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Within an ecological community, interactions between species in a community occur directly, through physical contact, and indirectly, via other species or through abiotic environmental modification. Genetic variation within a species has the ability to alter the outcome of interactions between species. In other words, the specific genotypes of the interacting individuals are important for the outcome of the interaction. In this thesis, I begin by showing that indirect interactions between a soil rhizobacteria (Pseudomonas aeruginosa) and a parasitoid wasp (Aphidius rhopalosiphum) are mediated by genotypic interactions between the two linking species, aphids (Sitobion avenae) and barley (Hordeum vulgare). This means that the magnitude and direction of the indirect effect of rhizobacteria (presence/absence) on the wing size of the parasitoid wasp was different, depending on the combination of interacting aphid and barley genotypes. If such interactions were to have an evolutionary effect, there would need to be non-random association between the aphid and barley genotypes. In the next chapter, I demonstrated that different aphid genotypes actively choose (no effect on reproductive rate) to colonize particular barley genotypes. I then showed that host preference of an aphid genotype could be altered by the presence, and sometimes genotypic identity, of another competing aphid genotype. I confirmed that these interactions were indirect, via the plant, by showing that aphid growth rate can be reduced when a plant is pre-conditioned with a different aphid genotype. Further investigation, using microarrays, showed that the different aphid genotypes induced differential gene expression in a single barley genotype. Many of these sequences belonged to known plant defense pathways and suggest a possible mechanism for the observed genotypic interactions between aphid and barley. In order to further understand the influence of within-species genetic variation on species interactions there is a need to consider these interactions in a natural system. I therefore investigated the influence of genetic variation within a single tree species on the associated plant and invertebrate communities in a complex, natural tropical ecosystem. I found that more closely related trees were host to more similar communities of epiphytic plants, leaf litter invertebrates around the base of the tree and trunk-dwelling invertebrates. This shows that even in a highly diverse, naturally occurring ecosystem the effect of genetic variation within a species can be an important factor for the structure of associated communities of both plants and animals.Research on the influence of within-species genetic variation on species interactions at a community level has wide applications for understanding how communities and ecosystems function, which can benefit agriculture, disease management and conservation practices.

Ecological communities – groups of interacting species – are subject to a variety of disturbances. Understanding responses to these disturbances is a primary goal of community ecology. The structural complexity of the community and the traits of the community’s constituent species are both known to have a significant impact on a community’s response to a disturbance. In this thesis, we investigated how these two scales – the community level and the species level – interactively affected community responses to both short and long term disturbances. Our first hypothesis was that interaction strength would be weaker in species with many interactions when compared to species with fewer interactions. To test this hypothesis, we used simulated food webs and found that, in locally stable food webs, species with many interactions tended predominantly to have interactions with predators or with prey. While these many predator or prey interactions were weak, they tended to be balanced by a few interactions of the opposite type (with prey or predators) which were stronger than average. The structure of the network, where species had predominantly one type of interaction, was essential for this relationship between the number and strength of interactions to arise. Our second study investigated how food webs of varying size and connectance respond to press and pulse disturbances. Many studies of food web stability only focus on the response to short term or “pulse” disturbances, however, as anthropogenic impacts on food webs increase, it is important to increase our understanding of food web responses to long term or “press” disturbances and determine whether they follow the same pattern as pulse disturbances. We found that more species rich and connected food webs were less stable to both types of disturbance and the more stable a food web was to a pulse disturbance, the more stable it was to a press disturbance as well. We also found that the traits – trophic level and number of interactions – of the disturbed species impacted a food web’s resistance to a press disturbance. Food webs were less resistant to the disturbance of species with many interactions or low trophic level than species with few interactions or high trophic level. The strength of species’ effects on stability was also moderated by the structural complexity of the food web. Together the work that makes up this thesis suggests that, to understand the stability of food webs to any kind of disturbance, we should consider both the structure of the network and the traits of the species embedded within it. While we found that networks were more vulnerable to disturbance of certain species than others, this observation also depended on the structure and complexity of the community they existed in. This has important implications for communities subject to disturbances, especially those disturbances which alter the way in which communities are structured and species interact.

