Abstract
Invasive alien plants are of concern in South Africa. Pompom weed ( Campuloclinium macrocephalum) is currently invading the Grassland and Savannah biomes of South Africa and is likely to continue spreading in the southern African sub- region. Two possible biological control agents ( Liothrips tractabilis and Cochylis campuloclinium) have been identified for control of pompom weed. We used ecological niche modelling to predict which areas in southern Africa are likely to be suitable for pompom weed and the two potential biological control agents. The overlap between areas predicted to be highly suitable for pompom weed and areas suitable for the biological control agents was assessed. Methods of reducing sampling bias in a data set used for calibrating models were also compared. Finally, the performance of models calibrated using only native range data, only invaded range data and both were also compared. Models indicate that pompom weed is likely to spread across a greater region of southern Africa than it currently occupies, with the Savannah and Grassland biomes being at greatest risk of invasion. Poor overlap was found between the areas predicted to be highly suitable for pompom weed and those areas predicted to be suitable for the biological control agents. However, models of the potential distribution of the biological control agents are interpreted with caution due to the very small sample size of the data set used to calibrate the models. Models calibrated using both native range and invaded range data were found to perform best whilst models calibrated using only native range data performed the worst. There was little difference found between models that were calibrated using spatially reduced (selecting only one record per 30 min grid cell) and randomly reduced (randomly selecting 50% of available records) biased data sets.
Highlights
Invasive species have resulted in a worldwide conservation and ecological crisis as they enter and alter communities across the globe (Gurevitch and Padilla, 2004)
The aims of this study were: (1) to predict the potential distributions of pompom weed and the two potential biological control agents in southern Africa using ecological niche models; (2) to assess the overlap in regions predicted to be highly suitable for pompom weed and the two agents; (3) to compare the performance of models for pompom weed that were calibrated with native records only, invaded range records only and a combination of both and (4) to compare two methods of reducing sampling bias in the dataset of occurrence records obtained from the invaded range of pompom weed
Once the occurrence records had been randomly reduced by 50% (NIr treatment), 45 occurrence records remained in the invaded range for pompom weed
Summary
Invasive species have resulted in a worldwide conservation and ecological crisis as they enter and alter communities across the globe (Gurevitch and Padilla, 2004). In the USA Pimmentel et al (2005) estimated the economic damage of the effects of alien invasive species to be $120 billion/year. South Africa seems to be vulnerable to invasive plant species, having one of the biggest problems with alien plant invaders in the world (Richardson and Van Wilgen, 2004). Pompom weed is currently invading the grassland biome of South Africa (Henderson et al, 2006), which is already the biome with the most invasive plant species in South Africa (Richardson and Van Wilgen, 2004). Pompom weed is thought to first invade disturbed areas before spreading to natural grasslands where it is likely to displace native species, resulting in reduced biodiversity and carrying capacity (Henderson et al, 2006)
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