Invasive species are significant drivers of global environmental change, altering the stability and functioning of numerous ecosystems. The exotic shrub Lonicera maackii is an aggressive invader throughout much of the eastern United States. While much is known about its population and community impacts, little is known about effects on ecosystem processes. This dissertation documents changes in ecosystem processes associated with L. maackii growing beneath three native tree species (Fraxinus quadrangulata, Quercus muehlenbergii, Carya ovata) in a savanna in Kentucky. Like many invasive plants, L. maackii litter decomposed and lost nitrogen (N) rapidly, especially in comparison with native tree litter. In comparison to the soils beneath the trees where the exotic shrub was absent, soils beneath L. maackii had a lower bulk density, elevated soil organic matter, C:N, and total soil N and a modified soil microbial community. Inorganic N deposition from spring throughfall was also altered by L. maackii, with higher NO3-N deposition beneath shrubs located beneath the tree canopy relative to canopy locations without L. maackii. While many exotic plant species have been shown to alter ecosystem processes, their impact is often not uniform. This variability is attributed to among-site differences (soil, climate, plant community): within site variability is often ignored. While many of L. maackii’s alterations to ecosystem processes were uniform across the site, several were dependent upon interactions between the exotic and the native tree species. Litter from L. maackii decomposed and lost N more rapidly under C. ovata than under the other native tree species. Soils beneath L. maackii shrubs located under C. ovata also had a greater fungal:bacterial ratio and a greater abundance of the saprophytic fungal lipid biomarker 18:1ω9c. These results demonstrate that L. maackii’s impact extends to ecosystem processes and suggests that invasive plants may have variable effects within a given environment depending on their interactions with the dominant native species. Identifying native species or communities that are more vulnerable to alterations of ecosystem function upon invasion may prove useful to land managers and foster a better understanding of the role that community dynamics play in moderating or enhancing invasive species impacts.

<p>Species invasions are more prevalent than ever before. While the addition of a species can dramatically change critical ecosystem processes, factors that mediate the direction and magnitude of those impacts have received less attention. A better understanding of the factors that mediate invasion impacts on ecosystem functioning is needed in order to target which exotic species will be most harmful and which systems are most vulnerable. The role of invasion on nitrogen (N) cycling is particularly important since N cycling controls ecosystem services that provision human health, e.g. nutrient retention and water quality.</p><p>We conducted a meta-analysis and in-depth studies focused on the invasive grass species, Microstegium vimineum, to better understand how (i) plant characteristics, (ii) invader abundance and neighbor identity, and (iii) environmental conditions mediate the impacts of invasion on N pools and fluxes. The results of our global meta-analysis support the concept that invasive species and reference community traits such as leaf %N and leaf C:N are useful for understanding invasion impacts on soil N cycling, but that trait dissimilarities between invaded and reference communities are most informative. Regarding the in-depth studies of Microstegium, we did not find evidence to suggest that invasion increases net nitrification as other studies have shown. Instead, we found that an interaction between its abundance and the neighboring plant identify were important for determining soil nitrate concentrations and net nitrification rates in the greenhouse. In field, we found that variability in environmental conditions mediated the impact of Microstegium invasion on soil N pools and fluxes, primarily net ammonification, between sites through direct, indirect, and interactive pathways. Notably, we detected a scenario in which forest openness has a negative direct effect and indirect positive effect on ammonification in sites with high soil moisture and organic matter. Collectively, our findings suggest that dissimilarity in plant community traits, neighbor identity, and environmental conditions can be important drivers of invasion impacts on ecosystem N cycling and should be considered when evaluating the ecosystem impacts of invasive species across heterogeneous landscapes.</p> / Dissertation

