversity, at different levels of biological organisation. This research examined the distribution and ecology of echolocating bat species (commonly referred to as microbats) in a fire-prone region of south-eastern Australia, to answer key questions of wider relevance to other fire-prone ecosystems. I explored the influence of the spatial pattern of different fire histories in the landscape – fire mosaics – on different levels of biological organisation, to draw inference on how fire-generated mosaics might affect the distribution of bat communities, individual species, populations, and genetic diversity. The research aims of this thesis were explored using a landscape scale field study, in the temperate forests and woodlands of south-western Victoria, Australia, which collected two types of microbat data. The first, an acoustic survey by means of passive acoustic detectors of 136 sites, spanning a range of fire histories, conducted over two seasons. The second a genetics study of 146 sites, also with a range of fire histories, over two seasons, which captured 647 individual bats. Passive acoustic detectors are often used to monitor echolocating bat species. However, identification of calls collected at large scales is hindered by substantial variation within and between species, and the considerable time investment needed to manually identify acoustic data. A pilot study designed to confirm the range of species present in the study area, along with the level of sampling needed to address the research aims, confirmed that the acoustic signature for many species overlapped. This meant that existing approaches for call identification would not be suitable for either automated or semi-automated identification of species from acoustic data. Therefore, as a first step, I developed a new method that allowed for much improved identification of species with overlapping call structures. This improvement then allowed for a semi-automated call identification approach, important for analysing large volumes of acoustic data collected across landscape scales. I built a machine learning classifier (Random Forest) from field collected data, to better differentiate acoustically overlapping species. The method improved overall classification success, including a 60% improvement for bats that navigate in open spaces (Chapter 2). The classifier was used to identify to species, a dataset containing 687,377 recordings over 1,632 detector nights collected at 136 sites. The results were used to model responses of species relative occurrence and specie richness to fire mosaic variables (Chapter 3). In Chapter 3, I determined how the amount, diversity, and configuration of fire influenced species richness and the relative occurrence of 13 individual species. Sites were primarily stratified to represent a range of fire age-classes: recently burnt (0-3 years post fire), early successional (4-10 years), mid successional (11-34 years) and late successional (>34 years). I built regression models for 13 species and species richness against different measures of the fire mosaic across six spatial scales (500 m – 5 km). Bat species richness responded positively to the diversity of fire age-class at all spatial scales, as did 9 out of 12 species, at one or more scales. Models of individual species responses to fire diversity indicated that the scale at which fire is measured can be important. The amount of a fire age-class was also an important determinant of relative occurrence for five species over a range of scales. Contrasting responses to fire configuration, displayed by closely related species, revealed that even subtle differences in wing morphology can contribute to differing responses of otherwise similar species. Next, I determined the influence of the fire mosaic on the genetic structure, connectivity, and diversity of two ecologically different species (Chapter 4). High resolution genetic data was derived from tissue samples collected from 280 individuals across 71 sites. This research employed emerging techniques in landscape genetics, to identify genetic variations among individuals within populations of two ecologically distinct species. I used a multi-step analytical approach. Firstly, for both species, a spatial admixture model was employed to establish likely ancestral populations. Secondly, these ancestral populations were then incorporated as random factors in Generalized Linear Mixed Models (GLMMs) to explore the correlation between genetic distance (indicative of genetic connectivity) and a range of fire metrics representing gradients of spatial patterns in fire history, availability of woodland habitat, and spanning six scales. Finally, GLMMs were used to determine the connection between individual genetic diversity and the same fire metrics across these six scales. High resolution genetic research yielded insights on the interplay between species' genetic patterns and the fire mosaic. The phylogenetic structure of Chalinolobus morio uncovered in this study supported the presence of male and female philopatry, a notable finding considering that such philopatry is rare among mammals. Conversely, the phylogenetic structure of Vespadelus vulturnus, coupled with decreased genetic connectivity and diversity, indicated that external constraints related to the fire mosaic likely disrupt the connectivity of habitat for this species. In summary, this research developed a new approach to surveying and monitoring echolocating bats species at large spatial scales. The approach is flexible and applicable to acoustic surveys more broadly. I quantified the influence of the spatial pattern of fire on insectivorous bats to define fire mosaics that benefit bat conservation. Areas of long unburnt vegetation were found to be important for several species. Importantly, the configuration and diversity of fire age-classes were also important drivers, in addition to the amount of habitat. Specific to this landscape patchy burns at the current scale of management will promote bat diversity. Specific to individual species conservation, this study found that maintaining ‘enough’ of the extent of an age-class to support roosting and foraging habitat varied among species. Genetic signatures uncovered differences between individual habitat selection and muti generational gene flow, important for the persistence of populations in the face of changing climates. Moreover, variation in the spatial scale or buffer zone that produced the strongest models highlighted the variety of ways that bat species perceive and interact with the landscape. This thesis highlights how this knowledge can be used to improve fire managed landscapes for conservation. This research frames the response of occurrence and genetic data to ecological understanding of species responses, and by doing so is applicable more generally to other fire managed ecosystems.
