Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Norma Fowler. Vita. Includes bibliographical references. Available also from UMI Company.

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Habitat loss, habitat fragmentation and species invasions have been recognized as three of the leading threats to biodiversity. I examined the effects of habitat loss and fragmentation on native and invasive plants in central Texas. During the last century, the density and abundance of woody plants has been increasing in the savannas of eastern Edwards Plateau. This process, known as woody plant encroachment, not only reduces the amount of open herbaceous habitat but also fragments that habitat creating smaller and more isolated patches. In three studies, I investigated the consequences of this habitat loss and fragmentation for plants which do not occur under the cover of woody plants including native grasses and forbs and the invasive Eurasian bunchgrass, Bothriochloa ischaemum (King Ranch Bluestem). In the first study, I show that woody plant encroachment reduces native herbaceous species richness (the number of species in a given area). Using a collection of historical aerial photographs, I demonstrate that current native herbaceous species richness was most strongly related to recent habitat amount, but to the degree of habitat fragmentation at least 50 years ago. In a second study, I show that the presence of B. ischaemum was negatively related to the degree of fragmentation in the surrounding landscape. Finally, I found that B. ischaemum had higher rates of germination and growth in experimental plots where the species commonly lost with woody plant encroachment were removed than in unmanipulated control plots. Together, this work suggests that woody plant encroachment is directly slowing the spread of an invasive species while indirectly facilitating its establishment. / text

Invasive species are the second-leading cause of the decline of native species, making it a critical global environmental issue (Pimentel 2005). The goal of this study was to identify which non-native invasive plant species are common in eastern Travis County and which environmental factors associate with their establishment and spread in this area. Five parks and preserves were sampled, as they are intended for the conservation of native species and are more accessible for educational use. Six of the 15 species searched for, Bothriochloa ischaemum, Lactuca serriola, Ligustrum lucidum, Lonicera japonica, Melia azedarach, and Sorghum halepense, were each found in at least one of the five sites. The presences and absences of these species were compared to a suite of ecosystem properties, including habitat, disturbance, the average soil depth, and the horizontal and vertical distances to the nearest pond, lake, stream, or river. None of these six species appears to need observable recent disturbance. All of these six species, both woody and herbaceous, can tolerate shallow soils. The woody invasive species Ligustrum lucidum and Melia azedarach seem to prefer more mesic habitats. The invasive grass species Bothriochloa ischaemum and Sorghum halepense seem to prefer habitats that are more open. Several invasions of potential conservation concern were identified. / text