Les interactions trophiques plante-herbivore sont indirectes lorsque la consommation d’une plante par un herbivore est influencée par la présence d’une plante voisine. L’influence des plantes environnantes sur la sélection des herbivores a été décrite par des études sur l’approvisionnement des herbivores, mais aussi par des études sur les effets associatifs entre plantes. À cause de leur évolution parallèle, certains thèmes communs dans les études sur l’approvisionnement des animaux ont eu peu d’échos dans la compréhension des effets associatifs entre les plantes. L’objectif général de cette thèse est de comprendre les effets indirects des communautés végétales sur l’utilisation des plantes par les grands herbivores, dans le contexte des facteurs affectant l’approvisionnement. Premièrement, je me suis intéressée à la distance à laquelle une plante environnante influence la sélection, car l’approvisionnement est un processus spatialement hiérarchique. Grâce à une méta-analyse, je révèle que les plantes environnantes influencent l’utilisation d’autres plantes par les grands herbivores jusqu’à des centaines de mètres carrés. En étudiant les effets associatifs à l’île d’Anticosti (Québec, Canada), je démontre que les plantes environnantes peuvent augmenter ou diminuer le broutement du cerf de Virginie sur le sapin baumier, selon l’échelle considérée. Deuxièmement, j’ai testé l’effet de facteurs susceptibles d’influencer l’approvisionnement des herbivores sur les effets associatifs. Par exemple, la qualité nutritive des plantes environnantes augmentait le broutement sur les sapins. De plus, l’abondance relative des plantes de la communauté pouvait créer des effets associatifs : en Outaouais (Québec, Canada), les pins blancs étaient moins broutés dans les parcelles plus diversifiées où l’abondance relative des espèces préférées était plus faible. Ma thèse met en valeur l’importance d’incorporer une perspective animale, qui tient compte des objectifs et des contraintes qui génèrent les choix des herbivores, à l’étude des effets associatifs. De plus, elle propose des avenues de recherches prometteuses, comme incorporer la variabilité intraspécifique dans les traits fonctionnels des plantes, tel que leur qualité nutritive, et considérer les effets associatifs à plusieurs échelles spatiales. Ces travaux améliorent notre compréhension des interactions trophiques indirectes, en démontrant l’influence de facteurs comme l’abondance des ressources sur ces interactions. De plus, je propose un mécanisme potentiel, soit les comportements d’approvisionnement des herbivores, ce qui ouvre la voie à une généralisation des effets associatifs à différents systèmes. / Plant-herbivore trophic interactions can be indirect, such as when the consumption of a plant by an herbivore is influenced by the presence of a neighbouring plant. The influence of neighbouring plants on herbivores’ selection has been described in studies of foraging but also in studies of associational effects between plants. Because of their parallel evolution, some common theme in the studies of animal foraging has been seldom addressed in the understanding of associational effects among plants. The general objective of this thesis is to understand the indirect effects of vegetation communities on plant use by large herbivores, in the context of factors affecting foraging. First, I investigated at which distance a neighbouring plant can influence herbivores’ selection, because foraging is a spatially hierarchical process. By a meta-analysis, I uncovered that neighbouring plant can influence the use of other plant species by large herbivores up to hundreds square metres. By a study of associational effects on Anticosti Island (Québec, Canada), I also demonstrated that neighbouring plants can increase or decrease browsing by white-tailed deer on balsam fir, depending on the scale considered. Second, I tested the relative contribution on associational effects of factors susceptible to influence foraging by herbivores. For example, the nutritional quality of neighbouring plants increased browsing on firs. Moreover, the relative abundance of plants in the community can generate associational effect: in the Outaouais region (Québec, Canada), white pines were less browsed in diversified plots where the relative abundance of preferred species was lower. My thesis supports the importance of incorporating an animal perspective, which takes into account the objectives and constraints on herbivores foraging choices, in the study of associational effects. Moreover, it offers promising research perspective, like including plant intraspecific variability in functional traits, such as nutritional value, and considering associational effects at multiple spatial scales. This work improves our understanding of indirect trophic interactions, by demonstrating the influence of factors such as resource abundance on these interactions. It also proposes a mechanism, foraging processes of herbivores, which paves the way to a generalization of associational effects in different systems.

