Ailanthus altissima : an assessment of its distribution at different spatial scales and options for management in South Africa

Walker, Gareth Alexander (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.


ENGLISH ABSTRACT: The constant flow of goods into and out of cities has resulted in the proliferation of invasive species in urban areas and, because of this, cities are viewed as ‘hotspots’ for invasive species. The rise of global trade and long distance transportation will ensure that further introductions will take place in urban ecosystems. Conditions that have been altered due to human activities enable some alien species to proliferate rapidly throughout urban centres, often also spreading into natural and semi-natural areas at the urban/wildland interface. Evaluating the current distribution of an invader by analysing potential patterns in its distribution, as well as determining the potential distribution of a species over multiple landscapes and identifying factors that promote its spread, is essential for determining the full extent of an invader. Understanding 1) where a species is; 2) where a species may spread to; and 3) the factors that promote the spread of a species are all needed to gain an understanding of the biological and biogeographical preferences that facilitate the spread of the species. This is best achieved by using a multi-scale analysis that studies all aspects of an invasive species over a variety of spatial scales. This study focusses on a notorious northern hemisphere invader - Ailanthus altissima – that, although having been introduced into South Africa in 1834, is yet to replicate similar levels of invasion success. I adopted for a multi-scale analysis of the species evaluating the current and potential distribution of the species at a global scale (Chapter 1), national scale (Chapter 2) and local scale (Chapter 3). Each chapter addresses a central aim in which a number of research questions are evaluated. In Chapter 1 I assess the biogeography, distribution, and habitat suitability of A. altissima by evaluating the current and potential distribution of the species (based on climatic suitability) at both a global scale and a national scale (in South Africa). The main conclusions of this chapter are that 1) online databases (although outdated at times) provide an accurate representation of the distribution of invasive species and should therefore be accessed. This was ascertained by cross referencing a number of online databases to evaluate whether they were largely in agreement with one another in terms of the distribution of an invader; and 2) modelling the potential distribution of an invasive species based on climatic suitability serves as a good first approximation of potential species spread. Although this may be true, it was also suggested in this chapter that to accurately quantify the potential distribution of an invasive species, other distribution drivers (such as land use and human mediated disturbances) need to be incorporated into the design of a species distribution model. Chapter 2 evaluates the degree of range filling exhibited by A. altissima in South Africa and identifies areas at risk of future invasion. This chapter also identifies specific factors that promote the spread of the species at a global and national scale. I determined that at the global scale, climate had the highest influence on the distribution of A. altissima whereas at the national scale, human mediated disturbances exerted a higher influence on the distribution of the species. This chapter builds on the concepts proposed in Chapter 1 by highlighting the importance of incorporating different environmental variables at various spatial scales to identify potential invasion ‘hotspots’. I conclude that the multi-scale approach presented in this chapter enables the early detection of invasive species, preventing damage associate with their potential spread. Also, the novelty of this approach is particularly effective when it comes to analysing urban invaders. The third chapter evaluates the distribution pattern of A. altissima within the City of Cape Town by using the suburb of Newlands as a case study. In addition, it determines potential options for management at a city scale. It can be concluded that fine-scale analysis of invasive species distributions is best achieved using Geographic Information System (GIS) techniques and that such these ultimately help to identify potential areas susceptible to future invasion while delimiting unsuitable areas. I determined that A. altissima has a preference for affluent urban areas with podzolic soils that are exposed to high levels of rainfall within the City of Cape Town. Using this information, I determined that there is a large capacity for further spread by the species throughout the city. The systematic approach that we used in Newlands is an effective way of gauging the full extent of an invader and is especially effective for evaluating the population structure of a particular invasive species. Future studies should adopt this approach in conjunction with remote sensing techniques to achieve the best possible results. The final chapter of this thesis provides the overall conclusions in which all the main findings are highlighted. Altogether, the approach presented in this thesis is an effective method that could be used on other emerging, urban invaders globally. The systematic, multi-scale analysis proposed in this project showed high levels of success and it is my belief that this project achieved its desired aims.