The introduction of trout has been implicated in the declines in native fish fauna in New Zealand and worldwide. Since the introduction of brown (Salmo trutta) and rainbow (Oncorhynchus mykiss) trout to New Zealand in 1867, their distribution has spread and they have been implicated in the fragmentation of native fish distributions, particularly native non-migratory galaxiids. However, in the Upper Waimakariri basin the co-occurrence of trout and galaxiid populations is relatively common, even in streams where trout reach sizes known to be piscivorous. To investigate mechanisms that may regulate trout and galaxiid co-occurrence, I investigated differences in antipredator behaviour and growth rate between stream types with varying levels of trout presence. Using quantitative survey data collected between 1997 and 2006, I found that trout abundance was low and varied annually in frequently disturbed sites compared their high abundance in stable streams. This finding was used to classify streams into three population types, barrier (trout absent), disturbed (trout presence intermittent) and sympatric (constant trout presence). Using this classification, I tested the effects of trout chemical cues on galaxiid activity and refuge use in artificial channels. There were no differences in activity or refuge use between trout odour and there were no effects of population type or galaxiid size during both the day and the night. Using otolith weight-fish length relationships in galaxiids collected from each population type, I found that galaxiid growth rate was higher in disturbed streams than in stable streams either with or without trout. An experiment manipulating trout size and presence, over two months in a natural stream, found galaxiids from treatments without trout grew slower than those with trout. Slow growth rates in galaxiids above trout-migration barriers and in sympatry, combined with low growth rates in treatments without trout suggest that the mechanisms that regulate galaxiid growth are more complex than previously thought.

The introduction of trout has been implicated in the declines in native fish fauna in New Zealand and worldwide. Since the introduction of brown (Salmo trutta) and rainbow (Oncorhynchus mykiss) trout to New Zealand in 1867, their distribution has spread and they have been implicated in the fragmentation of native fish distributions, particularly native non-migratory galaxiids. However, in the Upper Waimakariri basin the co-occurrence of trout and galaxiid populations is relatively common, even in streams where trout reach sizes known to be piscivorous. To investigate mechanisms that may regulate trout and galaxiid co-occurrence, I investigated differences in antipredator behaviour and growth rate between stream types with varying levels of trout presence. Using quantitative survey data collected between 1997 and 2006, I found that trout abundance was low and varied annually in frequently disturbed sites compared their high abundance in stable streams. This finding was used to classify streams into three population types, barrier (trout absent), disturbed (trout presence intermittent) and sympatric (constant trout presence). Using this classification, I tested the effects of trout chemical cues on galaxiid activity and refuge use in artificial channels. There were no differences in activity or refuge use between trout odour and there were no effects of population type or galaxiid size during both the day and the night. Using otolith weight-fish length relationships in galaxiids collected from each population type, I found that galaxiid growth rate was higher in disturbed streams than in stable streams either with or without trout. An experiment manipulating trout size and presence, over two months in a natural stream, found galaxiids from treatments without trout grew slower than those with trout. Slow growth rates in galaxiids above trout-migration barriers and in sympatry, combined with low growth rates in treatments without trout suggest that the mechanisms that regulate galaxiid growth are more complex than previously thought.

Interactions are important drivers of selection and community structure, which makes the study of multi-species interactions critical for understanding the ecology and evolution of organisms. This dissertation includes four data chapters that examine the biotic and abiotic mechanisms that shape multi-species interactions in both tropical and temperate ecosystems. The first three data chapters (Chapters 2–4) were completed within a Neotropical rainforest in Costa Rica and focus on one plant genus, Piper (Piperaceae). The final data chapter (Chapter 5) was conducted within a working landscape of soybean (Glycine max) fields in eastern Maryland, USA. In Chapter 2, I explore intra- and inter-specific dietary niche partitioning of Piper fruits among three frugivorous bats, illustrating the importance of fine-scale mechanisms that facilitate species coexistence and influence plant–animal interactions. In Chapter 3, I demonstrate how the chemical ecology of a Neotropical shrub, Piper sancti-felicis, shapes fruit interactions with antagonists (fruit fungi) and mutualists (frugivorous bats and birds), developing a foundation for understanding evolutionary ecology of plant chemical traits based on phytochemical investment patterns. In Chapter 4, I describe the direct