159 research outputs found
Classical biological control: exploiting enemy escape to manage plant invasions
Practitioners of classical biological control of invasive weeds are confronted with a dual expectation: to achieve successful control of plant invaders and to avoid damage to nontarget plants and adverse indirect effects. In this paper we discuss key issues that we consider to be crucial for a safe, efficient, and successful classical biological control project, and that have also caused some recent controversy. These include selection of effective control agents, host specificity of the biological control agents, implications of the genetic population structure of the target populations, and potential impact on native food webs. With regard to improving the success rate of biological control of plant invaders, we first emphasize the importance of a clear a priori definition of success and a more ecosystem-based approach to better document both negative effects of the invasive plant as well as potential positive and negative effects of introducing biological control agents. Secondly, pre-release impact assessment could be improved by better focusing on how to reach high densities of the control agents and by including tolerance to and compensation of herbivory. Thirdly, we advocate a reinforced effort to integrate and combine biological control in combination with existing or potential management options. Finally, we propose various ecological and evolutionary hypotheses in the framework of our topic to document that biological control programmes against plant invaders also offer a great opportunity to gain new insights into basic processes in ecology and evolution
The Relation Between Unpalatable Species, Nutrients and Plant Species Richness in Swiss Montane Pastures
In agriculturally marginal areas, the control of unpalatable weeds on species rich pastures may become problematic due to agricultural and socio-economic developments. It is unclear how increased dominance of unpalatable species would affect the botanical diversity of these grasslands. We investigated whether there was any relationship between plant species diversity and the abundance of unpalatable species and whether soil conditions affected this relationship. In three species-rich montane pastures in western Switzerland, we related plant species richness to soil attributes, the relative cover of all unpalatable species and the relative cover of the locally dominant, toxic Veratrum album in 25 plots of 4m2. We furthermore determined species richness in small transects through patches of V. album. Species richness was significantly lower in and near (†0.3m) patches of V. album. At the field scale, plant species richness was best described by total soil N:P ratio (positive relation) in one site and the relative abundance of unpalatable species (negative relation) and soil N:P ratio (positive relation) in a second site. In the third site, species richness was not significantly related to any measured variable. Vegetation diversity (Simpson's D) was negatively related to the relative abundance of unpalatable species in one site and positively related to pH in another site. The results suggest that no single factor can explain plant species richness and diversity in montane pastures. At very high densities unpalatable species can have adverse effects but soil nutrient status appears to be a more general determinant of plant species richness. Conservation efforts should give priority to the prevention of intensification of these pasture
Classical biological control: exploiting enemy escape to manage plant invasions
Practitioners of classical biological control of invasive weeds are confronted with a dual expectation: to achieve successful control of plant invaders and to avoid damage to nontarget plants and adverse indirect effects. In this paper we discuss key issues that we consider to be crucial for a safe, efficient, and successful classical biological control project, and that have also caused some recent controversy. These include selection of effective control agents, host specificity of the biological control agents, implications of the genetic population structure of the target populations, and potential impact on native food webs. With regard to improving the success rate of biological control of plant invaders, we first emphasize the importance of a clear a priori definition of success and a more ecosystem-based approach to better document both negative effects of the invasive plant as well as potential positive and negative effects of introducing biological control agents. Secondly, pre-release impact assessment could be improved by better focusing on how to reach high densities of the control agents and by including tolerance to and compensation of herbivory. Thirdly, we advocate a reinforced effort to integrate and combine biological control in combination with existing or potential management options. Finally, we propose various ecological and evolutionary hypotheses in the framework of our topic to document that biological control programmes against plant invaders also offer a great opportunity to gain new insights into basic processes in ecology and evolutio