AFRIKAANSE OPSOMMING: Die voordurende vloei van goedere in en uit stede het tot gevolg dat indringerspesies vinnige in stedelike gebiede versprei, en stede dus as “brandpunte” vir indringerspesies beskou word. Die toename in internasionale handel en langafstand vervoer dra verder by tot die verspreiding van nuwe indringerspesies in stedelike gebiede. Omgewingstoestande wat verander het weens menslike aktiwiteite, stel sommige uitheemse spesies in staat om vinnig te floreer in stedelike sentrums. Dit stel hierdie spesies ook dikwels in staat om te versprei na omliggende natuurlike en semi-natuurlike gebiede. Om die volle omvang van ʼn indringer te bepaal is dit belangrik om die huidige verspreidingspatrone van die indringer, sy potensiële toekomstige verspreiding oor ekosisteme, asook na die faktore wat sy verspreiding kan bevorder, in ag te neem. Om die biologiese en biogeografiese voorkeure wat ʼn spesie se verspreiding sal fasiliteer te verstaan, moet daar begrip verkry word vir 1) waar ʼn spesie is; 2) waarheen ʼn spesie kan versprei; en 3) die faktore wat sy verspreiding kan bevorder. Dit word bepaal deur gebruik te maak van ʼn multi-skaal analise wat na alle aspekte van ʼn ndringerspesie oor verskeie ruimtelike skale bestudeer. Hierdie studie fokus op Ailanthus altissima wat ʼn aggressiewe indringer in die Noordelike halfrond is. Hoewel A. altissima reeds sedert 1834 in Suid-Afrika voorkom, het hierdie indringer nog nie dieselfde vlakke van indringing soos in die Noordelike halfrond, bereik nie. Ek het ʼn multi-skaal ontleding opgestel om die spesie se huidige en potensiële verspreiding op globale skaal (hoofstuk 1), nasionale skaal (hoofstuk 2) en plaaslike skaal (hoofstuk 3) te bepaal. Elke hoofstuk het ʼn sentrale doelstelling waarin ʼn aantal navorsingsvrae geëvalueer word. In die eerste hoofstuk bepaal ek die biogeografie, verspreiding en habitatgeskiktheid van A. altissima deur die evaluering van die huidige en potensiële verspreiding van die spesie (gebaseer op klimaatstoestande geskiktheid) op beide 'n globale skaal en 'n nasionale skaal (Suid-Afrika). Die belangrikste gevolgtrekkings van hierdie hoofstuk is dat 1) aanlyn databasisse (hoewel verouderd by tye) verskaf 'n akkurate voorstelling van die verspreiding van indringerspesies en moet dus volgehou word. Dit was vasgestel deur kruisverwysings in 'n aantal van die aanlyn databasisse. Die databasisse is geëvalueer om te bepaal of hulle grootliks ooreenstem in terme van die verspreiding van indringerspesies; 2) en die modellering van die potensiële verspreiding volgens die geskiktheid van klimaatstoestande. Om die potensiële verspreiding van 'n indringerspesies meer akkuraat te bepaal, is daar aanbeveel dat, ander verspreiding faktore soos grondgebruik en mensgemaakte versteurings, in die verspreidingsmodel ingebou moet word. Hoofstuk 2 evalueer tot watter mate die gebiede wat deur die model voorgestel is reeds deur A. altissima beset is, en identifiseer gebiede wat die risiko van toekomstige indringing beloop. Hierdie hoofstuk identifiseer ook spesifieke faktore wat die verspreiding van hierdie spesie op 'n globale en nasionale skaal bevorder. Daar is vasgestel dat klimaat die grootse invloed op die spesie se verspreiding op ʼn globale skaal het. Op nasionale skaal het mensgemaakte versteurings ʼn groter invloed op die verspreiding van die spesie uitgeoefen. Hierdie hoofstuk bou voort op die konsepte in hoofstuk 1, deur klem te lê op die belangrikheid van die integrasie van verskillende omgewingsveranderlikes op verskeie ruimtelike skale om potensiële ‘brandpunte’ van indringing te identifiseer. Die multi-skaal benadering van hierdie hoofstuk stel ons in staat tot vroeë opsporing van indringerspesies wat sodoende gepaardgaande skade kan voorkom. Hierdie benadering is veral effektief in die ontleding van indringerspesies in stedelike gebiede. Die derde hoofstuk evalueer die verspreidingspatroon van A. altissima binne die Stad Kaapstad deur die woonbuurt Nuweland as studiegebied. Dit bespreek ook die moontlike opsies vir bestuur op ʼn stedelike skaal. Daar kan tot die gevolg kom dat Geografiese Inligtingstelsel (GIS) tegnieke effektief aangewend kan word om fynskaalse ontleding van indringerspesies verspreiding te doen. Dit is effektief om gebiede wat vatbaar vir toekomstige indringing te identifiseer, terwyl ongeskikte gebiede geëlimineer kan word. Ek het vasgestel dat in die Stad Kaapstad, A. altissima gegoede stedelike gebiede met “podzolic” gronde verkies wat blootgestel word aan hoë vlakke van reënval. Hierdie inligting dui daarop dat daar ʼn hoë kapasiteit is vir verdere verspreiding van die spesie in die stad. Die sistematiese benadering wat in Nuweland gebruik is, is 'n doeltreffende manier om die volle omvang van 'n indringer te bepaal en is veral effektief vir die evaluering van die bevolking struktuur van 'n bepaalde indringerspesie. Toekomstige studies behoort hierdie benadering in samewerking met afstandswaarneming tegnieke toe pas vir die beste moontlike resultate. Die laaste hoofstuk van hierdie tesis bied algehele gevolgtrekkings waarin al die belangrikste bevindings uitgelig word. Alles in ag geneem, kan die benadering in hierdie tesis as ʼn effektiewe metode toegepas word op ander opkomende, stedelike indringers wêreldwyd. Die sistematiese, multi-skaal analise wat voorgestel word in hierdie projek toon hoë vlakke van sukses en dit is my oortuiging dat hierdie projek die gewenste doelwitte bereik het.

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