Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Since its introduction in 1898 into South Africa, the Argentine ant, Linepithema humile [Mayr 1868 (Hymenoptera: Formicidae)], has invaded human-occupied areas (i.e. urban and agricultural areas) and natural areas characterised by few, if any, anthropogenic disturbances. However, compared to other countries in which the Argentine ant has been recorded, and until the past few decades, very little research had been done on this invasive ant in South Africa. Consequently, several issues concerning its ecological and social effects are still under-researched. The first of these issues concerns the lack of knowledge about the distribution of the Argentine ant in the natural areas, particularly the protected areas (PAs), of South Africa. In order to determine how many PAs are occupied by this invasive ant, a study was conducted in the Western Cape Province (WCP). It was found that, of the 614 PAs documented for WCP, ten have a known presence and nine known absence records of the Argentine ant. The remainder of the PAs have no known occupancy records for this ant. A second issue concerns the seasonal bait preference of the Argentine ant in a fynbos habitat. Six bait treatments (two carbohydrate and protein baits, a mixture of the carbohydrate and protein treatments, and a control) were applied in a fynbos habitat across a sampling grid in four different Latin Square designs, i.e. once for every season. Based on these experiments, it was determined that the Argentine ant prefers the mixture of carbohydrate and protein treatments, and that this preference does not change according to season. Furthermore, previous studies conducted in Jonkershoek Nature Reserve [JNR (in WCP)] determined the existence of a distribution boundary of Argentine ants in an area known as Swartboschkloof. Therefore, the third issue concerned the exact location of the distribution boundary and possible reasons for its establishment. This distribution boundary of the Argentine ant was found to be present 450 m from Swartboschkloof hiking trail. A combination of several explanatory variables may contribute to the maintenance of this boundary, i.e. a change in the horizontal and vertical vegetation distribution, as well as in the slope and aspect across the distribution boundary. With these explanatory variables, the increasing presence of an indigenous ant species, Anoplolepis custodiens, from this boundary may also have contributed to the distribution boundary. In addition, the short-term effect of a fire that swept through this area was also assessed, which revealed that Argentine ants are severely negatively affected by fire (at least over the short-term), i.e. their abundance decreased considerably after the fire and their local distribution range contracted. The final issue concerns the public perceptions of invasive alien species (IAS) in general and the Argentine ant specifically, at JNR. This study revealed that the majority of visitors to JNR were aware of the presence of IAS in South Africa and in its PAs, while very few visitors knew about the Argentine ant. This study also revealed that future research concerning South Africans perceptions of IAS might play a strong contributing role in conservation. / AFRIKAANSE OPSOMMING: Sedert die Argentynse mier, Linepithema humile [Mayr 1868 (Hymenoptera: Formicidae)], in 1898 in Suid-Afrika ingevoer is, het dié mier mens-bewoonde gebiede (soos stedelike en landbou gebiede) en natuurlike gebiede, gekenmerk deur min, indien enige, antropogeniese versteuringe, binnegedring. Nietemin, in vergelyking met ander lande waar die Argentynse mier opgeteken is, en tot die onlangse paar dekades, is min navorsing oor hierdie indringermier in Suid-Afrika onderneem. Gevolglik is daar verskeie kwessies rondom die mier se ekologiese en sosiale uitwerkinge wat nog nie nagevors is nie. Die eerste kwessie het betrekking op die gebrek aan kennis oor die Argentynse mier se verspreiding in die natuurlike gebiede, veral die beskermde gebiede (BG), van Suid-Afrika. Om te bepaal hoeveel BG deur hierdie indringermier beset word, is ʼn ondersoek in die Wes-Kaapprovinsie (WKP) uitgevoer. Daar is bevind dat, van die 614 BG gedokumenteerd in die WKP, het tien bevestigde aanwesigheid- en nege bevestigde afwesigheidrekords van hierdie mier. Die oorblywende BG het geen bekende besettingsrekords van hierdie mier nie. ʼn Tweede kwessie het betrekking op die seisoenale lokaasvoorkeur van die Argentynse mier in ʼn fynbos habitat. Ses lokaas-behandelings (twee koolhidraat en proteïen lokaas, ʼn mengsel van die koolhidraat en proteïen behandelings, en ʼn kontrole) is aangewend in ʼn fynbos habitat, oor ʼn steekproefruitgebied, in vier verskillende Latyns-kwadraatpatrone (“Latin Sqaure designs”), d.i. een vir elke seisoen. Op grond van hierdie eksperimente is vasgestel dat die Argentynse mier die mengsel van koolhidrate en proteïne