Neighbour origin and ploidy level drive impact of an alien invasive plant species in a competitive environment
Our understanding of the potential mechanisms driving the spread and naturalization of alien plant species has increased over the past decades, but specific knowledge on the factors contributing to their increased impact in the introduced range is still urgently needed. The native European plant Centaurea stoebe occurs as two cytotypes with different life histories (monocarpic diploids, allo-polycarpic tetraploids). However, only tetraploids have been found in its introduced range in North America, where C. stoebe has become a most prominent plant invader. Here, we focus on the ploidy level of C. stoebe and origin of neighbouring community in explaining the high impact during the invasion of new sites in the introduced range. We conducted a mesocosm experiment under open-field conditions with the diploid (EU2x) and tetraploid (EU4x) cytotype of Centaurea stoebe from its native European (EU) range, and with the invasive tetraploid (NA4x) cytotype from the introduced North American (NA) range in competition with EU (old) or NA (new) neighbouring plant communities. In the presence of competition, the biomass of EU neighbouring community was reduced to a comparable level by all three geo-cytotypes of C. stoebe. In contrast, the biomass of the NA neighbouring community was reduced beyond when competing with tetraploid, but not with diploid C. stoebe. The fact that the biomass of all three geo-cytotypes of C. stoebe was correlated with the biomass of the EU neighbouring community, but not with that of the NA neighbouring community suggests that different mechanisms underlie the competitive interactions between C. stoebe and its old vs. new neighbouring communities, such as competition for the same limiting resources at home vs competition through novel allelo-chemicals or differential resource uptake strategies in the introduced range. We therefore caution to simply use the ecosystem impact assessed at home to predict impact in the introduced rang
Aboveground environment type, soil nutrient content and arbuscular mycorrhizal fungi explain establishment success of Centaurea jacea on ex-arable land and in late-successional grasslands
We studied the relative importance of the aboveground and belowground environment for survival and growth of emerged seedlings of Centaurea jacea to better understand the general difficulty of establishing late-successional species at restoration sites on ex-arable land. Potted seedlings growing on soil from six late-successional grasslands and from six ex-arable (restoration) sites were reciprocally exchanged, and survival and relative growth rate of the seedlings monitored. In addition, we assessed aboveground herbivory and colonization of roots by arbuscular myccorhizal fungi of all plants, as well as nutrient availability, and microbial biomass and community composition using PLFA techniques in all twelve soils. Seedling survival was higher in restoration habitat and soil than in grassland habitat and soil, but growth did not differ between the aboveground and belowground environment types. Shoot growth rate was initially correlated with soil nutrient content, and later in the experiment with mycorrhizal colonization levels. Our results indicate that arbuscular mycorhizal fungi may be important for the successful establishment of C. jacea and that abiotic soil factors, like K availability and N:P ratio, can promote mycorrhizal colonization. Hence, the belowground environment should be considered when selecting sites for restoring species-rich grassland
Seed bank persistence of clonal weeds in contrasting habitats: implications for control
The ability of weeds to form a seed bank is important for their population dynamics and management because it provides a refuge enabling reinvasion after established target plants have died. However, knowledge of the differential seed behaviour of individual species over multiple years and varying environmental conditions is surprisingly rare but necessary for effective control of diverse weed populations. We established a seed burial experiment in alpine habitats differing in management regime (i.e., forest, hay meadow and pasture) to determine whether seeds of the unpalatable perennial weeds, Veratrum album (white hellebore) and Gentiana lutea (yellow gentian) were able to delay germination and remain viable over 3years. Our study shows that both species formed a