and indirect impacts of elevated temperature and CO2 concentration on the plant traits and interactions in Piper generalense, improving our understanding of the effects of climate change on a Neotropical plant–herbivore system. In Chapter 5, I explore the biotic (herbivore-induced plant volatiles) and abiotic (fine-scale weather conditions) drivers affecting insectivorous bat foraging in soybean fields in eastern Maryland, providing a pathway to further investigate new strategies for integrated pest management. As a collective work, this dissertation disentangles the nuances of multi-species interactions, exploring foundational mechanisms underlying biodiversity maintenance as well as answering applied questions to address a changing climate and aid sustainable agriculture. / Doctor of Philosophy / Everything in nature is connected, so studying ecological interactions requires us to view them from many different angles. As with most relationships, ecological interactions are multi-faceted and context-dependent. In this dissertation, I describe both tropical and temperate systems, collecting a variety of measurements from plants, microbes, and animals to explore the complicated relationships that exist between them. In Chapter 2, I explore how three species of fruit-eating bats may divide the use of a shared food resource (tropical pepper fruits in the genus Piper) to maintain separate populations and how those foraging differences may affect Piper plant populations. In Chapter 3, I characterize a chemical compound found in the fruits of a Piper plant species and test the effect of that compound on fruit fungi and fruit-eating bats and birds, leading to a better understanding of the selective pressures affecting fruit chemistry. In Chapter 4, I describe the direct and indirect effects of climate change on a Piper plant in the first study to measure the responses of tropical understory plants to treatments that mimic climate change using active warming and CO2 supplementation. In Chapter 5, I explore the fine-scale drivers of bat activity in soybean fields, including how weather conditions and the specific compounds emitted by insect-damaged plants may affect bat activity. As a collective work, this dissertation describes the complex relationships among plants and their many interactors, exploring questions from biodiversity maintenance to integrated pest management strategies.

The production of cereals, especially maize, which is a staple food in sub-Saharan Africa, is challenged by pests and diseases. In addition, climate change will exacerbate the magnitude of these challenges and agriculture in general. Lepidopteran stemborers are major pests of cereal crops in sub-Saharan Africa. Two indigenous noctuids, Busseola fusca and Sesamia calamistis and one exotic crambid, Chilo partellus, occur as single and mixed-species communities infesting cereal crops in East and southern Africa. The composition of these communities however varies with altitude and over seasons. Interactions between the species especially when in combination, can affect the level of damage to cereal crops. The objective of this study was to describe the intraspecific and interspecific interactions between the three stemborer species when they make use of the same resources. This study involved field surveys in maize fields in major agro-ecological zones and also different experiments under greenhouse and laboratory conditions to describe the interactions between the species as well as the effect of temperature, species density and duration of the period of competition on outcomes of these interactions. Results showed that stemborer communities are composed of single and mixed species of B. fusca, S. calamistis and C. partellus, which varied with the different agro-ecological zones and also along altitudinal gradients. Temperature was the most important abiotic factor that influenced the composition of stemborer communities. However, infestation patterns varied with season. This study also showed that female moths of the three species did not avoid oviposition on plants that were previously infested by stemborers. Busseola fusca showed a high preference for heterospecific-infested plants, while C. partellus preferred conspecific-infested plants. Sesamia calamistis did not show a significant preference for one plant over the other. This study further showed that both intra- and interspecific competition characterise communities of these stemborer species which ultilise the same resources. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuid species. Temperature had a significant influence on the competitive outcomes between the three species. Finally the study showed that densitydependent effects and the duration of the interactions are also important factors which influence the level of competitive outcomes between the species. Climate change is likely to influence the interactions and composition of stemborer communities. / PhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

