School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemImpacts of short-interval wildfires on tree demography and forest structure in temperate Australia( 2019)Fire is a powerful agent of disturbance in terrestrial ecosystems, and it shapes vegetation composition and patterns globally. This is particularly true in south-eastern Australia where forests are dominated by species of the genus Eucalyptus, many of which have the capacity to recover from high-severity fire by resprouting from epicormic or basal buds. Climate change is predicted to yield more severe fire weather and lengthen fire seasons in temperate Australia, leading to increased wildfire frequency in these forests. While increased fire frequency – resulting in wildfire intervals of under a decade – are known to negatively affect fire-sensitive eucalypts (obligate seeders which have a juvenile period of 10 – 15 years) less is known about how such changes impact fire-tolerant, resprouting eucalypts. This Thesis examines the impacts of recent wildfires in south-eastern Australia, where a series of large wildfires burned over four million hectares of land, leading to the burning of different types of fire-tolerant eucalypt forests (basal resprouters, epicormic resprouters) by high-severity wildfires once, twice, and sometimes three times between 2003 and 2013. In the context of this massive natural experiment in the landscape, my overarching aim was to quantify the impacts of short-interval wildfire on eucalypt tree demography and regeneration, to improve understanding of potential fire-related changes to the structure and resilience of fire-tolerant forests. Short-interval high-severity wildfires significantly increased whole-tree mortality and decreased the abundance of both resprouts and seedlings in basally resprouting eucalypt forests. In these sub-alpine forests, dominated by snow gum (E. pauciflora), more frequent wildfire (two and particularly three short-interval fires) also increased the cover of grasses at the expense of shrubs. In mixed-species eucalypt forests, which occupy extensive tracts of low elevation landscapes in south-eastern Australia, resprouting occurs from both basal and epicormic buds. In these forests, the dynamics of both topkill (i.e. stem, but not whole-tree, mortality) and whole-tree mortality have important ramifications for forest structure. After a single high-severity wildfire, small-diameter stems were typically topkilled; after two short-interval wildfires, the diameter of stems topkilled increased. Additionally, the overall likelihood of either basal or epicormic resprouting decreased after two short-interval wildfires. This decline in resprouting capacity indicated that the size class most vulnerable to ‘resprout failure’ after multiple wildfires was intermediate sized stems (in the vicinity of 20 - 30 cm DBHUB), rather than smaller or larger stems. Seedling regeneration also decreased in these forest types after short-interval wildfires, suggesting that, as for fire-sensitive forests, immaturity risk may be a relevant factor for fire-tolerant forests. Short-interval wildfires reduced the total and aboveground carbon stocks of mixed-species forests, while also increasing the proportion of carbon mass in the dead pool, indicating that resprouter forests might not be perpetually secure carbon stocks under emerging fire regimes. My Thesis highlights that fire-tolerant forests may not be as invulnerable to changes in fire frequency as widely assumed, and that management interventions will likely be required to counteract increasing tree mortality and decreasing tree regeneration if predictions of more frequent and severe wildfires in temperate Australia are realised.