short-term persistent seed bank; in the third-year, the soil seed banks of both species were nearly depleted, having declined to <5% of their original size. Both species had strikingly different germination strategies: G. lutea seeds mainly germinated in their first-year, whilst the majority of V. album seeds germinated in their second-year. The fraction of dormant G. lutea seeds increased with seed age, indicating that seeds remained viable after forgoing germination in the previous year. Habitat-specific differences in seed germination increased with seed age, with germination fractions being lowest in moist hay meadows. This suggests that the negative effects of anoxic conditions became more pronounced as seeds aged in hay meadows. Conversely, seed dormancy was equal among habitats. The absence of a long-term persistent seed bank has important implications for the management of both nuisance and endangered-plant populations. In the case of V. album and G. lutea, re-colonization of habitats from the seed bank is unlikely after established plants have been remove
Plant neighbours rather than soil biota determine impact of an alien plant invader
1. Various factors have been shown contributing to the ecosystem impact of invasive alien plants, but their relative importance remains unclear. We focused on the effects of neighbouring plant community and soil biota as these biotic factors have been repeatedly put forward to explain invasion success (e.g. as components of the novel weapons and of the biotic release hypothesis).2. To assess their relative importance in explaining the high impact of Centaurea stoebe during the invasion of new sites in the introduced range, we conducted a greenhouse experiment with both European (EU) and North American (NA) tetraploid C. stoebe competing with/without EU vs. NA neighbouring community and with the two neighbouring communities growing without C. stoebe. Plants were grown in sterilized commercial soil inoculated with naĂŻve soil (from rhizosphere of plants other than C. stoebe) originating either from the home EU or the introduced NA range and half of which was sterilized to remove soil biota.3. In the competition pots, relative competitive ability (difference between the relative growth rates of C. stoebe and neighbouring community) and impact level (biomass of the neighbouring community relative to that in non-competition pots) of C. stoebe was significantly higher when grown with the NA than with the EU neighbouring community, although growth rates of EU and NA neighbouring communities did not differ in the non-competition pots. Both soil origin and C. stoebe origin had no effect on these processes. Soil sterilization increased growth of both C. stoebe and neighbouring communities, but had only a moderate effect on impact level and type, and no effect on the relative competitive ability of C. stoebe.4. These results suggest that during the colonization of new sites in North American grasslands, the impact of C. stoebe is strongly driven by reduced competitive ability of NA neighbours compared with EU neighbours, while altered biotic soil conditions in the introduced range and post-introduction evolutionary changes in the invader are of less importance. This differential impact appears to be due to inherently different mechanisms underlying the competitive interactions between EU and NA neighbouring communities when grown with C. stoebe
Life-history variation in contrasting habitats: flowering decisions in a clonal perennial herb (Veratrum album)
Quantifying intraspecific demographic variation provides a powerful tool for exploring the diversity and evolution of life histories. We investigate how habitat-specific demographic variation and the production of multiple offspring types affect the population dynamics and evolution of delayed reproduction in a clonal perennial herb with monocarpic ramets (white hellebore). In this species, flowering ramets produce both seeds and asexual offspring. Data on ramet demography are used to parameterize integral projection models, which allow the effects of habitat-specific demographic variation and reproductive mode on population dynamics to be quantified. We then use the evolutionarily stable strategy (ESS) approach to predict the flowering strategyâthe relationship between flowering probability and size. This approach is extended to allow offspring types to have different demographies and density-dependent responses. Our results demonstrate that the evolutionarily stable flowering strategies differ substantially among habitats and are in excellent agreement with the observed strategies. Reproductive mode, however, has little effect on the ESSs. Using analytical approximations, we show that flowering decisions are predominantly determined by the asymptotic size of individuals rather than variation in survival or size-fecundity relationships. We conclude that habitat is an important aspect of the selective environment and a significant factor in predicting the ESSs