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ItemThe effects of fire and landscape structure on animal communities, species, and connectivity( 2023-04)The loss and fragmentation of habitat associated with land use change is the primary driver of global biodiversity declines. Changes to fire regimes that alter habitat suitability also threaten a range of animal taxa. Fire has been increasingly recognised as an important ecological process and is now used to manage fire-prone landscapes around the world, but important questions remain about the effects of fire regimes on animals, especially in fragmented landscapes. The aim of this thesis is to determine the influence of landscape structure (the composition and configuration of landscape elements) on animals in heathy woodlands in southern Australia in terms of fire, fragmentation, and interactions between them. First, I explored post-fire growth stage and land use together as components of landscape structure and assessed their relative and interacting effects on mammal communities. I used camera traps to collect mammal presence-absence data in 2019-20 and analysed it using ordination and linear modelling. I found that land cover composition was the primary influence on community composition. The composition of the fire mosaic had a secondary, weaker effect and one that may change depending on land cover composition. Second, I explored habitat structure as a mechanism by which fire regimes may affect mammal species, using a species activity index derived from the same camera trap data. Post-fire growth stage (a categorical representation of time since fire) was not a direct predictor of any species’ activity levels, but some habitat structure attributes were linked to certain growth stages and were therefore a mediating influence on animals. Finally, I assessed how the growth stages influence functional connectivity for a litter-dwelling skink. I used genetic data, landscape resistance modelling, and circuit theory-based mapping to find the relative connectivity of land use types and growth stages. Functional connectivity varied little with growth stage, with the primary influence on connectivity being the matrix of pasture and forestry plantation. Overall, I did not find direct effects of growth stage on animal communities, species, or connectivity. However, less obvious effects such as the composition of the fire mosaic beyond the site-scale and indirect effects through habitat structure are important to consider in future fire management. The extent of native heathy woodland was also vital for native mammal communities and functional connectivity; remaining native vegetation must be protected and expanded for the best outcomes for native diversity and species persistence.
