Thesis (MSc )--Stellenbosch University, 2010. / ENGLISH ABSTRACT: The patterns of spread of non-indigenous species and the factors influencing their distribution have been studied infrequently on Southern Ocean Islands, where the prevention and control of biological invasions is a priority for conservation management. Owing to its remoteness and relatively small size, sub-Antarctic Marion Island provides an ideal opportunity to investigate the patterns of spread of invasive species and the factors likely influencing the distribution of alien species. Therefore, this study provides a spatially explicit documentation of the alien and invasive vascular plant species on Marion Island, the change in their distribution patterns through time, an assessment of the correlates of the current distributions, documentation of species rich areas, and provides recommendations for control based on these data and life history data available for each species. To ensure comprehensive coverage for the current estimate of distributions, a combination of a systematic (spatially explicit) survey and an ad hoc data collection method was used to examine the abundance and occupancy of each alien plant species across the island. The spatially explicit survey was used to identify sampling sites on a ½ minute by ½ minute scale (926 m × 926 m) which resulted in 348 sites across the island, whereas 2356 additional presence records were added during the ad hoc sampling. The residence time of each species was also estimated from the first records reported in various literature. While it has been documented in many studies that residence time is an important factor explaining species distribution, this study showed that distribution of alien species on Marion Island is not explained by residence time. The alien plant species on Marion Island differ markedly in their occupancy, distribution and rate of expansion. Agrostis stolonifera, Cerastium fontanum, Poa annua and Sagina procumbens are all expanding their ranges, whereas the range of Stellaria media appears to be contracting. Cerastium fontanum is the most widespread species on Marion Island while Sagina procumbens is the most abundant (number of records) and most rapidly expanding species on both Marion and Prince Edward Islands. Generalized linear models demonstrated that altitude, distance to human disturbed sites, and nearest neighbour presence are all significant and often strong correlates of spatial variation in alien vascular plant species richness and individual species presences on Marion Island. In particular, alien species richness on Marion Island declines sharply with increasing elevation, and that the distribution of the most widespread species is spatially aggregated. The surroundings of the Meteorological station and Mixed Pickle hut exhibit high richness and a high likelihood of the presence of individual species. None of the models developed for this study resulted in habitat type being significant explanatory variable for the distribution of any of the alien plant species, although from the records alone it is clear that the salt spray vegetation type is avoided. Together with assessments of the life history of the species, the current distribution data suggest that few alien plant species can be easily eradicated. Most are already too widespread already for cost effective eradication. Moreover, given the current rate of spread of Agrostis stolonifera, Cerastium fontanum, Poa annua and Sagina procumbens, and their apparent lack of habitat preference, it seems likely that within the next few decades many lowland habitats will be dominated by these species. By contrast, Rumex acestosella, Agrostis gigantea, Agropyron repens, Juncus effusus, Luzula cf. multiflora and the newly established shrub species should be the subject of control or eradication efforts. This thesis provides comprehensive baseline data on alien vascular plant distributions as a benchmark for future assessments. / AFRIKAANSE OPSOMMING: Die verspreidingspatrone van uitheemse spesies en die faktore wat hulle verspreiding beïnvloed is selde op Suidelike Oseaan Eilande bestudeer, waar die voorkoming en beheer van biologiese indringing ‘n prioriteit is vir bewaringsbestuur. Te danke aan sy afgeleëndheid en relatiewe klein grootte, voorsien sub-Antarktiese Marion Eiland ‘n ideale geleentheid om die patrone van verspreiding van indringer spesies en die faktore wat moontlik die verspreiding van uitheemse spesies beïnvloed te ondersoek. Daarom voorsien hierdie studie ‘n ruimtelik eksplisiete dokumentering van die uitheemse en indringer vaatplant spesies op Marion Eiland, die verandering in hulle verspreidingspatrone oor tyd, ‘n bepaling van die korrelasies van die huidige verspreidings, dokumentering van spesiesryke areas, en voorsien aanbevelings vir beheer gebasseer op hierdie data en die lewensloopdata beskikbaar vir elke spesie. Om omvattende dekking vir die huidige skatting van vespreidings te verseker, is ‘n kombinasie van sistematiese en ad hoc metodes gebruik om die talrykheid en besetting van elke uitheemse spesie oor die eiland te ondersoek. Die ruimtelik eksplisiete opname is gebruik om moniteringsplotte op ‘n ½ minuut by ½ minuut skaal (926 m × 926 m), met die resultaat van 348 plotte oor die eiland, terwyl 2356 addisionele voorkomsrekords bygevoeg is tydens die ad hoc versameling. Die verblyftyd van elke spesie is ook beraam vanaf die eerste rekords in die literatuur. Terwyl dit aangeteken is dat verblyftyd ‘n belangrike faktor is om spesie verspreiding te verduidelik, wys hierdie studie dat verspreiding van uitheemse spesies op Marion Eiland nie verduidelik word deur verblyftyd nie. Die uitheemse spesies op Marion Eiland verskil aansienlik in hulle besetting, verspreiding en tempo van uitbreiding. Agrostis stolonifera, Cerastium fontanum, Poa annua en Sagina procumbens brei almal hulle areas uit, terwyl die area van Stellaria media bleik te krimp. Cerastium fontanum is die mees wydverspreide spesie op Marion Eiland, terwyl Sagina procumbens die oorvloedigste (aantal rekords) en die vinnigste uitbreidende spesie is op beide Marion en Prince Edward Eiland. Veralgemeende lineêre modelle het aangetoon dat hoogte, afstand na menslike versteurde plotte, en naaste buurman teenwoordigheid almal betekenisvol is en gereeld sterk korreleer met ruimtelike variasie in uitheemse vaatplant spesierykheid en individuele spesie voorkoms op Marion Eiland. Uitheemse spesierykheid op Marion Eiland neem in besonder sterk af met ‘n toename in hoogte en die verspreiding van die mees wyduitgebreide spesies is ruimtelik saamgesteld. Die omgewing om die Weerstasie en Mixed Pickle hut toon hoë rykheid en ‘n hoë waarskynlikheid van die voorkoms van individuele spesies. Nie een van die modelle wat vir hierdie studie ontwikkel is toon ‘n belangrikheid in habitat tipe as ‘n verduidelikbare veranderlike vir die verspreiding van enige van die uitheemse plant spesies nie, alhoewel vanuit die rekords alleen is dit duidelik dat soutsproei plantegroei nie verkies word nie. Tesame met ondersoeke oor die lewensloop van die spesies, stel die huidige verspreidingsdata voor dat min uitheemse plant spesies maklik uitgeroei kan word. Die meeste is eenvoudig te wydverspreid vir koste effektiewe uitroeiing. Voorts, gegewe die huidige tempo van uitbreiding van Agrostis stolonifera, Cerastium fontanum, Poa annua en Sagina procumbens en hulle skynbare gebrek aan habitat voorkeur, is dit hoogs waarskynlik dat baie van die laerliggende habitatte binne die volgende paar dekades gedomineer sal word deur hierdie spesies. In teenstelling, Rumex acestosella, Agrostis gigantea, Agropyron repens, Juncus effusus, Luzula cf. multiflora en die nuutgevestigde struik spesie moet die fokus wees van beheer/uitroeiing pogings. Hierdie tesis voorsien omvattende basislyn data oor uitheemse vaatplantverspreidings as ‘n standaard vir toekomstige ondersoeke. / This project was funded by the South African National Antarctic Programme of the National Research Foundation (NRF – SANAP). Additional funding was provided by The Prince Edward Islands book fund.