Phenotypic plasticity and genetic differentiation in ruderal and agricultural populations of the weed Senecio vulgaris L.: implications for its biological control
La plasticitĂ© phĂ©notypique et la diffĂ©rentiation gĂ©nĂ©tique de la mauvaise herbe annuelle Senecio vulgaris L. dans le habitats rudĂ©raux et agricoles ont Ă©tĂ© Ă©tudiĂ©es afin dâĂ©valuer les implications pour son contrĂŽle biologique avec la rouille Puccinia lagenphorae Cooke en utilisant lâapproche dite âsystem managementâ. Les Ă©tudes dĂ©mographiques des mauvaises herbes sont une nĂ©cessitĂ© pour toute stratĂ©gie de contrĂŽle des mauvaises herbes. Pour cela, la dynamique des populations de S. vulgaris et P. lagenphorae se trouvant dans des habitats rudĂ©raux et agricoles a Ă©tĂ© examinĂ©e (chapitre 2). LâĂ©tablissement de plantes germinales semble ĂȘtre un facteur important influençant la dynamique des populations de S. vulgaris. Cette derniĂšre peut ĂȘtre qualifiĂ©e de stratĂšge rudĂ©ral sur la base de lâimportance de lâĂ©tablissement de plantes germinales, sur lâobservation que cinq semaines sont nĂ©cessaires pour une gĂ©nĂ©ration ainsi que sur le large spectre de taille des plantes se trouvant Ă lâĂ©tat reproductif. Il a Ă©tĂ© observĂ© que la dynamique des populations est diffĂ©rente entre les habitats rudĂ©raux et agricoles. Les plantes de lâhabitat rudĂ©ral hivernent, donc, au printemps les populations sont plus importantes, avec une majoritĂ© des plantes en fleurs et produisant des semences. Par contre, dans lâhabitat agricole, S. vulgaris nâhiverne quasiment pas, ceci Ă cause des mesures de contrĂŽle des mauvaises herbes. Par consĂ©quent, les populations sont moins nombreuses au printemps et la plupart des plantes se trouvent Ă lâĂ©tat vĂ©gĂ©tatif. La diffĂ©rente dynamique des populations de S. vulgaris rĂ©sulte en une dynamique diffĂ©rente de P. lagenophorae dans les habitats rudĂ©raux et agricoles. Dans lâhabitat rudĂ©ral, la rouille a dĂ©jĂ Ă©tĂ© observĂ©e au mois dâavril alors quâon ne la trouve pas avant le mois dâaoĂ»t dans lâhabitat agricole. Ceci indique que la rouille hiverne dans lâhabitat rudĂ©ral sous forme de mycĂ©lium dans son hĂŽte alors quâelle ne peut hiverner dans lâhabitat agricole au vu de faible nombre dâhĂŽtes hivernants. En comparaison avec lâhabitat rudĂ©ral, les Ă©pidĂ©mies printaniĂšres de rouilles dans lâhabitat agricole semblent non seulement ĂȘtre empĂȘchĂ©es par la faible quantitĂ© dâinoculum mais aussi par une majoritĂ© de plantes se trouvant dans un jeune stade. Cependant, dans le deux habitats, les plantes au stade reproductif sont plus sensible envers lâinfection par la rouille que les plantes Ă lâĂ©tat vĂ©gĂ©tatif. La manipulation de la dynamique de lâhĂŽte au travers de lâhiver pour favoriser les Ă©pidĂ©mies de rouille pourrait par consĂ©quent ĂȘtre utilisĂ©e comme moyen de contrĂŽle biologique. Les S. vulgaris hivernant pouvant servir non seulement de source dâinoculum au printemps mais fournissant aussi des stades de dĂ©veloppement sensibles Ă la rouille, ce qui favoriserait le dĂ©veloppement des Ă©pidĂ©mies de rouille. La plasticitĂ© phĂ©notypique et la diffĂ©rentiation gĂ©nĂ©tique de S. vulgaris provenant dâhabitats rudĂ©raux et agricoles ont Ă©tĂ© examinĂ©es lors dâune expĂ©rience en serre (chapitre 3). Les caractĂ©ristiques vĂ©gĂ©tatives et reproductives des plantes des habitats rudĂ©raux et agricoles confrontĂ©s Ă un milieu nutritif riche et pauvre, ainsi que se trouvant confrontĂ© ou non Ă lâinfection par la rouille ont Ă©tĂ© comparĂ©es. Il est Ă signaler que les plantes de habitats rudĂ©raux et agricoles sont gĂ©nĂ©tiquement diffĂ©rentes. Les plantes de lâhabitat agricole se caractĂ©risent par une surface foliaire importante ainsi quâune reproduction Ă©levĂ©e. Leurs caractĂ©ristiques reproductives sont aussi plus plastiques face Ă un apport nutritif Ă©levĂ©. Ces rĂ©sultats indiquent que la diffĂ©rentiation gĂ©nĂ©tique entre les habitats repose sur les diffĂ©rences dans lâapport nutritif. Dans lâhabitat agricole, lĂ oĂč les populations de S. vulgaris sont gĂȘnĂ©es par les mesures de culture, une reproduction plus Ă©levĂ©e pourrait ĂȘtre un avantage. La rĂ©serve de semences est pour S. vulgaris de moindre importance et câest pour cette raison que les populations doivent sâĂ©tablir Ă nouveau par recolonisation. Une reproduction plus Ă©levĂ©e augmente aussi la dispersion des semences de maniĂšre Ă ce que S. vulgaris soit capable de sâĂ©tablir rapidement dans de nouveaux endroits appropriĂ©s. Des familles de S. vulgaris gĂ©nĂ©tiquement diffĂ©rentes ont Ă©tĂ© trouvĂ©es seulement dans lâhabitat agricole. La plasticitĂ© des caractĂ©ristiques reproductives en rĂ©ponse Ă un apport nutritif plus Ă©levĂ© a Ă©tĂ© aussi gĂ©nĂ©tiquement