Tsuga canadensis (L.) Carr. (eastern hemlock), a long lived and shade tolerant coniferous tree species native to eastern North America, is currently threatened by the hemlock woolly adelgid (Adelges tsugae Annand, HWA). This exotic, invasive insect poses a serious threat to T. canadensis stands throughout their native range. The loss of this unique tree species is often coupled with numerous ecological consequences. HWA-induced mortality has exceeded 95% in some forest types, and will likely continue to decimate T. canadensis populations in the coming years. Shortly after HWA was found in Great Smoky Mountains National Park (GSMNP), long-term monitoring plots were established across a gradient of T. canadensis associated forest communities to monitor HWA impacts and stand decline. Our objectives were to (1) determine how T. canadensis populations have changed in conjunction with HWA infestation, and (2) document how HWA has affected vegetative community dynamics following five to six years of infestation in GSMNP. We assessed T. canadensis associated overstory communities and determined that crown density, top dieback, and T. canadensis stem density has significantly declined over the observational period. Furthermore, we found that midstory T. canadensis trees were experiencing greater rates of decline than those of the overstory. Even with these losses, overstory and midstory community composition did not significantly change from 2003 to 2008/09. Many studies have documented rapid pulses of understory vegetative productivity in conjunction with T. canadensis defoliation, which often allows increased amounts of solar radiation to reach the forest floor. Although we observed significant T. canadensis mortality at most plots sampled, understory species composition did not significantly change following HWA infestation. We attribute this lack of species response to the overwhelming abundance of Rhododendron maximum in the mid/understory, which restricts increased light from reaching the forest floor despite the formation of small overstory gaps. However, we found that understory composition significantly varied among ecological groupings, and differences were largely associated with soil properties and elevation. Overall, results of this project suggest that T. canadensis may largely disappear from southern Appalachian forests resulting from infestations of HWA. Although community composition of T. canadensis associated stands has not yet been altered, changes are likely to occur as duration of infestation progresses.

Made available in DSpace on 2014-06-11T19:26:42Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-28Bitstream added on 2014-06-13T18:55:01Z : No. of bitstreams: 1 casimiro_acr_me_botib.pdf: 201221 bytes, checksum: c6760921b16474071278a9ee89b7359f (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O Reservatório da UHE Escola Mackenzie (Represa de Capivara) Rio Paranapanema foi constituído em 1977 e representa o maior reservatório em extensão e volume ocupado na bacia hidrográfica do Rio Paranapanema e nas primeiras décadas (80 e 90) após sua formação foi palco de constantes introduções acidentais e intencionais dos mais diversos organismos, sendo uma destas espécies a Cichla monoculus. Neste contexto, diversos pesquisadores concluíram que espécies do gênero Cichla representam importante impacto à bacia do alto Rio Paraná onde foi introduzida, o que originou a presente proposta desse estudo. O objetivo central desse projeto foi realizar uma análise do processo de introdução da espécie Cichla monoculus no reservatório de Capivara, como forma de avaliar biologicamente se a mesma pode ser considerada como uma invasora ―latu senso‖. Foram compilados dados de captura dos anos de 1991 a 1999 (relativos ao projeto Tibagi, trecho reservatório). E as coletas relativas a este estudo foram realizadas trimestralmente correspondendo ao período de outono de 2001 a verão de 2002, outono de 2002 a verão de 2003 e outono de 2009 a verão de 2010 finalizando 3 ciclos (I, II e III, respectivamente). Os resultados nos possibilitaram verificar que após 3 anos da primeira introdução da espécie (1997 a 1998), a mesma já alcançou proporções de abundancia equivalente as nativas e com acréscimo populacional subseqüente até 2003, com forte indicativo de um processo de estabelecimento e conseqüente invasão no reservatório, com modesta depleção durante o ultimo ciclo de estudo (Ciclo III). Os dados de densidade (CPUEn) demonstram uma tendência ao decréscimo e o comprimento padrão médio não apresentou alterações significativas. A proporção sexual apresentou grande... / The UHE Escola Mackenzie (Capivara Reservoir) Paranapanema River, was build in 1977 and nowadays is de biggest reservoir in extension and volume in the Paranapanema River Basin. In the first two decades after your construction occurred several introduction (accidental or on purpose) of none native fishes in this area. One of this species was Cichla monoculus. In this context, several researches concluded that species belonging to the Cichla gender cause important impact to the Parana River Basin where it has been introduced. The idea of this project came from these observations. The aim of this study was to analyze the introduction process of the species Cichla monoculus in the Capivara reservoir and to evaluate biologically if this species is established or not in the reservoir. Data from an old project realized in the same area was used covering a period from 1991 to 1999 (Tibagi Project). The samples related to this study