Aquatic plants (macrophytes) are important components of freshwater ecosystems and serve numerous purposes, physical and biological, that help to structure aquatic communities. Although macrophytes represent an essential component of stable aquatic communities, invasive macrophytes may negatively alter ecosystem properties. Non-native, invasive species have been identified as a major cause of biodiversity loss and the increasing prevalence of invasive species has prompted studies to help understand their impacts and to conserve biodiversity. Studying mechanisms of invasion also gives insight into how communities are structured and assembled. This study examined mechanisms that contribute to macrophyte invasion. First, I reviewed literature concerning mechanisms of macrophyte invasion. Mechanisms identified with this review were then placed within the context of the invasion process and potential taxonomic biases were discussed. Second, a set of classic invasion hypotheses were tested, including biotic resistance, disturbance, and stress, using mixed-effects models on survey data collected from twenty-nine lakes across the United States. Finally, using the same survey data, I performed an observational test of Darwin’s Naturalization Hypothesis at a small (point) and large (lake) scale for two highly invasive macrophytes, Potamogeton crispus and Myriophyllum spicatum. Results of the first study indicated that many invasion mechanisms have been tested with fully aquatic macrophytes with varied levels of support. In addition, there is likely a taxonomic bias depending on geographic location of the invaded area. The second study indicated that biotic interaction, disturbance, and stress interact, often in non-linear ways to influence probability of an invasive species occurring at a location. However, models containing these variables explained a relatively low percentage of variation in probabilities. Finally, there was no support for Darwin’s naturalization hypothesis at either a point or lake scale. Future research should continue the search for mechanisms that allow introduced species to establish. It is likely that general principles do not exist, at least among comparisons across ecosystem types. However, ecologists should continue to search for general patterns within definable ecosystem units to increase understanding about factors contributing to invasibility.

The degree to which biotic interactions influence invasion success may partly depend on the evolutionary relationship between invaders and native species. In particular, since host-use by enemies such as invertebrate herbivores and fungal pathogens tends to be phylogenetically conserved, exotic plants that have close native relatives in the invaded range should be more likely to interact with enemies. In this thesis, I explore this idea using a series of experiments and field surveys at nested taxonomic levels. My results indicate that exotics from multiple plant families experience lower damage if their average phylogenetic distance from locally co-occurring native family members is higher. I then demonstrate that within the Asteraceae, foliar and capitular damage are lower on exotic compared to native species. Both damage types had a relatively large phylogenetic component, but did not decline with phylogenetic distance to native or exotic confamilials. Finally, I show that communities with versus without close relatives are unlikely to differ in resistance to the novel invader, Solidago virgaurea: biotic resistance imposed by competitors, generalist vertebrates, and specialist invertebrates resulted in similar patterns of damage and mortality regardless of the presence of congeneric natives. In some cases, effects of biota were positive: growth of S. virgaurea seedlings in soils collected near congeneric natives was enhanced more than in soils from communities where congenerics were absent. Overall, these results suggest that biotic interactions between exotic and native species can be phylogenetically structured, although trends based on distance measures tend to be weak. In some cases, damage does decline with phylogenetic distance to native species; however this trend is unlikely to be a strong force limiting invasion or structuring plant communities. These results have significant implications for current theories of invasion biology including the "Enemy Release Hypothesis" and "Darwin's Naturalization Hypothesis", as well as for community phylogenetics.

The degree to which biotic interactions influence invasion success may partly depend on the evolutionary relationship between invaders and native species. In particular, since host-use by enemies such as invertebrate herbivores and fungal pathogens tends to be phylogenetically conserved, exotic plants that have close native relatives in the invaded range should be more likely to interact with enemies. In this thesis, I explore this idea using a series of experiments and field surveys at nested taxonomic levels. My results indicate that exotics from multiple plant families experience lower damage if their average phylogenetic distance from locally co-occurring native family members is higher. I then demonstrate that within the Asteraceae, foliar and capitular damage are lower on exotic compared to native species. Both damage types had a relatively large phylogenetic component, but did not decline with phylogenetic distance to native or exotic confamilials. Finally, I show that communities with versus without close relatives are unlikely to differ in resistance to the novel invader, Solidago virgaurea: biotic resistance imposed by competitors, generalist vertebrates, and specialist invertebrates resulted in similar patterns of damage and mortality regardless of the presence of congeneric natives. In some cases, effects of biota were positive: growth of S. virgaurea seedlings in soils collected near congeneric natives was enhanced more than in soils from communities where congenerics were absent. Overall, these results suggest that biotic interactions between exotic and native species can be phylogenetically structured, although trends based on distance measures tend to be weak. In some cases, damage does decline with phylogenetic distance to native species; however this trend is unlikely to be a strong force limiting invasion or structuring plant communities. These results have significant implications for current theories of invasion biology including the "Enemy Release Hypothesis" and "Darwin's Naturalization Hypothesis", as well as for community phylogenetics.