diffĂ©rente entre des familles se trouvant dans lâhabitat agricole. Par contre, une diffĂ©rentiation gĂ©nĂ©tique par rapport Ă lâinfection avec la rouille nâa pu ĂȘtre dĂ©montrĂ©e. Dâautre part, les plantes soumises Ă© un apport nutritif Ă©levĂ© ont prĂ©sentĂ© une rĂ©action plus forte en rĂ©ponse Ă lâinfection avec la rouille. Cette derniĂšre a eu pour consĂ©quence une diminution da la surface foliaire, de la biomasse vĂ©gĂ©tative et de la reproduction. Par contre, il nây a eu aucune diffĂ©rence entre les rĂ©actions envers lâinfection entre les diffĂ©rentes familles rudĂ©rales et agricoles. Ceci indique que P. lagenphorae nâest prĂ©sentement pas un important facteur de sĂ©lection pour les populations de S. vulgaris. L âinoculation avec succĂšs des diffĂ©rentes familles avec une seule lignĂ©e de rouille montre le potentiel de ces lignĂ©es comme organismes de contrĂŽle biologique. Une expĂ©rience de transplantation rĂ©ciproque de semences a permis dâĂ©tudier les variations gĂ©nĂ©tique ainsi que celles dues Ă lâenvironnement sur le caractĂ©ristiques de vie de S. vulgaris provenant dâhabitats rudĂ©raux et agricoles ainsi que leur potentiel dâadaptation locale par sĂ©lection naturelle (chapitre 4). La germination, la croissance et la fertilitĂ© ont Ă©tĂ© dĂ©terminĂ©es selon les variations environnementales et gĂ©nĂ©tiques. Aucune indication dâadaptation locale nâa toutefois pu ĂȘtre Ă©tablie. La survie des plantes germinales jusquâĂ la reproduction nâa Ă©tĂ© influencĂ©e que par les facteurs environnementaux, ce qui souligne lâimportance des facteurs stochastiques pour la mortalitĂ© des plantes. Les plantes provenant des endroits rudĂ©raux prĂ©sentent une croissance rĂ©duite, produisant par contre des semences et se comportant par consĂ©quent comme des plantes tolĂ©rantes au stress. Les plantes se dĂ©veloppant dans la majoritĂ© des endroits agricoles agissent comme stratĂšges rudĂ©raux, pouvant utiliser un environnement productif pour une croissance rapide et une production maximale de semences. LâĂ©tablissement des plantes germinales, la survie, la croissance ainsi que la fertilitĂ© sont Ă©levĂ©es dans les endroits agricoles par rapport Ă les endroits ruderaux. Lâinfection avec P. lagenophorae a Ă©tĂ© prĂ©sente dans tous les endroits, avec le taux dâinfection le plus Ă©levĂ© dans les endroits agricoles. Il a Ă©tĂ© remarquĂ© quâen prĂ©sence de lâhĂŽte appropriĂ©, lâinfection est uniquement dĂ©pendante des bonnes conditions environnementales. Un taux dâinfection plus Ă©levĂ© nâa toutefois pas eu comme consĂ©quence un taux de maladie plus Ă©levĂ© sur la croissance et la reproduction des plantes. Cela nâa non plus pas influencĂ© la survie de S. vulgaris par rapport Ă lâinfection avec la rouille, ce qui indique que P. lagenophorae nâest pas un facteur de mortalitĂ© important pour les populations de S. vulgaris. Toutes les autres caractĂ©ristiques de vie corroborent les conditions pour la sĂ©lection naturelle. Cela donne donc Ă penser que S. vulgaris, en absence de facteurs de mortalitĂ© stochastiques, a le potentiel dâadapter Ă son environnement local. Finalement, les rĂ©sultats obtenus durant cette thĂšse ont Ă©tĂ© rĂ©sumes et discutĂ©s du point de vue du contrĂŽle biologique de S. vulgaris en gĂ©nĂ©ral et en particulier du point de vue de lâapproche dite âsystem mangementâ, ainsi que de la suite Ă donner de cette Ă©tude (chapitre 5). Comme la variation gĂ©nĂ©tique de S. vulgaris par rapport a lâinfection avec la rouille n âa pu ĂȘtre dĂ©montrĂ©e et comme la rouille semble prĂ©sentement ĂȘtre un facteur de sĂ©lection mineur, les rĂ©sultats soulignent le potentiel de lâapproche dite âsystem managementâ pour le contrĂŽle biologique de S. vulgaris. Cette mĂ©thode est conçue pour stimuler les Ă©pidĂ©mies de P. lagenophorae sur S. vulgaris, ce qui peut ĂȘtre atteint par les mesures suivantes i) lâhivernage de S. vulgaris infectĂ©es comme source dâinoculum, ii) lâutilisation de lignĂ©es unique de rouille, lâoptimisation des conditions dâinfection par arrosage et amendement lors de lâapplication de lâinoculum.Phenotypische PlastizitĂ€t und genetische Differenzierung des annuellen Unkrautes Senecio vulgaris L. aus ruderalen und landwirtschaftlichen Habitaten und ihre Relevanz in Bezug auf die biologische Kontrolle mit dem Rostpilz Puccina lagenophorae Cooke, unter Anwendung des 'System Management Approach', waren Gegenstand dieser Dissertation. Demographische Studien der Unkrautpopulation sind eine Voraussetzung fĂŒr jegliche UnkrautbekĂ€mpfungsmassnahme. Daher wurde die Populationsdynamik von S. vulgaris und P. lagenophorae aus ruderalen und landwirtschaftlichen Habitaten untersucht (Kapitel 2). Die Etablierung von Keimlingen scheint ein wichtiger Einflussfaktor auf die Populationsdynamik von S. vulgaris zu sein. Die Bedeutung der Etablierung von Keimlingen, sowie die beobachtete kurze Generationszeit