Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here I investigate how nitrogen availability and insect herbivory interact to alter above- and belowground plant community biomass in an old-field ecosystem. In 2004, 36 experimental plots were established in which soil nitrogen (N) availability (at three levels) was manipulated and insect abundance (at two levels) in a completely randomized plot design. In 2009, after six years of treatment, I measured aboveground biomass and assessed root production at peak growth. Overall, I found a significant effect of soil N availability on both above- and belowground plant biomass while insects affected only aboveground biomass of subdominant plant species and coarse root production; there were no statistical interactions between N availability and insect herbivory for any response variable. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be primarily limiting production in this ecosystem. Insect herbivory altered the aboveground biomass of the subdominant plant species and altered allocation patterns to coarse root production belowground. Overall, the results of six years of nutrient amendments and insect removals suggest strong bottom-up influences on total plant community productivity.

Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here I investigate how nitrogen availability and insect herbivory interact to alter above- and belowground plant community biomass in an old-field ecosystem. In 2004, 36 experimental plots were established in which soil nitrogen (N) availability (at three levels) was manipulated and insect abundance (at two levels) in a completely randomized plot design. In 2009, after six years of treatment, I measured aboveground biomass and assessed root production at peak growth. Overall, I found a significant effect of soil N availability on both above- and belowground plant biomass while insects affected only aboveground biomass of subdominant plant species and coarse root production; there were no statistical interactions between N availability and insect herbivory for any response variable. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be primarily limiting production in this ecosystem. Insect herbivory altered the aboveground biomass of the subdominant plant species and altered allocation patterns to coarse root production belowground. Overall, the results of six years of nutrient amendments and insect removals suggest strong bottom-up influences on total plant community productivity.

In Europe, fossil fruits and seeds of Rhodoleia (Hamamelidaceae) have been described from the Upper Cretaceous to the Miocene, whereas no fossil record of Rhodoleia has been reported in Asia, where the modern species occur. Herein, 21 fossil leaves identified as Rhodoleia tengchongensis sp. nov. are described from the Upper Pliocene of Tengchong County, Yunnan Province, Southwest China. The fossils exhibit elliptic lamina with entire margins, simple brochidodromous major secondary veins, mixed percurrent intercostal tertiary veins, and looped exterior tertiaries. The leaf cuticle is characterized by pentagonal or hexagonal cells, stellate multicellular trichomes, and paracytic stomata. The combination of leaf architecture and cuticular characteristics suggests that the fossil leaves should be classified into the genus Rhodoleia. The fossil distributions indicate that the genus Rhodoleia might originate from Central Europe, and that migrated to Asia prior to the Late Pliocene. Additionally, insect damage is investigated, and different types of damage, such as hole feeding, margin feeding, surface feeding, and galling, are observed on the thirteen fossil leaves. Based on the damage frequencies for the fossil and extant leaves, the specific feeding behavior of insects on Rhodoleia trees appears to have been established as early as the Late Pliocene. The high occurrence of Rhodoleia insect herbivory may attract the insect-foraging birds, thereby increasing the probability of pollination.

Insect herbivory on forest canopies strongly affects ecosystem nutrient cycling and availability in a variety of ways, e.g. by changing the quantity, quality and timing of nutrient input to forest soils. A qualitative method for measurements of ingestion, egestion and exuviation rates of the insect Chrysomela tremulae on leaves of the hybrid Populus tremula x tremuloides were tested in this study with the aim to detect differences in relative nutrient cycling efficiencies. The assimilation efficiency (AD), efficiency of conversion of digested food (ECD) and efficiency of conversion of ingested food (ECI) increased from 1st, through 2nd and 3rd instar larvae with generally higher efficiencies for nitrogen than carbon. Effects on nutrient limitations for the insect were also tested by increasing the C:N ratio of insect diet. A carbohydrate solution was painted onto leaves which resulted in a significant increase in C:N (p<0.0001). This lead to a trend of lengthened developmental time for each ontogenetic stage, as well as higher ingestion rate and lower egestion- and exuviation rates. However, a different method of increasing the C:N ratio is recommended in future experiments since the leaves never truly absorbed the solution.