The production of cereals, especially maize, which is a staple food in sub-Saharan Africa, is challenged by pests and diseases. In addition, climate change will exacerbate the magnitude of these challenges and agriculture in general. Lepidopteran stemborers are major pests of cereal crops in sub-Saharan Africa. Two indigenous noctuids, Busseola fusca and Sesamia calamistis and one exotic crambid, Chilo partellus, occur as single and mixed-species communities infesting cereal crops in East and southern Africa. The composition of these communities however varies with altitude and over seasons. Interactions between the species especially when in combination, can affect the level of damage to cereal crops. The objective of this study was to describe the intraspecific and interspecific interactions between the three stemborer species when they make use of the same resources. This study involved field surveys in maize fields in major agro-ecological zones and also different experiments under greenhouse and laboratory conditions to describe the interactions between the species as well as the effect of temperature, species density and duration of the period of competition on outcomes of these interactions. Results showed that stemborer communities are composed of single and mixed species of B. fusca, S. calamistis and C. partellus, which varied with the different agro-ecological zones and also along altitudinal gradients. Temperature was the most important abiotic factor that influenced the composition of stemborer communities. However, infestation patterns varied with season. This study also showed that female moths of the three species did not avoid oviposition on plants that were previously infested by stemborers. Busseola fusca showed a high preference for heterospecific-infested plants, while C. partellus preferred conspecific-infested plants. Sesamia calamistis did not show a significant preference for one plant over the other. This study further showed that both intra- and interspecific competition characterise communities of these stemborer species which ultilise the same resources. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuid species. Temperature had a significant influence on the competitive outcomes between the three species. Finally the study showed that densitydependent effects and the duration of the interactions are also important factors which influence the level of competitive outcomes between the species. Climate change is likely to influence the interactions and composition of stemborer communities. / PhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

A major goal of ecology is to understand the factors that shape interactions among species. In this study, I explored the little-known associations between beetles and macrofungal fruiting bodies to characterize patterns of beetle-fungus association and to investigate sources of variation in the structure of these trophic interactions. First, I characterized the composition and diversity of beetle-sporocarp associations at two sites in the Appalachian Mountains and foothills, and evaluated the extent to which beetle community structure varied with fungal species, sporocarp age, and sporocarp dry mass. My results showed that beetle abundance and diversity differed among fungal species and were positively associated with sporocarp age and dry mass. I also found evidence of a nested structure in beetle-sporocarp interactions, wherein specialists on both sides of the association interact preferentially with more generalized species. Next, I performed a field study of beetle-sporocarp associations over two summers to evaluate the factors related to interaction breadth in trophic associations. I found evidence that interaction breadth varies with the palatability of the food organism (as indicated by sporocarp toughness and sporocarp age) and showed that beetle interaction breadth was negatively correlated with sporocarp persistence. I found strong intraseasonal variation in interaction breadth, but no evidence that this variation was structured by precipitation or differences in beetle community composition. In my third chapter, I conducted a field experiment to investigate (1) the importance of an individual food organism's physical properties in determining its relative importance in the beetle-sporocarp interaction network and (2) whether the structure of the beetle-sporocarp interaction network cycles predictably with the time of day. My results show that size and density of individual food organisms may be important factors in determining their relative importance in an interaction network, and offer the first evidence of diurnal cycling in the structure of interaction networks.

O conhecimento das interações entre Brevipalpus phoenicis (Geijskes), outros ácaros fitófagos ou predadores e as plantas de cafeeiro ainda são insuficientes para se poder elaborar um programa adequado de manejo da cultura, para tentar solucionar os problemas causados pela mancha anular. Apesar da importância do problema, pouco se sabe sobre a diversidade de ácaros, assim como a dinâmica populacional do ácaro B. phoenicis e seus inimigos naturais na cultura cafeeira, bem como nas diversas cultivares comercialmente exploradas. As informações sobre o impacto de agroquímicos nas populações de ácaros em cafeeiro são praticamente inexistentes. Contudo, sabe-se que alguns inseticidas ou fungicidas podem afetar a população de ácaros predadores podendo favorecer o aumento populacional de ácaros pragas. Ao caracterizar a diversidade de ácaros em duas importantes regiões produtoras (Jeriquara e Garça), constatou-se que em Jeriquara a diversidade foi superior ao encontrado em Garça. Foram coletados no total 13.052 ácaros nos dois locais