von fĂŒnf Wochen, und die breite Streuung der Pflanzengrösse zu Beginn der Reproduktion klassifizieren S. vulgaris als einen r-Strategen Die Populationsdynamik von S. vulgaris zwischen dem ruderalen und dem landwirtschaftlichen Habitat war unterschiedlich. Pflanzen von S. vulgaris im ruderalen Habitat ĂŒberwinterten, was eine grössere Population sowie eine Mehrzahl an blĂŒhenden und Samen bildenden Pflanzen im FrĂŒhjahr zur Folge hatte. Im landwirtschafltichen Habitat konnte S. vulgaris aufgrund von Unkrautkontrollmassnahmen kaum ĂŒberwintern, sodass die Populationen im FrĂŒhjahr kleiner waren mit der Mehrzahl der Pflanzen im vegetativen Stadium. Der Unterschied in der Populationsdynamik von S. vulgaris zwischen den Habitaten bedingte eine unterschiedliche Dynamik von P. lagenophorae aus ruderalen und landwirtschaftlichen Habitaten. Im ruderalen Habitat trat der Rost bereits im April auf, wĂ€hrend er im landwirtschaftlichen Habitat nicht vor August anzutreffen war. Dies weist daraufhin, dass der Rost im ruderalen Habitat als Myzel innerhalb der Wirtspflanze ĂŒberwintert, wĂ€hrend er im landwirtschaftlichen Habitat aufgrund mangelnder Wirtspflanzen nicht ĂŒberwintern kann. Rost Epidemien im landwirtschaftlichen Habitat im FrĂŒhjahr scheinen nicht nur durch die geringe Menge an Inokulum verhindert zu werden, sondern auch durch die Mehrzahl an Pflanzen in jungen Entwicklungsstadien verglichen mit dem ruderalen Habitat. In beiden Habitaten waren reproduktive Pflanzenstadien anfĂ€lliger fĂŒr eine Infektion mit dem Rost als vegetative Stadien. Die Manipulation der Wirtspflanzendynamik ĂŒber Winter zur Förderung von Rost Epidemien könnte als biologische Kontollmassnahme eingesetzt werden. Ăberwinternde S. vulgaris Pflanzen dienen als Inokulum Quellen im FrĂŒhjahr von denen Rost Epidemien ausgehen und stellen reifere und somit fĂŒr den Rost anfĂ€lligere Entwicklungsstadien, was die Entwicklung von Rost Epidemien fördert. Phenotypische PlastizitĂ€t und genetische Differenzierung von S. vulgaris aus ruderalen und landwirtschaftlichen Habitaten wurden in einem GewĂ€chshaus Experiment bestimmt (Kapitel 3). Vegetative und reproduktive Merkmale von ruderalen und landwirtschaftlichen Pflanzen, die unter hohem oder niedrigem NĂ€hrstoffangebot und mit oder ohne Infektion mit P. lagenophorae angezogen waren, wurden verglichen. Pflanzen von S. vulgaris aus ruderalen und landwirtschaftlichen Habitaten waren genetisch unterschiedlich. Die landwirtschaftlichen Pflanzen zeichneten sich durch eine grössere BlattfĂ€che sowie eine höhere Reproduktion aus. Sie wiesen auch eine plastischere Reaktion der reproduktiven Merkmale auf ein höheres NĂ€hrstoffangebot auf. Diese Ergebnisse weisen daraufhin, dass die genetische Differenzierung zwischen den Habitaten auf Unterschiede im NĂ€hrstoffangebot beruht. Im landwirtschaftlichen Habitat, bedingt durch Kultivierungsmassnahmen, welche die Populationen von S. vulgaris störend beeinflussen, könnte eine erhöhte Reproduktion von Vorteil sein. Die Samenbank ist fĂŒr S. vulgaris von geringer Bedeutung und daher mĂŒssen sich die Populationen durch Re-Kolonisierung erneut etablieren. Eine erhöhte Reproduktion erhöht auch die Samenverbreitung, sodass S. vulgaris in der Lage ist sich rasch an dafĂŒr geeigneten Stellen neu zu etablieren. Genetisch unterschiedliche Familien von S. vulgaris wurden nur im landwirtschaftlichen Habitat gefunden. Die landwirtschaftlichen Familien unterschieden sich auch in Bezug auf die PlastizitĂ€t von reproduktiven Merkmalen auf ein höheres NĂ€hrstoffangebot. Eine genetische Differenzierung von S. vulgaris auf Grund von Infektion mit P. lagenophorae konnte nicht gefunden werden. Pflanzen mit einem höherem NĂ€hrstoffangebot zeigten eine stĂ€rkere Reaktion auf die Infektion mit dem Rost. Infektion mit dem Rost hatte eine signifikante Verringerung der BlattflĂ€che, der vegetativen Biomasse sowie der Reproduktion zur Folge, ohne Unterschiede der Reaktionen auf Infektion zwischen den unterschiedlichen ruderalen und landwirtschaftlichen Familien. Dies weist daraufhin, dass P. lagenophoarae gegenwĂ€rtig als Selektionsfaktor fĂŒr S. vulgaris Populationen von untergeordneter Bedeutung ist. Weiterhin zeigt die erfolgreiche Inokulation verschiedener Pflanzenfamilien mit einer Rostlinie das Potential einzelner Rostlininen als biologische Kontrollorganismen an. Die umweltbedingte und genetische Variation von lebensgeschichtlichen Merkmalen von S. vulgaris aus ruderalen und landwirtschaftlichen Habitaten im Feld und deren Potential zur lokalen Adaptation durch natĂŒrliche Selektion wurde in einem reziproken Samen Transplantations Experiment untersucht (Kapitel 4). Keimung, Wachstum und FertilitĂ€t wurden durch umweltbedingte als auch genetische Variation bestimmt. Jedoch konnten keine Hinweise auf lokale Adaptation gefunden werden. Das Ăberleben von Keimlingen