Les herbivores consomment les tissus végétaux et affectent la survie et la croissance des végétaux. Les ennemis naturels des herbivores peuvent réguler leurs populations et indirectement diminuer les dommages sur la végétation. Les relations entre plantes, herbivores et prédateurs sont liées aux caractéristiques intrinsèques des espèces végétales mais aussi influencées par la communauté végétale environnante. L'hypothèse de résistance par association stipule qu'une communauté végétale diversifiée entraîne une diminution des dégâts sur une plante cible par dilution, répulsion des herbivores et/ou favorisation des ennemis naturels. Inversement, les herbivores peuvent se concentrer sur la plante-cible dans une communauté végétale diversifiée (susceptibilité par association). L'objectif de cette thèse est d'évaluer 1) l'influence de la composition de la communauté végétale sur les herbivores et leurs dommages sur une plante-cible, 2) l'impact de la régulation des herbivores par les prédateurs et 3) les interactions entre herbivores, communauté végétale et prédateurs, et les conséquences pour les intensités d'herbivorie sur la plante-cible. Nous nous sommes focalisés sur les dommages causés par les insectes herbivores sur des plantules d'essences forestières feuillues (Betula pendula, Quercus robur et Q. ilex). La variété importante de communautés végétales, au niveau inter (essences dominantes) et intraparcellaire (présence, structure et composition), nous a permis de tester les effets du voisinage végétal sur les insectes herbivores et leurs dommages sur les plantules cibles. Les oiseaux insectivores sont les principaux prédateurs d'insectes et nous avons cherché à estimer les effets de leur exclusion sur les insectes et les conséquences pour les plantules, et ceci le long des gradients de composition du voisinage végétal. Enfin, le degré de spécialisation (spécialiste/généraliste) et le mode de vie (exo/endophyte) ont été caractérisés car ils conditionnent les réponses des insectes herbivores à la végétation voisine ainsi que la prédation avienne. Le voisinage végétal des plantules-cibles modifie la charge en herbivores et les dommages engendrés. Ces réponses sont liées à la spécialisation de l'insecte herbivore : la colonisation des plantules par les espèces spécialistes (mineuses de feuilles) est négativement affectée par la structure de la végétation locale (présence, recouvrement arbustif). Les dommages d'insectes généralistes augmentent avec la richesse spécifique des peuplements et dans les parcelles dominées par des espèces conspécifiques. Les abondances des insectes externes et leurs dommages sont régulés par les oiseaux insectivores mais cet effet indirect dépend de l'espèce de plantule-cible et de la composition de la communauté végétale. Les effets de cascade trophique sont les plus intenses dans les parcelles de pin maritime où les plantules constituent une ressource nouvelle pour les niveaux trophiques supérieurs. Plus localement, le voisinage végétal modifie la sensibilité des arthropodes à la prédation: les oiseaux insectivores bénéficient aux plantules lorsque les proies sont plus accessibles (végétation environnante supprimée).Pour conclure, une des principales richesses de ces résultats vient de la mise en évidence d'interactions très fortes existant entre les effets des oiseaux insectivores et la végétation environnante. Les effets indirects des prédateurs et directs de la végétation environnante sont très liés au degré de spécialisation des insectes phytophages et à leur mode de vie. / Herbivory is thought to depend on intrinsic plant resistance traits and negatively affect survival and growth of plants. Predators can depress herbivore populations and thereby indirectly limit the consumption of primary producers. The surrounding vegetation is also expected to modify the interactions between a focal plant, its herbivores and their natural enemies. The surrounding vegetation may disrupt the colonisation and the consumption of a focal plant by insect herbivores, providing associational resistance. Increasing diversity of neighbouring vegetation can decrease resource availability or enhance herbivores control by predators. By contrast, associational susceptibility may occur with higher levels of herbivory in a focal plant in more diverse plant communities. We tested 1) the effect of the neighbouring vegetation on the levels of herbivory on seedlings, 2) the indirect impact of predators on herbivores and subsequent herbivory, and 3) the interactions between surrounding vegetation, focal plants, herbivores and predators. We examined the damage caused by insect herbivores on three native broadleaved species. We then compared the importance of taxonomic similarity between seedlings and canopy tree species and the structure and composition of surrounding vegetation at both large- and small-scales. Birds are vertebrate predators likely to limit damage to plants and facilitate plant growth by consuming herbivorous insects. We estimated their effects by excluding them from focal plants, and compared their interactive effects with vegetation diversity or removal. The effects of surrounding vegetation and avian predation were also disentangled on concealed- vs. external-feeding guilds (predation sensitivity) and specialist- vs. generalist-insects.Surrounding vegetation of focal seedlings influences the abundance of insect herbivores and damage. Specialist insects (leaf miners) are affected by the understorey vegetation close to oak seedlings, decreasing with its presence or structural diversity. The damage caused by generalist insects depend on the large-scale composition of plant communities, and increase with tree species richness and cover or on conspecific forest habitats. Exclusion of insectivorous birds affects insect herbivory in a species-specific manner, and also greatly varies with forest habitats and presence of vegetation around focal plants. The indirect effect of bird predation on leaf damage is observed on seedlings beneath noncongeneric canopy trees. The removal of local surrounding vegetation affects the top-down effect of insectivorous birds on insect herbivores, according to their sensitivity to predation and the accessibility of preys. To conclude, our studies provide experimental evidences of interactive effects between bird predation and neighbouring vegetation on insect herbivores and levels of herbivory. These indirect effects are strongly related to the specialisation of herbivores and to their sensitivity to bird predation.

Fragmentation of forest habitat by urban and rural development has had profound effects on the distribution and abundance of many native species; however, little is known about the ecological processes driving patterns in community structure (species richness and composition) of host-specialised herbivores in modified habitats. I examined patterns in community structure of 9 specialist and 19 generalist invertebrate herbivores of cabbage trees (Cordyline australis Laxmanniaceae) across a highly-modified landscape. I found that, although species richness of specialists was highest in forest sites, the majority of host-specialised species were not restricted to forest habitats and were as widespread as many generalists. In terms of site occupancy, only two specialist and four generalist species were rare. I show that patterns of species occupancy and abundance reflect differing susceptibility to habitat modification, with landscape-level variation an important predictor of abundance for nearly all species. When species occurrences and life history traits were considered I did not find strong evidence for the importance of dispersal ability, which suggests that habitat variability had a stronger organising effect on the community. In a replicated common garden experiment, I found distinct regional patterns in the community structure of the specialist invertebrates occurring on different phylogenetic groups of C. australis. In contrast, community structure of generalist herbivores did not differ significantly among host genotypes. I speculate these patterns are due to historical changes in the distribution of cabbage trees in the Southern phylogenetic region that caused specialised herbivores to become locally adapted on populations of low genetic diversity following expansion after the last glacial maximum. However, this consistent selection pressure did not occur in the Northern region where C. australis habitat has been more consistently available over the past tens of thousands of years, reflected in higher host genetic diversity. This study has advanced our understanding of the patterns in community structure of an indigenous, host-specialised fauna in a highly modified and fragmented urban and rural landscapes.