No description available.

No description available.

No description available.

With an increase in the popularity of wildlife ranching in southern Africa has come the introduction of non-native (extralimital) mammalian herbivores. Financial gain has arguably been at the forefront of these introductions, with little or no assessment of the ecological consequences. The diet of three populations of introduced giraffe Giraffa camelopardalis was assessed by direct observation in the Eastern Cape Province, South Africa between January 2002 and October 2003, as the first step towards understanding the ecological role played by giraffe in the region. Similar to the diet of giraffe within their native range, a deciduous species from the genus Acacia (Acacia karroo) was the most important species in the diet. Giraffe in the Eastern Cape Province, however, consumed more evergreen species than those within their native range. The relative lack of deciduous species in the Eastern Cape Province provides a probable explanation for such a result. Seasonal variation in the consumption of the most important species in the diet was evident with members of the genus Rhus being more important in the winter months. This was attributed to the deciduous nature of A. karroo. The potential for giraffe to have a detrimental effect on the indigenous vegetation is discussed. We conclude that the study provides a much-needed list of plant species threatened by giraffe browsing in a region where the vegetation is thought to have evolved in the absence of such a browser.

No description available.

No description available.

No description available.

Theory predicts that, upon release from natural enemies in the new range, invasive species will evolve to allocate more resources towards traits that improve their ability to compete with native species. The motivation for most biological control programs is to re-introduce native enemies in order to oppose this effect and reduce or even reverse some of the negative impacts of invasive species on native and economically important ecosystems. In many cases, multiple biocontrol agents are introduced, often under the assumption that their impacts will be complementary. However, studies that attempt to quantify the nature of interactions among biocontrol agents tend to find unique outcomes: as yet, no general patterns have emerged. Here I describe the findings of a study of the impacts of the introduction of one species of biocontrol agent of invasive purple loosestrife, the leaf beetle Neogalerucella almariensis, on the reproductive success of a second biocontrol agent, the flower-feeding weevil Nanophyes marmoratus. Somewhat surprisingly, I found that more adult weevils tended to emerge from plants that were simultaneously infested with both types of beetles. This was true even when differences in flower number among plants were taken into account. Moreover, there were more aborted flowers on an inflorescence where both biocontrol agents were present. Finally, more pollinators visited the plants that were infested by both biocontrol agents. The observational data also show a positive correlation between the presence of the leaf beetle larvae and the number of adult flower weevils found on an inflorescence. I discuss various explanations, including the possibility that biocontrol-induced changes in flowering phenology and plant nutritional compounds could be, at least partially, responsible for these findings.

The dry weight of zooplankton is an important parameter conventionally used to estimate secondary production in aquatic ecosystems. Estimates of zooplankton weight vary considerably across studies. This study examines various factors that contribute to differences in the individual dry weight of freshwater herbivorous zooplankton. In the first chapter, I quantified and compared the individual length and weight changes of Daphnia catawba and Diaptomus minutus resulting from preservation in either 4% sugar-formalin, 70% ethanol solution or freezing over dry ice. The results indicate that the dry weight of both animals was significantly altered by chemical preservatives. The length of Diaptomus was also significantly reduced due to preservation. Site-specific differences in lakes, such as available food and the presence of predators, introduce another potential source of variation in herbivore dry weight. In Chapter 2, the food available to several herbivorous zooplankton in two softwater lakes was estimated using various methods. An index of "edible" phytoplankton biomass based on stomach content analyses was developed and compared to the traditional techniques used to estimate available food. The results indicate that the available was food different for each herbivore and that estimates of food available based on stomach content analyses were not significantly related to the traditional techniques used to estimate available food. In Chapter 3, I evaluate the relative impacts of "topdown" (predation) versus "bottom-up" (food) effects on the dry weight of several herbivores in two softwater lakes with contrasting food-web structures. The length-specific dry weights of Daphnia catawba, Diaptomus minutus and Holopedium gibberum varied seasonally and these changes were taxa-specific and unique to each lake. Herbivore weights were regressed against length, available food, clutch size, population density and temperature to determine if "bottom-up" effects could explain between-lake differences in herbivore weight. Length and population density were the prominent predictor variables in the resulting regression models for the herbivores in these lakes. Predator effects were determined by comparing if the between-lake differences in herbivore weight were consistent with the presence or absence of planktivores. Holopedium dry weight was lower in the presence of planktivores, while Diaptomus dry-weight differences exhibited no consistent trend with the presence or absence of planktivores. / Thesis / Master of Science (MS)