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ItemEdge effects in fire prone landscapes: ecological importance and implications for fauna( 2018)The overarching aim of this thesis was to investigate the ecological importance of fire edges, focusing on the influence of fire-induced edge effects on fauna in forested landscapes. Edges are ubiquitous environmental features, occurring in a wide range of ecosystems and across multiple spatial scales. Edges have been extensively researched in some contexts, particularly agricultural and urban landscapes. Accordingly, much of our understanding about how edges influence animals comes from highly modified ecosystems. Fire is an agent of edge creation and a globally important driver of biome distribution and community composition, yet little is known about how fire edges affect ecological processes in flammable ecosystems. In this thesis I review the literature on fire, fauna and edge effects to summarise current knowledge of faunal response to fire edges and identify knowledge gaps (Chapter 2). I developed a conceptual model for predicting edges effects in fire-prone landscapes, combining several drivers of faunal-fire responses. Fire-generated edge effects were found to differ from edges in modified systems, being temporally dynamic, spatially complex and characterised by the strength of the interaction between components of the disturbance regime and other biophysical factors. In Chapter 3 I investigated the response of ground-dwelling mammals to burnt/unburnt edges created by prescribed burning. I used a space-for-time substitution design to explore how species use of fire edges changes over time as the burnt side of the edge regenerates. I found that understorey complexity was reduced on the burnt side of edges for the first two years after fire. Larger animals with generalist resource requirements were more active at burnt edges immediately after fire, whereas small mammals were generally less active on burnt edges for up to 3 years. Species were not following patterns of temporal change in vegetation structure, with high usage during times of reduced understorey complexity and low usage when complexity was high. This suggests that habitat change is not a good predictor of animal use at fire edges and that other important processes are likely occurring. For example, foxes and cats were using the burnt side of edges immediately after fire, which may have important implications for the long-term persistence of native fauna if changes in habitat structure at fire edges cause predation rates to increase. In Chapter 4 I assessed the trade-off between deploying more detection units or extending the length of the sampling period on two frequently assessed variables in camera trapping studies – species richness and detection probability. The trade-off between these two factors is expected to affect data quality, but there is little information about their relative influence. I examined the trade-off between increasing deployment time or increasing the number of detection units on species richness and detectability (Chapter 4). I found that that increasing the number of cameras deployed per site was an effective method for increasing the detection of ground-dwelling mammals. Multiple cameras and longer deployment times were necessary to detect a high proportion of species present. Increasing the number of cameras or increasing deployment length resulted in high overall detectability for the more detectable species, but multiple cameras were required to achieve high detectability in a reasonable time frame (<50 days) for less detectable species. In Chapter 5 I investigated resource selection of a semi-arboreal mammal eight years after a major wildfire using GPS telemetry. Survival and persistence of animals after fire is largely driven by the abundance and distribution of remaining resources and the rate at which key habitat components regenerate or re-accumulate. I found that resource selection for the mountain brushtail possum (Trichosurus cunninghami) often depended on the sex of the animal and forest type, suggesting that considering spatial changes in resource availability and demographic class may be necessary to accurately determine patterns of resource selection after a major wildfire. This thesis adds to the body of knowledge on the ecological importance of fire edges and their implications for fauna, while providing several important conceptual and methodological advances in the study of ecology. Edges are pervasive and important environmental features that require further attention. Mechanistic approaches based on the strength of habitat associations and resource availability may help to clarify the nature and strength of edge effects in fire-prone landscapes and improve predictive models. A better understanding of fire edges will enable land managers to integrate the needs of biodiversity into future fire management planning.
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ItemInteractions between invasive predators, native mammals and fire in a forest ecosystem( 2016)A predator’s impact upon its prey depends not only on the evolutionary history and intrinsic characteristics of the two species, but also on the structure of the environment in which they interact. Fire is a major driver of vegetation structure, and there is growing concern that fire could increase the threat that invasive predators pose to native fauna. In this thesis, I investigated the interactions between fire, two invasive predators (red foxes Vulpes vulpes and feral cats Felis catus), and a suite of native mammal species. I used four different approaches to examine this problem within a fire-prone forest ecosystem of south-eastern Australia. At a landscape-scale, species distributions are often poorly predicted by time-since-fire. I developed a conceptual model of the potential interactions between fire