School of BioSciences - Theses
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ItemMove it or lose it: how and when to use targeted gene flow( 2023-04)A myriad of threats are currently assailing threatened and soon to be threatened populations. Habitat removal, climate change, wildlife disease and the invasion of non-native species are all placing increased pressure on the systems we undervalue for their role in providing clean air, water, carbon sequestration and natural beauty. Many populations, however, contain individuals expressing traits that enable them to survive and reproduce even in the presence of these existential threats. Unfortunately, many of these adaptive traits are rare in endangered populations. But useful variation in traits doesn't only occur within populations: geographic trait variation is ubiquitous and can arise from a number of different processes, including natural selection, spatial sorting, and drift. Natural selection allows populations to adapt to local environmental conditions and can lead to locally adapted variants. Spatial sorting, the spatial analogue to natural selection, can also lead to predictable geographic variation in traits across space, as invasion and recolonisation select for increased dispersal. Both of these forms of selection result in predictable trait variation that conservation biologists can harness to promote conservation benefits. Geographic variation might also arise through drift, but such variation is much less predictable. Once geographic variation exists, it is possible to harness such variation to affect conservation goals. This idea -- targeted gene flow -- has the objective of identifying and harnessing geographic trait variation to promote conservation benefit. Current work on targeted gene flow has focused on understanding how species respond to certain threats and on exploring the use of targeted gene flow to boost adaptive potential in threatened populations. In this thesis, I explore the utility of applying targeted gene flow to two different conservation problems: i) can targeted gene flow be deployed to facilitate evolutionary rescue in response to rapid (and flexible) environmental change? and ii) can targeted gene flow be deployed to directly mitigate a threatening process? To answer these questions, I use a blend of field studies and simulation models focused on the introduced pest, the Cane Toad (Rhinella marina). This thesis starts by providing the reader with background information and current applications of targeted gene flow, as well as identifying and framing the need for novel measures to reduce the impacts of cane toads in Australia. To explore this use case, we first need to understand how to apply targeted gene flow in an optimal fashion, as well as how to measure any benefits. In the opening data chapter of my thesis, I set out to understand how to optimally deploy targeted gene flow and build upon initial work in this area to examine how targeted gene flow should be deployed against differing threat profiles. I find that the optimal timing and size of a targeted gene flow action is highly sensitive to the maximum rate of change of the threat across time, and that if conducted correctly, targeted gene flow can provide enough adaptive potential to stave off extinction whilst retaining almost all of the genetic diversity of the population under threat. This measure, the expected benefit, is a novel metric to benchmark the effectiveness of targeted gene flow applications. In the subsequent chapters of this thesis, I extend the notion of targeted gene flow to a different context: reducing the dispersal ability of invasive species. Cane toads are one of the most harmful introduced species in Australia. Decades of sustained investment in cane toad control, research and management has unearthed a number of effective strategies for the local control of cane toad populations, but no landscape level solution currently exists. In chapter three, I quantify the financial benefit of keeping areas toad-free. I conduct field studies to generate estimates of toad density and detectability, before combining these with removal and cost models to provide an estimate of the value of cane toad quarantine across offshore islands as well as a potential toad-free haven on the mainland: the Pilbara region of Western Australia. My final chapter explores how targeted gene flow can be deployed to increase the effectiveness of a landscape barrier designed to contain the toad invasion and so create a toad-free haven over 265,000 km2 of the Australian mainland. This chapter provides the first case study of how targeted gene flow can be deployed to directly mitigate a threatening process. In doing so, I provide the first evidence that targeted gene flow can be used to directly reduce the negative impacts of an invasive species, through driving down their dispersal ability, and in doing so render landscape barriers substantially more effective. This thesis shares the process of exploring a new application of targeted gene flow, from theoretical conception to an applied trial. I provide the first evidence that targeted gene flow can be used to reduce the ability of an invasive species to move across the landscape, alongside extensions to the current framework surrounding how to optimally implement targeted gene flow to aid threatened populations. The resulting strategies are not limited to the impacts of cane toads but instead have application to a wide range of conservation scenarios. Generally, my thesis develops the under-appreciated idea that, by being creative with geographic trait variation, we have a powerful and cost-effective tool for conserving biodiversity.