Dissertations / Theses on the topic 'Predator-Prey'

Seasonal shifts in predation habits, from a generalist in the summer to a specialist in the winter, have been documented for the great horned owl (Bulbo virginialis) in the boreal forest. This shift occurs largely due to varying prey availability. There is little study of this switching behaviour in the current literature. Since season length is predicted to change under future climate scenarios, it is important to understand resulting effects on species dynamics. Previous work has been done on a two-species seasonal model for the great horned owl and its focal prey, the snowshoe hare (Lepus americanus). In this thesis, we extend the model by adding one of the hare's most important predators, the Canadian lynx (Lynx canadensis). We study the qualitative behaviour of this model as season length changes using tools and techniques from dynamical systems. Our main approach is to determine when the lynx and the owl may invade the system at low density and ask whether mutual invasion of the predators implies stable coexistence in the three-species model. We observe that, as summer length increases, mutual invasion is less likely, and we expect to see extinction of the lynx. However, in all cases where mutual invasion was satisfied, the three species stably coexist.

In this thesis, we study the population dynamics of predator-prey interactions described by mathematical models with age/stage structures. We first consider fixed development times for predators and prey and develop a stage-structured predator-prey model with Holling type II functional response. The analysis shows that the threshold dynamics holds. That is, the predator-extinction equilibrium is globally stable if the net reproductive number of the predator $\mathcal{R}_0$ is less than $1$, while the predator population persists if $\mathcal{R}_0$ is greater than $1$. Numerical simulations are carried out to demonstrate and extend our theoretical results. A general maturation function for predators is then assumed, and an age-structured predator-prey model with no age structure for prey is formulated. Conditions for the existence and local stabilities of equilibria are obtained. The global stability of the predator-extinction equilibrium is proved by constructing a Lyapunov functional. Finally, we consider a special case of the maturation function discussed before. More specifically, we assume that the development times of predators follow a shifted Gamma distribution and then transfer the previous model into a system of differential-integral equations. We consider the existence and local stabilities of equilibria. Conditions for existence of Hopf bifurcation are given when the shape parameters of Gamma distributions are $1$ and $2$.

The paper is focused on the Predator-Prey model modified in the case of harvesting one or both populations. Firstly there is given a short description of the basic model and the sensitivity analysis. The first essential modification is percentage harvesting. This model could be easily converted to the basic one using a substitution. The next modification is constant harvesting. Solving this system requires linearization, which was properly done and brought valuable results applicable even for the basic or the percentage harvesting model. The next chapter describes regulation models, which could be used especially in applying environmental policies. All reasonable regulation models are shown after distinguishing between discrete and continuous harvesting. The last chapter contains an algorithm for maximizing the profit of a harvester using econometrical modelling tools.

In the first half of this thesis, I have focused on predator ability to locate prey using olfaction and how prey aggregation and turbulence affect prey detection. In chapter 2 I investigate the ability of three spined sticklebacks to compensate for loss of visual cues by using olfaction and find that they can use olfactory cues but that these most likely help the fish detect prey rather than locate prey. In chapter 3 I explore the effect of prey aggregation as an anti-predator strategy when avoiding an olfactory predator and find that aggregated prey survive longer than do dispersed prey. In order to further investigate why this may be, I carried out an experiment using Gammarus pulex as the predator where I recorded search time as a function of prey group size. I found that similarly to detection distance, search time relates to the square root of the number of prey. Finally, I investigate the effect that turbulence in flowing water may have on prey group detection using three spined sticklebacks in a y-maze. I find that risk of detection increases with prey group size but that turbulence lowers this risk. This may mean that there are thresholds below which size prey groups can benefit from turbulence as a ‘sensory refuge’ thus avoiding predators. In the second part of my thesis I focus on the interactions between a cleaner fish and a parasite in an aquaculture setting focusing on whether said fish is useful as a cleaner in industry. I carry out experiments to investigate the use of lumpfish as salmon cleaners in terms of cleaning efficiency and behaviour. I find that while some lumpfish do clean salmon, the required circumstances are still unknown and that further work including selective breeding, personality and effects of tanks is necessary.

Thesis (Ph. D.) -- University of Maryland, College Park, 2009.
Thesis research directed by: Dept of Entomology. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Phenotypic plasticity is an environmentally based change in phenotype and can be adaptive. Often, the change in an organism's phenotype is induced by the presence of a predator and serves as a defense against that predator. Defensive phenotypes are induced in freshwater physid snails in response to both crayfish and molluscivorous fish. Alternative morphologies are produced depending on which of these two predators snails are raised with, thus protecting them from each of these predators' unique mode of predation. Snails and other mollusks have been shown to produce thicker, differently shaped shells when found with predators relative to those found without predators. This production of thicker, differently shaped shells offers better protection against predators because of increased predator resistance. The first study in this thesis explores costs and limits to plasticity using the snailfish- crayfish system. I exposed juvenile physid snails (using a family structure) to either early or late shifts in predation regimes to assess whether developmental flexibility is equally possible early and late in development. Physid snails were observed to produce alternative defensive morphologies when raised in the presence of each of the two predators. All families responded similarly to the environment in which they were raised. Morphology was found to be heritable, but plasticity itself was not heritable. Morphology was found to become less flexible as snails progressed along their respective developmental pathways. In the second study, I raised physid snails with and without shell-crushing sunfish and examined the differences in shell thickness, shell mass, shell size and shell microstructural properties between the two treatment groups. Shells of snails raised with predators were found to be larger, thicker and more massive than those raised without predators, but differences in microstructure were found to be insignificant. I conclude that the observed shell thickening is accomplished by the snails' depositing more of the same material into their shells and not by producing a more complex shell composition.