À l’intérieur d’un réseau trophique, les différents niveaux trophiques réagissent différemment aux changements de la température, car certains organismes sont plus sensibles que d’autres. Dû à l’interdépendance entre les différents niveaux, même le plus petit changement de la température peut déclencher des différences en cascade de la performance de toutes les composantes du réseau. Ceci peut mener à l’effondrement partiel ou total du système. Dans le cadre de mon projet, je me suis intéressée aux effets de la température et des changements climatiques sur la performance relative d’un réseau tritrophique (plante-herbivore-parasitoïde). Les objectifs généraux étaient de déterminer : a) Quel niveau trophique est le plus sensible à l’augmentation de la température? et b) Quels sont les effets des changements climatiques sur l’ensemble du réseau trophique? Afin de pouvoir évaluer l’effet de la température sur l’ensemble du réseau trophique, j’ai développé trois paramètres de la performance relative en utilisant la biomasse comme monnaie commune : la productivité nette générationnelle (NGP), le taux de performance relative d’un réseau bitrophique (φh/p), et le taux de performance relative d’un réseau tritrophique (φ3t). En général, mes résultats suggèrent que la fenêtre thermique de la performance relative de chaque niveau trophique est plus large à la basse du réseau (c.-à-dire pour la plante) et qu’elle est réduite d’environ 4 °C pour chaque niveau trophique subséquent. Aussi, les valeurs de φh/p suggèrent que les pucerons sont plus performants que la plante à basse température, mais ils sont incapables de se reproduire au-delà de 28 °C, ce qui donne un avantage aux plantes. Néanmoins, cet avantage ne peut être maintenu longtemps dû aux effets négatifs des températures élevées sur la biologie de la plante. Les valeurs de φ3t suggèrent que la performance du réseau tritrophique est gouvernée par des cascades trophiques du type « top-down », mais la variation inter et intraspécifique de la plante-hôte joue un rôle majeur dans la productivité relative du système. Les résultats des expériences de régime thermique simulé pour l’horizon 2050 suggèrent que : malgré une fenêtre thermique plus réduite, le parasitoïde a la performance relative la plus grande du système, dans les trois scénarios testés. Ceci suggère, possiblement, une plasticité thermique plus grande que prévue à ce niveau trophique. Néanmoins, en absence du parasitoïde, l’herbivore domine le système. La réponse de la performance relative du réseau tritrophique est la même dans trois scénarios testés, malgré une différence d’environ 4 °C entre eux. Par contre, l’exposition à long terme aux températures élevées a un effet négatif sur l’accumulation de la biomasse sur les trois composantes du réseau au niveau individuel et collectif. Ceci est la première étude à évaluer de façon empirique et exhaustive les effets de la température sur autant d’interactions plante-herbivore-parasitoïde, afin de déterminer la performance relative d’un système tritrophique de façon holistique. / Each trophic level of a food-web reacts differently to changes in temperature, because some species are more sensitive than others. Because of the interdependence between the different trophic levels, even the smallest change in temperature could trigger cascading effects throughout the food-web. This may cause a partial or total collapse of the system. As part of my project, I was interested in the effects of temperature and climate change over the relative performance of a tri-trophic food web system (plant-herbivore-parasitoid). The general objectives were to determine: a) which trophic level is more sensitive to an increase in temperature? and b) What are the effects of climate change on a food-web as a whole? In order to determine the effects of temperature on the whole food-web, I developed three parameters to measure the relative performance, by using biomass as common currency between the three trophic levels. The developed parameters are: net generational productivity (NGP), the bi-trophic food-web ratio (φh/p), and the tri-trophic food web ratio (φ3t). In general, my results suggest that the thermal window of the relative performance of each trophic level has a wider span at the base of the food-web (e.g. the plant) and it is reduced by about 4 °C for each subsequent trophic level. Also, the (φh/p) values obtained, suggest that the aphids have the highest performance at low temperatures, but they are incapable of reproducing beyond 28°C, which gives the plant a competitive advantage. Nonetheless, this advantage cannot be maintained for long, due to the negative effects of temperature on the biology of the plant. The φ3t values suggest that the performance of the food-web is