bis zur Samenproduktion wurde allein durch UmwelteinflĂŒsse bedingt, was die Bedeutung von stochastischen Faktoren fĂŒr die MortalitĂ€t von Pflanzen hervorhebt. Pflanzen, die aus den ruderalen Standorten stammten, wiesen ein reduzierten Wachstum auf, produzierten jedoch Samen, und verhielten sich somit wie stress-tolerante Pflanzen. Pflanzen, die in den landwirtschaftlichen Standorten wuchsen, zeigten das Verhalten von r-Strategen, welche eine produktive Umwelt fĂŒr ein rasches Wachstum und eine maximale Produktion von Samen nutzen. Etablierung von Keimlingen, Ăberleben, Wachstum und FertilitĂ€t waren in den landwirtschaftlichen Standorten im Vergleich zu den ruderalen erhöht. Infektion mit P. lagenophorae trat in allen Standorten auf, mit der höchsten Infektionsrate in den landwirtschaftlichen Standorten, und war rein von der Umwelt abhĂ€nging. Es scheint, dass Pilzsporen fĂŒr eine Infektion weit verbreitet sind und in Anwesenheit geeigneter Wirte, die Infektion von gĂŒnstigen Umweltbedingungen fĂŒr die Sporenkeimun abhĂ€ngig ist. Eine höhere Infektionsrate wirkte sich jedoch nicht in einem erhöhten Krankheitsdruck auf Pflanzenwachstum-und reproduktion aus, noch wurde das Ăberleben von S. vulgaris Pflanzen durch Infektion mit dem Rost beeinflusst, was daraufhin weist, dass P. lagenophorae kein wichtiger MortalitĂ€tsfaktor fĂŒr Populationen von S. vulgaris ist. Alle anderen lebensgeschichtlichen Merkmale wiesen die Voraussetzungen fĂŒr natĂŒrliche Selektion auf. Daher ist anzunehmen, dass sich S. vulgaris, in der Abwesenheit von stochastischen MortalitĂ€tsfaktoren, potentiell an seine lokale Umgebung anpassen kann. Abschliessend wurden die Resultate dieser Dissertation zusammengefasst und in Bezug auf die biologische Kontrolle von S. vulgaris im allgemeinen und die Anwendung des âSystem Management Approachâ im speziellen diskutiert, sowie mögliche Folgestudien, die auf Ergebnissen dieser Dissertation beruhen, vorgestellt. (Kapitel 5). Da keine genetische Variation von S. vulgaris Pflanzen in Bezug auf Infektion mit P. lagenophoarae nachgewiesen werden konnte und der Rost gegenwĂ€rtig als Selektionsfaktor fĂŒr S. vulgaris Populationen von untergeordneter Bedeutung ist, unterstreichen die Ergebnisse dieser Dissertation das Potential des 'System Mangement Approach' fĂŒr die biologische Kontrolle von S. vulgaris. Diese Methode ist darauf ausgerichtet Epidemien von P. lagenophorae auf S. vulgaris zu stimulieren, was durch folgende Massnahmen erreicht werden kann: i) die Ăberwinterung von infizierten S. vulgaris Pflanzen als Inokulum Quellen, ii) die Verwendung einzelner Rostlinien als Inokulum und iii) die Optimierung von Infektionsbedingungen durch BewĂ€sserung und DĂŒngung zum Zeitpunkt der Ausbringung von Inokulum.Phenotypic plasticity and genetic differentiation of the annual weed Senecio vulgaris L. from ruderal and agricultural habitats were investigated to evaluate the implications for its biological control with the rust fungus Puccinia lagenophorae Cooke using the system management approach. Weed demographic studies are a prerequisite to any weed control strategy. The population dynamics of S. vulgaris and P. lagenophorae at ruderal and agricultural habitats was investigated (Chapter 2). Seedling establishment appeared to be a major factor influencing S. vulgaris population dynamics. The importance of seedling establishment, together with the observed short generation time of 5 weeks and the broad range of plant sizes at reproduction classified S. vulgaris as a ruderal strategist. The population dynamics of S. vulgaris differed between the ruderal and the agricultural habitat. Plants of S. vulgaris at the ruderal habitat survived over winter, resulting in greater population sizes and a majority of plants in flower and seed set in spring. At the agricultural habitat, where only a few S. vulgaris winter due to weed control measures, population sizes in spring were smaller with mainly vegetative plants. The observed difference in the population dynamics of S. vulgaris resulted in divergent dynamics of P. lagenophorae in ruderal and agricultural habitats. At the ruderal habitat the rust occurred already in April, while at the agricultural habitat it did not appear before August. This suggests that the rust survives over winter as mycelium within the host at the ruderal habitat, whereas it could not survive over winter at the agricultural habitat due the relatively low number of wintering hosts. The build-up of P. lagenophorae epidemics at the agricultural habitat in spring seems not only inhibited by a limited amount of inoculum, i.e. wintering mycelium, but also by the abundance of S. vulgaris plants in relatively young stages compared to the ruderal habitat. In both habitats mature plant stages were more susceptible to rust infection than vegetative stages. Manipulating the dynamics of