>Magister Scientiae - MSc
In principle, an enrichment of resources in predator-prey systems show prompts destabilisation of a framework, accordingly, falling trophic communication, a phenomenon known to as the \Paradox of Enrichment" [54]. After it was rst genius postured by Rosenzweig [48], various resulting examines, including recently those of Mougi-Nishimura [43] as well as that of Bohannan-Lenski [8], were completed on this problem over numerous decades. Nonetheless, there has been a universal none acceptance of the \paradox" word within an ecological eld due to diverse interpretations [51]. In this dissertation, some theoretical exploratory works are being surveyed in line with giving a concise outline proposed responses to the paradox. Consequently, a quantity of di usion-driven models in mathematical ecology are evaluated and analysed. Accordingly, piloting the way for the spatial structured pattern (we denote it by SSP) formation in nonlinear systems of partial di erential equations [36, 40]. The central point of attention is on enrichment consequences which results toward a paradoxical state. For this purpose, evaluating a number of compartmental models in ecology similar to those of [48] will be of great assistance. Such displays have greater in uence in pattern formations due to diversity in meta-population. Studying the outcomes of initiating an enrichment into [9] of Braverman's model, with a nutrient density (denoted by n) and bacteria compactness (denoted by b) respectively, suits the purpose. The main objective behind being able to transform [9]'s system (2.16) into a new model as a result of enrichment. Accordingly, n has a logistic- type growth with linear di usion, while b poses a Holling Type II and nonlinear di usion r2 nb2 [9, 40]. Five fundamental questions are imposed in order to address and guide the study in accordance with the following sequence: (a) What will be the outcomes of introducing enrichment into [9]'s model? (b) How will such a process in (i) be done in order to change the system (2.16)'s stability state [50]? (c) Whether the paradox does exist in a particular situation or not [51]? Lastly, (d) If an absurdity in (d) does exist, is it reversible [8, 16, 54]? Based on the problem statement above, the investigation will include various matlab simulations. Therefore, being able to give analysis on a local asymptotic stability state when a small perturbation has been introduced [40]. It is for this reason that a bifurcation relevance comes into e ect [58]. There are principal de nitions that are undertaken as the research evolves around them. A study of quantitative response is presented in predator-prey systems in order to establish its stability properties. Due to tradeo s, there is a great likelihood that the growth rate, attack abilities and defense capacities of species have to be examined in line with reviewing parameters which favor stability conditions. Accordingly, an investigation must also re ect chances that leads to extinction or coexistence [7]. Nature is much more complex than scienti c models and laboratories [39]. Therefore, di erent mechanisms have to be integrated in order to establish stability even when a system has been under enrichment [51]. As a result, SSP system is modeled by way of reaction-di usion di erential equations simulated both spatially and temporally. The outcomes of such a system will be best suitable for real-world life situations which control similar behaviors in the future. Comparable models are used in the main compilation phase of dissertation and truly re ected in the literature. The SSP model can be regarded as between (2018-2011), with a stability control study which is of an original.

Once regarded as little more than simple model systems, microbial communities are now understood to drive major earth processes and patterns of biodiversity. In order to more fully understand them, there is a need to investigate the mechanisms that determine the structure and function of microbial systems. Predator-prey interactions have been studied for a long time in macroecological systems and are known for their role in determining community structure and dynamics. Functional diversity has come to the fore of ecology to take a central role in understanding what links species to biodiversity and ecosystem function. In this thesis I investigated the biodiversity and ecosystem functioning of microbial predator-prey communities. I used naturally occurring bacterial communities in combination with co-occurring heterotrophic protists to perform manipulative experiments to investigate: a) the functional diversity of heterotrophic protists and their interactive contributions to ecosystem functioning; b) the effects of bacterial prey community diversity and composition on these attributes of protists; and c) the role of functional diversity in maintaining protist community diversity. I found that not only is it possible to classify functional diversity by species interactions, but that the effects of prey community diversity and composition highlight the flexible nature of this property, particularly in predators. I also found evidence of negative biodiversity-functioning relationships and strong competitive interactions among heterotrophic protists. However, their functional diversity allowed them to maintain higher diversity in disturbed environments. Taken together, my thesis has advanced the understanding of the role of predator-prey interactions and functional diversity in regulating interactions and ecosystem functioning in microbial systems.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico
Em todo o mundo lavouras de tomate sofrem o ataque de pragas. Uma das principais pragas do tomate é o ácaro vermelho Tetranychus evansi Baker & Pritchard (Acari: Tetranychidae). Uma característica marcante de T. evansi é a sua alta produção de teia sobre suas plantas hospedeiras. Essa teia pode afetar diretamente os inimigos naturais, reduzindo a sua eficiência de predação ou indiretamente alterando seu comportamento de busca. No entanto, alguns ácaros predadores da família Phytoseiidae podem ser bem adaptados para lidar com a teia dos ácaros fitófagos. Para selecionar agentes de controle biológico eficientes é importante compreender as interações diretas e indiretas entre os organismos envolvidos no agroecossistema em questão. Nesta dissertação, foi investigado o papel da teia produzida por T. evansi como mediadora de interações diretas e indiretas com seus inimigos naturais. No Capítulo 1 foram investigados os efeitos da teia produzida por T. evansi em suas interações com o ácaro predador Phytoseiulus longipes Evans (Acari: Phytoseiidae). Foi avaliado se fêmeas de T. evansi reconhecem sinais químicos do predador e se estes sinais podem induzir os ácaros a produzirem mais teia. Os resultados mostram que a presa não produz mais teia em resposta a estímulos de risco de predação, no entanto, passa a depositar uma maior parcela de seus ovos suspensos na teia, longe da superfície da folha. Esses ovos suspensos sofreram uma menor predação por P. longipes do que os ovos que se encontravam na superfície da folha. Entretanto a taxa de predação de P. longipes sobre ovos de T. evansi em discos com e sem teia não diferiram, indicando que esse predador não é afetado negativamente pela teia de sua presa. No segundo capítulo foi estudado se a teia produzida por T. evansi pode afetar o comportamento de forragemamento de P. longipes modulando a sua preferência entre ambientes e presas. Estudou-se também se ocorre predação intraguilda entre P. longipes e outro ácaro predador Phytoseiulus macropilis (Banks) (Acari: Phytoseiidae), que podem coocorrerem sobre as mesmas plantas. Observou-se que P. longipes prefere as metades de discos de folhas que apresentavam uma estrutura mais complexa (mesmo quando eles não tinham alimento) em relação às metades dos discos com ovos e sem teia de T. evansi. Esta preferência por ambientes de estrutura complexa pode ser explicada pela menor taxa de predação intraguilda de P. macropilis observada em discos de tomate com teia de ácaros. A presença da teia reduziu a capacidade predatória de P. macropilis. A diferença entre ambos os predadores para lidar com a teia de ácaros fitófagos pode ser explicado em parte pelas diferenças no comprimento das setas dorsais j3, z2 e z4, que são mais longas em P. longipes. Com relação à preferência de P. longipes por espécies de presas, observou-se que os ácaros predadores escolheram T. evansi a T. urticae, tanto em presença quanto ausência de teia. Esses resultados indicam que a teia de T. evansi pode mediar o comportamento de forrageamento de ácaros da família Phytoseiidae de acordo com seus níveis de adaptações morfológicas e comportamentais. Em linhas gerais, pode-se concluir que os ácaros predadores P. longipes são bem adaptados para lidar com a grande quantidade de teia produzida por T. evansi. Além de não evitar plantas com elevada quantidade de teia, esse ácaro predador pode se beneficiar desse ambiente complexo, reduzindo a competição com outros predadores. No entanto, a presença do predador estimula T. evansi a mudar seu comportamento de oviposição, reduzindo a eficiência predatória de P. longipes. Assim, ao alterar o seu comportamento de oviposição em resposta aos sinais dos predadores, as fêmeas de T. evansi utilizam mais eficientemente sua teia para protegerem sua prole.
Worldwide, tomato suffers from herbivory. One of the most important tomato pests is the red spider mite Tetranychus evansi Baker & Pritchard (Acari: Tetranychidae). A striking characteristic of T. evansi is its high production of web over its host plants. The web can directly affect natural enemies by reducing its predation efficiency or indirectly by changing its foraging behaviour. However some Phytoseiidae can be well adapted to cope with spider mite webbing. To select efficient biologicol control agents, it is important to understand the direct and indirect interactions between the organims involved in the agroecosystem. In this dissertation, I investigated how the web produced by T. evansi mediates direct and indirect interactions with its natural enemies. In Chapter 1 I investigate the effects of the web produced by T. evansi on its interactions with the predatory mite Phytoseiulus longipes Evans. We tested whether spider mite females recognize predator cues and whether these can induce the spider mites to produce denser web. We found that the prey did not produce denser web in response to such cues, but laid more eggs suspended in the web, away from the leaf surface. These suspended eggs suffered less predation by P. longipes than eggs that were laid on the leaf surface, under the web. However the predation rate of P. longipes on eggs of T. evansi on discs with and without web was not different, indicating that this predatory mite is not negatively affected by the spider mite web. In the second Chapter I studied whether the complex web produced by spider mites could affect the foraging behaviour of P. longipes modulating its preference between patches and prey. Was also studied if there is intraguild predation between the predatory mites P. longipes and Phytoseiulus macropilis (Banks) (Acari: Phytoseiidae), which can co-occur on the same plants. We observed that P. longipes always preferred the leaf discs halves with more complex environmental structure (even when there was no food on it) to discs half with eggs and no web of T. evansi. The strong preference for patches with complex structure could be explained by the reduced rate of intraguild predation by P. macropilis observed on tomato discs with spider mite web. As observed, the presence of spider mite web reduced the predatory efficiency of P. macropilis in intraguild predation experiment. The difference between both predatory mites to cope with the web of spider mites could be explained in part by the differences in length of the setae j3, z2 and z4 that are longer in P. longipesi, but is needed manipulation of these traits. With respect to the preference of P. longipes by prey species, we observed that the predator chose T. evansi to T. urticae, both in the presence and absence of web. These results indicate that spider mite web can mediate the foraging behaviour of phytoseiid mites according to their level of morphological and behavioral adaptation to webbing. In general, we conclude that the predatory mite P. longipes is well adapted to cope with the higher web densities produced by T. evansi. Besides not avoid thus plants with high amounts of web, P. longipes can benefit from this complex environment by reducing competition with others predators. However the presence of the predatory mite stimulates T. evansi to change its oviposition behaviour, reducing the predation efficiency of P. longipes. Thus, by altering their oviposition behaviour in response to predator cues, females of T. evansi make better use of their web to protect their offspring.