were spread in three different cycles (I, II and III): autumn 2001 to summer 2002, autumn 2002 to summer 2003 and autumn 2009 to summer 2010, respectively. The data shown that three years after the C. monoculus accidental escaping the species reach abundance proportions equivalent to de native species and further increase to 2003, indicating a establishment process and consequent invasion of the reservoir system, with small depletion during the last cycle (Cycle III). The CPUEn shown a decrease tendencies and the standard length did not present significant alterations. The sexual proportion presents variations and was attributed to the environmental factors. During the study, juveniles were sampled mainly (86.98% Cycle I; 81.81% Cycle II e 83.84% Cycle III) indicating a recruitment success. More than 55% of the specimens collected were between 6 to 11 centimeters of standard... (Complete abstract click electronic access below)

Twenty-first-century globalization has led to an extraordinary rise in international trade and transit. Consequentially, invertebrates, plants, and mammals are displaced more frequently, which has catalyzed a historic rise in biological invasions. The Joro Spider (Araneae: Trichonephila clavata) recently established from Asia in a landlocked region of southern Appalachia. Its range continues to expand; its cold tolerance is expected to favor northward invasion. As a large-bodied orbweaver that forms extensive webs and aggregations, the Joro spider is likely capable of inducing fundamental change to community structure via spatial competition. A valuable first step in estimating any invader’s economic or biological impact is to hypothesize regions susceptible to invasion using species distribution models. Recent work also shows that comparing global and regional distribution models yields insight into different stages of invasion. To examine potential spread and niche utilization differences in the Joro spider, one global and two regional models were developed. Maximum Entropy models were trained using open-source citizen science occurrence data and six bioclimatic variables at 2.5-arcminute resolution. An AUC-weighted ensemble model was used to produce each of the 3 global suitability projections. To compare invasive stage differences, projections were then translated to presence-absence maps using a 50% suitability threshold. The Asia-regional model predicts widespread suitability in eastern North America. However, the US-regional model reflects local adaptation to a climate niche that does not occur in the spider’s historic Asian range. Permutation feature importance shows the US-regional model was driven mainly by precipitation seasonality (64%) and annual oscillations in daily temperature range (29.1%). The Asia-regional model was instead driven by mean temperature of the driest quarter (34.9%), maximum temperature of the warmest month (23.6%), and precipitation of the warmest quarter (20.1%). The introduced Joro spider has invaded a North American niche that it is naïve to, but which co-occurs spatially with a niche akin to its historic Asian niche. If the Asia-regional climatic niche is indeed exploitable in North America, then conservative estimates show the bounds of range suitability should approach the 95th meridian and the 28th and 50th parallels. A total of 1,231,711 km2 within North America was predicted above 50% suitability. Altogether, these findings suggest niche versatility and plentiful suitable habitat favors successful North American invasion by the Joro spider.

Les méthodes traditionnelles de gestion des renouées asiatiques (Fallopia spp.), plantes invasives parmi les plus menaçantes au monde, ont des résultats mitigés et coûteux. Certains gestionnaires s'orientent donc vers des solutions alternatives comme le génie végétal. Ordinairement utilisé contre l'érosion des berges de cours d'eau, il permet également d'installer un couvert végétal rapidement et durablement. Un des enjeux pour le contrôle des renouées asiatiques est donc d'optimiser ces méthodes afin de reconstituer des communautés végétales capables de les réguler. Ce travail de thèse tente de répondre à cet enjeu en apportant une meilleure compréhension du rôle des interactions biotiques dans la régulation des renouées asiatiques.Dans ce projet, nous nous sommes intéressés aux populations déjà installées de renouées asiatiques et donc aux facteurs qui interviennent en tant que régulateur de leur performance. Les renouées asiatiques étant plus rares en forêts structurées et étant connues comme des espèces pionnières dans leur aire d'origine, l'hypothèse est que la compétition, en particulier pour la lumière, joue un rôle central dans la régulation de leur performance. L'objectif du travail de thèse a été de vérifier cette hypothèse, en évaluant le rôle de la lumière dans la performance des renouées asiatiques (chapitre 1), en étudiant le rôle des interactions biotiques de type compétition (chapitre 2) et en évaluant l'effet inhibiteur de la renouée du Japon sur des boutures de Salicaceae en fonction de la lumière disponible (chapitre 3). Le premier