the host over winter may enhance rust epidemics in spring serving as biological control of S. vulgaris. Survival of S. vulgaris over winter provides inoculum sources inducing new rust epidemics in spring with mature plant stages being more susceptible to infection, enhancing rust epidemics. Phenotypic plasticity and genetic differentiation of S. vulgaris from ruderal and agricultural habitats were assessed in a greenhouse experiment (Chapter 3). Vegetative and reproductive responses of ruderal and agricultural plants grown at two nutrient levels (high/low) and two rust environments (presence/absence) were compared. Plants of S. vulgaris from ruderal and agricultural habitats were genetically different with agricultural plants having a larger leaf area and a higher reproductive output. They also showed a stronger plastic response of reproductive traits to nutrients. Results suggest that genetic differentiation among habitats is nutrient specific. An increased reproductive output might me of advantage in the agricultural habitat with its disturbance due to cultivation practices. Due to the minor importance of the soil seed bank, populations have to re-establish through re-colonisation. An increased reproductive output improves seed dispersal enabling S. vulgaris to quickly re-establish new patches at favourable sites. Genetically different families of S. vulgaris were only detected at the agricultural habitat. Plasticity of reproductive characters in response to nutrients was also genetically different among agricultural families. Genetic differentiation of S. vulgaris due to infection by P. lagenophorae could not be observed. Plants with additional nutrient application showed a stronger response to rust infection. Rust infection significantly decreased vegetative biomass and reproductive output with no differences in the response to infection between plant families, suggesting that P. lagenophorae is not an important selection factor in S. vulgaris populations at present. Successful infection of different plant families with one rust line also suggests the potential of using a single rust line as a biological control agent. A reciprocal seed transplant experiment was conducted to determine environmental and genetic variation of life history traits in S. vulgaris from ruderal and agricultural habitats in the field and their potential for local adaptation through natural selection (Chapter 4). Emergence, growth and fecundity showed environmental as well as genetic variation. However, there was no indication of local adaptation. Seedling survival was solely under environmental control stressing the importance of stochastic events for plant mortality. Plants originating from ruderal sites mainly performed like stress tolerators showing reduced growth, but nevertheless reaching reproduction. Plants at most agricultural sites behaved like r-strategists exploiting a productive environment for rapid plant growth maximis
Polyploidy and invasion success: trait trade-offs in native and introduced cytotypes of two Asteraceae species
Invasion success is favoured by the introduction of pre-adapted genotypes. In addition, novel pressures in the introduced range may lead to phenotypic changes related to fitness or competitive ability of introduced plants. Polyploidy appears to be over-represented in invasive plants, but differences between cytotypes in growth strategies including trade-offs among plant traits have received little attention so far in the context of biological invasions. We grew Centaurea stoebe L. and Senecio inaequidens D.C. in a greenhouse experiment to test for differences in fitness (shoot biomass, reproductive output) and competitive ability (vegetative size, specific leaf area, leaf dry matter content, root-shoot ratio) between diploid and polyploid cytotypes as well as between native and introduced plants. For both species, diploid and tetraploid genotypes occur in the native range, whereas only tetraploids are present in the introduced range. In the native range of both species, diploid and tetraploid genotypes had different growth strategies. Tetraploid genotypes of C. stoebe and S. inaequidens had, respectively, higher specific leaf area and stem height than diploid ones. Thus, for both species, native tetraploids appeared more competitive than native diploids, which could explain, at least partially, the invasion success of the pre-adapted tetraploid genotypes. The comparison of native and introduced tetraploid genotypes revealed differences in traits linked to competitive ability, which could be linked to novel selection in the new environment. In S. inaequidens, we found evidence for a competition-colonisation trade-off, whereas persistence of C. stoebe in the new range seemed to be linked to a competition-defence trade-of
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