Anthropogenic stressors such as habitat loss, extreme weather events, and acidification can change predator-prey interactions. An understanding of the mechanisms by which these stressors impact predator-prey interactions may elucidate the fate of bivalves in the face of global change. My dissertation research informs management of marine resources in Chesapeake Bay, which has experienced substantial seagrass and oyster reef loss, increased storm activity, and combined estuarine and atmospheric CO2 acidification. In my dissertation, I used field survey data, field caging experiments, laboratory mesocosm experiments, time-series analysis, and density-dependent mathematical models to assess the role of habitat, major storm events, acidification, and predators on bivalve distribution in lower Chesapeake Bay, with a special focus on the commercially important, thin-shelled clam species Mya arenaria, which has declined significantly in the past few decades.;In field surveys, seagrass supported one additional bivalve functional group (based on bivalve morphology and feeding mode) than all other habitat types, and bivalve diversity was 2754% higher in seagrass than in shell hash, oyster shell, coarse sand, and detrital mud habitats. The odds of finding M arenaria were higher in seagrass than in all other habitats. Predators likely consumed seasonal pulses of juveniles each year. In field caging experiments, blue crabs Callinectes sapidus were likely responsible for most of the mortality of juvenile M arenaria, which was 76.6% higher for caged juveniles than for uncaged individuals over 5 d. In mesocosm feeding trials, M arenaria maintained a low-density refuge from predation by blue crabs, and had higher survival in oyster shell or shell habitats as compared to sand or seagrass habitats. Time series analysis suggested M arenaria was subjected to a storm-driven phase shift to low abundance in 1972, which has been maintained by blue crab predation. Density-dependent predator-prey models parameterized with data from laboratory and field experiments confirmed the presence of a coexistence steady state at low densities of M arenaria, providing the theoretical proof-of-concept that M arenaria can exist in a low-density stable state in the face of blue crab predation. Acidification altered behavior of both predator (C. sapidus) and prey (M arenaria), resulting in no net change in proportional mortality of clams between acidified and control feeding trials.;My dissertation examined multiple lines of evidence to address the importance of structured habitat, extreme weather events, and acidification in the mediation of predator-prey dynamics. For the crab-bivalve predator-prey interactions examined here, predation exacerbated the effects of some anthropogenic stressors (habitat loss, extreme weather events) and ameliorated the impacts of other stressors (acidification) on bivalve prey. An understanding of density-dependent predation is a necessary component of an adaptive management strategy that can cope with climate change.

This dissertation explores how natural ecosystem can be integrated with economic system through two case studies of multiple species interactions, or predator-prey relations. By the inclusion of biological, ecological and economic aspects, the integrated approaches aim at more clearly understanding of how regional ecosystem and economy interact with each other, given threats of resource extinction and environmental shock. I also explain strategies and policy regimes that can be considered to achieve efficient and sustainable ecosystem management in those circumstances. The first case study focuses on a predator-prey relation in the Pacific Ocean between the United States and Canada, where endangered/threatened predators feed primarily on commercially valuable species as prey. Accounting for the importance of those predators as critical natural resources for whale watching industry, this case study synthesizes the species biological and the regional economic systems, and analyzes possible management strategies for both ecosystem conservation and sustainable economic growth. A long-term drought and fragmented management has been one of the critical issues in the Great Salt Lake (GSL) ecosystem that is linked with its regional economy in Utah. For this issue, the second case study builds an integrated model for describing how the lakes main natural resources, such as water, brine shrimp, and migratory birds, are related to primary industries in the region including agriculture, mining, fishery, and recreation. With the model framework, the study presents how the prolonged drought affects both the GSL ecosystem and its rigional economy, and suggests economic management strategies for the lakes ecosystem recovery in the presence of drought.

This thesis concerns the predator-prey interactions of three raptor species in a Swedish arctic community: the gyrfalcon (Falco rusticolus), the rough-legged buzzard (Buteo lagopus) and the golden eagle (Aquila chrysaetos).