estudados, sendo 7.155 em Jeriquara e 5.897 em Garça. De um total de 108 espécies encontradas, 45 espécies foram observadas em ambos os locais avaliados, que apresentaram similaridade de 56%. Os predadores mais freqüentes nas folhas foram Euseius citrifolius Denmark & Muma, E. concordis (Chant) e Agistemus brasiliensis Matioli, Ueckermann & Oliveira. Foram observadas correlações significativas a 0,05% (Pearson) entre as populações E. concordis e B. phoenicis; Zetzellia malvinae Matioli, Ueckermann & Oliveira e B. phoenicis; E. concordis e A. brasiliensis; entre outras. Em relação à diversidade em diferentes cultivares de cafeeiro (Coffea canephora cv Apoatã e de C. arabica cultivares Mundo Novo, Icatu Vermelho, Icatu Amarelo e Catuaí Amarelo) realizado em Garça, verificou-se que a maior riqueza de espécies e o maior número de indivíduos na superfície das folhas foram observados para Apoatã. A cultivar Icatu Vermelho foi a que apresentou maior uniformidade na distribuição das espécies de ácaros e Apoatã a que apresentou menor uniformidade. Em relação à preferência hospedeira, B. phoenicis foi encontrado em maior abundância na cultivar Apoatã, representando 61% de todos os indivíduos. E. citrifolius ocorreu em maior número na cultivar Mundo Novo e E. concordis na Apoatã. A. brasiliensis ocorreu em maior quantidade na cultivar Icatu Vermelho e Z. malvinae ocorreu sem diferença estatística em todas as cultivares. Em relação aos efeitos de pesticidas sobre a diversidade de ácaros, constatou-se que no tratamento com triadimenol + disulfoton apresentou a menor , enquanto que no deltametrina + triazophos a maior. B. phoenicis apresentou redução no número de indivíduos no tratamento com aldicarb e um aumento de cerca de duas vezes nos tratamentos com triadimenol + disulfoton e no thiamethoxam. Os tratamentos que apresentaram as maiores semelhanças na composição das espécies foram: testemunha e cartap, e thiamethoxam e ethion; e os tratamentos de menor similiaridade foram: testemunha e aldicarb, aldicarb e cartap, e aldicarb e deltametrina + triazophos. Reduções significativas na população de A. brasiliensis nos tratamentos com aldicarb e thiamethoxam e de E. citrifolius no tratamento com cartap foram detectadas. / The knowledge on the interactions among Brevipalpus phoenicis (Geijskes), other phytophagous or predaceous mites and coffee plants is still insufficient to establish an adequate management program for this crop, in order to minimize the problems caused by coffee ringspot virus (CoRSV), transmitted by B. phoenicis. In spite of the problem, little is known on the diversity of mites, as well as on the population dynamics of B. phoenicis and its natural enemies on coffee plants of several important commercial cultivars. Information on the impact of agrochemicals on the population of mites on coffee plantations are practically inexistent. However, it is known that some insecticides and fungicides can affect the population of predaceous mites inducing pest mite population increases. Characterizing the diversity of mites on two important coffee-producing areas of the state of São Paulo (Jeriquara and Garça), it was observed that the diversity of mites was higher in Jeriquara county than in Garça. A total of 13,052 mites was collected in both studied areas, of which 7,155 in Jeriquara and 5,897 in Garça. Of the total of 108 species identified in this study, 45 species were observed in both localities (Garça an Jeriquara), which presented 56% of similarity. The most frequent predaceous mites on leaves were Euseius concordis (Chant), E. citrifolius Denmark & Muma and Agistemus brasiliensis Matioli, Ueckermann & Oliveira, among others. The study on the diversity of mites in different coffee cultivars (Coffea canephora cv. ‘Apoat㒠and C. arabica cv. ‘Mundo Novo’, ‘Icatu Vermelho’, ‘Icatu Amarelo’ and ‘Catuai Amarelo’), carried out in Garça, showed the highest species richness and the highest number of specimens for the leaf surface of ‘Apoatã’. ‘Icatu Vermelho’ was the cultivar which presented the highest uniformity of distribution of mite species and ‘Apoat㒠was the cultivar with the lowest uniformity. The species B. phoenicis was found in higher abundance in ‘Apoatã’, representing 61% of all specimens collected. E. citrifolius occurred in higher number on the cultivar ‘Mundo Novo’ and E. concordis in ‘Apoatã’. The stigmaeid mite A. brasiliensis occurred in higher number on the cultivar ‘Icatu Vermelho’ and Z. malvinae was present in similar populations on all coffee cultivars. The study on the effect of pesticides on the mite diversity, showed the lowest diversity for the treatment with triadimenol + dissulfoton, and the highest diversity was observed for treatment with deltamethrin + triazophos. B. phoenicis presented population reduction for the treatments aldicarb, but an increase of around two times for treatments with triadimenol + disulfoton and with thiamethoxam. The treatments with the highest similarities in species composition were: cartap and control (without pesticide), and thiamethoxam and ethion; and the treatments with the lowest similarities were: control and aldicarb, aldicarb and cartap, and aldicarb and deltamethrin + triazophos. A significant reduction in A. brasiliensis population was detected for the treatments with aldicarb and thiamethoxam. E. citrifolius population was affected significantly by cartap.