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ItemLandscape-scale disturbance history and the dynamics of the conifer taxa of Bidoup–Nuiba National Park, Vietnam( 2023-03)Tropical forests are complex and poorly understood ecosystems. In recent decades large-scale, long-term forest dynamics plots have been providing important insights into basic demographic processes such as recruitment, growth, and mortality. However, this information offers little insight into long-term forest dynamics and the role of infrequent disturbances that may occur at the scale of decades to centuries. Understanding how species-rich tropical forests will respond to global change requires understanding these long-term dynamics. Dendroecology, the study of tree rings, has provided the foundation for understanding forest dynamics in temperate forests in the northern hemisphere and parts of the southern hemisphere. It has been little applied in the tropics where most tree species do not have annual growth rings. The overarching aim of this study was to better understand the ecology and dynamics of Vietnam’s Central Highlands, and, in particular, the rich conifer taxa that are found there. To address this, my thesis involves the application of dendroecological approaches to: (1) reconstruct historical fire regimes in the Central Highlands of Vietnam; (2) understand the historical dynamics of Pinus kesiya forests and (3) the spatial and temporal patterns of recruitment and growth of the endemic conifer taxa in mixed conifer-angiosperm forests, and (4) identify angiosperm species with significant potential for future tree-ring based studies. A detailed reconstruction of how disturbances have varied across the Central Highlands landscape over recent centuries will provide important insights into the response of these forests to past disturbances and climate variability, as well as potential future trajectories of development under a changing climate. My thesis presents one of the most comprehensive reconstructions of fire in tropical forests using tree rings of Pinus kesiya in the Bidoup Nuiba National Park (BNNP) in the Central Highlands of Vietnam. A tree ring-based fire reconstruction showed that fire occurrence increased after 1900 and was highly correlated with climate indices (ENSO and PDSI) during the period 1900 – 1960. However, after 1964 the relationship between climate and fire disappeared due to the overwhelming pressure of human ignition sources, which have effectively eliminated climate as a factor limiting fire in these landscapes. While climate change is seen as a potential driver of changing fire regimes in many parts of the world, the role of local human populations may be equally or more important in shaping them. A detailed history of fire in these forests provides a baseline against which recent and future changes can be assessed. Dendrochronological analysis also indicated that extreme droughts, fire, and canopy disturbance have been associated with tree regeneration, growth, and canopy recruitment for centuries. The age distributions of Pinus dalatensis, Keteleeria evelyniana and Fokienia hodginsii show a distinct pulse of recruitment during the 1750s and during the period 1800-1850 that consistently coincided with extreme regional droughts. Periods of sudden and sustained growth release amongst these endemic conifers were associated with regional mega-droughts in the late 18th and 19th centuries. Results from superposed epoch analysis between Palmer Drought Severity Index and discrete tree release events indicated that extreme droughts and abrupt changes in moisture availability may have contributed to increased canopy disturbance rates and tree-level mortality. The effects of drought on tree growth were typically recorded in the tree rings 1-5 years after the drought event. This indicated that extreme drought conditions may drive large-scale canopy disturbances in BNNP. My results also confirmed that growth releases occurred within ~5 years of recorded fire events. This relationship between growth releases and fire events is consistent over 250 years of tree-ring records (1770-2020). Finally, my thesis demonstrated that a number of the angiosperm tree species occur in BNNP have significant potential for tree-ring studies. This is the first study to systematically examine the potential of a large number of angiosperm species for dendrochronological study in Vietnam. Demographic information derived from tree-rings of angiosperms could provide well-dated, long-term data that complements the tree-ring record from the conifers at BNNP. This would further advance our understanding of forest dynamics and the impacts of climate change in these species-rich tropical landscapes.
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ItemDynamics of a flammable species in a forest landscape: A case study on forest wiregrass Tetrarrhena juncea R.Br.( 2022)Species abundance often determines the extent of influence of a species to ecosystem function and processes. Typically, the abundance of a species is constrained by environmental factors within its habitat. However, there are instances where native species becomes prolific and the shift in abundance greatly impacts the ecosystem. Such is the case when a flammable species becomes prolific within its range and alters the flammability of the ecosystem. This is a concern with climate change, as conditions might be tipped in favour of such species. Hence, it is crucial to understand the drivers of abundance to understand how native species can be released from environmental constraints of abundance to become prolific within their own range, and to predict the potential effect of changing environmental conditions on their abundance. Thus, the overarching aim of this thesis was to understand how a flammable native species can become prolific within