Browsing by Subject "Alien invader"

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    Environmental factors influencing the distribution of Opuntia stricta, an invasive alien plant in the Kruger National Park, South Africa
    (2007) Hoffmann, J H; Foxcroft, L C; Viljoen, J J; Kotze, J J
    Opuntia stricta (Cactaceae), an alien weed, has invaded an area of more than 35 000 ha in the Skukuza region of the Kruger National Park [KNP]. The distribution of the plant and features of the environment were used to identify biotic and abiotic factors which may be affecting the distribution and density of the plant. A Canonical Community Analysis revealed that none of the environmental factors that were monitored influenced the distribution or abundance of O. stricta in KNP. There were no apparent natural barriers that might limit the occurrence of O. stricta within the KNP. The study provides insight into the relationship between the invader and host environment, showing that there is a high probability that, unless there is appropriate intervention, O. stricta will eventually colonise the entire KNP.
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    Factors influencing the distribution of Cactoblastis cactorum, a biological control agent of Opuntia stricta in Kruger National Park, South Africa
    (2007) Hoffmann, J H; Foxcroft, L C; Viljoen, J J; Kotze, J J
    Cactoblastis cactorum, a phycitid moth, has been used with considerable success as a biological control agent against several different cactus species in many parts of the world, including Opuntia stricta in Kruger National Park [KNP], South Africa. Although the moth has become widespread and abundant in KNP, its populations have not reached levels where it is able to provide the desired level of control on the alien species which has invaded more than 35 000 ha of the park and continues to spread. Although several factors, especially acquired predators and parasitoids, are known to curb the abundance of C. cactorum in South Africa, the extent to which this applies in KNP is unknown. Logistic regression and Canonical Community Analysis were used to determine the biotic and abiotic factors affecting the distribution and abundance of C. cactorum in KNP. To do this, maps of the ranges of the insect and host plant were constructed and integrated with other environmental features. The logistic regression showed that O. stricta density had no influence on whether or not C. cactorum was present in an area. The Canonical Community Analysis suggested that a high fire frequency, the herbicide control programme and the presence of small host plants may be important in limiting the distribution of C. cactorum, either directly or indirectly, through impacts on associated predators or parasitoids.