The gastrointestinal nematode parasite Haemonchus contortus is responsible for substantial global disease and production loss in small ruminants. These losses may be exacerbated by climate change, and by increasingly widespread resistance to anthelmintics. Development of successful integrated-sustainable parasite control (IPC) requires adequate knowledge of how climate drives the population dynamics of H. contortus and the seasonal occurrence of ovine haemonchosis. Therefore a simple model based on climate that is able to predict future challenge and risk resulting from H. contortus could greatly contribute to sustainable control. This thesis set forth to develop a simple, universally-applicable and useful model of H. contortus transmission to sheep; with focus on the effects that changes in climate have on the availability of infective larvae. The study aimed to (i) analyse and predict the effect(s) that changes in climate will have on H. contortus infection pressure in sheep across different geo-climatic zones; (ii) map the risk and geographical distribution of H. contortus infection pressure over time; and (iii) provide farmers with useful information on risk of infection to make cost-effective farm management decisions for sustainable control of H contortus. South Africa and the United Kingdom are the study locations. Time series analyses (TSA) was first used, as a purely statistical approach and starting point to assess the seasonal forcing influence that climate (rainfall and temperature) has on the pattern and incidence of haemonchosis. This aimed to find out if a statistical approach can identify valid climatic predictors of the risk of haemonchosis across different geo-climatic zones. Thereafter, a second model, based on the basic reproduction quotient (Qo), which is a process-based mechanistic approach, was employed to understand the effects of changes in climate on the population dynamics (i.e. transmission potential) of the free-living stages, and to predict E-[. contortus infection pressure across different geo-climatic zones. The model tries to replicate and summarize the underlying mechanisms that drive the response of parasite populations to changes in prevailing climatic variables. Finally, the use of the Qo model as a decision SUppOlt tool on farms was assessed by comparing predictions to observed faecal egg counts in south-west England over the course of a grazing season. Results suggest that TSA is able to predict the relationship between prevailing climatic conditions and the incidence of haemonchosis in a given area. However, the climatic predictors and best-fit-model were not transferable across different geo-climatic zones. Local data are needed in order to estimate coefficients for climatic predictors, such that extrapolation beyond the observed range becomes problematic and cumbersome. The Qo model, on the other hand, was able to capture the effects of seasonal variation in the prevailing climate on the pattern and incidence of haemonchosis across different geoclimatic locations. The model was spatially extended within a geographic information system (GIS) to produce Qo -based haemonchosis risk maps. The risk maps display the capability of Qo as a spatial predictor of haemonchosis risk across different geo-climatic zones over time. These risk maps have potential as spatial platforms for decision support systems, in support of integrated, sustainable control of H. contortus.

The Microsporidia form a phylum of obligate intracellular parasites known to cause disease in humans and a diverse range of economically important animal species. Once classified as ‘primitive’ eukaryotes, it is now recognised that the peculiarities of microsporidian genomics and cell biology are, in fact, the consequence of extreme reduction allowed by an intimate relationship with the host cell. Excluding survival as an extracellular spore, microsporidia are in direct contact with the host throughout their developmental lifecycle, from entry to egress. Host cell manipulations have been described in morphological terms, but despite this, characterisation of such processes at the molecular level remains challenging. The logistics of the microsporidian lifecycle suggest secreted proteins and membrane proteins with extracellular domains may be involved in virulence and implicated in host cell manipulation. This study employs bioinformatic tools to predict secreted proteins in diverse microsporidia and comparative genomics to identify conserved proteins which may be required for host cell manipulation, pathogenicity and lifecycle progression. The protein complement secreted into the extracellular environment during microsporidian spore germination, a lifecycle stage required for host cell invasion, is identified experimentally. This analysis suggests that novel microsporidian specific hypothetical proteins, that is, proteins with no functional annotation or domain, play a significant role during parasite invasion of the host and provides the first identification of potential microsporidian effector proteins. Aiming to address microsporidian pathogenicity during intracellular stages, candidate virulence factor proteins, namely a hemolysin and a protein tyrosine phosphatase are also characterised and localised in situ. Lastly, an animal-derived horizontal gene transfer event is used in conjunction with both the fossil record and molecular dating approaches to add timescale to the microsporidian diversification. This work suggests that microsporidia radiated recently, achieving extreme cellular diversity, acquiring a novel infection mechanism and undergoing vast speciation in a short evolutionary timescale, likely within the last 200 million years.

It is generally believed that tree species growing in mixed forest stands are less susceptible to insect herbivore damage than if grown in monocultures, but previous studies have been largely observational and focussed mainly on tree species richness effects. In this thesis, I examined effects of three components of forest diversity (tree species richness, intraspecific genotypic diversity and functional diversity) on insect herbivores using three long-term forest diversity experiments in Finland and Germany. I have also explored the sources of variation in and the mechanisms behind the effects of tree diversity on insect herbivores. I found that all three components of forest diversity significantly influenced insect herbivore abundance and damage. Tree species richness effects depended on the insect herbivore feeding guild, but also changed within season and between years. As a result, silver birch (Betula pendula) experienced both associational resistance (reduced damage in mixed stands) and associational susceptibility (higher damage in mixed stands) to different insect herbivores and in some instances this altered temporally. In contrast, tree species richness effects on insect herbivory were spatially consistent and not mediated by tree size (physical apparency), physical properties of leaves or natural enemies. Interestingly, tree species richness and genotypic diversity had opposite effects on leaf miners; leaf miner abundance and species richness were lower in species-rich stands, but higher in mixtures containing several genotypes of silver birch. To test the effects of tree functional diversity, I created a functional diversity index based on constitutive emissions of monoterpenes and isoprene by different tree species and showed that tree species which emitted low levels of volatiles experienced associational resistance in stands with high diversity of volatile emissions. This suggests that increasing chemical complexity in mixed stands may interfere with host finding ability of herbivores.

