Thesis ( Ph.D. ) -- University of Texas at Arlington, 2008.

The effects on vegetation of three small mammalian herbivores were determined experimentally in a subalpine meadow in southwestern Colorado. Pikas (Ochotona princeps) inhabit talus and forage on surrounding vegetation. Their foraging was consistent with predictions of central place foraging theory. Amount of foraging decreased with distance from talus, whereas selectivity increased. These patterns are expected to influence the resources used by pikas, and plant abundance and species richness increased with distance from talus, as predicted. Experimental exclusion of pikas demonstrated that a significant portion of the lower vegetational cover and species richness was due to pikas. A model postulating that the effects of pikas on vegetation resulted from a gradient in herbivory, and a corresponding gradient in the relative importance of herbivory and competition among plants, was generally supported. Total vegetational abundance and species richness increased in the absence of pikas. The increase in pika exclosures compared to controls was greatest near talus and decreased with distance. The initial effect of pika exclusion on cushion plants (assumed to be the poorest competitors among the plants in the absence of herbivory) was positive near talus, but was insignificant and tended to be negative at greater distance from talus. The prediction of eventual decline of cushion plant populations in the absence of pikas was supported in two of three sites after three years. The effects on vegetation of two co-occurring herbivores, pocket gophers (Thomomys talpoides) and voles (Microtus montanus), were evaluated using a factorial exclusion experiment. Non-independence of effects was assessed as presence of a significant interaction term in 2-way ANOVA. Gopher x vole interaction terms were at least marginally significant for many analyses. Results only weakly suggested direct foraging interactions between gophers and voles, but showed considerable importance of indirect, plant-plant interactions. Exclusion of either gophers or voles significantly increased vegetational abundance, however, no additional change in abundance resulted when both were excluded simultaneously. Exclusion of either increased forb abundance, and exclusion of gophers increased grass abundance. Only simultaneous exclusion of both, however, altered proportional abundances of plant groups.

No description available.

Thesis (Ph. D.)--University of Florida, 1982. / Description based on print version record. Typescript. Vita. Includes bibliographical references (leaves 249-265).

No description available.

L’hypothèse selon laquelle l’accumulation des métaux lourds par les plantes a évolué comme mécanisme de défense contre les herbivores a été testée avec l’hyperaccumulatrice de zinc Thlaspi caerulescens (Brassicaceae). En utilisant l’écotype métallicole (poussant sur sols métallifères) et l’écotype non métallicole (sols normaux) de T. caerulescens, nos résultats ont conduit à rejeter l’hypothèse de défense par accumulation de métaux: les plantes ont été consommées indépendamment de leur concentration en Zn dans toutes les situations expérimentales examinées (conditions contrôlées, jardin expérimental, populations naturelles). Par contre, les herbivores ont montré une préférence systématique pour les plantes de l’écotype métallicole, quelle que soit leur concentration en Zn. Lorsque l’on mesure les concentrations en métabolites secondaires défensifs (glucosinolates) des écotypes métallicole et non métallicole de T. caerulescens, on constate que les individus d’origine métallicole produisent constitutivement moins de glucosinolates que les individus non métallicoles, tant dans les populations belges que dans les populations françaises. Par ailleurs, sur les sites métallifères où ont évolué les populations métallicoles, on constate à la fois une plus faible pression d’herbivorie sur les plantes (moins de dégâts) et une plus faible densité de gastéropodes que dans les sites normaux. La diminution des défenses chez l’écotype métallicole serait la conséquence d’un relâchement de la pression d’herbivorie sur les sites métallifères. En outre, nous avons montré que la chenille spécialiste d’Issoria lathonia (Nymphalidae) est capable de se développer sur les feuilles riches en Zn de l’accumulatrice de zinc Viola calaminaria (Violaceae) en excrétant efficacement le Zn dans leurs fèces. L’ensemble de nos résultats suggère donc que l’hyperaccumulation des métaux lourds n’a pas évolué en tant que mécanisme de défense contre les herbivores.