verkies, en dat hierdie voorkeur nie seisoenaal verander nie. Boonop, vorige ondersoeke wat in die Jonkershoek Natuurreservaat [JNR (in die WKP)] uitgevoer is, het ʼn verspreidings-grens van Argentynse miere ontdek in ʼn gebied bekend as Swartboschkloof. Gevolglik het die derde kwessie betrekking op die presiese ligging van hierdie grens en moontlike redes waarom dit gevestig het. Dié verspreidings-grens van die Argentynse mier is 450 m vanaf die Swartboschkloof voetslaanpad gevind. ʼn Kombinasie van verskeie verklarende veranderlikes kon tot hierdie grens bygedra het, d.i. ʼn verandering in die horisontale en vertikale plantegroeiverspreiding, sowel as in die helling en ligging oor die verspreidings-grens van die Argentynse mier. Tesame met hierdie verklarende veranderlikes, kon die toenemende teenwoordigheid van ʼn inheemse mier, Anoplolepis custodiens, vanaf hierdie grens ook tot die verspredings-grens bygedra het. Daarbenewens is die korttermyn-effek van ʼn vuur wat deur die area beweeg het, ook bestudeer. Die ondersoek het getoon dat die Argentynse mier (ten minste oor die korttermyn) erg negatief deur vuur beïnvloed is, d.i. hul volopheid het ná die vuur aansienlik verminder en hul plaaslike verspreidings-grens het gekrimp. Die finale kwessie het betrekking op openbare persepsie van uitheemse indringerspesies (UIS) oor die algemeen en spesifiek die Argentynse mier, by JNR. Hierdie ondersoek het aan die lig gebring dat die meerderheid van besoekers aan JNR bewus was van die teenwoordigheid van UIS in Suid-Afrika en in dié se BG, terwyl baie min egter van die Argentynse mier geweet het. Hierdie ondersoek het ook aan die lig gebring dat toekomstige navorsing rakende Suid-Afrikaners se persepsie van UIS ʼn sterk bydra tot bewaring kan maak.

<p>Biological invasions are an important issue of global change and an increased understanding of invasion processes is of crucial importance for both conservation managers and international trade. In this thesis, I have studied the invasion of the brown seaweed <i>Fucus evanescens</i>, to investigate the fate and effect of a perennial, habitat-forming seaweed introduced to a coastal ecosystem. A long-term study of the spread of <i>F. evanescens</i> in Öresund (southern Sweden) showed that the species was able to expand its range quickly during the first 20 years after the introduction, but that the expansion has been slow during the subsequent 30 years. Both in Öresund and in Skagerrak, the species is largely restricted to sites where native fucoids are scarce. Laboratory experiments showed that the restricted spread of <i>F. evanescens</i> cannot be explained by the investigated abiotic factors (wave exposure and salinity), although salinity restricts the species from spreading into the Baltic Sea. Neither did I find evidence for that herbivores or epibiota provide biotic resistance to the invader. On the contrary, <i>F. evanescens</i> was less consumed by native herbivores, both compared to the native fucoids and to <i>F. evanescens</i> populations in its native range, and little overgrown by epiphytes. Instead, the restricted spread may be due to competition from native seaweeds, probably by pre-occupation of space, and the establishment has probably been facilitated by disturbance. </p><p>The studies provided little support for a general enemy release in introduced seaweeds. The low herbivore consumption of <i>F. evanescens</i> in Sweden could not be explained by release from specialist herbivores. Instead, high levels of chemical anti-herbivore defence metabolites (phlorotannins) could explain the pattern of herbivore preference for different fucoids. Likewise, the low epibiotic colonisation of <i>F. evanescens </i>plants could be explained by high resistance to epibiotic survival. This shows that colonisation of invading seaweeds by native herbivores and epibionts depends on properties of the invading species. The large differences between fucoid species in their quality as food and habitat for epibionts and herbivores imply that invasions of such habitat-forming species may have a considerable effect on a number of other species in shallow coastal areas. However, since <i>F. evanescens</i> did not exclude other fucoids in its new range, its effect on the recipient biota is probably small.</p>

