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Local-scale impacts of an invasive cactus (Opuntia stricta) on beetle and spider diversity in the Kruger National Park

An important topic of invasion biology research is to measure and understand the impacts that biological invasions have on biodiversity. Although many studies have been undertaken, there is still a poor understanding of the impacts of invasive plants on invertebrates. It is important to measure these impacts because invertebrates make a large contribution to global species diversity and are important in regulating many key processes across the world. Populations of invasive organisms need to be carefully managed to ensure that they have the lowest possible impact on biodiversity. It is important to understand the level of management that is required to reduce the impacts of invasive organisms to acceptable levels.

Opuntia stricta invasion in Kruger National Park

An Opuntia stricta invasion in the Skukuza region of the Kruger National Park (Photo: Berndt van Rensburg)

In the Kruger National Park (KNP) invasive alien plants have been identified as one of the most serious threats to biodiversity. Opuntia stricta var. dillenii (Cactaceae) is the most widespread of these invasive plants in the KNP. It was first recorded in 1953 and has invaded 35,000 ha (2%) of KNP’s surface area. A biological control programme, initiated in 1988, has successfully reduced the density of the plant. This study aimed to assess the impact of different densities of O. stricta on beetle and spider assemblages and to determine whether these densities could be considered to be having an impact on beetle and spider diversity.

Fieldwork was conducted in the Skukuza region of the KNP. Unbaited pitfall traps were used over a 12-month period to capture beetles and spiders in four treatments of varying O. stricta density. Spiders were sampled using tree-beating and active searching across the treatments.

A total of 72 beetle and 128 spider species were collected during the sampling period. The results suggest that current densities of O. stricta are not having a major impact on spider assemblages. However, there was a significant impact on beetle assemblages. Beetle and spider species richness and species density were not significantly affected. The most likely explanation for these findings is that the current level of O. stricta invasion, maintained by the use of biological control, is insufficient to transform the structure of the vegetation to the extent that it significantly alters spider assemblages but that it is sufficient to alter beetle assemblages. We suggest that higher densities of O. stricta, resulting in greater habitat transformation, could have a greater impact on beetles and spiders and that the current control programme should aim to prevent densities from increasing.

This study was conducted by Kyle Harris as part of his MSc research funded by the Centre for Invasion Biology.




Read the paper: Robertson, M.P., Harris, K.R., van Rensburg, B.J., Coetzee, J., Foxcroft, L., Dippenaar-Schoeman, A.S. 2011. Assessing local scale impacts of Opuntia stricta (Cactaceae) invasion on beetle and spider diversity in the Kruger National Park, South Africa. African Zoology. 46: 205-223.

For further details, please contact Prof. Mark Robertson at mrobertson@zoology.up.ac.za