Current and future vulnerability of South African ecosystems to perennial grass invasion under global change scenarios

Rahlao, Sabataolo John (2010-03)

Thesis (PhD (Conservation Ecology and Entomology))--University of Stellenbosch, 2010.

Thesis

ENGLISH SUMMARY: Climate change and biological invasions are major components of global change induced by human activity and are considered major drivers of global biodiversity decline in terrestrial ecosystems. These drivers interact synergistically and render ecosystems vulnerable to invasion by invasive alien species. Grasses are a group of invasive plants that easily respond to global changes and alter native plant community structure and ecosystem processes, such as fire frequency. To date there have been few studies that assess the ecological drivers and effects of invasive grass species on arid and semi-arid ecosystems of South Africa. Fountain grass (Pennisetum setaceum) is a widely distributed invasive alien perennial grass from North Africa and the Middle East, valued by horticulturalists worldwide. It spreads along the edges of roads on the outskirts of most towns, and is common on mine spoil in many areas throughout South Africa. Occasionally, it escapes into natural vegetation along drainage lines or after fires. This grass can be a costly problem for agriculture and biodiversity conservation as it is unpalatable and increases fire risk. Understanding its distribution and invasive dynamics will contribute to better management and control practices. The main objective of this study was to understand the ecology and invasion processes of P. setaceum across South African environmental gradients, and use it as a model to understand the synergistic relationships between biological invasions and other global change (climate and land use) scenarios. This study provides the first assessment of how P. setaceum overcomes different invasion barriers in South Africa as an emerging invader, in comparison with other parts of the world where it has already become problematic. A number of management and control options for this grass and other similar perennial grasses result from this study. Specifically, the study provides comprehensive understanding of: 1) the distribution and habitat preferences of P. setaceum in arid and semi-arid parts of South Africa, 2) environmental resources and habitat conditions that promote its invasive potential, 3) growth and reproductive performance across environmental gradients, 5) the response of an arid ecosystem to P. setaceum invasion and fire promotion, and 6) the predicted future distribution of this grass in South Africa under scenarios of climate change and spread. The study has found that road-river interchanges are ideal sites for P. setaceum growth and that these sites should be targeted for management and control of this species. P. setaceum was found to persist under diverse biome environments, which is attributed to local adaptation. Disturbance was found to be a major factor promoting fountain grass invasion into semi-natural areas away from roadsides. Major clean-ups of this grass should focus on disturbed areas, especially in the fertile parts of the fynbos region where the grass has high climatic suitability. Management and control should also focus on areas with high nutrients and extra water, as these areas facilitate growth and reproduction. The dynamics of P. setaceum invasion and spread makes it a good model for management of similar emerging invasive perennial grasses in similar ecosystems.

AFRIKAANSE OPSOMMING: Klimaatsverandering en biologiese indringing is grootskaalse komponente van mensgedrewe globale veranderinge, en kan ook gesien word as prominente drywers van die huidige afname in globale biodiversiteit van terrestriële ekosisteme. Hierdie drywers werk saam in sinergie, en laat sodoende ekosisteme kwesbaar vir indringing deur uitheemse indringerplante. Grasse is ‘n groep indringerplante wat maklik reageer ten opsigte van globale veranderinge en verander inheemse plantgemeenskapstruktuur en ekosisteem prosesse, soos byvoorbeeld veldbrand frekwensie. Tot op hede is daar wynig studies gedoen wat die ekologiese drywers en effekte van indringergras spesies in droë en semi-droë ekosisteme van Suid-Afrika assesseer. Pronkgras (Pennisetum setaceum) is ‘n wyd verspreide meerjarige uitheemse indringergras, oorspronklik vanaf Noord-Afrika en die Midde-Ooste, en word waardeer deur tuinboukundiges wêreldwyd. Dit versprei al langs padrande aan die buitewyke van meeste dorpe, en is algemeen op ou mynhope in verskeie dele van Suid-Afrika. Somtyds ontsnap hierdie gras langs dreineringskanale of na veldbrande, en beland so in die omringende natuurlike plantegroei. Hierdie gras kan ‘n duur probleem word vir landbou asook biodiversiteit bewaring omdat dit onsmaaklik is vir diere, en dit verhoog veldbrand risiko. ‘n Beter begrip van pronkgras verspreiding en indringingdinamika sal bydra tot verbeterde bestuur en kontrole praktyke. Die hoofdoel van hierdie studie was om die ekologie en indringing prosesse van P. setaceum regoor Suid-Afrikaanse omgewingsgradiente te begryp, en dit dan te gebruik as ‘n model om die sinergistiese verhoudings tussen biologiese indringing en ander globale veranderinge (klimaat en grondgebruik) te verstaan. Die studie verskaf die eerste assessering van hoe P. setaceum verskillende indringing hindernisse in Suid-Afrika oormeester as ‘n opkomende indringer, in vergelyking met ander dele van die wêreld waar dit al klaar problematies is. Hierdie studie verskaf uiteindelik ‘n aantal bestuur en kontrole opsies vir hierdie en ander soortgelyke meerjarige grasse. Die studie verskaf spesifiek ‘n deeglike verstaan van: 1) die verspreiding en habitat voorkeure van P. setaceum in droë en semi-droë areas in Suid-Afrika, 2) omgewingshulpbronne en habitat toestande wat hierdie plant se indringing potensiaal verhoog, 3) groei- en voortplantingsvertoning oor verskillende omgewingsgradiente, 4) die reaksie wat ‘n droë ekosisteem toon teenoor P. setaceum indringing en veldbrand verhoging, en 5) die toekomstige verspreiding van hierdie gras in Suid-Afrika onder voorspellings van klimaatsverandering en verspreiding. Die studie het gevind dat pad-rivier tussengange ideale liggings is vir P. setaceum vestiging, en dat hierdie liggings dus geteiken moet word vir bestuur en kontrole doeleindes. P. setaceum toon volharding onder diverse bioom omgewings, wat toegeskryf word aan plaaslike aanpassing. Daar is gevind dat versteuring ‘n groot faktor is in die verhoging van pronkgras indringing in semi-natuurlike areas weg van padskouers. Grootskaalse pronkgras uitroeiing projekte moet fokus op versteurde gebiede, veral in die vrugbare dele van die fynbos streek, waar hierdie gras hoë klimaatsgeskiktheid toon. Bestuur en kontrole programme moet ook fokus op areas met hoë nutriente inhoud en ekstra water, aangesien hierdie areas groei en voortplanting vergemaklik. Die dinamika van P. setaceum indringing en verspreiding maak dit ‘n goeie model vir die bestuur van soortgelyke opkomende meerjarige indringergrasse in soortgelyke ekosisteme.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/3987
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