influenced by trophic cascades in a « top-down » fashion; but both the inter- and intra-specific variation of the host plant plays a major role in the productivity of the system. The results of the experiments about climate change suggest that: in all three tested climate change scenarios, the parasitoid has the largest relative performance of the system in spite of having the smallest thermal window. This suggests a greater thermal plasticity than previously thought. Nonetheless, in the absence of parasitoids, the herbivore dominates the system. Although there was a 4 °C difference between the three climate change scenarios that were tested, the the performance of the tri-trophic food-web was not significantly affected. In comparison, under two 2050 climate change scenarios, the long-term exposure to high temperatures has a negative effect on the accumulation of biomass for the three components of the food web, both individually and collectively. This is the first study to evaluate empirically and exhaustively the effects of temperature over a great range of plant-herbivore-parasitoid interactions, in order to determine the relative performance of the system in a holistic way.

South Africa’s growing elephant populations are concentrated in relatively small enclosed protected areas resulting in the over utilisation of the available food sources. Elephants and other herbivores as well as other natural disturbances such as fires and droughts play an important role in maintaining savannah environments. When these disturbances become too concentrated in a particular area the vegetation composition may be negatively affected. Excessive damage to the vegetation would result from exceeding the capacity of a protected area to provide food resources. The effect of the 120 elephants on the vegetation of Welgevonden Private Game Reserve, is not known. The rugged terrain of this reserve makes it a difficult, time consuming and labour intensive exercise to conduct ground studies. Satellite images can be used as a monitoring tool for vegetation change and improve the quantity and quality of environmental data to be collected significantly, allowing more informed management decision-making. This study evaluated the use of satellite imagery for monitoring elephant induced vegetation change on Welgevonden Private Game Reserve. The LANDSAT Thematic Mapper multispectral images, acquired at two yearly intervals from 1993 until 2007 were used. However, no suitable images were available for the years 1997, 2001 and 2003. A series of vegetation change maps was produced and the distribution of water sources and fire occurrences mapped. The areas of change were then correlated with the spatial distribution of water points and fire occurances, with uncorrelated areas of change. This was analysed using large animal population trends, weather data and management practices. On the visual comparison of the vegetation maps, it was seen that over this time period there was some decrease and thinning of woodland, but the most notable change was the increase of open woodland and decrease in grasslands. Using only the digital change detection for the period 1993 to 2007, a general increase in vegetation cover is seen. But this generalisation is misleading, since comparing the digital change detection to the vegetation maps indicates that while vegetation cover may have increased, significant changes occurred in the vegetation types. Most of the areas of significant change that were identified showed a strong positive correlation with burnt areas. The distribution of the water sources could not be directly linked to the vegetation change although rainfall fluctuations seemed to have accelerated vegetation changes. Years with high game counts, such as 1999, also coincide with very low rainfall making it difficult to differentiate between the effects of heavy utilisation of vegetation and low rainfall. Furthermore, many of the initial vegetation changes could be the result of land use changes due to the introduction of browsers, selective grazers and elephants that allow for more natural utilisation of the vegetation. Remote sensing makes it possible to successfully track changes in vegetation and identify areas of potential elephant induced vegetation change. Vegetation changes caused by disturbances, such as fire and anthropogenic activities, can be accounted for but it is not possible to conclude with a high level of certainty that the further changes seen are solely a result of elephant damage. Further work is required to reliably isolate elephant induced vegetation changes, as well as to establish the effects these changes have on the ecosystem as a whole. / Environmental Sciences / (M. Sc. (Environmetal Sciences))

No description available.