Au XIX ème siècle, la construction d'un réseau de digues autour du delta du Rhône a conduit à l'établissement de nouveaux milieux colonisés par de nouvelles espèces végétales et animales. Ces nouveaux milieux caractérisés, entre autre, par des marais temporaires ont vite été utilisés comme pâturage par le bétail et les chevaux durant la période estivale. Le Scirpe maritime (Scirpus maritimus L.) présent dans ces terres fertiles est alors devenu une des espèces végétales les plus appétantes pour ces herbivores qui en consomment les parties aériennes. Au début des années 80, les Oies cendrées (Anser anser anser) ont commencé à s'installer en quartier d'hivernage dans ces marais temporaires et leur ef'fectif' n'a cessé d'augmenter pour atteindre 1300 individus au cours de l'hiver 1999/2000. En se nourrissant des tubercules de Scirpe maritime, les oies occasionnent des déconnexions physiologiques dans les chaînes rhizomateuses que forment les parties souterraines de Scirpus maritimus. De telles déconnexions peuvent perturber l'allocation des ressources, le mode de reproduction et par conséquent la dynamique des scirpaies. Nous avons déterminé les tailles optimales de quadrat et de carotte pour échantillonner les parties aérienne et souterraine de Scirpus maritimus. Par la suite, nous avons élaboré des relations allométriques sur Scirpus maritimus. Un total de 115, 120 et 60 tiges a été échantillonné en 2002 et 2003 dans un marais brouté et pâturé par les oies et le bétail et en 2003 dans un marais non fréquenté par ces herbivores, respectivement. Ces relations nous ont permis d'estimer la biomasse aérienne du Scirpe maritime dans chacun de ces marais. Nous avons également déterminé la relation entre les biomasses aérienne et souterraine, Nous avons utilisé les précédentes données de biomasses et de densités (aériennes et souterraines) pour discerner si les plants de Scirpe allouaient différemment leurs ressources dépendamment que ceux-ci sont broutés et pâturés par les deux herbivores. Des données sur la salinité de l'eau, la durée de submersion et la densité d'oies en alimentation ont également été intégrées dans les analyses. Les quadrats de 15 x 15 cm et les carottes de 15 x 15 x 15 cm se sont avérés être les tailles optimales pour échantillonner les parties aérienne et souterraine. La hauteur et le diamètre ont expliqué plus de 95% de la variation de la biomasse des tiges mais les modèles allométriques différaient entre les années et entre les marais. Malgré une sous-estimation des relations allométriques sur la biomasse obtenue par l'échantillonnage destructif, les résultats sont encourageants dans le sens où ils nous permettent de voir qu'il est possible d'estimer de manière non destructive les biomasses aérienne et souterraine dans des milieux perturbés; la biomasse de tubercules et la biomasse souterraine totale ayant été prédites avec justesse par la biomasse aérienne. Même si la biomasse souterraine est deux fois plus importante dans le marais brouté, il semble que les oies et le bétail n'aient pas eu un impact important sur la structure et la dynamique des scirpaies étudiées. Il est possible que les ef'fets des herbivores aient été tamponnés par l'étroit niveau d'intégration physiologique des genets de scirpe. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Relations allométriques, Échantillonnage non destructif, Scirpus Maritimus, Herbivorie, Allocation des ressources, Plante clonalc, Biomasse, Scirpe maritime, Marais.