The gyrfalcon behaved like a highly specialised ptarmigan (Lagopus spp.) predator. Gyrfalcon’s functional response to ptarmigan was close to density independent, and ptarmigan remained the dominating prey even in areas with the lowest ptarmigan density. The gyrfalcon did not respond functionally to microtine rodents (i.e. lemmings and voles) and it was clear that the gyrfalcon did not use microtines as an alternative prey category to ptarmigan. As the gyrfalcons did not switch to any alternative prey when ptarmigan was scarce, their reproductive success seemed to be directly dependent on the amount of ptarmigan available in the breeding territories. Of the two ptarmigan species in the study area, rock ptarmigan (L. mutus) dominated gyrfalcon’s diet. Locally, the proportion of rock ptarmigan in gyrfalcons’ diets showed a positive relationship to the expected availability of rock ptarmigan in the breeding territories, indicating a density dependent utilisation.

The rough-legged buzzard behaved like a highly specialised microtine rodent predator and Norwegian lemming (Lemmus lemmus) was its preferred microtine species. The buzzards showed a type 2 functional response to lemmings. Surprisingly though, they also had a type 3 functional response to grey-sided voles (Clethrionomus rufocanus). We present an optimal diet model where a central place forager, during good food conditions, benefits from partial prey preference, which renders separate functional responses to each prey category. We discuss how the double functional responses of the buzzard affect the population dynamics of sympatric vole species, on both temporal and spatial scales.

The golden eagle behaved like a generalist predator, and it preyed on all major prey categories in the study area: microtines, ptarmigan, mountain hare, (Lepus timidus) and reindeer (Rangifer tarandus). It seemed to respond functionally to microtine rodent fluctuations with an increased consumption of lemmings during a peak year in the microtine rodent cycle. The golden eagle showed a numerical response to its main prey, the ptarmigan.

Ptarmigan, microtine rodents and hares seemed to have synchronized population fluctuations in the study area. Such synchronized population fluctuations are believed to be generated by predation. Although the three raptors are the main predators of their community, their predation patterns fail to explain the observed prey population dynamics in the study area.

Creating models for simulating real-life situations can be a difficult task because of all the small factors that can impact the outcome of an event. One model aimed to accurately predict flocking behaviour for animals is Boids flocking model. In this study, we aim to answer if the model is adequate for modeling a predator vs. prey situation. And if it is not, we aim to conclude what factors is needed to increase the accuracy of the model. The conclusion is that flocking is mainly a defensive tool, and that the Boids flocking model does not model predator actions accurately. To adequately model predator actions, a factor of teamwork and/or coordination is needed. Flocking in offensive situations makes predators act too much like a single unit and decrease their effectiveness. The advantages of hunting in a group are lost. The number of dead animals did not change depending on if prey flocked or not.
Det är svårt att skapa simuleringar över verkliga händelser eftersom det finns en mängd mindre faktorer som drastiskt kan påverka händelsens utgång. En modell som änmar efterlikna riktigt flockbeteende är Boids flockningsmodell. Denna studie undersöker om denna modell är tillräcklig för att modellera en verklighetstrogen jaktsituation mellan rov- och bytesdjur. Vidare undersöks vilka extra faktorer som behövs för att öka modellens realism. Resultaten visar att flockbeteende främst är ett defensivt verktyg samt att Boids modell inte ensamt kan användas för att simulera ett rovdjurs beteende. Det behövs faktorer såsom sammarbete och koordination för att förbättra rovdjurens situation. I offensiva sammanhang agerar rovdjuren för mycket som en tät grupp, vilket resulterar i minskad effektivitet då de låser varandra - fördelen av att vara i grupp går förlorad. Antalet levande och döda djur ändrades inte signifikat om bytesdjuren flockade eller ej.

Both predator and prey have evolved to maximise reproductive success by balancing food intake with risk. There has been a bias in predator-prey studies, where prey behaviour has been examined in detail, yet predators are assumed to follow simplistic rules. I use three-spined sticklebacks predating upon invertebrate prey to test a range of ways in which prey risk was hypothesised to be affected by predator behaviour. The relationship between encounter rate and prey density has been recently shown not to be directly proportional, and theoretical arguments have been made that predator search behaviour can explain this trend. I test these arguments, and show acceleration of a predator's search path can in fact lead to the observed less-than-directly proportional relationship between prey density and encounter rate. The perceptual constraints of predators can have major impacts on prey risk. Once encountered, an attack was more likely when prey were encountered late in a search, probably due to a decrease in anti-predator vigilance as the fish became more habituated to the arena. In a subsequent study, larger groups of prey were more quickly found, as were larger numbers of groups. This led to the conclusion that the field of attention is a subset of the total visual field, and this is also supported by denser prey being more conspicuous. Although the predator responded to increased prey group size and density with a reduced time to detect and attack prey, attacks on such groups were less successful due to the confusion effect. Interestingly, I show the effect of prey density to be sensitive to spatial scale, where a large-scale measure of density affected conspicuousness and a small-scale measure affected attack success. This was explained by a reduction in the total number of prey in the visual field as a group of prey is approached and attacked. In the final chapter, I turn my attention to differences in temperament within a predator population, and how this affects prey risk. As expected, bolder fish represented a greater risk to the prey. However, as larger fish tended to be more bold, suggesting boldness was driven by their perception of predation risk, a within community behaviourally-mediated trophic cascade may occur. where the risk to prey is driven by their predators' own perceived risk of predation. This shows that optimal foraging decisions under the threat of predation, as well as perceptual constraints, can mediate the effect of predator behaviour on prey risk.

Phenotypic plasticity in response to environmental stimuli is exceedingly common across systems and taxa. For instance, predation risk in many gastropods can induce a variety of defenses including growing thicker shells, growing shells of different shapes, and developing apertural teeth. However, the role of coevolution between species that produce these defense responses and their consumers is not well known. This thesis examines the responses of an ovoviviparous gastropod (Littorina saxatilis) with low dispersal from three different habitats (marsh habitat, rocky habitat, and cobble stone habitat) to the presence of chemical cues from a native (Dyspanopeus sayi) and nonnative (Hemigrapsus sanguineus) crab predator. This work tested the potential role of coevolution in shaping phenotypically plastic responses, and whether responses to both a native and a nonnative predator differed for snails from different source sites. The morphological responses I tested for included axial growth, width growth, whorl growth, changes in total mass, and shell shape changes. I found that many measures of growth were needed in the investigation of plastic responses. Overall snails exposed to native predator cues had a similar response as those in the reduced diet treatment in both growth measurements and shell shape change, indicating a behavior response of reduced feeding in the presence of the native predator. Snails from the marsh and rocky habitats displayed a reduced response to cues from the nonnative predator, suggesting that they recognized this predator as a risk, but did not show as strong of a response as they did to the native predator. Snails in the rocky habitat, which live in barnacle tests, also had a slower growth rate than snails from the other two source sites. These results suggest the possibility of local adaptation and genetic differences between snails in these different source sites.

