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- Samnegård, Ulrika, 1985-
(author)
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The impact of forest on pest damage, pollinators and pollination services in an Ethiopian agricultural landscape
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The distribution of wild biodiversity in agroecosystems affect crop performance and yield in various ways. In this thesis I have studied the impact of wild biodiversity, in terms of trees and forest structures, on crop pests, pollinators and the pollination services provided in a heterogeneous landscape in southwestern Ethiopia. Coffee, Coffea arabica, is a forest shrub native to Ethiopia and is grown in most wooded areas in the landscape where I conducted my studies. Wild coffee is still found in remote parts of the forests in the landscape. For my first paper, I surveyed pest damage on coffee in coffee forest sites, where some sites were situated in continuous forest and some in isolated forest patches. I found the variation in pest damage frequency to mainly be among coffee plants within a site, rather than among sites, which indicates the importance of local processes. However, some pests were clearly connected to the forest habitat, such as the olive baboon.In my second study, I surveyed pollinators visiting coffee flowers across a gradient of shade-tree structures. I found the semi-wild honeybee to be the dominating flower visitor. The abundance of the honeybee was not related to shade-tree structures, but to amount of coffee flower resources in the site. On the other hand, other pollinators, which included other bee species and hoverflies, were positively affected by more shade trees in the site.In my third study I investigated how the forest cover affected local bee communities in the agricultural landscape. Moreover, I investigated if this relationship differed between the dry and rainy season. The distribution of food resources for bees changes between the seasons, which may affect the bees. Most trees, fruit trees and coffee, which are patchy resources, flowers in the dry season, whereas most herbs and annual crops, which are more evenly spread resources, flowers during the rainy season. I found a clear turnover in bee species composition between the dry and rainy season, with more mobile species in the dry season. Increased forest cover in the surrounding landscape had a positive impact on bee abundance and species richness. However, the impact did not change between seasons.In my fourth study I evaluated the pollination success and pollen limitation of a common oil crop in the landscape in relation to forest cover. I found severe pollen limitation across the landscape, which may be related to the observed low bee abundances. The pollen limitation was not related to surrounding forest cover.In conclusion, I have found the forest and wooded habitats to impact several mobile animals and pathogens in our study landscape, which in turn affect people. However, there is large complexity in nature and general relationships between forest structures and all crop related organisms may be unlikely to find. Various species are dependent on different resources, at different spatial scales and are interacting with several other species. To develop management strategies for increased pollination services, for reduced pest damage or for conservation in the landscape, more species-specific knowledge is needed.
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| 2. |
- Verschut, Thomas Alexander, 1988-
(author)
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Searching for food in complex environments : Integrating processes at multiple spatial scales
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Resources are often unevenly distributed through the environment, resulting in a challenging task for insects to locate food, mates and oviposition sites. Consequently, there is an ongoing need to unravel how insects rely on behavioural and sensory traits while searching for resources in heterogeneous environments. In the first part of this thesis, I addressed this issue by studying how neighbouring resources can affect the likelihood of insects finding their preferred host resources. These effects of neighbouring resources are commonly referred to as associational effects, and are expected to result from limitations in the sensory physiology of insects. Such limitations constrain the insect’s ability to correctly evaluate resource quality at the different steps involved in insect search behaviour. Furthermore, I determined whether the physiological state of an insect, and sensory experiences made during larval stages, can affect host search behaviour in heterogeneous environments.By comparing the behaviour of Drosophila melanogaster in environments with single and multiple resources, I found that the presence of neighbouring recourses increased the selection rates for attractive resources, while it decreased the selection rates for less attractive resources. These effects are referred to as associational susceptibility and associational resistance respectively. Furthermore, by studying oviposition behaviour, I found that during these small-scale behavioural decisions, associational effects are mainly governed by gustatory mediated selection and less by olfactory mediated selection. The oviposition assay eliminated potential misinterpretations of resource quality along the different steps of search behaviour, hence the results suggested that associational effects rely on distinctive selection behaviour between resource types rather than on sensory constraints.In the second part of this thesis I determined whether natal experiences can be used by insects as sensory shortcuts to find host resources, and whether this leads to better larval performance on those selected host resources. For this purpose, I studied the interactions between the larval parasitoid Asecodes lucens and the oligophagous leaf beetle Galerucella sagittariae. The results showed that the relationship between oviposition preference and larval performance, of both insect species, depends on an interactive effect between the insects’ natal origin and the quality of the different host resources. Moreover, I found that the natal origin was a better predictor for the adult host preference, rather than for larval performance. This suggests that, aside from the actual quality of the host resources, locating any suitable host might be even more limiting for the female’s fitness.
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| 3. |
- Fors, Lisa, 1976-
(author)
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Ecology and evolution in a host-parasitoid system : Host search, immune responses and parasitoid virulence
- 2015
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Doctoral thesis (other academic/artistic)abstract
- In host-parasitoid systems, there is a continuous coevolutionary arms race where each species imposes a strong selection pressure on the other. The host needs to develop defence strategies in order to escape parasitism and the parasitoid must evolve counter-defence strategies in order to overcome the host’s immune defence and successfully reproduce. This makes host-parasitoid systems excellent model systems for understanding evolutionary processes underlying host race formation and speciation. In order to gain a better understanding of the complexity of host-parasitoid interactions several aspects must be considered, such as search behaviour and host selection in the parasitoid, the development of immune responses in the host and counter-defence strategies in the parasitoid. In this thesis, I investigate interactions and coevolution in a natural host-parasitoid system, consisting of five species of Galerucella leaf beetles and three species of Asecodes parasitoids, by combining behavioural ecology with chemical ecology and immunology. In the studies performed, I found that pheromone production and responses in the beetles are connected to the phylogenetic relatedness between the Galerucella species (Paper I). I found no evidence that Asecodes exploits the adult pheromone to locate host larvae, but observed an ability in the parasitoids to distinguish a better host from a less suitable one based on larval odors (Paper II). The studies also revealed large differences in immune competence between the Galerucella species, which were linked to differences in hemocyte composition in the beetle larvae (Paper III, IV). Further, the results suggest that parasitism success in polyphagous Asecodes is strongly affected by former host species of the parasitoid (Paper IV). In conclusion, the results of this thesis suggest an on-going evolution in both parasitoid virulence and host immune responses in the Asecodes-Galerucella system.
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