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Non-native species are recognized as a major component of global environmental change. Their ecological impacts are numerous and include the alteration of whole ecosystem processes as well as the loss of native biodiversity. As such, understanding the processes that drive the invasion of non-native species is essential for the control and management thereof. Numerous research approaches have been used to provide insight on the history and ecology of non-native species invasions. However, recent approaches employing molecular techniques have greatly helped in solving taxonomic issues associated with some of these species; identifying sources of invasions; and shedding light on colonization dynamics. Lantana camara, a globally invasive and highly variable species complex, is one of the most notorious plant invaders in South Africa. The species has been associated with negative impacts in agricultural areas, decreased invertebrate diversity, livestock mortality, and where it occurs along riparian areas- decreased water quality and obstruction to accessing water sources. This project aimed to review L. camara invasions in South Africa and to unravel patterns of spread in L. camara along the Sabie-Sand catchment in South Africa’s flagship protected area, the Kruger National Park, using a molecular approach. The findings of the first part of the study highlight that L. camara has successfully spread across South Africa with only four known introduction events, and this can be attributed to the species’ broad ecological tolerance, its use in the horticultural industry, and a variety of dispersal vectors (birds, humans and rivers). Furthermore, although sale of the species is prohibited in the country, it will continue to spread naturally and also has the potential to expand its distribution under changing climate scenarios. The second part of the study highlights that spread along the Sabie-Sand catchment is primarily river-driven and that the Sand tributary is the invasion source in the system. Because the Sand tributary originates outside the Kruger National Park, and only a small portion is under park management, the implications for spread are important. / AFRIKAANSE OPSOMMING: Nie-inheemse spesies word erken as 'n belangrike komponent van die globale omgewing verandering. Hulle ekologiese impak is talle en sluit in die verandering van ekosisteem prosesse asook die verlies van inheemse biodiversiteit. As sodanig, die begrip van die prosesse wat lei tot die inval van 'n nie-inheemse spesies is noodsaaklik vir die beheer en bestuur daarvan. Talle navorsingsbenaderings is gebruik om insig te gee oor die geskiedenis en ekologie van nie-inheemse spesies invalle. Onlangse benaderings soos die gebruik van molekulêre tegnieke, help in die oplossing van taksonomiese kwessies wat verband hou met 'n paar van hierdie spesies,in die identifisering van bronne van invalle, en om lig te werp op die kolonisasie dinamika. Lantana camara, 'n wêreldwye indringende en spesie kompleks, is een van die mees berugte plantindringer in Suid-Afrika. Die spesie is geassosieer met negatiewe gevolge in landbou gebiede, afgeneem ongewerwelde diversiteit, vee mortaliteit, en waar dit voorkom saam oewer gebiede - afgeneem kwaliteit van die water en obstruksie tot waterbronne. Hierdie projek is daarop gemik om om L. camara invalle in Suid-Afrika te hersien en patrone van verspreiding te ontrafel in L. camara langs die Sabie-Sand-opvanggebied in Suid-Afrika se vlagskip beskermde gebied, die Kruger Nasionale Park, met behulp van 'n molekulêre benadering. Die bevindinge van die eerste deel van die studie wys dat L. camara het suksesvol versprei oor die hele Suid-Afrika met slegs vier bekende inleiding gebeure, en dit kan toegeskryf word aan die spesie se breë ekologiese verdraagsaamheid, die gebruik daarvan in die hortologie bedryf, en 'n verskeidenheid van die verspreiding vektore (voëls, mens en riviere). Verder, hoewel die verkoop van die spesie in die land verbied word, sal dit natuurlik voortgaan om te versprei en het ook die potensiaal om uit te brui onder veranderende klimaat scenario's. Die tweede deel van die studie wys dat versprei langs die Sabie-Sandopvanggebied hoofsaaklik rivier-gedrewe is en dat die Sand sytak die inval bron is. Omdat die Sand sytak buite die Kruger Nasionale Park ontstaan, en slegs 'n klein gedeelte onder park bestuur is, is die implikasies vir verspreiding belangrik. / Financial support from the DST-NRF Centre of Excellence for Invasion Biology and the Working for Water (WfW) Programme through their collaborative project on “Research for Integrated Management of Invasive Alien Species” and Stellenbosch University’s Subcommittee B Young Researcher's Fund

A invasão por gramíneas africanas é uma das principais ameaças à diversidade biológica e funcional das formações savânicas do cerrado. Uma questão central em invasões por plantas é entender como espécies oriundas de sistemas com diferentes pressões seletivas são capazes de adquirir e utilizar recursos em um novo ambiente. O objetivo deste estudo foi de comparar os padrões de uso luz e água de gramíneas nativas e invasoras em duas condições de cobertura arbórea ao longo de um gradiente de invasão em uma área de cerrado localizada no Parque Estadual do Juquery - SP. O efeito da presença da cobertura sobre os indivíduos foi investigado através de amostragens em fisionomia de campo (dossel ausente) e de campo cerrado (dossel presente). A hipótese de que o desempenho no uso de