The main aim of mathematical ecology is to explore interactions among organisms and the environment where they live, and predator-prey interaction is one of the major type of interactions observed in nature. Models of predator-prey systems - mathematically described by ODEs, PDEs or integro-differential equations - have a long and illustrious history starting from the seminal works by Lotka and Volterra. However, despite a large number of existing publications in the literature, some fundamental questions related to this type of systems still remain open. For example, the spatial heterogeneity of the environment and its role in stabilisation of predator-prey dynamics and persistence of species is still not well understood. Another major challenge is the effect of external forcing (e.g. daily, seasonal, or other variation of model parameters) on long-term dynamics of the predator-prey or host parasite models. Finally, the parameterisation of model functions describing species interactions, for instance, formulation of the functional response of predator, can play a crucial role in the model outcomes. In the present dissertation, we explore the three above challenging issues (i.e. space heterogeneity, external forcing and model parametrisation) on the patterns of spatio-temporal dynamics of predator-prey or/and host-parasite systems and their stability. In particular, we revisit the famous paradox of enrichment which is classical in mathematical biology and explain how the spatial heterogeneity and animal movement on various time scales can stabilise the system characterised by an infinitely large carrying capacity (Chapter 2). Mathematically, we use a system of integro-differential equations and consider a tri-trophic planktonic system as a case study. In the two next chapters, we consider the role of daily and seasonal variation of temperature on the control of pathogenic bacteria by their predators: bacteriophages (i.e. bacterial viruses). As an important ecological case study, we explore seasonable dynamics of the infectious bacteria causing the lethal disease Melioidosis in Thailand. In the beginning we model interaction in the top water of a rice field (Chapter 3). Here we build two different models of host-parasite interactions based on ODEs and DDEs (delay differential equations). In Chapter 4, by using reaction-diffusion framework, we extend the previous model of bacteria-phage interactions to consider bacteria-phage dynamics in soil. Using our modelling approach we can make predictions about disease management of Melioidosis in tropic environments.

Predator-prey modelling involves computationally simulating predator-prey relationships which are observable in nature. Predator species hunt and consume prey species, forming an ecosystem where the population sizes of the various species are interdependent. The models take the form of mathematical equations, where a set of coupled equations model the effect that predator and prey species have upon each other, with the modelling either taking place on a continuous or discrete time basis. Generally to date, the majority of research into the modelling of predator-prey ecosystems has concentrated on a single predator species interacting with a single prey, especially in the case of discrete time implementations. Historically, the modelling of large scale multiple species predator-multiple prey ecosystems would have been too computation ally intensive to be modelled. As computers have increased in power, the execution of these larger ecosystems has become possible. This thesis investigates large discrete time multiple species predator-prey models with the introduction of mutation amongst the various predators and prey species that occupy it. It consists of three main studies. First it was necessary to model and discuss a discrete time multiple species predator-prey model without mutation. This was achieved by the modification and execution of a discrete time single predator- single prey model taken from current literature. A focus here is placed on the number of surviving species when executing the multiple species model for a varying numbers of initial predators and prey species (between 2 and 10,000 species), and the effect of the control parameters varying upon them. Two main studies are undertaken. Firstly the unimodal discrete time map that governs the prey dynamics is varied, with this it was shown that the choice of unimodal map has a great effect on the survival of the predator and prey species. Secondly a study into the effect of predation strategies, which define how the predator hunts the prey species, is presented. Each strategy is outlined and conclusions are drawn comparing each strategy, assessing how effective each of them are in allowing predator and prey species survival in the model. The next step was to detail how mutation could be introduced into the ecosystem, with a Coupled Map Lattice (CML) approach being identified. The second study details the dynamics of this CML without predation. A focus was placed upon a CML where the parameters are linearly scaled across the set of maps. The number of maps in the CML is varied between 10 and 10,000, Comments are made upon how changing the mutation rate affected the dynamics of the CML and conclusions are drawn upon how the number of maps within the CML impacts upon these dynamics. Finally the Finally the two studies are merged, attaining and presenting a multiple species discrete time predator-prey model with CML based mutations amongst the various species that occupy the ecosystem. Again various unimodal maps are usedfor the underlying dynamics of the prey species, and different predation strategies are utilised. The system is first observed in a simple form, with a single predator and multiple mutating preys, and then in more complex form , with both mUltiple mutating predators and multiple mutating prey. Conclusions are drawn on how varying the parameters govern the underlying behaviour and survival of the species within the various models. It is observed, that in such a complex system, with both multiple mutating predators and multiple mutating preys, a large range of non-linear dynamics are possible.

This thesis analyses temporal and spatio-temporal modified Holling-Tanner predator-prey models with alternative food for predators. Different types of functional responses and a density-dependent phenomenon called Allee effect(s) on the prey were considered. By using analytical and numerical analysis, the stability of the equilibrium points for the different combinations of modifications were proven. The necessary conditions for the models to undergo a different type of bifurcation were illustrated. Additionally, this thesis has provided numerical evidence where the Turing instability leads to spatio-temporal patterns in a specific version of the model.

