Species of Prosopis (Mimosaceae), or mesquite, are native to the Americas and introduced in South Africa as agro forestry trees to provide wood, fodder and shade in the late 1800s. In the Northern Cape Province, these trees have been particularly used for their pods by local farmers and local industry. However due to Prosopis’ ability to spread rapidly there has been increasing pressure to step up control of the tree. Due to the costs associated with most control methods biological control has been singled out as the most viable control method worth pursuing. This thesis attempts to explain and shed light on the effects that increased biological control of Prosopis would have on the farming community and Prosopis based businesses in the area. Two hypothetical scenarios were used, the first being a possible 50% reduction in available pods and the other a possible 100% reduction. In an effort to explain these effects data was collected from a series of collection activities in the form of a pilot study and two unique questionnaire surveys. Twenty-seven farmers and one Prosopis based business were interviewed at random with the data analysed and documented. Results showed that the business community is the main user of pods incurring crippling losses in the event of a 50% or 100% decrease in available pods. Using control costs from working for water De Aar long term costs of the presence of Prosopis were estimated and compared to the estimated provincial cost of losing pods giving a cost comparison ratio of (α)270 million : Y(N(15.7million)). Only 48% of the respondents valued pods and 11% had traded in pods, the top concern resulting from the invasion was the loss of underground water. With a 50% decline in available pods, all respondents indicated unchanged effects. However empirical results showed that with a 100% decline in pods estimate losses of R 5 818 per farm are expected. The cost comparison ratio showing the losses farmers bore with Prosopis in the area and the losses they would suffer with a 100% decline in pods was R11 389 + (K): R5 818. In light of the empirical results, policy recommendations that support the introduction of more effective biological control agents have been made with areas for further research identified and discussed.

Thesis (MScConEcol (Conservation Ecology and Entomology))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: I evaluated the impact of Prosopis invasion and clearing on ecological structure, function and agricultural productivity in heavily grazed Nama Karoo rangeland on two sheep farms near the town of Beaufort West in the Western Cape Province of South Africa. My aims were to (1) determine the effects of invasion and clearing on rangeland vegetation composition, diversity (alien and indigenous species richness) and structure (alien and indigenous species cover), soil vegetation cover (plant canopy and basal cover) and agricultural productivity (grazing capacity), (2) describe the vegetation processes that underlay the invasion and clearing impacts and (3) evaluate the success of clearing in facilitating unaided restoration of ecological structure, function and agricultural productivity in formerly invaded rangeland. I hypothesised that invasion would significantly change rangeland vegetation composition and structure, leading to greater alien species richness and cover and lower indigenous species richness and cover while clearing would lead to lower alien species diversity and cover and greater indigenous species richness and cover. In addition I hypothesized that invasion would reduce rangeland plant canopy and basal cover and grazing capacity while clearing would substantially increase them. Finally I predicted that vegetation composition, alien and indigenous species cover and richness, plant canopy and basal cover and grazing capacity would revert to pre-invasion status and levels within four to six years of clearing. My results suggest that in heavily grazed Nama Karoo rangeland Prosopis invasion (~15 percent canopy cover) and clearing can significantly change rangeland vegetation composition, with invasion leading to greater alien species cover and lower indigenous species richness, while clearing leads to lower alien species richness and cover and greater indigenous species richness and cover. However invasion seems to have no effect on alien species richness and overall indigenous species cover. Clearing appears to facilitate the spontaneous restoration of alien species cover and indigenous species richness within four to six years but not species composition, alien species richness and indigenous species cover. In addition my results also indicate that Prosopis invasion can lower rangeland plant canopy and basal cover and grazing capacity while clearing, even under heavy grazing, can substantially raise them. Clearing however does not seem to facilitate the restoration of rangeland plant canopy and basal cover and grazing capacity to pre-invasion levels within four to six years after clearing. / AFRIKAANSE OPSOMMING: Ek het die impak van Prosopis indringing en verwydering van indringers op ekologiese struktuur, funksie en landbou produktiwiteit in ‘n swaar beweide Nama Karoo gebied op twee skaapplase naby Beaufort-Wes in die Wes-Kaap provinsie van Suid-Afrika geëvalueer. My doelwitte was om (1) te bepaal wat die gevolge van die indringing en verwydering van indringers op die natuurlike plantegroei samestelling, diversiteit (uitheemse en inheemse spesiesrykheid) en struktuur (uitheemse en inheemse spesies bedekking) sal wees, sowel as die effek op plantegroei bedekking (kroon en basalebedekking) en landbou produktiwiteit (weidingkapasiteit), (2) die plantegroei prosesse te beskryf wat onderliggend deur die impakte van indringing en verwydering van indringers veroorsaak word, en (3) die sukses van die verwydering van indringers te evalueer deur die fasilitering van blote restorasie van ekologiese struktuur en funksie en landbou produktiwiteit in voorheen ingedringde gebiede. My hipotese is dat indringing ‘n aansienlike verandering in natuurlike plantegroeisamestelling en struktuur sal veroorsaak, wat sal lei tot groter uitheemse spesiesrykheid en bedekking met minder inheemse spesiesrykheid en bedekking, terwyl die verwydering van indringers sou lei tot minder uitheemse spesie diversiteit en bedekking met 'n groter inheemse spesiesrykheid en bedekking. Verder vermoed ek dat indringing die natuurlike kroon- en basalebedekking en weidingkapasiteit sal verminder, terwyl die verwydering van indringers dit aansienlik sal verhoog. Ten slotte voorspel ek dat plantegroei samestelling, uitheemse en inheemse spesiesbedekking en -rykheid, kroon- en basalebedekking en weidingkapasiteit sou terugkeer na voor-indringing status en vlakke binne vier tot ses jaar na die verwydering van indringers. My resultate daarop dat die indringing van Prosopis (~ 15 persent kroonbedekking) en die verwydering van indringers in swaar beweide Nama Karoo gebiede ‘n aansienlike verandering in die gebied se natuurlike plantegroei samestelling toon, waar indringing gelei het tot groter uitheemse spesiesbedekking en minder inheemse spesiesrykheid, terwyl die verwydering van indringers lei tot minder uitheemse spesiesrykheid en groter inheemse spesiesrykheid en - bedekking. Dit lyk egter of indringing geen effek op uitheemse spesiesrykheid en algehele inheemse spesiesbedekking het nie. Die verwydering van indringers blyk om die spontane herstel van indringerbedekking en inheemse spesiesrykheid binne vier tot ses jaar te fasiliteer, maar nie spesiesamestelling, uitheemse spesiesrykheid of inheemse spesiesbedekking nie. Benewens dui my resultate ook aan dat Prosopis indringing die natuurlike kroon- en basalebedekking sowel as weidingskapasiteit verlaag, terwyl die verwydering van indringers, selfs onder swaar beweiding, die bedekking aansienlik kan verhoog. Verwydering van indringers lyk egter nie asof dit die herstel van die gebied se natuurlike kroon- en basalebedekking en weidingkapasiteit na voor-indringing vlakke toe kan fasiliteer binne vier tot ses jaar na die verwydering van indringers nie.