chapitre met en évidence, par des mesures in situ, l'importance de la quantité de lumière sur la production de biomasse aérienne des renouées asiatiques, comparativement à d'autres facteurs liés aux perturbations ou aux conditions édaphiques (partie 1). Une expérimentation en conditions semi-contrôlées dans laquelle des renouées asiatiques ont été cultivées et soumises à un gradient de lumière complète ces résultats (partie 2). Elle met en évidence leur plasticité phénotypique ainsi que l'existence de différents seuils de lumière pour la production de biomasse, l'allocation des ressources ou leurs effets compétitifs.Le deuxième chapitre s'intéresse plus spécifiquement aux interactions compétitives et en particulier à la compétition pour la lumière. La première partie démontre, par l'évaluation in situ de la performance des renouées asiatiques le long de gradients environnementaux, le rôle prépondérant de la structure fonctionnelle de la communauté végétale. La deuxième partie s'appuie sur une expérimentation en microcosme et montre les effets compétitifs respectifs du saule des vanniers (espèce prometteuse pour le contrôle des renouées asiatiques) et de la renouée du Japon. Enfin, le dernier chapitre de la thèse s'intéresse plus spécifiquement aux effets allélopathiques de la renouée du Japon sur des boutures de différentes espèces de Salicaceae. Une première analyse met en évidence des effets phytotoxiques différentiels entre les espèces de Salicaceae (partie 1). La deuxième analyse répète cette expérimentation mais dans des conditions de lumières limitées, mettant ainsi en évidence les compromis de ressources chez la renouée du Japon qui privilégie l'acquisition de la lumière au détriment de la production de composés inhibiteurs.Cette thèse, située à l'interface de l'écologie des invasions, de l'écologie des communautés et de l'écologie fonctionnelle, est centrée sur un complexe d'espèces hautement invasives, les renouées asiatiques. Elle propose des éclairages sur leur écologie permettant une meilleure compréhension de ses capacités invasives dans la perspective d'améliorer leur gestion. / Traditional methods for the management of Asian knotweeds (Fallopia spp.), among the worst invasive plants worldwide, are fastidious and expensive. Managers try to develop alternative technics such as bioengineering. Habitually used for riverbank protection against erosion, bioengineering also promotes the rapid recovery of plant communities. One issue for the control of Asian knotweeds is to improve such methods in order to restore competitive plant communities able to regulate the invasive populations. In this context, this work aims at better understanding the role of biotic interactions in the regulation of Asian knotweed populations. In this PhD project, we focused on Asian knotweed populations already established, trying to highlight factors that control their performance. Asian knotweeds are rare in mature and structured forests and are known as pioneer species in their native range. Our hypothesis is that competition and particularly competition for light, has a central role in the control of their performance. The objective of this work was to test this hypothesis through i) the evaluation of the role of light in the performance of Asian knotweeds (chapter 1), ii) the study of the role of competitive interactions (chapter 2) and iii) the investigation on the inhibitory effect of Asian knotweed on Salicaceae cuttings in different light conditions (chapter 3). The first chapter highlights, through field measurements, the importance of light quantity filtered by plant canopy relatively to other factors like disturbance and soil resources on aerial biomass production (part 1). Another experiment in semi-controlled conditions, where Asian knotweeds were grown under a light gradient, shows their phenotypic plasticity and puts forward different light thresholds considering biomass production, resource allocation or capacity to filter light.The second chapter focuses more specifically on biotic interactions like competition for light. The first part demonstrates, through the field evaluation of knotweed performance along environmental gradients, the major role of the functional structure of plant community. The second part studies more precisely the respective competitive effects between Salix viminalis (a good candidate for the control of Asian knotweeds) and Asian knotweed. Finally, the last chapter deals with allelopathic effect of Asian knotweed on cuttings of different Salicaceae species. A first analysis shows differential inhibitory effects between Salicaceae species (part 1). A second analysis repeats the first one in different light conditions and highlights resource trade-offs in Asian knotweed which favors light acquisition to secondary compound production. This PhD thesis, at the crossing point of invasion ecology, community ecology and functional ecology, is focused on the highly invasive Asian knotweeds. This work proposes new insights on their ecology allowing a better understanding of their invasive capacities in order to improve their management.

