Patterns and processes of alien plant invasions in small urban areas in South Africa : the Berg River catchment as a case study

McLean, Phil (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Many studies in invasion biology focus on the negative consequences of invasive alien plant species in natural areas. In South Africa, national legislation relating to invasive species focusses mainly on the management of such species in areas that provide strategic water and/or biodiversity resources. However, urban centres are host to many alien plant species, specifically those associated with the very popular activities of gardening and the pet trade. Urban environments can facilitate plant invasions because alien plants are cultivated in large numbers and are nurtured, and there are often sites of regular disturbance that provide favourable conditions for colonisation which allow some species to become naturalized and invasive. Small urban centres are more numerous than large cities and are often more deeply embedded in the landscape. This, combined with their higher proportional perimeter-to-area ratio, means they could be launching invasions into their surrounding areas. I investigated one such small town in detail to determine the patterns of spread of alien plant species. I then surveyed, in less detail, an additional 11 towns within the Berg River Catchment. Lastly, I compared the type and abundance of alien plant species found in towns to data on invasive alien plant species found specifically outside urban areas in the same catchment. I found a large number of alien plant species within small urban areas, with a high proportion of listed invasives. Most of the total alien plant diversity resides in gardens, but the most abundant alien plant species in all land-use types are either listed as invasive in national legislation, or are noted as problematic species in the regional literature. The extremely high species heterogeneity between gardens means that detailed, time-consuming surveys and high levels of taxonomic expertise are needed to ensure accurate results. However, reasonable assessments of a town’s invasive plant species component can be made by surveying gardens and roadsides in low-income areas and in town centres (with the exception of the Main road, as commercial activity often render these areas hostile to plants). All urban areas surveyed were equally capable of hosting a high proportion of invasive plant species, irrespective of their location within the catchment. By comparing abundant alien plants to regional lists of invasive plant species, I was able to determine the suite urban species which have naturalization records in this catchment and have thus ‘jumped the garden fence’ to become invasive in the surrounding natural areas. Most species in this group were introduced for ornamental horticulture, highlighting the risks associated with this pathway. Small urban areas are difficult to survey comprehensively due to extreme context speceficity, but contain a high diversity of alien plant species. The most abundant species are typically also naturalized, if not invasive, in the region, highlighting that small towns are important for launching plant invasions into surrounding natural areas.

AFRIKAANSE OPSOMMING: Binne die wetenskap van Indringer Biologie is daar baie studies wat fokus op die negatiewe impak van indringerplante op die natuurlike omgewing. Die nasionale wetgewing op indringerspesies van Suid Afrika fokus hoofsaaklik op indringerbestuur in strategiese wateropvang- gebiede en areas met hoë biodiversiteit. Stede huisves egter baie uitheemse spesies, spesifiek dié wat verband hou met tuinmaak en troeteldierhandel. Stedelike omgewings fasiliteer hierdie indringerplant verspreidings maklik, omdat uitheemse plante in groot getalle gekweek en versorg word. Daar is ook dikwels plekke met gereelde versteuring wat gunstige toestande vir kolonisering bied, dit stel sommige spesies in staat om te naturaliseer en indringers te word. Daar is meer kleiner dorpies as groot stede, en hulle is dikwels meer verweef met die landskap. In kombinasie hiermee het hul ‘n hoër proporsionele omtrek-tot-area-verhouding, wat beteken dat klein dorpies die bron van infestasies kan wees. Ek het een so 'n klein dorpie in detail bestudeer om die verspreidingspatrone van uitheemse plantspesies te bepaal. Daarna het ek 11 bykomende dorpe binne die Bergrivieropvanggebied minder volledig ondersoek. En laastens het ek die soort en hoeveelheid van uitheemse plantspesies wat in dorpe gevind is, vergelyk met data oor indringerspesies wat spesifiek buite stedelike gebiede in dieselfde opvanggebied voorkom. Ek het 'n groot aantal uitheemse plantspesies gevind in klein stedelike gebiede, waarvan ‘n groot persentasie reeds gelyste indringerspesies was. Tuine het die grootste hoeveelheid en diversiteit uitheemse plante, maar die mees oorheersende plantsoorte in alle grondgebruik afdelings word reeds as indringers in nasionale wetgewing gelys, of word as problematiese spesies in die streeksliteratuur beskou. Die uiters hoë spesies heterogeniteit tussen tuine beteken dat uitheemse plantopnames in klein dorpies benodig gedetailleerde, tydrowende opnames en hoë vlakke van taksonomiese kundigheid om akkurate resultate te verseker. Redelike assessering van 'n dorp se indringerplant-komponent kan gemaak word deur ‘n opname van; tuine, padreserwes in lae-inkomste-gebiede, en in die middedorp (met die uitsondering van die hoofpad, wat gewoonlik bar is van enige plante weens kommersiële aktiwiteite). Die stedelike gebiede wat ondersoek was, het almal ‘n gelyke moontlikheid gehad om 'n hoë persentasie indringerspesies te berg, ongeag hul ligging binne die opvanggebied. Deur die volop uitheemse plante te vergelyk met streekslyste van indringerspesies, kon ek die groep stedelike spesies wat genaturaliseerde rekords in hierdie opvanggebied het, bepaal. Hulle het dus ‘ontsnap’ uit tuine om indringers in die omliggende natuurlike omgewing te word. Die meeste spesies in hierdie groep is bekend as sierplante vir tuinbou, wat dan ook die risiko’s verbonde aan hierdie roete van verspreiding beklemtoon. As ‘n gevolg van uiterste konteks-spesifisiteit, is dit moeilik om voledige opnames in klein stedelike gebiede te maak, maar dit bevat 'n hoë diversiteit van uitheemse plante. Die volopste spesies is tipies ook genaturaliseerd, of reeds ‘n indringer in die streek, en beklemtoon die feit dat klein dorpies ‘n belangrike faktor is vir die verspreiding van indringerplante in die omliggende natuurgebiede.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103295
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