No description available.

Neste estudo foi avaliado o grau de proteção conferido à Chamaecrista debilis a partir das formigas visitantes dos seus nectários extraflorais (NEFs), que são estruturas não envolvidas diretamente na polinização e que representam uma importante fonte alimentar para vários grupos de artrópodes. Foram utilizados dois experimentos, no primeiro foram escolhidas e marcadas 20 plantas, sendo 10 destinadas ao controle, permanecendo em seu estado natural, e outras 10 denominadas tratamento, onde as formigas presentes foram retiradas, sendo aplicada uma resina na base do caule, afim de impedir o acesso das formigas à planta. Em cada planta de ambos os grupos, foram marcadas 6 folhas, contando¬se seus folíolos para acompanhar a herbivoria. No segundo experimento, foram escolhidas 6 plantas, sendo marcados dois ramos em cada uma, um destinado ao controle e outro destinado ao tratamento, onde foi aplicada a resina em sua base e as formigas foram excluídas. Foram marcadas 3 folhas em cada ramo, sendo contado o número de folíolos para acompanhar a herbivoria. No primeiro experimento não objetivou¬se determinar a defesa diferencial, dependendo das espécies das formigas associadas aos NEFs, enquanto que no segundo experimento foi avaliada especificamente a defesa conferida por Camponotus cingulatus. A fenologia foi acompanhada quinzenalmente, avaliando¬se a porcentagem das estruturas da plantas. Formigas, visitantes florais, herbívoros e outros insetos presentes na planta foram coletados. Foram encontradas 15 espécies de formigas associadas aos NEFs da planta, enquanto que os principais herbívoros foram os proscopídeos (Orthoptera). Quinze espécies de visitantes florais foram relatadas, e vários outros grupos estiveram associados à planta, especialmente em relação às galhas. Os resultados dos experimentos demonstraram que a presença das formigas determina uma redução significativa da herbivoria em Ch. debilis, nos dois experimentos, sendo que esta defesa amplia¬se com o passar do tempo. Todavia, este mutualismo parece ser facultativo, como a maioria destas relações envolvendo plantas com NEFs e organismos associados. / In this study was valued the protection degree provided to Chamaecrista debilis from ants visiting its extrafloral nectaries (EFNs), that are structures not involved directly to pollination and that are a important food resource to various arthropod groups. Were performes two experiments, in the first one were choosed and marked 20 plants, where 10 were control plants, remaining in to their natural conditions, and the other 10, labeled as treatement, where the ants presents were excluded and applied a resin at the basis of the stem, to prevent the access of the ants. In wich plant of both groups were marked 6 leaves, counting the number of folioles to value the herbivory rates. In the second experiment was choosen six plants, where were marked two branches, one destined to control and the other one to the treatment, where the resin was applied at the basis, to prevent the ants? access to these branches, from where the ants were excluded. Were marked 3 leaves in wich branch counting the number of folioles to value the herbivory rates. In the first experiment was not evalueted the differencial degree of protection, depending on the ants? species while in the second one was valueted specifically the defense from Camponotus cingulatus. The phenology was evalueted each 15 days, considering the porcentage of the plants? structures. Ants, flower visitors, herbivores and other insects presents on the plant were colected. Was found 15 ant species associated to EFNs, while the main herbivore were the Proscopiidae. Fifteen specis of flower visitors were recorded and another various groups were associeted to plant, specially to the galls. The results of the two experiments showed that the presence of ants provide a significative reduce to herbivory rates in Ch. debilis, and this defense grows along the time. However, this mutualism seems to be facultative, like the majotity of these relationships involving EFN plants and associated organisms.

Abstract not available

Thesis (Ph. D.)--University of Chicago, Dept. of Ecology and Evolution, August 1999. / Includes bibliographical references. Also available on the Internet.