Afin de quantifier les risques d’invasion chez le goujon Asiatique Pseudorasbora parva, un petit cyprinidae d’eau douce, j’ai tout d’abord testé une approche corrélative de modélisation de la niche climatique dans laquelle j’ai intégré un proxy relatif à la probabilité d’introduction. Cette approche s’appuie sur des assomptions relatives à l’adaptation des organismes aux conditions environnementales locales ou régionales. J’ai ensuite comparé la niche climatique mesurée de deux lignées génétiques majeures à la fois dans les aires natives et envahies. Puis dans un second temps, par une approche expérimentale j’ai comparé la réponse aux variations de température, des traits d’histoire de vie de populations vivant dans des zones climatiques contrastées. Finalement, je me suis intéressé à la dispersion des goujons Asiatiques afin de mieux comprendre si les populations situées sur les fronts d’invasions avaient des capacités de colonisation plus importante. J’ai donc quantifié et comparé le long d’un gradient d’invasion, le potentiel de dispersion des individus ainsi que des traits pouvant y être liés (activité et morphologie). L’étude des risques d’invasions a montré que de nombreuses zones – au delà des zones déjà envahies - étaient climatiquement favorables au goujon Asiatique. C’était le cas en particulier pour certaines zones comme l’Australie, l’Amérique du Sud et du Nord, indiquant que l’invasion de cette espèce pourrait encore s’étendre. Après avoir pris en compte les probabilités d’introduction, cette étude a montré que l’Amérique du Nord était la zone la plus à risques. Par contre, la niche climatique entre les différentes lignées dans les zones géographiques envahies est très similaire, ce qui laisse à penser qu’il n’y a pas de patrons d’adaptation locale chez cette espèce. Pour autant, la niche climatique observée dans la zone envahie est très différente de celle observée dans la zone native, ce qui suggère un shift climatique important au cours de l’invasion. Les réponses thermiques des traits d’histoire de vie du goujon Asiatique testés expérimentalement n’ont pas varié significativement entre les populations originaires de conditions climatiques continentales et maritime-tempérées. Par exemple, l’effort reproductif global des femelles n’a pas varié entre les températures testées (15-25°C) mais la stratégie temporelle de reproduction a beaucoup varié. L’effort reproductif était plus cours et plus intense à forte température, alors qu’il était plus étalé et avec des pics reproductifs moins forts à faible température. Pour ailleurs, il semble qu’il existe un gradient morphologique fort entre les populations situées à différentes distances du front d’invasion ce qui suggère une forte plasticité morphologique mais qui ne serait pas liée à la capacité de dispersion de ces populations. En effet, cette dernière ne variant pas significativement le long du gradient d’invasion. La capacité de dispersion serait principalement liée à la taille du corps de goujon Asiatique, les individus les plus grands ayant une probabilité plus élevée de disperser.Bien que les prédictions générales du modèle de niche puissent être affectées par de potentielles adaptations à l’échelle de la population ou de la lignée évolutive, les résultats suggèrent qu’une certaine incertitude liée à ces prédictions persiste puisque la distribution native ne prédisait que très mal la distribution actuelle dans les zones envahies. Par ailleurs, mes travaux expérimentaux à plus fine échelle suggèrent que cette espèce est extrêmement adaptable et tolère une large gamme environnementale, ce qui pourrait expliquer son caractère invasif. Les connaissances produites au cours de cette thèse constituent donc des ressources extrêmement pertinentes pour développer des stratégies de gestion visant à contrôler les invasions futures du goujon Asiatique. / In this thesis I set out to quantify the risk of invasion from the invasive freshwater fish, Pseudorasbora parva, at a global extent, using traditional correlative ecological niche modelling approaches with the integration of surrogate data representing introduction likelihood (Chapter I). These correlative approaches rely upon key assumptions relating to the presence or absence of local or regional adaptations, and so I subsequently tested for evidence of such adaptations in genetic lineages and in individual populations. This was achieved through analyzing climatic niche differentiation of key genetic lineages in the native and invasive ranges (Chapter II) and by conducting lab experiments comparing thermal responses of important life history traits in populations from contrasting climates (Chapter III). The initial risk assessment did not account for a key factor in invasions; namely, natural dispersal. Natural dispersal has been observed to be subject to selection in vanguard populations of invasive species, and adaptation of dispersal traits can infer additional invasive vigor, allowing the species to spread across the landscape quicker. For this reason, I quantified dispersal, activity and morphological differences, often associated with differential dispersal ability, in populations along a distance-gradient from an invasion front, in order to identify if P. parva is capable of such adaptations.The initial risk mapping study showed that large areas, beyond the current distribution of the species, are climatically suitable. These areas are mainly in North and South America, Australia and New Zealand, and constitute significant scope for spread and impact of this species. When introduction likelihood was included, N. America appears most at risk. I found no evidence to suggest