and other drivers of faunal occurrence (including predation), and then used non-parametric Bayesian networks to quantify these relationships for terrestrial native mammals. I found that critical-weight-range mammals were more likely to occur at long unburnt sites with high habitat complexity, and in wetter forest types. In contrast, large macropods preferred less complex habitats and younger or drier forest. Species distributions were generally more strongly associated with habitat complexity than time-since-fire or invasive predator occurrence. Yet, because Bayesian networks captured the relationships between proximal and distal drivers, models could effectively predict the distributions of most species using only mapped and remote-sensed data. At a finer-scale, I used a before-after control-impact experiment to investigate the short-term effects of a prescribed fire on understorey vegetation cover, native mammal occurrence, and invasive predator occurrence and diet. Associations between species occurrences and vegetation cover in unburnt forest indicated that fire was likely to promote invasive predators but disadvantage small- and medium-sized native mammals. After the fire, there was a five-fold increase in invasive predator occurrence at burnt sites, relative to the control. Concurrently, red foxes increased their consumption of medium-sized native mammals, and selected more strongly for long-nosed bandicoots Perameles nasuta and short-beaked echidnas Tachyglossus aculeatus. The occurrence of several native mammals declined after the fire, but it was difficult to distinguish the effects of the fire from seasonal variation. I used GPS-tracking collars to investigate whether forest-dwelling red foxes selected for human-modified habitats (including recently-burnt forest). There was substantial variation in fox behaviour, highlighting the importance of considering individual variation in habitat selection studies. At a broad-scale, however, red fox habitat selection tended to vary with proximity to the forest edge. Most foxes selected for human-modified habitats such as reservoirs, roads and forest-farmland edges in their fine-scale movements, particularly at night. Two foxes whose home-ranges overlapped a burn-block intensified their use of the block immediately after fire. Yet other nearby foxes showed little response, suggesting that fire responses are highly localised. Finally, I used an agent-based simulation model to explore how changes in vegetation cover and predator abundance after fire could affect a critical-weight-range mammal. The model confirmed that fire and predation can have synergistically negative impacts on native mammal populations in burnt forest, and that local access to unburnt refuges substantially reduces these effects. Invasive predators are highly opportunistic, wide-ranging and thoroughly integrated into this flammable forest ecosystem. Lethal control programs for foxes need to consider fox movement across land-tenures, and could selectively target habitat features such as roadsides, forest-farmland edges and recently-burnt forest. Habitat-based management approaches might also reduce invasive predator impacts on native mammals, for example by preserving dense vegetation in unburnt refuges, or removing anthropogenic resources that subsidise predator populations within forests. Evidence-based, integrated management of threatening processes is vital to conserving native biodiversity.
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ItemThe influence of fire on forest birds at multiple scales( 2014)Improved understanding of the impact of fire on fauna is required because the frequency and severity of fire are predicted to increase under climate change, and the implications for biodiversity are largely unknown. To better understand the characteristics of fire regimes that sustain avian diversity, my thesis tests two overarching hypotheses: (i) that bird diversity increases with fire-mediated landscape heterogeneity; and (ii) that bird diversity increases with fine-scale heterogeneity in vegetation structure and plant species diversity. To test my first hypothesis, I examined bird responses to inter-patch variation in fire age class and vegetation type using landscape sampling units at a large spatial scale (60,000 ha). At a smaller scale (400 ha), I used a before-after control-impact experiment to investigate the effects of intra-patch variation in fire severity on bird diversity and the occurrence of individual species. To test my second hypothesis, I used measurements of vegetation structure and plant diversity to explain patterns in taxonomic diversity, functional diversity and species’ occurrence. Birds were surveyed across a 70-year chronosequence spanning four broad vegetation types, from heathland to wet forest. Results provided some support for both hypotheses. First, bird diversity was positively associated with landscape heterogeneity at the inter- and intra-patch levels. Second, bird functional evenness was positively related to fine-scale structural heterogeneity, and 13 of 15 modelled species responded to elements of habitat structure measured at fine scales. Only four of the 13 species responded to time since fire, indicating that time is unlikely to be a useful surrogate for bird occurrence in systems characterised by variable rates of post-fire structural development. Although I identified positive relationships between bird diversity and fire-mediated heterogeneity at multiple scales, results indicate that older vegetation is of disproportionate importance to the region’s birds, and that the preservation of old vegetation is paramount. Management strategies that use controlled application of patchy, low-severity fire to break up large areas of mature vegetation are likely to enhance avian diversity, ecosystem function and resilience, while conserving species reliant on older vegetation.