Questa tesi riguarda modelli differenziali preda-predatore, trattati inizialmente nel caso spazialmente omogeneo e successivamente considerando la diffusione spaziale. Dal punto di vista matematico pertanto vengono considerati sistemi di equazioni differenziali ordinarie e di equazioni differenziali alle derivate parziali di tipo parabolico. In particolare, nella prima parte viene studiato un modello preda-predatore spazialmente omogeneo, retto da due equazioni differenziali ordinarie, in cui sono presi in considerazione un effetto Allee forte nella crescita delle prede e una risposta funzionale predatore dipendente. Il punto di forza dello studio risiede nel fatto che le funzioni che descrivono questi processi non hanno un'espressione esplicita, ma sono caratterizzate solo da alcune proprietà comuni a funzioni specifiche utilizzate in letteratura. Tali proprietà sono sufficienti per effettuare l'analisi qualitativa del sistema, con riguardo all'esistenza degli equilibri e alle loro proprietà di stabilità mediante i criteri di Lyapunov, utilizzando due parametri di biforcazione che caratterizzano il processo di predazione. Il modello presenta dei punti di biforcazione di codimensione 2 quali una biforcazione Bogdanov-Takens e una di tipo cuspide, non legati alla particolare realizzazione scelta per le funzioni del modello. Lo studio è stato proseguito numericamente fissando un'espressione per la funzione di crescita delle prede e per la funzione trofica che soddisfano le proprietà considerate e utilizzando il software di continuazione Matcont per Matlab. Tale studio ha mostrato l'ulteriore presenza di biforcazioni globali che determinano la sparizione dei cicli limite, mediante la formazione di orbite omocline ed eterocline. Inoltre è stato individuato una biforcazione di Hopf generalizzata, un altro punto di biforcazione di codimensione 2. Le biforcazioni di codimensione 2 individuate sono tutte e sole quelle ammesse da un sistema a due equazioni differenziali. La seconda parte della tesi verte invece sullo studio di due sistemi preda-predatore con diffusione in cui vengono dedotte in un opportuno limite due tipi di risposte funzionali classiche come termine reattivo e un termine diffusivo non lineare. In dettaglio, vengono considerati due livelli trofici, le prede e i predatori. Questi ultimi sono suddivisi in due classi, searching predators e handling predators: i primi sono i predatori effettivamente impegnati nella predazione, mentre i secondi non sono attivi in tale processo. Ne deriva un sistema composto da tre equazioni differenziali alle derivate parziali, in cui la diffusione è modellizzata in modo classico, mediante un termine lineare in forma di Laplaciano e l'interazione tra prede e predatori è inizialmente del tipo Lotka-Volterra. Mediante una approssimazione quasi steady-state è possibile ridurre il sistema di partenza, ottenendo un sistema di due PDE, una per le prede e una per la totalità dei predatori, in cui la risposta funzionale è del tipo Holling-II, in particolare preda-dipendente, e che presenta una non-linearità nel termine di diffusione. Questa classe di modelli non dà luogo a instabilità di Turing. Viene quindi considerata nel modello a tre equazioni una competizione tra i predatori che permette di ricavare, mediante un'approssimazione quasi steady-state, un sistema preda-predatore con risposta funzionale del tipo Beddington-DeAngelis nel termine di reazione e ancora una non-linearità nel termine di diffusione. Vengono quindi ricavate condizioni sui parametri che permettono di avere instabilità di Turing e confrontati i risultati sia nel caso di diffusione lineare che in quello non-lineare.
Predator-prey models, homogeneous in space or with spatial diffusion, play a central role in this thesis. Indeed, from a mathematical view point, we investigate stability in systems of ordinary differential equations and of partial differential equations of parabolic type. First, we deal with a predator-prey model, described by a system of two ODEs, in which a strong Allee effect on the prey growth and a predator-dependent trophic function are taken into account. The main strength of this part is that these functions are not specified by analytical expressions, but only characterized by some biologically meaningful properties determining their shapes. On the basis of these properties we are able to perform the stability analysis of the system, using the predation efficiency and a measure of the predator interference as bifurcation parameters. The system admits codimension-two bifurcations points, such as a Bogdanov-Takens and a cusp point; it is worthwhile to notice that they are independent of the particular expression of the model functions. The numerical investigation is further carried on choosing for the model equations some analytical expressions well known in literature, which satisfied the assumed properties, and using Matcont, a continuation Matlab toolbox. This investigation, in addition, has shown the presence of global bifurcations that determine the disappearance of limit cycles through the formation of homoclinic and heteroclinic orbits involving some equilibrium points. Moreover, we have detected a further codimension-two bifurcation point, a Generalized-Hopf. Together with the cusp and the Bogdanov-Takens bifurcation points, these three types of codimension-two bifurcations are the only admissible by a planar system of ordinary differential equations. The second part of this thesis focuses on the study of two predator-prey models with diffusion that justify, in a suitable limit, two classical types of functional responses in the reaction part and present a cross-diffusion term. In detail, two trophic levels are considered, preys and predators which are further divided into searching predators and handling predators. The former are predators active in the predation process, the latter are resting individuals. Then, we start from a system of three partial differential equations, with a standard linear diffusion in terms of Laplacian, and with a Lotka-Volterra reaction term. Through a quasi steady-state approximation we end up with a system of two PDEs with prey and total predator densities as unknowns, in which an Holling-type II functional response appears together with a cross-diffusion term in the predator equation. It is proved that this class of predator-prey models can not give rise to Turing instability. Then we modify the starting model inserting a competition among predators. With this change we end up after a quasi steady-state approximation with a system of two PDEs for prey and total predator densities, characterized by a Beddington-DeAngelis-type functional response and a cross-diffusion term in the predator equation. We look for conditions on the parameters values which lead to Turing instability and we compare these Turing instability regions with the ones obtained when the cross-diffusion term is substituted by a linear diffusion.

The logistic Lotka-Volterra predator-prey equations with diffusion based on Luckinbill's experiment with Didinium nasutum as predator and Paramecium aurelia as prey, have been solved numerically along with a third equation to include prey taxis in the system. The effect of taxis on the dynamics of the population has been examined under three different non-uniform initial conditions and four different response functions of predators. The four response functions are Holling Type 2 Response, Beddington Type Response or Holling Type 3 Response, a response function involving predator interference and a modified sigmoid response function. The operator splitting method and forward difference Euler scheme have been used to solve the differential equations. The stability of the solutions has been established for each model using Routh - Hurwitz conditions, variational matrix. This has been further verified through numerical simulations. The numerical solutions have been obtained both with and without prey-taxis coefficient. The effect of bifurcation value of prey-taxis coe�cient on the numerical solution has been examined. It has been observed that as the value of the taxis coefficient is increased significantly from the bifurcation value chaotic dynamics develops for each model. The introduction of diffusion in predator velocity in the system restores it back to normal periodic behaviour. A brief study of coexistence of low population densities both with and without prey-taxis has also been done.

Odour plays an important role in many predator/prey interactions. In the northern hemisphere, many mammalian prey species have been shown to respond to predator odours. It is also widely assumed that mammalian predators utilise odours to locate their prey. This thesis explores the importance of odour in Australian mammalian predator/prey interactions. Responses of native Australian species to the faecal odour of two predators; the native tiger quoll Dasyurus maculatus and the introduced red fox Vulpes vulpes, were evaluated through live-trapping and focussed behavioural studies of captive animals. Tiger quoll responses to prey olfactory cues were investigated in a captive experiment. Native rodents (bush rats Rattus fuscipes, swamp rats R. lutreolus and eastern chestnut mice Pseudomys gracilicaudatus) equally avoided traps scented with either quoll or fox faeces, and in captive experiments, bush rats and swamp rats reduced their average speed in response to both predator odours. Of the marsupial species, northern brown bandicoots Isoodon macrourus and common brushtail possums Trichosurus vulpecula were captured more frequently in quoll-scented traps than unscented traps or foxscented traps, while captures of brown antechinus Antechinus stuarttii, long-nosed bandicoots Perameles nasuta and southern brown bandicoot I. obesulus were unaffected by the either predator odour. In captive experiments, brown antechinus, long-nosed and northern brown bandicoots decreased their foraging in response to both predator odours, and spent less time in areas scented with quoll faeces. Tiger quolls didn't appear to detect odour sources from a distance of >65 cm, but they did follow scent trails and spent more time in areas scented with the urine and faeces of potential prey. Chemical analysis revealed no common components in fox and quoll odour which prey species could be responding to. Therefore, these native species have evolved to respond to fox odour since foxes were introduced to Australia 130 years ago. The stronger response of native rodents to fox odour may be a legacy of their co-evolution with canid predators prior to entering Australia. A better understanding of how odour is utilised in Australian predator/prey interactions may lead to a greater ability to protect Australia's unique mammalian fauna from introduced predators.

