Date of Award

Summer 7-18-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology & Evolutionary Biology

First Advisor

Nichole Barger

Second Advisor

Michael Cramer

Third Advisor

Timothy Seastedt

Abstract

Alien tree invasions cause widespread impacts to ecosystem structure and function in many regions across the globe. Limiting the spread of these invasions into untransformed native ecosystems is often a management priority. Understanding the mechanisms by which these native ecosystems are able to resist tree invasion can not only enhance our understanding of invasion processes but may also aid in predicting invasion risk and informing management decisions on monitoring, removal and restoration. Australian Acacia species are particularly invasive in the uniquely biodiverse Cape Floristic Region of South Africa. The generalist reproductive biology of these species results in the ability to reproduce and spread widely in invaded regions. However, little is understood of the ability of native ecosystems to constrain the establishment and growth of these tree invaders. I thus investigated the ability of an untransformed South African shrubland to resist the establishment and growth of an invasive tree species, Acacia cyclops, in order to gain a mechanistic understanding of ecosystem resistance. My results indicate that herbivory by an abundant indigenous rodent resulted in mortality of 60-100 % of transplanted A. cyclops seedlings. Rodent behaviour, rather than densities influenced herbivory levels since rodents were less likely to forage in sparsely vegetated sandy areas. A separate experiment found that belowground competition reduced A. cyclops seedling survival by ca. 1.4–1.8-fold and biomass accumulation by 4–5-fold. A review of available evidence from published literature led me to conclude that if able to navigate early constraints and establish ecophysiological traits such as heteroblasty, N2-fixation and strong allocation to belowground biomass aid invasive Australian Acacias to overcome abiotic constraints. These findings indicate that native ecosystems constrain the survival and growth of an invasive tree species. Mechanisms of resistance may be useful in predicting invasion risks to different native communities, identifying targets for ecosystem restoration and informing decisions on management and clearing operations so as to maximize ecosystem resistance.

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