Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item Ecological interactions of the cadmium- and zinc-hyperaccumulating plant, Thlaspi caerulescens, and their implications for phytoremediation(2011) Che-Castaldo, Judy P; Inouye, David W; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The success of invasive species can be attributed to a combination of abiotic factors, such as abundant resources and favorable climate, and biotic factors, such as low levels of competition and predation or herbivory, at the introduced location. While studies have demonstrated the effects of these factors on known invasive species, the degree to which these factors affect a non-native species can be used to predict its likelihood of becoming invasive. The metal-hyperaccumulating plant Thlaspi caerulescens (Brassicaceae) is potentially useful for remediating soils that are moderately contaminated with Cd and Zn, and has been experimentally introduced to contaminated sites outside of its native range for phytoremediation. To assess the ecological risks involved in introducing metal-hyperaccumulating plants for phytoremediation, including their potential invasiveness, I have performed three studies to examine the abiotic and biotic factors that could influence the establishment of T. caerulescens at three contaminated sites near the Rocky Mountain Biological Laboratory in Gothic, Colorado. In the first two studies, I test the effects of soil metal concentrations and interspecific competition on plant performance, and in the third study I examine the strength of herbivore pressure on this plant. Results from these studies show that the growth rate of T. caerulescens in field conditions is generally low, but higher where there are high concentrations of soil Zn and low concentrations of soil Cu. Interspecific competition between T. caerulescens and a native congener is weak overall, and herbivory pressure from a native Lepidopteran herbivore is also low. Therefore, abiotic conditions are more limiting to T. caerulescens than biotic interactions, and would likely prevent T. caerulescens from becoming invasive or spreading outside of contaminated soils at these sites. In the fourth chapter, I use a long-term dataset to describe the demography of Frasera speciosa (Gentianaceae), a long-lived monocarpic plant. Results show that the population is stable, and despite the low elasticity values for the reproductive stages, masting events must be observed to describe accurately the population dynamics of this species.Item The roles of nutrients and herbivory in controlling the microbioerosion of tropical reefs(2007-08-31) Silva, Marina Carreiro; Mihursky, Joseph A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Coral reefs worldwide are suffering degradation from increasing fishing pressure, pollution, diseases, and coral bleaching. One important ecological consequence of such degradation is an increase in biological erosion, or bioerosion, of the coral framework by boring and grazing organisms. Therefore, it has become essential to understand the factors that control the processes and agents of bioerosion. The aim of my dissertation is to understand how organic and inorganic nutrients and herbivory affect the bioerosion of carbonate substrates by microbial endolithic organisms (bacteria, fungi and algae), an often overlooked component of bioerosion processes in coral reefs. Results of controlled experiments using herbivore-exclusion cages and fertilizers at Glovers Reef, Belize consistently showed significant effect of nutrients in stimulating microbial endoliths' substrate colonization and bioerosion rates of Strombus gigas shells. The addition of inorganic nutrients increased bioerosion rates by a factor of 8 to 15 in comparison to control treatments. Changes in nutrient ratios changed microbial endolithic community structure. The addition of nitrogen alone or in combination with phosphorus stimulated green algae, the addition of phosphorus alone stimulated cyanobacteria, and the addition of organic matter alone stimulated fungi. The inclusion of herbivores reduced observed bioerosion rates by half, demonstrating the importance of herbivory in modifying bioerosion processes. Field experiments on the relationship between water quality and the amount of microbioerosion in Lambis chiragra shells in nine coral reefs in East Africa demonstrated that other factors within reefs may interact with nutrients in determining bioerosion rates. Results suggested that epilithic algal cover, particularly crustose coralline algae, may decrease microborer colonization and bioerosion rates by reducing light conditions within substrates, so that no direct effects of nutrients on bioerosion rates are detected. A critical review of the evidence for nutrients as a primary control of bioerosion by different bioeroder groups (microborers, macroborers, and grazers) suggests that macroborer abundances reflect increases in nutrient conditions and may therefore represent a useful indicator of eutrophication and coral reef "health". This dissertation contributes to a better understanding of the factors affecting bioerosion by microbial endolithic organisms, which are important but often overlooked agents of bioerosion in coral reefs.Item Ecological Effects of the Biocontrol Insects, Larinus Planus and Rhinocyllus Conicus, on Native Thistles(2005-05-11) Dodge, Gary Jonathan; Inouye, David W; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biological control of invasive weeds is, by nature, a delicate balance between introducing effective biological control agents and not introducing another invasive species. A disconcertingly similar suite of traits is used to describe invasive insect species and to identify appropriate biological control agents (or candidates): good control agents and invasive exotic species are good dispersers, they are good colonizers, they have high reproduction rates, and they are suited for broad distribution. It shouldn't come as a surprise that two previously released weed biological control agents can now be characterized as invasive species. The purpose of this dissertation is twofold: first, it is to explore the ecological relationship between predispersal seed predation and plant population dynamics, and second, it is to elucidate the risks to native plants involved with introduction and redistribution of exotic species. Rhinocyllus conicus and Larinus planus are Eurasian seed-head weevils, introduced and redistributed broadly across the western U.S. to control exotic thistles. Exclusion experiments on native thistles, including one that is rare and imperiled, at sites in Colorado present strong evidence that a decrease in seed production due to herbivory by both R. conicus and L. planus has lead to a reduction in recruitment of the thistles. The density of seedlings in both cases, even in the excluded units, was far below where density dependent effects may play a role in the dynamics of the thistle. Further, a survey of eight western states demonstrates established populations of L. planus and effects on seed production in multiple native species in four states. It also documents the near ubiquity and broad diet breadth of R. conicus. The process for approval of phytophagous biocontrol agents has become more cautious and more efforts are made to prevent nontarget herbivory. Nevertheless, land managers still routinely redistribute previously approved, non-regulated agent insects that appear to pose a higher risk to the native flora. The results of this research will benefit resource managers who wish to consider use of phytophagous insects as biological control agents as well as help ecologists and environmental managers understand the risk probabilities of biological control applications.