Predation by exotic predators (cats Felis catus and foxes Vulpes vulpes) is believed to be one of the factors that has contributed to the decline of medium-sized mammals in arid Australia. Other factors include habitat degradation by introduced herbivores (rabbits Oryctolagus cuniculus and grazing stock) and altered fire regimes after Aboriginal people moved into permanent settlements. In general, the impact of exotic predators on arid zone mammals is believed to be significant only when predator numbers have been elevated by increased food availability from exotic prey species (rabbits, house-mice Mus domesticus, cattle carcasses) or when native prey populations have already been dramatically reduced by competition from introduced herbivores. In much of the spinifex grasslands of the central Australian deserts, pastoralism never occurred, rabbit colonisation was extremely patchy and in some areas, traditional burning was still being practised when the extinctions commenced. None of the current models of mammalian extinctions adequately explain the declines in this environment. In this study I examined predator-prey interactions in two areas of the Tanami Desert to investigate whether predation by exotic predators may be a primary agent of extinction in its own right, capable of causing mass declines even in the absence of other human-induced perturbations. If this were the case then the following would be expected: (i) cats and foxes would eat medium-sized mammals when they are available, but be able to survive on alternative prey when mammals are scarce; (ii) populations of cats and foxes would be buffered against the declines of mammals during droughts, or would be able to recover more quickly than medium-sized mammals after droughts; (iii) medium-sized mammals would be more vulnerable to predation by cats and foxes than by dingoes Canis lupus dingo and other native predators, and (iv) there would be a correlation between the timings of the extinctions and the colonisation (or sudden increase) of cats and foxes. These predictions were investigated by monitoring the diets and relative abundance of cats, foxes and dingoes in relation to fluctuating prey availability in two areas of the Tanami Desert at latitudes separated by approximately 400 km. Mean annual rainfall is higher and more reliable in the northern study area which was situated in the centre of bilby Macrotis lagotis distribution within the Northern Territory, whereas the southern study area was located on the southern edge of the bilby�s range. Within each study area, monitoring occurred at three sites, approximately 20 km apart. Each site contained a sub-plot in each of two habitat types. Field work was conducted between September 1995 and December 1997. When the study began, the southern study area was experiencing drought conditions, however both study areas received significant rainfall in early 1997. The population dynamics of a variety of potential prey groups were monitored to examine their resilience during droughts, patterns of recolonisation after rainfall, and use of two habitat types: the ubiquitous sandplain, and the moister, nutrient enriched palaeodrainage habitat which is believed to have provided a refuge for medium-sized mammals during droughts. Native mammals were uncommon throughout the study period. Bilbies and macropods were significantly more abundant in the northern study area, and tended to occur more frequently in palaeodrainage habitat than sandplain. However, the palaeodrainage habitat did not appear to provide adequate refuge for the medium and large mammals during drought conditions in the southern study area, as they disappeared from the study sites altogether. Small mammals were significantly more abundant in the southern study area but densities remained low (less than 2% trap success) throughout the study, and showed little response to improved seasonal conditions. In contrast, the abundance and species richness of birds showed a marked increase following rainfall in the southern study area. Flocks of nomadic birds arrived within several months of drought-breaking rains, increasing the relative abundance of birds from 9.3 per km of transect in December 1996 to 49/km in July 1997. Reptiles were the most resilient prey group during the drought conditions. Both varanids and smaller reptiles were equally abundant in the wet and dry years and showed no difference in abundance between study areas. However, reptiles showed marked temperature-related patterns in activity, with many species becoming inactive in the winter months. A total of 142 cat scats, 126 fox scats and 75 dingo scats were analysed to investigate predator diets in the two study areas. Unlike cat, fox and dingo diets elsewhere in Australia (and the world), mammalian prey did not dominate. Reptile was the prey category that was most frequently consumed by cats and foxes in �summer� (October-April) and by dingoes throughout the year, and was identified as a �seasonal staple� prey type for all three predators in the Tanami Desert. When biomass of prey was taken into account, the varanids (predominantly the sand goanna Varanus gouldii) were the most important prey sustaining predators in the two study areas. Birds were an important part of the diets of cats and foxes in winter when reptiles were less active. Small mammals were consumed by cats and foxes throughout the study, in proportion to their field abundances. Invertebrates were a major component of the diets of foxes, representing 31% of prey items consumed. There was considerable overlap in the diets of the three predator species, but dingoes ate more medium (100-999 g) and large (greater than 1000 g) prey than cats and foxes did. The scarcity of medium-sized mammals in the study areas provided little opportunity to find evidence of predation events on such prey. However, bilby remains were found in two cat scats and one dingo scat in the northern study area, mulgara Dasycercus cristicauda remains occurred in several cat and fox scats from the southern study area, and there were fourteen occurrences of marsupial mole Notoryctes typhlops in predator scats during the study, primarily in fox scats. Elsewhere in Australia, there is ample evidence that cats and foxes regularly consume medium-sized mammalian prey (e.g. rabbits and ringtail possums Pseudocheirus peregrinus) when it is available. Overall cats were the most abundant eutherian predators in the two study areas, and they were significantly more abundant in the northern study area than the southern study area. Surveys revealed that cats can persist into droughts by feeding on reptilian prey. When the study commenced, cats occurred on five of the six sub-plots in the southern study area, despite six consecutive years of below-average rainfall. However, by the end of the first year, they could only be detected on one sub-plot. Recolonisation of the sites rapidly occurred after significant rainfall (260 mm in 2 months), when nomadic birds colonised the sites and provided a plentiful food source. Foxes also declined to very low densities during drought in the southern study area, but they had recolonised all sites by the winter of 1997. This coincided with the increase in abundance of birds, which became their most frequently consumed prey item. Overall, foxes were equally abundant in the two study areas, but statistical analyses revealed a significant interaction between latitude and habitat because in the southern study area foxes tended to utilise the palaeodrainage habitat more than the sandplain, whereas in the northern study area the majority of fox sign was detected in the sandplain habitat. This may have been due to the abundance of dingoes in the palaeodrainage habitat in the northern study area. Dingoes were significantly more abundant in the northern study area than the southern, where they were usually only present at one of the three sites. The northern study area had higher densities of macropods (supplementary prey for dingoes) and more reliable access to drinking water, which persisted in the palaeodrainage channels for up to 6 months after significant rain events. Dingo numbers were relatively stable throughout the study and did not increase in response to improved seasonal conditions in the southern study area in 1997. This study revealed that the distribution of foxes extends further north into the Tanami Desert than has previously been reported, and is not necessarily tied to the distribution of rabbits in the Northern Territory. Furthermore, discussion with Aboriginal people who lived a traditional lifestyle in the area until the 1940s, revealed that foxes were already present in the northern Tanami desert at that time, before the disappearance of many medium-sized mammal species. The patterns of medium-sized mammalian extinctions in the northern and western deserts between 1940 and 1960 is thus consistent with the colonisation of the fox. Although cats had been present in central Australia for at least 50 years before the mammalian declines occurred, this does not discount them from contributing to the extinction process. It is postulated that during the early decades of their colonisation of the arid interior, cat populations may have been maintained at low levels by predation from dingoes and also Aboriginal people (for whom cats were a favoured food). But between 1920 and 1960 the western deserts were depopulated of Aboriginal people, and human hunting of cats diminished. This coincided with the introduction of the dingo bounty scheme, which encouraged many Aboriginal people to continue making regular excursions into the deserts to collect dingo scalps. In this study, cat remains occurred in 9% of dingo scats, suggesting that dingoes may be an important predator of cats. Thus, there may have been an increase in the cat population between 1930 and 1960, producing a more significant impact on native mammal populations than had previously occurred. Information collected during this study was used to construct a new model of mammalian extinctions in the spinifex grasslands of central Australia that promotes predation by cats and foxes as the primary agent of extinction. The model proposes that cats and foxes will eat medium-sized mammals when they are available, but are capable of subsisting on naturally occurring alternative prey when mammals are scarce. Thus, cats and foxes can persist into drought periods by feeding on reptilian prey, which remains an abundant resource regardless of rainfall (at least during the warmer months). Predator populations eventually decline after a series of dry winters. When the drought breaks, the rapid response of nomadic birds provides a readily available food source for cats and foxes as they recolonise areas and commence breeding. Predation by cats and foxes thereby has the potential to exacerbate the declines of native prey populations during droughts and delay their recovery when seasonal conditions improve. In this way, introduced predators are capable of causing local extinctions of medium-sized mammals when populations contract during drought periods, even in the absence of introduced herbivores and altered fire regimes. Although dingoes also prey upon medium-sized mammals, dingoes did not cause extinctions of medium-sized mammals in the spinifex grasslands because (i) they are more reliant on drinking water than foxes and cats, thus waterless areas would have provided some degree of predation refugia, and (ii) their social structure and territoriality prevent high densities accumulating, even when resources are abundant. If further extinctions of medium-sized mammals (such as the bilby) are to be prevented, it may be necessary for wildlife managers to establish a series of predation refugia where fox and cat populations can be controlled without extinguishing local dingo populations. This could be achieved with a combination of predator-proof enclosures, zones in which foxes are killed through poison baiting and areas where Aboriginal people are employed to utilise traditional hunting methods to control introduced predators.