Thesis (PhD)--Stellenbosch University, 2010. / ENGLISH ABSTRACT: Biological invasions are a growing threat to biodiversity, trade and agriculture in South Africa. Though alien reptiles and amphibians (herpetofauna) are not currently a major issue, escalating problems worldwide and increased trade in South Africa suggest a possible increase in future problems. In this thesis I explore practical measures for risk assessment implementable under national legislation. I began by documenting record-keeping and legislative differences between provinces in South Africa. This revealed some serious deficiencies, complicating attempts to compile accurate inventories and discern import trends. International trade data, however, revealed an exponential increase in the number of imports to South Africa over the last 30 years. Characterising the abundance of species in this trade is important as species introduced in large numbers pose a higher establishment risk. In South Africa, I found a tendency for venomous and expensive species to be traded in low numbers, whereas species that are easy to breed and handle, or that are colourful or patterned are traded in higher numbers. Unlike South Africa, California and Florida have had a large number of well-documented herpetofaunal introductions. These introductions were used to verify the role of several key predictors in species establishment. I first evaluated the role of each variable separately. I examined different approaches for bioclimatic modelling, the predictive power of different sources of distribution data, and methods of assigning a climate-match score. I also present the first test of Darwin’s naturalization hypothesis for land vertebrates using two new phylogenies inferred for native and introduced reptiles in California and Florida. I then used boosted regression trees (BRT) to infer the relative contribution of each factor to species establishment success. Results from the BRTs were incorporated into a user-friendly spreadsheet model for use by assessors inexperienced in complex modelling techniques. Introduction effort was found to be the strongest contributor to establishment success. Furthermore, species with short juvenile periods were more likely to establish than species that started breeding later, as were species with more distant relatives in regional biotas. Average climate match and life form were also important. Of the herpetofaunal groups, frogs and lizards were most likely to establish, while snakes and turtles established at much lower rates, though analysis of all recorded herpetofaunal introductions shows slightly different patterns. Predictions made by the BRT model to independent data were relatively poor, though this is unlikely to be unique to this study and can be partially explained by missing data. Though numerous uncertainties remain in this field, many can be lessened by applying case by case rules rather than generalising across all herpetofaunal groups. The purpose for import and potential trade volume of a species will influence the threat it poses. Considering this in conjunction with a species’ environmental tolerances and previous success of species with similar life histories, should provide a reasonable and defendable estimate of establishment risk. Finally, a brief summary of the potential impacts of introduced alien herpetofauna is provided in the thesis. / AFRIKAANSE OPSOMMING: Indringer spesies hou ‘n al groter bedreiging in vir die biodiversiteit, handel en landbou van Suid- Afrika. Alhoewel uitheemse reptiele en amfibieërs (herpetofauna) tans nie ‘n groot bedreiging in Suid-Afrika is nie, dui groeiende probleme wêreldwyd asook 'n toename in plaaslike handel op moontlike toekomstige probleme. In hierdie tesis, ondersoek ek praktiese metodes vir risikobepaling wat onder nasionale wetgewing toegepas kan word. Ek begin deur die verskille in stoor van rekords en wetgewing tussen provinsies te dokumenteer. Hierdie proses het ernstige tekortkominge uitgewys, wat pogings om akkurate inventarisse saam te stel en invoertendense te bepaal, bemoeilik. Internasionale handelsdata het egter getoon dat daar ‘n eksponensiële toename in die hoeveelheid invoere na Suid-Afrika oor die laaste 30 jaar was. Die hoeveelheid spesies in hierdie handel is belangrik omdat spesies wat in groot hoeveelhede ingevoer word, ‘n hoër vestigingsrisiko het. In Suid-Afrika is ‘n tendens gevind vir handel in giftige en duur spesies teen lae hoeveelhede, terwyl spesies wat maklik teel, maklik hanteer kan word en kleurvol is of mooi patrone het, in groter hoeveelhede mee handel gedryf word. Kalifornië and Florida, in teenstelling met Suid-Afrika, het ‘n hoë aantal goed-gedokumenteerde gevalle van herpetofauna wat in die natuur vrygestel is. Hierdie introduksies was gebruik om die rol van verskeie belangrike faktore in die vestiging van populasies te bepaal. Eerstens het ek die rol van elke faktor apart ondersoek. Ek het verskillende benaderinge vir bioklimatiese model-bou ondersoek, die akuraatheid van verskillende bronne van distribusiedata getoets en drie metodes om ‘n “climate match score” te bereken, voorgestel. Ek bied ook die eerste toets van Darwin se naturalisasie-hipotese vir landwerveldiere aan, deur gebruik te maak van twee nuwe filogenieë wat ek gebou het vir inheemse en ingevoerde reptiele in Kalifornië en Florida. Ek het verder gebruik gemaak van “boosted regression trees” (BRT) om die relatiewe bydrae van elke faktor tot die vestigings-potensiaal van spesies te bepaal. Resultate van hierdie BRTs was ingekorporeerd in ‘n gebruikersvriendelike ontledingstaat wat deur bestuurders, onervare in komplekse modelboutegnieke, gebruik kan word. Invoer-hoeveelheid was die faktor wat die sterktste bygedra het tot suksesvolle vestiging. Verder is spesies met kort jeugperiodes en dié met verlangse familie in streeks-biotas, meer geskik om suksesvol te vestig. Gemiddelde klimaatooreenstemming en lewensvorm was ook belangrik. Paddas en akkedisse was die mees waarskynlikste van die herpetofauna groepe om populasies te vestig, terwyl slange en skilpaaie teen laer tempo’s populasies gevestig het, alhoewel analise van alle gedokumenteerde gevalle van herpetofauna-invoerings wêreldwyd effens verskillende tendense toon. Voorspellings wat deur die BRT-model vir onafhangklike data gemaak is was redelik swak, alhoewel hierdie resultaat onwaarskynlik nie uniek aan die studie is nie, en word gedeeltelik verduidelik deur die gebrek aan data. Alhoewel talle onsekerhede steeds bestaan, kan dié verminder word deur geval-tot-geval reëls toe te pas eerder as om vir herpetofauna as ‘n groep te veralgemeen. Die doel van invoer en potensiële handel-volumes van ‘n spesie, sal die bedreiging wat die spesie toon, bepaal. Hierdie faktore moet saam met omgewingstoleransie en voorafgaande sukses van spesies met soortgelyke lewenswyses oorweeg word, om ‘n aanvaarbare en verdedigbare beraming van vestigingsrisiko te gee. Laastens, word ‘n kort opsomming van die effekte wat uitheemse herpetofauna mag hê, verskaf.