Determining the invasive status of Australian Acacia species in South Africa, and the potential for eradicating species with limited distributions

Magona, Nkoliso (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: While widespread invasions of Australian acacia species (wattles) have been fairly well documented, very little is known about species that have no substantial commercial value or those that are not well-established invaders yet. South Africa has the highest number of invasive wattle species in the world. These have had negative impacts on the environment and socio-economy. However, the last detailed inventory of the group in South Africa was based on data collated forty years ago. In addition, there are several species with small naturalised populations that might pose a future risk. A recent study quantified different aspects of this “invasion debt” for wattles, both for South Africa and globally and found out that southern Africa has a large invasion debt. In Chapter 2 I aimed to determine how many Australian Acacia species are known to have been introduced to South Africa, which species are still present and what their status is. I visited herbaria, arboreta, botanical gardens and conducted field surveys in order to compile a list of introduced wattles, and used DNA barcoding to confirm the identity of these species. I found records for 114 wattle species introduced into South Africa, but I found the presence of only 50 species. Seventeen of these species are invasive (16 are in category E, one in category D2 in the Unified Framework for Biological Invasions); eight species have naturalised (category C3); and 25 species are present but are not known to produce seed in South Africa (category C1). Four of these occur in the Western Cape (three on the Cape Peninsula, A. piligera, A. retinodes and A. viscidula; 1 near Paarl, A. adunca) and two species, A. cultriformis, A. fimbriata in Grahamstown in the Eastern Cape. In Chapter 3, I focus on the potential to eradicate these six naturalised wattle species from South Africa. I carried out a systematic survey of populations and the surrounding areas. For each plant, I recorded plant canopy, height, stem basal diameter, presence or absence of reproductive structures and GPS coordinates. I then cut or pulled out the plants. I assessed the risk posed by these species using Australian weed risk protocol and lastly, I determined the current size of the seedbank for these species. Risk assessment showed that all of these species have high potential impact, hence, they should be considered as a threat. All of these species except A. retinodes can reach reproductive maturity within a year and three of these species have large seedbanks. If control efforts can continue to prevent reproduction, eradiation will be a matter of reducing the seed banks across the limited distributions for these species. I conclude that eradicating five of the species is feasible and annual clearing resurveys are recommended in order to prevent production of seeds. Acacia cultriformis was clearly at some point used in the ornamental plant trade and there are many isolated populations. This makes it difficult to find all plants and eradication is unfeasible. I conclude with Chapter 4, where I provided recommendations for listing and management.

AFRIKAANSE OPSOMMING: Terwyl wydverspreide indringing van Australiese akasia-spesies (wattels) redelik goed gedokumenteer is, is baie min bekend oor spesies wat geen beduidende kommersiële waarde het nie of die wat nog nie gevestigde indringers is nie. 'n Onlangse studie het verskillende aspekte van die "indringingskuld" vir wattels gekwantifiseer, beide vir Suid- Afrika en wêreldwyd, en het uitgevind dat Suider-Afrika 'n groot indringingskuld het, selfs vir wattels wat nog nie wydverspreid is nie. Dit beteken dat daar 'n beduidende toename in die algehele ekologiese en ekonomiese impakte van wattels sal wees. Suid-Afrika het die grootste aantal indringer wattel spesies in die wêreld, en dit het negatiewe impakte op die omgewing en sosio-ekonomie. Tog was die laaste gedetailleerde inventaris van die groep in Suid-Afrika gebaseer op data wat veertig jaar gelede ingesamel is. Daarbenewens is daar verskeie spesies met klein genaturaliseerde bevolkings wat waarskynlik 'n toekomstige risiko kan veroorsaak. Met hierdie studie het ek gepoog om vas te stel: hoeveel Australiese Acacia spesies is ingebring na Suid-Afrika, watter spesies is nog steeds teenwoordig en wat hul status is (Hoofstuk 2). Ek het herbaria, arboreta en botaniese tuine besoek, ook is veldopnames gedoen om 'n lys van ingevoerde wattels saam te stel. DNA-kodering is gebruik om die identiteit van hierdie spesies te bevestig. Ek het rekords gevind vir 114 wattle spesies wat in Suid-Afrika ingebring is, maar ek kon slegs 50 spesies steeds vind. Sewentien van hierdie spesies is indringers (16 is in kategorie E, een in kategorie D2 in die ‘’Unified Frame Work for Biological Invasions’’); 8 spesies is genaturaliseer (kategorie C3); en 25 spesies is teenwoordig, maar is nog nie waargeneem om saad in Suid-Afrika te produseer nie (kategorie C1). Ek het op ses genaturaliseerde wattel spesies uit vorige populasie opnames gedoen. Hiervan het 4 spesies in die Wes-Kaap voorgekom (3 Kaapse skiereiland: A. piligera, A. retinodes en A. viscidula; 1 naby Paarl: A. adunca) en twee spesies kom voor in die Oos- Kaap (Grahamstad: A. cultriformis en A. fimbriata). In Hoofstuk 3 fokus ek op die moontlikheid om genaturaliseerde wattel spesies uit te wis in Suid-Afrika. Ek het 'n sistematiese opname gedoen oor bevolkings en hul omliggende gebiede. Vir elke plant het ek die volgende aangeteken; kroon deursnee, hoogte, basale stam deursnee, teenwoordigheid of afwesigheid van reproduktiewe strukture en GPS koördinate. Dan trek ek die plant uit of kap dit af. Deur die Australiese onkruidrisiko-protokol te gebruik, is die risiko van hierdie spesies geassesseer en laastens is die huidige saadbank grootte per spesie bepaal. Risikobepaling het getoon dat al hierdie spesies 'n hoë potensiële risiko-impak het, daarom moet hulle as 'n bedreiging beskou word. Al hierdie spesies kan reproduktiewe volwassenheid bereik binne 'n jaar en drie van hierdie spesies produseer ' groot hoeveelhe saad. In Hoofstuk 4 het ek aanbeveel dat hierdie wattels gelys moet word en bestuurstrategieë word verskaf. Aangesien daar nie meer volwasse plante is nie, net hul saadbank, en beperkte lokale verspreidings, het ons tot die gevolgtrekking gekom dat die uitroeiing van hierdie spesies uitvoerbaar is, en dat jaarlikse opvolg her-opnames aanbeveel word vir die voorkoming van nuwe saadproduksie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103839
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