recursos de invasoras supera o de nativas em presença de cobertura arbórea foi testada através da medição de aspectos ligados à capacidade de interceptação da luz e ao desempenho de uso da luz e água associado ao potencial de ganho de carbono. As gramíneas africanas apresentaram padrões de arranjo espacial das folhas e estrutura foliar que diferem dos de nativas. As copas das invasoras apresentaram maior grau de sobreposição entre as folhas. As folhas das invasoras exibiram menores inclinações, menor espessura e menor razão de massa seca por área. A presença do dossel arbóreo favoreceu o transporte fotoquímico de elétrons de invasoras e desfavoreceu o de nativas. As principais diferenças nas dinâmicas de trocas gasosas entre nativas e invasoras foram observadas quando as razões foram expressas por massa seca foliar. Diferenças nas relações entre as variáveis indicam que a relação do padrão de investimento em tecidos foliares com o potencial fotossintético pode implicar em respostas contrastantes de nativas e invasoras aos regimes de irradiância nas fisionomias de campo sujo e campo cerrado / Invasions by African grasses constitutes one of the greatest threats to the biological and functional diversity of the cerrado savannas. A central question to understand plant invasions is how species originating from systems with distinct selective pressures are able to acquire and utilize resources in their new environments. The objective of this study was to compare native and invasive grasses with respect to their light and water use patterns in two sites with differing canopy tree cover along an invasion gradient in a cerrado area in the Juquery State Park, SP. The effect of the canopy tree cover on the grass individuals was investigated through sampling in the cerrado physiognomies of \'campo sujo\' (tree cover absent) and \'campo cerrado\' (tree cover present). The hypothesis that invaders outperform natives under the presence of canopy tree cover in terms of resource-use was tested through field measurements of traits related to light interception capacity and to the performance in light and water use associated with carbon gain potentials. Invasive African grasses showed distinct patterns of crown architecture and leaf structure when compared to natives. Invaders showed a greater degree of leaf overlapping in the crown. Invaders leaves were thinner, relatively more planophile, and showed smaller leaf dry mass per area ratios. The presence of canopy tree cover favored the invaders photochemical electron transport while disfavored natives. The main differences concerning native and invaders gas-exchange dynamics were observed on a leaf dry mass basis. The relation between patterns of leaf tissue investment and photosynthetic potentials could imply in contrasting responses of native and invasive grasses to the irradiance regime in differing cerrado vegetation types

As invasões por espécies exóticas vêm acarretando a homogeneização das comunidades naturais tornando-se; atualmente, a segunda principal causa da perda da biodiversidade. Entre os domínios brasileiros mais afetados está o Cerrado, que possui mais de 11 fitofisionomias e 33% de toda biodiversidade vegetal do país. As alterações pela atividade humana foram facilitadas em decorrência das áreas de Cerrado estarem dispostas em quase todo o território nacional e, na fisionomia de campo sujo, em particular, pela facilidade de mecanização e consequentemente pelo emprego da agricultura em larga escala. Dessa forma, foram inseridas gramíneas exóticas africanas para a melhoria da qualidade e produtividade do forrageio. Essas gramíneas se ajustaram tão bem em áreas de campo sujo que se expandiram desordenadamente e conduziram a um desequilíbrio estrutural e funcional destas áreas. O objetivo deste estudo foi investigar os padrões de utilização de recursos em gramíneas nativas e invasoras em um campo sujo levando em conta os períodos de maior e menor disponibilidade hídrica e os possíveis efeitos das variáveis microclimáticas sobre as respostas fisiológicas de gramíneas nativas e invasoras. A área de estudo localiza-se no Parque Estadual do Juquery, município de Franco da Rocha, Estado de São Paulo. As espécies utilizadas para o estudo foram as nativas: Tristachya leiostachya, Imperata brasiliensis, Axonopus pressus e Axonopus siccus. E as invasoras: Urochloa brizantha e Melinis minutiflora. As respostas fisiológicas frente às flutuações entre os períodos chuvoso e seco foram obtidas através de análises de trocas gasosas, análise de fluorescência e de medições das variáveis microclimáticas ao longo de 23 meses entre 8:00 e 12:00. Os resultados indicaram que gramíneas nativas investem 16% menos em fotoproteção do que gramíneas invasoras, sendo estas mais sensíveis aos fotodanos. No entanto, gramíneas nativas investem 13% a mais na síntese de clorofila total e se equivalem na capacidade de assimilação de carbono (A) e