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Wagneingen University (2010)

Savanna aliens

Masocha Mhosisi

Titre : Savanna aliens

Auteur : Masocha Mhosisi

Université de soutenance : Wagneingen University

Grade : PhD thesis 2010

Résumé
The invasion of alien plants into African savannas poses a threat to native biodiversity and alters ecosystem functioning. Our current understanding of the factors and mechanisms causing invasion in these ecosystems is poor. Yet, this knowledge is critical for the development of successful strategies for controlling invasive species and conserving native biodiversity. In this thesis, field measurements, a greenhouse experiment, field experiments, a long-term burning experiment, remote sensing, and Geographical Information System (GIS) techniques were used to understand the mechanisms of invasion and ecological factors controlling the susceptibility of African savanna systems to invasion by alien species (invasibility). In a nutrient-limited Zimbabwean savanna (southern Africa), native termites, which are widely distributed in the tropics, enhanced alien plant invasion by boosting nutrient concentrations in top soils around their mounds and creating spatial heterogeneity in microsite availability. This is the first time that the role of termites in facilitating alien plant invasion in a savanna has been reported. In a semi-arid savanna in southern Zimbabwe, the rate of spread of an invasive alien shrub was controlled by rainfall. During years of aboveaverage rainfall, the mean annual rate of spread of the invasive shrub Lantana 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 a novel finding suggesting that in water-limited African savanna systems, pulses in rainfall may accelerate the spread of invasive alien species. In a humid savanna in central Zimbabwe, frequent burning promoted invasion by alien plant species. The interaction of human disturbance and overgrazing by cattle increased the level of invasion of a degraded savanna in southern Zimbabwe. These results combined lead to the general conclusion that the invasibility of an African savanna system tends to increase 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 to colonisation by alien plant 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 invasive alien species and conserving native biodiversity in African savannas.

Mots clés : savannas / ecosystems / invasions / weeds / woody weeds / degraded land / habitat destruction / isoptera / burning / geographical information systems / ecological disturbance / disturbed land / monitoring / africa / terrestrial ecosystems / environmental monitoring

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Page publiée le 8 octobre 2010, mise à jour le 14 janvier 2018