The impacts of global change on ecosystems from climate change and invasive species are likely to be complex. Rising atmospheric CO2 concentrations, the associated climate forcing and greater frequency of extreme weather are serious challenges to natural ecosystems. In tandem with climate change, globalisation has led to the spread of invasive alien species around the globe that threaten to interrupt food web dynamics. Advancing understanding of the effects of global change on trophic interactions therefore requires study of interspecific and multi-trophic interactions. The aim of this thesis was to examine how host-plant heterogeneity, native–invasive species interactions and climate change effects (elevated atmospheric CO2 (eCO2) or drought) influence trophic interactions. An experimental approach was used which centred on a study system comprising the European raspberry (Rubus idaeus), the herbivorous large raspberry aphid (Amphorophora idaei) and coccinellid beetle predators (native species: Adalia bipunctata, Coccinella septempunctata; invasive alien species Harmonia axyridis). Under eCO2, R. idaeus resistance to A. idaei was unchanged for two cultivars (Glen Clova, Glen Ample) partially susceptible to A. idaei, but significantly reduced for another (Octavia) with complete resistance in ambient climatic conditions. The inclusion of a coccinellid predator, however, mitigated the reduction in the resistance of Octavia by reducing aphid abundance. Behavioural responses to predation by A. idaei were also impaired under eCO2 after feeding on Glen Ample. The role of natural enemies in controlling herbivore abundance in future climates is therefore crucial. Native coccinellid species are currently declining in much of Europe, attributed to the occurrence of the invasive species, H. axyridis. Despite the declines in native coccinellid species, it was found that behavioural modification to feeding by both native and invasive coccinellid species can, theoretically, result in coexistence. Plant resistance in a future climate is likely to be modified significantly. Reduced resistance to aphid herbivory demonstrated here mirrors previous studies, highlighting the future importance of natural enemies to control aphid abundance. Changes to the abundance and behaviour of aphid prey and intraguild predators will modify the effectiveness of native and invasive natural enemies. Further mechanistic research is required to understand multi-trophic interactions in dynamic environments.

Elevations in atmospheric carbon dioxide (CO2) are anticipated to lead to the acidification of the world’s oceans over the next century. This thesis sought to understand the fate of the predator-prey interactions between the endemic predator, the Mulberry whelk Tenguella marginalba (Blainville, 1832) and their prey the native Sydney rock oyster Saccostrea glomerata (Gould, 1850) and the recently introduced Pacific oyster Crassostrea gigas (Thunberg, 1793) in a marine environment increasingly affected by elevated pCO2. It was predicted that predator-prey relationships will be altered by exposure to elevated pCO2 because 1) the energetic costs for T. marginalba to survive in an acidified environment will increase causing them to compensate by increasing their consumption rate of prey; 2) growth and physiological defences of S. glomerata and C. gigas will decrease and greater energy will be required for the maintenance of acid-base balance. It was also predicted that responses will vary between oyster species and within populations of oysters. This study provides evidence that alterations in predator-prey relationships will be complex. Responses of oysters to elevated CO2 were variable and dependent on the species, family line, ploidy and size which in some cases interacted with the presence of the whelk. This thesis provides evidence that utilising triploid breeding programs to produce oysters which can divert a greater proportion of their energy budget into growth and acid-base balance, may be a viable option to reduce the predicted impacts of elevated pCO2 on oyster aquaculture over this century. Preliminary evidence for selecting oyster family lines that are resilient to both elevated pCO2 and predation suggests that this may be a challenge and more research is required to determine whether this is a feasible option to help ‘climate-proof’ aquaculture industries and oyster populations in Australia and around the world.