its own range. This is achieved using a case study species – forest wiregrass Tetrarrhena juncea R.Br. (hereafter wiregrass) – an understorey native species that is of high importance to flammability in the eucalypts forests of south-eastern Australia and grows prolifically under certain conditions. The overarching aim of the thesis was addressed using a mix of research methods to identify the key drivers of wiregrass distribution and abundance. Firstly, a database of the current distribution for wiregrass were analysed using species distribution modelling to identify highly suitable habitat for wiregrass (Chapter 2). Temperature seasonality, precipitation of the driest month, rainfall seasonality, annual mean temperature, the minimum temperature of the coldest month and soil pH were strongly associated with the suitable habitat of wiregrass. The high importance of climatic factors indicates the distribution of wiregrass may be sensitive to climate change. Highly suitable habitats do not necessarily harbor abundant wiregrass because site-specific factors can also control abundance. Hence, Chapter 3 sought to identify the factors most important to wiregrass abundance in the highly suitable habitat of Mountain Ash-dominated forest. Wiregrass cover was assessed in a field survey across a chrono-sequence of 126 sites with contrasting disturbance histories. Canopy cover and net solar radiation were the most important predictors of wiregrass abundance, with wiregrass cover highest in recently disturbed areas with sparse canopy cover, high light levels, and low precipitation. The final component of the thesis used a glasshouse experiment to quantify causal links between resource availability and wiregrass abundance. Wiregrass growth was more sensitive to water availability than light, whereas biomass allocation and leaf morphology were more sensitive to light availability. Collectively, the results showed that, where wiregrass is present (distribution), three key conditions will greatly favour its prolific growth (abundance): (i) non-limiting water resource; (ii) reduced canopy cover and increased light; and (iii) recent disturbance. These key results strongly suggest wiregrass can become prolific when resources are increased, and the vegetation community is substantially disturbed. Under such conditions, increased wiregrass abundance could create a window of increased flammability for the forest ecosystem. Since climate change can alter resource availability and disturbance regime, shifts in wiregrass abundance are likely to occur under future climate scenarios.
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ItemPredicting future fire regimes and the implications for biodiversity in temperate forest ecosystems( 2022)Fire regimes are changing around the world. Fire seasons are lengthening, high severity fires are occurring more often and in unexpected places. Relationships among fire, climate, and vegetation are varied, dynamic, and under-examined in many ecosystems. While some studies have explored links between fire, climate, and vegetation such as species distributions or future fire weather under changing climate, relatively few have considered the dynamic interactions among all three simultaneously. In this thesis, I develop and apply modelling approaches to predict future fire regimes in south-eastern Australia and explore the implications for fire-responsive functional plant types. In the first quantitative chapter of my thesis (Chapter 2), I develop a new fuel model for south-eastern Australia. I use edaphic, climatic, and fire variables to build a predictive fuel model that is independent of vegetation classes and their future distributions. In Chapter 3, I use my fuel model in a landscape fire regime simulator, alongside multiple predictions of future climate, to examine the immaturity risk to an obligate seeder tree species (Eucalyptus delegatensis). My simulations indicate that this species will be under increased immaturity risk under future fire regimes, particularly for those stands located on the periphery of the current distribution, closer to roads or surrounded by a drier landscape at lower elevations. In Chapter 4, I expand the application of the above simulation approach to examine the relative importance of future fuel and future climate to changing fire regimes in six case study areas across temperate south-eastern Australia. My results indicate that the direct influence of climate on fire weather will be the principal driver of changes in future fire regimes (most commonly involving increased extent, decreased intervals, and an earlier start to the fire season). The indirect influence of climate on vegetation and therefore fuel was also important, acting synergistically or antagonistically with weather depending on the area and the fire regime attribute. Finally, in my fifth chapter, I consider future climate and fire impacts on plant persistence by combining the landscape fire regime simulator with spatially explicit population viability analyses. Obligate seeder species were at risk of population extinction or reduction in more simulation scenarios than facultative resprouters. However, my approach highlighted that the resilience of facultative resprouters might also be tested by climate related changes in demographic processes and fire regimes. Overall, my research has provided new methods and scientific insights into the changing nature of fire regimes in temperate south-eastern Australia. Some negative impacts on biodiversity from a changing fire regime, particularly on more vulnerable plant functional types like obligate seeders, appear inevitable. Further understanding of the complex interactions among fire, climate, and vegetation will enable improved integration of risks to people, property, and biodiversity into land and fire management planning.