quanto as taxas de transporte de elétrons (ETR) indicando uma aprovável equivalência no uso da luz que pode estar associada a maiores taxas de eficiência no uso do nitrogênio em nativas tanto no período chuvoso (70%) quanto no seco (70%), além da ausência de diferença estatística no conteúdo de fósforo foliar por massa seca (Pmg/ms) e na eficiência no uso deste recurso (EUP). A condutância estomática (gs) e transpiração (E) foram respectivamente 22 e 60% maiores nas gramíneas nativas, por isso, a eficiência no uso da água foi 60% menor nestas em comparação às invasoras, tanto no período chuvoso quanto no seco. Tal regulação da condutância provavelmente esteve relacionada ao conteúdo de nitrogênio foliar que foi 100% maior nas invasoras no período chuvoso e 85% maior no período seco. Como esperado, gramíneas nativas apresentaram respostas de menor sensibilidade à seca em comparação às invasoras. Gramíneas invasoras, por sua vez, não corroboraram a expectativa de maior oportunismo em relação às nativas no período chuvoso quanto a captação de recursos, assim como, diferentemente do esperado, não se mostraram tão sensíveis à seca devido a um comportamento conservador no uso da água / Invasions by alien species are causing the homogenization of natural communities, becoming; currently the second leading cause of biodiversity loss. And among the most affected areas are the Brazilian Cerrado, which has more than 11 vegetation types and 33 % of all the country\'s biodiversity. The changes were facilitated by human activity due to the Cerrado areas are arranged in almost all the national territory. And in the face of dirty field, in particular, the ease of mechanization and consequently the employment of agriculture. Thus, African exotic grasses to improve the quality and productivity of foraging were inserted. These grasses have adjusted so well in areas of Cerrado dirty field that expanded wildly and led to a structural and functional imbalance of these areas. The aim of this study was to investigate the patterns of resource use of native grasses and weeds in a field Cerrado dirty taking into account periods of high and low water availability and the possible effects of microclimatic variables on the physiological responses of native and invasive grasses . The study area is located in the Juquery State Park, city of Franco da Rocha, State of Sao Paulo. The species used for the study were native: Tristachya leiostachya , Imperata brasiliensis , Axonopus pressus and Axonopus siccus. And the invaders: Urochloa brizantha and Melinis minutiflora. The physiological responses to fluctuations between wet and dry periods were obtained from analyzes of gas exchange, fluorescence analysis and measurements of microclimate variables over 23 months between 8:00 am and 12:00 am. The results indicate that native grasses Invest 16 % less than photoprotection grass weeds, which are more sensitive to photodamage. However, native grasses invest 13 % more in the synthesis of chlorophyll and are equivalent in capacity to carbon assimilation (A) and on the rate of electron transport (ETR) indicating an approvable equivalence in the use of light that can be associated with higher rates of nitrogen use efficiency in native both in the rainy season (70%) and the dry (70 %). Besides the lack of statistical difference in the content of phosphorus of leaf dry mass (Pmg/ms) and efficient use of this resource (EUP). The stomatal conductance (gs ) and transpiration ( E) were respectively 22 and 60 % higher in native grasses , so ; efficiency in water use was 60 % lower compared to these invasive both in the rainy season as in the dry. Such regulation conductance probably was related to leaf N content that was 100 % higher in invasive in the rainy season and 85 % higher in the dry season. As expected, responses of native grasses showed lower sensitivity to drought compared to invasive. Invasive grasses in tur ; does not support the expectation of greater opportunism in relation to native in the rainy season as fundraising , as well as , unlike expected, were not as sensitive to drought due to a more conservative behavior in water use

The red imported fire ant, Solenopsis invicta, is a South American native introduced in Alabama in the early 20th century. This predatory species has rapidly spread throughout the southeastern US and parts of the West Indies, inflicting great ecological and economic damage. For example, Solenopsis invicta is known to attack the eggs and hatchlings of ground nesting birds and reptiles. The ants swarm into the nests attacking hatchlings and diminishing their chance for survival. My thesis research aimed to survey the distribution of ants on sea turtle nesting beaches in South Florida and St. Croix, USVI, and to evaluate the possible threat of Solenopsis invicta and other predatory ants to sea turtle hatchlings. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2013.