that native genetic lineages represented local adaptations to their respective native climates - there was little or no differentiation of the lineages’ climatic niches in the invasive range. It was also apparent, from the niche comparisons, that the climatic niche in the invaded range constituted a significant shift, compared to the native range. The thermal responses of P. parva life history traits did not differ significantly between populations from a strongly seasonal continental climate and a mild temperate maritime climate. The overall reproductive output of females did not vary according to breeding season temperature, however, temporal reproductive strategy showed a strong response, with lower temperatures inducing a protracted breeding season and higher temperatures inducing rapid and intense reproductive output. The dispersal and morphology-related study identified a strong gradient of morphological change, corresponding with distance from invasion front. This demonstrates a high degree of plasticity in P. parva’s morphology in an invasion context, however this was not linked to either dispersal or activity levels, neither of which was significantly linked to distance from invasion front. Dispersal was best explained by body size, with larger fish more likely to disperse further.Whilst I found no evidence to suggest that the model predictions (Chapter I) were hampered by differentiation at either lineage or population levels, the findings of Chapter II do highlight the uncertainties surrounding the degree of conservatism in such predictions, mainly owing to the fact that past, native, distribution did not accurately predict the current invaded distribution. The results of Chapters II-IV show broad tolerances and great plasticity in P. parva, which likely underpin this species success as a pan-continental invader. The knowledge produced in this thesis provides a useful new resource for the development of management strategies for P. parva and could be usefully enhanced by the additional of analogous studies on native populations, which could help elucidate the source of the observed plasticity.

Understanding what promotes invasiveness of species outside their native range and predicting which ecosystems and under which conditions will be invaded is an ultimate goal of the field of invasion ecology. Obtaining general answers to these questions requires synthesis of extensive yet heterogeneous empirical evidence, coupled with a solid theoretical background. In this dissertation, I sought to provide insight into the drivers of non-native plant invasions through combining and synthesizing ecological data from various sources using advanced statistical techniques. The results of this work are presented as three independent research studies. In the first study, I aimed to understand what determines competitive advantage of non-native over native plants: the ability to suppress other plants, tolerate them, or both. For this, I collected data from 192 studies on plant competition and analyzed them within a Bayesian multilevel meta-analytic framework. I showed that non-native plants outperform their native counterparts due to the high tolerance of competition, as opposed to strong suppressive ability. Competitive tolerance ability of non-native plants was driven by neighbor’s origin and was expressed in response to native species and not to other non-native species. This synthesis demonstrates that non-native plants are competitively distinct from native plants and challenges the common notion that neighbor suppression is the primary strategy for plant invasion success. In the second study, I quantified the extent to which regional, landscape and local environmental factors individually and jointly affect understory non-native invasive plants across northern US forests. I used boosted regression trees and Bayesian nonlinear regressions to analyze forest inventory data spanning 14 northern US states in combination with data on climate, land use, and disturbance. Regionally, the highest level of plant invasion was observed in hotter regions with lower annual precipitation and climate seasonality and higher summer precipitation. Locally, young forests with moist to wet soils and relatively flat topography in open, human-altered landscapes at low elevation were most susceptible to invasion. Climate and land use strongly interacted in their effect on plant invasions. This study refines the understanding of the non-native plant invasion process in northern US forests and the obtained models can be used to generate predictions under current and future environmental regimes to inform management. In the third study, I tested the relationship between the long-term history of recurrent canopy disturbance by a non-native invasive defoliator, the gypsy moth (Lymantria dispar), and the level of non-native plant invasion in northeastern US forests. I reconstructed 46 years (1970–2015) of gypsy-moth defoliation history and quantified the cumulative effect of defoliation on understory non-native invasive plant species using multivariate techniques and Bayesian nonlinear regressions. Contrary to what is commonly expected, the cumulative severity of gypsy moth defoliation tended to be negatively associated with the presence and richness of invasive plant species, although this association was weak. This study suggests that the effect of biotic disturbance on forest plant invasions may vary in both the magnitude and direction depending on characteristics of disturbance regime and its effect on resident biota, and this needs to be explicitly taken into account when predicting future plant invasions.