South Africa’s growing elephant populations are concentrated in relatively small enclosed protected areas resulting in the over utilisation of the available food sources. Elephants and other herbivores as well as other natural disturbances such as fires and droughts play an important role in maintaining savannah environments. When these disturbances become too concentrated in a particular area the vegetation composition may be negatively affected. Excessive damage to the vegetation would result from exceeding the capacity of a protected area to provide food resources. The effect of the 120 elephants on the vegetation of Welgevonden Private Game Reserve, is not known. The rugged terrain of this reserve makes it a difficult, time consuming and labour intensive exercise to conduct ground studies. Satellite images can be used as a monitoring tool for vegetation change and improve the quantity and quality of environmental data to be collected significantly, allowing more informed management decision-making. This study evaluated the use of satellite imagery for monitoring elephant induced vegetation change on Welgevonden Private Game Reserve. The LANDSAT Thematic Mapper multispectral images, acquired at two yearly intervals from 1993 until 2007 were used. However, no suitable images were available for the years 1997, 2001 and 2003. A series of vegetation change maps was produced and the distribution of water sources and fire occurrences mapped. The areas of change were then correlated with the spatial distribution of water points and fire occurances, with uncorrelated areas of change. This was analysed using large animal population trends, weather data and management practices. On the visual comparison of the vegetation maps, it was seen that over this time period there was some decrease and thinning of woodland, but the most notable change was the increase of open woodland and decrease in grasslands. Using only the digital change detection for the period 1993 to 2007, a general increase in vegetation cover is seen. But this generalisation is misleading, since comparing the digital change detection to the vegetation maps indicates that while vegetation cover may have increased, significant changes occurred in the vegetation types. Most of the areas of significant change that were identified showed a strong positive correlation with burnt areas. The distribution of the water sources could not be directly linked to the vegetation change although rainfall fluctuations seemed to have accelerated vegetation changes. Years with high game counts, such as 1999, also coincide with very low rainfall making it difficult to differentiate between the effects of heavy utilisation of vegetation and low rainfall. Furthermore, many of the initial vegetation changes could be the result of land use changes due to the introduction of browsers, selective grazers and elephants that allow for more natural utilisation of the vegetation. Remote sensing makes it possible to successfully track changes in vegetation and identify areas of potential elephant induced vegetation change. Vegetation changes caused by disturbances, such as fire and anthropogenic activities, can be accounted for but it is not possible to conclude with a high level of certainty that the further changes seen are solely a result of elephant damage. Further work is required to reliably isolate elephant induced vegetation changes, as well as to establish the effects these changes have on the ecosystem as a whole. / Environmental Sciences / (M. Sc. (Environmetal Sciences))

Although herbivory and other types of plant damage typically are viewed as detrimental to plant survival and performance, vigorous regrowth, greater seed set, and fitness benefits may be possible when damage to the apical meristem, or actively growing stem terminal, is involved. Such damage releases apical dominance, or the hormonal suppression of lateral buds, activates dormant lateral buds, and enables lateral shoots to grow. Since in plants with terminal flowers, each stem may bear a flower, removal of the apical meristem may result in stem bifurcation and ultimately increase the number of flowers and seeds, thereby increasing potential fitness. In the current study, possible overcompensation in response to apical meristem damage caused by simulated herbivory (clipping) and the gall midge Asphondylia borrichiae Rossi and Strong (Diptera: Cecidomyiidae) (galling) was investigated in the native coastal halophyte, sea oxeye daisy Borrichia frutescens (L.) DC. (Asteraceae), in relation to nutrient supplementation. Results suggest a strong correlation between stem count and gall count at the study site; moreover, apical dominance was relatively weak early in the growing season and stronger in short plants that were shaded by taller neighbors later in the season. Results also indicate that overcompensation or even full compensation is an unlikely response to apical meristem damage in B. frutescens. Stem count was similar across all stem treatments, but increased significantly with nutrient supplementation, which all supports weak apical dominance in sea oxeye daisy. Nearly all measures of fitness also were either slightly or significantly lower when clipped and galled compared to plants with stems intact, while seed count responded positively to nutrient supplementation.