[Truncated abstract] Mediterranean southwest Australia, a global biodiversity hotspot, has nutrient deficient soils, exacting climatic conditions and is species rich with 7380 native vascular plant species, of which 49% are endemic. The region is expected to experience one of the world's highest degrees of biodiversity loss and change in the coming decades, with introduced species presenting a major threat. Limited knowledge is available on the mechanisms of ecosystem change associated with invasion and fire in this biodiversity hotspot region. Banksia woodland, an iconic complex species-rich natural ecosystem is one of the major vegetation types of the coastal sandplain, extending from 15 to 90 km inland and 400 kms along the west coast. The following hypothesis was tested to explore the ecological impacts of invasion: Is invasion of Banksia woodland by the introduced species Ehrharta calycina and Pelargonium capitatum accompanied by an alteration in ecosystem properties and processes, whereby the degree of change is related to fire frequency and abundance of introduced species? Different vegetation conditions, i.e. Good Condition (GC), Medium Condition (MC), Poor Condition invaded by Ehrharta calycina (PCe) and Poor Condition invaded by Pelargonium capitatum (PCp) were utilized for field assessments. ... In the soil seed bank, species numbers and germinant density decreased significantly for native and seeder (fire sensitive) species between GC sites and invaded sites. Surprisingly 52% of germinants at GC sites were from introduced species, with much of the introduced soil seed bank being persistent. Native species were dominated by perennial shrubs, herbs and sedges, while introduced species were dominated by perennial and annual grasses and herbs. Invasion by introduced species, associated with frequency of fire, altered the ecosystem, thus disadvantaging native species and improving conditions for even greater invasion within the Banksia woodland. Significantly higher soil phosphorus P (total) and P (HCO3) were found at PCe and PCp sites compared to GC sites. Leaf nutrient concentrations of phosphorus were significantly higher, and potassium and copper significantly lower in PCe and PCp sites, with introduced species having significantly greater concentrations than native species (except Manganese). This study demonstrated the key role of phosphorus in the Banksia woodland, in contrast to other research which identified nitrogen as the major nutrient affected by invasion. Higher levels of soil and leaf phosphorus, loss of species diversity and function, changes in fire ecology and canopy cover and a limited native soil seed bank make restoration of a structural and functional Banksia woodland from the soil seed bank alone unlikely. Without management intervention, continuing future fire is likely to result in a transition of vegetation states from GC to MC and MC to PC. The knowledge gained from this study provides a better ecological understanding of the invasive process. This enhanced understanding will enable the development of adaptive management strategies to improve conservation practices within a biodiversity hotspot and reduce the impact of the key threatening process of invasion.

