Thesis (PhD (Conservation Ecology and Entomology))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Natural and semi-natural ecosystems and human communities worldwide are under siege
from a growing number of destructive invasive alien species. Alien species are those
whose presence in an area is due to intentional or accidental introduction as a result of
human activities. Some alien species become invasive, and some cause tremendous
destruction to the ecosystem and their stability, but we do not yet understand fully the
many factors that determine the levels of invasiveness in alien species. However,
management of alien plants requires a detailed understanding of the factors that make
them invasive in their new habitat. The aim of this study was to explore in detail the
processes and potential for invasion of Schinus molle (Peruvian pepper tree) into semiarid
savanna in South Africa and to examine the potential for this species to invade
further in these ecosystems, and in other South African biomes.
In this thesis I explored the patterns and processes of invasion of S. molle in semiarid
savanna using small-scale experiments to investigate physical and ecological barriers
to invasion that prevent or accelerate the invasion of this species. I examined factors such
as pollination; seed production; seed dispersal; seed predation and viability, all known to
contribute to invasiveness. I highlighted the critical role of microsite conditions
(temperature, humidity, water availability) in facilitating S. molle seedling establishment
in semi-arid savanna and demonstrated that microsite type characteristics need to be
considered for management and monitoring of the species in South Africa. I
demonstrated the ability of S. molle to out-compete indigenous woody plants for light and
other resources and also showed that disturbance of natural ecosystems was not a
prerequisite for invasion, although human activities such as tree planting have played a
major role in disseminating this species in South Africa.
Predicting the future distribution of invasive species is very important for the
management and conservation of natural ecosystems, and for the development of policy.
For this reason, I also assessed the present and potential future spatial distribution of S.
molle in South Africa by using bioclimatic models and a simulation-based spread model.
I produced accurate profiles of environmental conditions (both biophysical and those related to human activities) that characterize the planted and naturalized ranges of this
species in South Africa, by linking species determinants, potential habitat suitability and
likely spread dynamics under different scenarios of management and climate change. All
those components provided insights on the dynamics of invasions by fleshy-fruited
woody alien plants in general, and on S. molle invasions in South Africa in particular. I
developed a conceptual model that described S. molle population dynamics leading to an
understanding of the processes leading to the invasive spread of this species in South
Africa. This work also emphasized the need for policy review concerning the invasive
status of S. molle in South Africa, and recommendations are made for future research. / AFRIKAANSE OPSOMMING: Die natuurlike and half-natuurlike ekosisteme sowel as menslike gemeenskappe
wereldwyd word bedreig deur ‘n groeiende hoeveelheid indringerplantspesies.
Indringerplantspesies (daardie spesies wie se teenwoordigheid toegeskryf kan word aan
opsetlike of toevallige inbringing deur menslike toedoen) is ‘n bedreiging nie net vanweë
die massiewe verwoesting van die ekosisteme en ekosisteemstabilitiet nie, maar ook
omdat ons nog nie ten volle verstaan hoe hulle van skaars in hul natuurlike omgewing tot
dominant in hul nuwe habitat gaan nie. Bestuur van indringer plante vereis ’n begrip van
biologise en ekologiese faktore wat lei tot hulle indringing in die nuwe habitat. Die
primêre doel van hierdie studie was om in detail uit te vind wat die prosessesse en
potensiaal is vir die indringing van S. molle (die Peruviaanse peper boom) in droë
savanna en om indringingspatrone in droë savanna met huidige en potensieële
toekomstige patrone in ander Suid-Afrikaanse biome te vergelyk.
In hierdie tesis ondersoek ek die patrone en prosesse wat verband hou met die
indringing van S. molle in droë savanna deur gebruik te maak van kleinskaalse
eksperimente om fisiese en ekologise hindenisse te ondersoek wat indringing van S. molle
in Suid-Afrika voorkom of versnel. Dit sluit faktore in wat bekend is om by te dra tot
indringing van plant spesies, soos bestuiwing, saadproduksie, saadpredasie en
kiemkragtigheid. Ek het die kritieke rol beklemtoon van mikroomgewingskondisies
(temperatuur, humiditeit, waterbeskikbaarheid) in die fasilitering van S. molle
saailingvestiging in droë savanna en het gedemonstreer dat die tipe
mikroomgewingskarakteristieke in ag geneem moet word by betuur en monitering van
die verspreiding van S. molle in Suid-Afrika. Ek het die vermoë van S. molle om
inheemse plante te uitkompeteer gedemonstreer, en het gewys dat versteuring van
natuurlike ekosisteme nie ’n voorvereiste vir S. molle indringing was nie, hoewel
menslike aktiwiteite soos boomaanplantings ’n groot rol speel deur by te dra tot
indringing van hierdie spesie in Suid-Afrika. Voorspelling van toekomstige verspreiding van indringerspesies is baie belangrik
vir die bestuur en bewaring van natuurlike ekosisteme, sowel as vir ontwikkeling van
wetgewing. Daarom is die huidige en potensiele toekomstige ruimtelike verspreiding van
S. molle in Suid-Afrika bereken deur inkorporering van bioklimaatsmodelle en simulering
gebasseer op ’n verspreidingsmodel. Ek het derhalwe akkurate profiele van
omgewingstoestande (beide fisiese en daardie wat verband hou met menslike aktiwiteite)
wat die aangeplante en natuurlike omvang van die spesie in Suid-Afrika kenmerk
geproduseer deur spesiedeterminante, potensieële geskiktheid van habitatte en moontlike
verspeidingsdinamika onder verskillende bestuursscenarios en kimaatsverandering te
koppel. Al hierdie komponente verskaf insig in die dinamika van die indringing van
houtagtige plante met vlesige vrugte oor die algemeen en S. molle in besonder in Suid-
Afrika. Hierdie werk beklemtoon ook die behoefte vir hersiening van beleidsrigtings wat
betrekking het op die indringerstatus van S. molle in Suid-Afrika en maak aanbevelings vir toekomstige navorsing.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/3975 |
| Date | 03 1900 |
| Creators | Midoko Iponga, Donald |
| Contributors | Milton, Suzanne J., Richardson, David M., University of Stellenbosch. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Rights | University of Stellenbosch |
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