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Climate change is causing many species to expand their ranges northwards, potentially impacting on the communities that they join through direct and indirect interactions. This thesis focuses on temporal and spatial variation in interactions in a single study system, lepidopteran hosts feeding on stinging nettles and their parasitoids, through collection of caterpillars fi'om sites in the north west of England. Quantitative food webs revealed two distinct sub-compartments for moths and butterflies. Strong links between the two most abundant butterfly species, Aglais urticae and A. io and their two parasitoid species, a tachinid, Pelatachina tibialis, and an ichneumonid, Phobocampe conjilsa, indicated potential for indirect interactions. The range-expanding species, A. io, was not found to be experiencing enemy release in recently colonised areas. The resident species, A. urticae, experienced higher parasitism rates at sites recently colonised by A. io than at sites where both species had been present for a long duration. At the site level, presence and parasitism rates of the butterfly hosts were not related to habitat features, however, both host species experienced higher parasitism with low connectivity to the other host species. Whilst parasitism rates by Ph. conjilsa were higher in more isolated nests, parasitism by P. tibialis was higher in well connected nests. With reference to the natural history of the parasitoid species, it is proposed that Ph. conjilsa uses isolated hosts to avoid P. tibialis, a potentially superior competitor. Finally, it was found that coexistence of the two host species and the two parasitoid species in this system is likely to persist through differential host use by the parasitoids, mainly mediated through differing phenologies. This thesis provides new and valuable information on the spatial and temporal variations in the interactions between co-occurring common lepidopteran species and their parasitoids in the context of climate change.

All organisms are to some degree parasitised and display some form of social behaviour. Host and parasite social interactions are incredibly diverse and occur in all natural populations, therefore understanding such fundamental interactions is of profound importance - particularly from a public health perspective. The work in this thesis is developed in the context of bacteria and virus interactions, using the bacterium Pseudomonas fiourescens and bacteriophage SBW2502 for experiments. Mathematical models are developed in close conjunction with empirical data, with the aim of maintaining a high level of biological relevance of any theoretical undertakings. Chapter 1 demonstrates that selection on non-social traits can limit the invasion of social cheats. This is because beneficial mutations are most likely to arise in the numerically dominant cooperator population, and clonal sweeps would thus purge any genetic diversity. Chapter 2 is a theoretical development of chapter 1, which includes coevolution between hosts and parasites. The model governing the underlying host-parasite infection genetics (Matching Alleles; MA, Gene-for-Gene; GFG, or somewhere in between) has a big impact on the outcome of host social behaviour. Host cooperation is more favourable under a MA model, due to the more frequent switching of host genotypes, which acts to purge genetic diversity and re-establish homogenous cooperating host populations. Chapter 4 explores how host-parasite interactions shape the evolution of parasite diversity.

Myrmica ruginodis ants and the gorse species U. europaeus have a national distribution in the U.K. and are considered important keystone species within British lowland heaths. These two species boost and maintain biodiversity by increasing habitat heterogeneity through ecosystem engineering of the soil physical and chemical environment, and are generally considered Keystone species in lowland heaths. Both species have been shown to increase limiting nutrients in the soil such as Nitrogen and Phosphorus. Here, for the first time in a single study, the extent to which these species augment soil chemistry and the resulting impacts on microbial communities is quantified. It was hypothesized that abiotic changes brought about by the action of keystone species directly and indirectly will lead to differences in both bacterial and fungal community diversity and composition, and such changes will be likely to modify ecosystem function. Analysis of soil cores taken from three U.K. lowland heath habitats representing a national distribution found that dissimilarities in microbial community structure (determined by T-RFLP) were largely attributable to differences in soil pH. However, pH was an imperfect predictor. To fully understand abiotic drivers and to identify the soil nutrients involved in augmenting microbial communities in heathlands, a data mining exercise was conducted using the large Countryside Survey 2007 bacterial data. The findings from this showed that as the soil pH gradient was shortened by focusing on low soil pH, and then by looking at a single type of vegetation cover (dwarf shrub heath), soil pH became less effective in explaining differences in the communities; other soil chemical properties then become more important, such as Carbon to Nitrogen (C:N) ratio and soil Phosphorus (P). The identification of these chemical attributes and the known impacts that the keystone species have on them led to the development of a controlled mesocosm experiment using Aberdeenshire lowland heath soil. In this experiment keystone species were isolated and found to impact upon soil chemistry and microbial diversity. Soil pH was identified as having an important defining role on communities, however soil P and C:N and Potassium (K) were also related to microbial community composition. Differences in respiration were also linked to the presence of the keystone species. A final field experiment was performed to test the mesocosm observations in the field site, but sampling specific soil regions (ant nest structures and rhizospheres). Less marked microbial community differentiation was found compared with the initial field experiment. Fungi had no responses to soil chemistry except for Dorset communities which responded to soil pH. Bacteria responded to all soil measures of chemistry for pooled region data and for Aberdeenshire. Cumbria communities responded P and Dorset to C:N. Soil chemistry and microbial communities were not affected by the same keystone species in the same way in any of the regions. Finally, community T-RFLP profiles for fungi and bacteria were directly compared for.any correlation; interaction was found to be limited, partly due to fundamental differences in the bacterial 16S rDNA and fungal ITS rDNA T-RFLP profiles and partly due to these microbes responding in different ways to their environment.