Les invasions biologiques sont aujourd'hui un enjeu majeur en termes de biodiversité et de bien-être. Qu'il s'agisse de prévention ou de limitation des dommages, la gestion des invasions biologiques présente les caractéristiques d'un bien public et, comme telle, relève de l'analyse coût-avantage. Dans cette optique, la thèse traite du problème du contrôle d'une espèce invasive dénuée de valeur marchande entrant en compétition spatiale avec une espèce native exploitée commercialement. La première partie propose un modèle bioéconomique théorique représentant l'interaction entre les deux espèces et la contribution des exploitants à la diffusion de l'invasion, dans lequel les variables de commande sont les efforts de prélèvement sur chaque espèce et la fonction-objectif est la rente de ressource de l'espèce native diminuée du coût de contrôle de l'espèce invasive. L'analyse du modèle à l'équilibre débouche sur une alternative radicale entre quasi-éradication et laisser-faire face à l'invasion. L'optimisation dynamique du modèle montre ensuite qu'il existe une trajectoire conduisant à un état stationnaire optimal combinant l'exploitation rentable du stock natif et la stabilisation de l'invasion à un niveau intermédiaire. Cet état existe si les coûts unitaires de l'effort de pêche du stock natif et de l'effort de nettoyage des zones envahies, les coefficients de dispersion naturelle et anthropique de l'espèce invasive, et le taux d'actualisation ne sont pas trop élevés. Cet état peut être atteint si le niveau d'invasion initial n'est pas trop important. La seconde partie est consacrée au cas d'étude de l'invasion de la pêcherie de coquille Saint-Jacques de la baie de Saint-Brieuc par la crépidule. L'analyse coût-avantage du cas de la baie de Saint-Brieuc s'effectue à l'aide d'un modèle structural d'exploitation de la coquille Saint-Jacques couplé à un modèle d'invasion spatiale binaire, à partir desquels un programme de contrôle de l'invasion est étudié.

Durant cette thèse, nous avons cherché à (1) déterminer le rôle du climat et de facteurs biotiques associés à celui-ci dans la sélection d'adaptations locales chez des parasitoides de drosophiles (2) comprendre comment la plasticité phénotypique des traits d'histoire de vie a évolué en réponse à l'environnement et aux stratégies de maturation de ces organismes. Contrairement aux consommateurs primaires pour lesquels la température d'origine affecte directement l'évolution des histoires de vie, il semblerait que les facteurs biotiques dépendants du climat comme la distribution des hôtes et la compétition interspécifique soient le moteur principal de la sélection naturelle chez les parasitoides. La distribution des hôtes expliquerait les très fortes variations géographiques observées sur des échelles fines, comme l'existence de populations proovogéniques ou synovogéniques au sein d'une même espèce, ainsi que de populations capables ou non de lipogenèse à l'age adulte. Cette dernière variation aurait notamment affecté les compromis entre traits et l'évolution du taux de métabolisme. La force de la plasticité phénotypique présente également de fortes variations géographiques. Celles-ci peuvent être attribuées à la variabilité de l'environnement d'origine et non aux histoires de vie des organismes. Prédire l'évolution des parasitoides en réponse au réchauffement global nécessite donc d'intégrer non pas seulement un effet direct du climat, mais également les facteurs biotiques et la variabilité environnementale associés au climat.

Biological invasions are an important issue of global change and an increased understanding of invasion processes is of crucial importance for both conservation managers and international trade. In this thesis, I have studied the invasion of the brown seaweed Fucus evanescens, to investigate the fate and effect of a perennial, habitat-forming seaweed introduced to a coastal ecosystem. A long-term study of the spread of F. evanescens in Öresund (southern Sweden) showed that the species was able to expand its range quickly during the first 20 years after the introduction, but that the expansion has been slow during the subsequent 30 years. Both in Öresund and in Skagerrak, the species is largely restricted to sites where native fucoids are scarce. Laboratory experiments showed that the restricted spread of F. evanescens cannot be explained by the investigated abiotic factors (wave exposure and salinity), although salinity restricts the species from spreading into the Baltic Sea. Neither did I find evidence for that herbivores or epibiota provide biotic resistance to the invader. On the contrary, F. evanescens was less consumed by native herbivores, both compared to the native fucoids and to F. evanescens populations in its native range, and little overgrown by epiphytes. Instead, the restricted spread may be due to competition from native seaweeds, probably by pre-occupation of space, and the establishment has probably been facilitated by disturbance. The studies provided little support for a general enemy release in introduced seaweeds. The low herbivore consumption of F. evanescens in Sweden could not be explained by release from specialist herbivores. Instead, high levels of chemical anti-herbivore defence metabolites (phlorotannins) could explain the pattern of herbivore preference for different fucoids. Likewise, the low epibiotic colonisation of F. evanescens plants could be explained by high resistance to epibiotic survival. This shows that colonisation of invading seaweeds by native herbivores and epibionts depends on properties of the invading species. The large differences between fucoid species in their quality as food and habitat for epibionts and herbivores imply that invasions of such habitat-forming species may have a considerable effect on a number of other species in shallow coastal areas. However, since F. evanescens did not exclude other fucoids in its new range, its effect on the recipient biota is probably small.