In this study, I examined the effect of watershed urbanization on the invasion of alien woody species in riparian forests. This study was conducted in three major steps: 1) estimating the degree of watershed urbanization using impervious surface maps extracted from remote sensing images; 2) examining the effect of urbanization on hydrologic regime; and 3) investigating a relationship between watershed urbanization and ecosystem invasibility of a riparian forest. I studied twelve riparian forests along urban-rural gradients in Austin, Texas. Hydrologic regimes were quantified by transfer function (TF) models using four-year daily rainfall-streamflow data in two study periods (10/1988-09/1992 and 10/2004-09/2008) between which Austin had experienced rapid urbanization. For each study period, an impervious surface map was generated from Landsat TM image by a support vector machine (SVM) with pairwise coupling. SVM more accurately estimated impervious surface than other subpixel mapping methods. Ecosystem invasibilities were assessed by relative alien cover (RAC) of riparian woody species communities. The results showed that the effects of urbanization differ by hydrogeologic conditions. Of the study watersheds, seven located in a hillslope region experienced the diminishing peakflows between the two study periods, which are contrary to current urban hydrologic model. I attributed the decreased peakflows to land grading that transformed a hillslope into a stair-stepped landscape. In the rest of the watersheds, peakflow diminished between the two study periods perhaps due to the decrease in stormwater infiltration and groundwater pumpage that lowered groundwater level. In both types of watersheds, streamflow rising during a storm event more quickly receded as watershed became more urbanized. This study found a positive relationship between RAC and watershed impervious surface percentage. RAC was also significantly related to flow recession and canopy gap percentages, both of which are indicators of hydrologic disturbance. These results suggest that urbanization facilitated the invasion of alien species in riparian forests by intensifying hydrologic disturbance. The effects of urbanization on ecosystems are complex and vary by local hydrologeologic conditions. These results imply that protection of urban ecosystems should be based on a comprehensive and large-scale management plan.

The invasive cactus moth, Cactoblastis cactorum, poses a threat to opunitoid cacti species of North America. The following work contains four separate studies investigating C. cactorum host plant preference and performance, predation and parastitism of C. cactorum, effects of C. cactorum on local Opuntia populations, and associational effects of host and non-host plants on C. cactorum and native Opuntia-feeding herbivores. We found that, among southwestern and Mexican opuntioid taxa, moths preferred O. engelmannii var. linguiformis and var. engelmannii for oviposition, while Consolea rubescens and O. streptacantha were superior larval hosts. Oviposition was best predicted by number of cladodes and degree of spininess; epidermal toughness was a significant predictor of most larval fitness parameters. In general, oviposition preference was not correlated with larval performance. A lack of co-evolutionary history between C. cactorum and North American opuntioid species may help explain this disconnect. We placed irradiated C. cactorum eggsticks and pupae on Opuntia plants in the field to test for predation. We found evidence of predation, most likely from ants, on ~16% of eggsticks and ~18% of pupae. Predation rates, ant abundance, and cladode growth were higher, and C. cactorum damage lower, on Opuntia located near the extrafloral nectar-producing legume Chamaecrista fasciculata. We attribute these associational effects to the ability of C. fasciculata to attract ants to its extrafloral nectar. Over the course of six years, ~78% of nearly 600 tagged Opuntia were attacked by C. cactorum at least once and ~76% of the plants survived. Two separate studies found that Opuntia stricta was more likely to be attacked by C. cactorum than O. humifusa; we also found that O. stricta was more likely to die following an attack. A plant's odds of survival decreased as C. cactorum attack frequency increased, but plants that did survive had positive growth rates, regardless of attack status. We did not find significant evidence of associational effects for O. humifusa and O. stricta, either for C. cactorum or native Opuntia-feeding herbivores. It could be that present herbivore densities are low enough, and host plants plentiful enough, to avoid mechanisms that usually lead to associational resistance or susceptibility. Overall, our results suggest that the presence and spread of C. cactorum should be taken seriously, especially for rare opuntioids and the Opuntia-rich deserts of North America. However, for more common opuntioid host species, there may be enough resistant or tolerant individuals, and sufficient top down control through ant predation, for populations to persist at current C. cactorum densities. We acknowledge that information on Opuntia reproduction and recruitment rates are needed to confirm this suggestion, and see this as an excellent opportunity for future research