Summary of PhD thesis to fulfil the requirements for the degree of Doctor on the authority of the Rector Magnificus of Wageningen University Prof. Dr. M.J. Kropff to be publicly defended on Wednesday 31 March, 2010 at 15:00 hrs in the auditorium at ITC, Enschede, The Netherlands by Mhosisi Masocha.
Numerous alien plant species are invading African savannas causing loss of biodiversity and altering ecosystem functioning. The ecological factors and underlying mechanisms causing these invasions are poorly understood. This hinders invasive species management and biodiversity conservation. In this thesis, a range of approaches (i.e., field measurements, a greenhouse experiment, field experiments, a long-term burning experiment, remote sensing, and Geographical Information System (GIS) techniques) was used to understand how the availability of two key resources limiting primary productivity in African savannas (water and nutrients) and how major disturbances (i.e., fire, grazing) determine the invasion of these systems by alien plant species.
In a nutrient-limited Zimbabwean savanna, native termites facilitated alien plant invasion by boosting nutrient concentrations in top soils around their mounds and creating a patchy distribution of microsites in the landscape. Alien plant species richness increased threefold on termite mounds compared to surrounding off-mound areas. In addition, two invasive alien shrubs, Lantana camara and Duranta erecta, clustered around termite mounds without any occurring beyond the periphery of the nearest termite mound. These invasive shrubs grew more rapidly and doubled dry matter production in termite mound soils compared to the surrounding non-mound soils. Field experiments revealed that the germination rate of the invasive alien shrub D. erecta in mound soils was twice as high as in soils in the surrounding areas undisturbed by termites. In addition, seedling survival of D. erecta was consistently higher on termite mounds than away from mounds. These findings combined provide the first quantitative evidence that termites, which are widely distributed in tropical savannas, facilitate plant invasion by creating patches in the landscape with increased nutrient and microsite availability suitable for colonisation by alien species.
Rainfall had a profound effect on the rate of spread of an invasive alien species. In a semi-arid savanna in southern Zimbabwe, during years of above-average rainfall, the mean annual rate of spread of the invasive L. camara was at least twice that of native shrub encroachers, whereas in other years natives spread at the same rate as the alien shrub. This is the first time that, over a relatively long period, the spatio-temporal patterns of spread of invasive and native encroaching shrub species have been quantified in savanna systems and the link with rainfall variation tested. These results suggest that in semi-arid savannas, pulses in rainfall may accelerate the spread of invasive alien species.
Data from a long-term fire experiment performed at a humid savanna site in central Zimbabwe, in which plots were burnt at 1-, 2-, 3-, and 4-year intervals for more than 50 years and compared with unburnt controls, revealed that frequent burning of savannas promotes invasion. Alien species accounted for a significantly higher proportion of plant species richness in regularly burnt plots than in the unburnt control plots. The proportion of alien plant species was highest in the annually burnt plots, followed by plots burnt biennially, triennially, and quadrennially. Plots protected from fire contained the lowest proportion of alien invaders. Also, alien forbs occurred more frequently in regularly burnt plots than in the unburnt control plots and their abundance increased as the intervals between fires in years became shorter. These results suggest that frequent burning of mesic savannas enhances invasion by alien plants, with short intervals between fires favouring alien forbs. Previous results published on this fire experiment showed that regular burning had a significant effect on vegetation structure but the effect of fire frequency on alien plant invasion had not been evaluated. Hence, this thesis sheds light on the role of fire in promoting alien plant invasion in savannas.
When the susceptibility to invasion by alien plants of a southern African savanna degraded by human activity and continuous grazing by cattle was compared with that of an adjacent protected savanna, it was found that alien taxa represented a significantly higher proportion of the total species richness in the human-disturbed savanna than in the protected one. While alien plant species associated positively with the human-disturbed savanna and negatively with the protected one, their native confamilial species exhibited a positive association with the protected savanna, and a negative association with the human-disturbed savanna. The most abundant and widely distributed invasive species, L. camara, covered a significantly higher proportion of the human-disturbed savanna than of the protected savanna. These results clearly suggest that anthropogenic disturbance increases the invasibility of African savannas.
The findings of this study lead to the general conclusion that the susceptibility of African savanna systems to invasion by alien species increases when the availability of key limiting resources (water and nutrients) coincides with disturbances, like cattle grazing and fire, which open up an intact plant assemblage for colonisation by alien invaders. This is consistent with ecological theory and implies that manipulating resource availability and reducing the level of disturbance may be the keys to controlling the spread of alien species across savanna landscapes.
Finally, to improve invasive species mapping, conventional image classifiers were combined with a simple GIS expert system to yield a hybrid classifier. It was demonstrated for the first time that this hybrid classifier can map the cover of invasive plant species from satellite imagery across the entire landscape with greater statistical accuracy than either the standard image classifiers or the GIS expert system alone. Hence, it has the potential to address the data needs of natural resource managers, who require accurate information about the occurrence and abundance of invasive species in order to make invasive species management more cost effective.
Mhosisi Masocha was born on the 9th of May 1975 in Masvingo rural district, Zimbabwe. He did his Ordinary level education at Rukovo secondary school (1988-1991) and Advanced level education at Hama high school (1993-1994). From June 1995 to February 1997, he worked for the Ministry of Education as a secondary school teacher. In March 1997, he enrolled with the University of Zimbabwe to read for an honours degree in Geography and Environmental Science, which he completed in 2000. In January 2001, he joined the University of Zimbabwe as a research assistant in the Department of Geography and Environmental Science. In 2002, while working as a research assistant, he started reading for a master of research degree at the University of Zimbabwe. He completed the master’s degree in 2004 and was appointed a teaching assistant. In July 2005, the University of Zimbabwe appointed him a full-time lecturer in the Department of Geography and Environmental Science. In August 2005, he was awarded a fellowship by the Netherlands Organisation for International Cooperation in Higher Education (NUFFIC) to support his PhD studies with the International Institute for Geo-information Science and Earth Observation (ITC) and Wageningen University in the Netherlands. He commenced his PhD research in January 2006, culminating in the writing of this doctoral thesis. He is still with the University of Zimbabwe, teaching Spatial Ecology, Environmental Management, Remote Sensing, and Geographical Information Systems (GIS). His main research interests include: invasive species, spatial patterns in ecological data; and vegetation mapping and monitoring.