Biology Theses and Dissertations
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Item COMPETITIVE RESPONSE OF INVASIVE PLANT SPECIES TO NEIGHBOR PRESENCE, IDENTITY, AND PHENOLOGY ACROSS TWO GROWING SEASONS(2013) Barry, Kevin; Dudash, Michele; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Exotic plant species invade many native communities, yet some communities are less susceptible than others. Community properties that determine resistance to or influence on invasive species performance are less studied. The target-neighbor experimental design, originally used for studies of root and shoot competition (Chapter 1), is now commonly used to determine effects of communities of species on a target plant of interest. However, intensity of competition varies between species both within and across seasons, therefore interactions between species are not static. Thus I compared the competitive response of two widespread but relatively little studied invasive species, early flowering Hesperis matronalis and later flowering Nepeta cataria. Both invasives were introduced into native species neighbor communities composed of either early, late, or a mixture of early and late growth and flowering phenologies and measured over the course of two years (Chapter 2). Additionally, invasive species introduction time into native communities was manipulated by either coestablishing the invasive with the neighbors, or introducing the invasive into established neighbor communities (Chapter 3). I predicted that target invasives would experience the most intense competition (1) when sharing the same growth and flowering phenology as their surrounding native community, due to temporal niche overlap (2) when introduced into an established native community versus when coestablished with native neighbors, and also that (3) the overall effect of neighbor presence and neighbor identity would vary with the age of the competitors. Finally, I utilized field soil probes and greenhouse plants labeled with 15N isotopes to determine the relationship between phenology and nitrogen uptake to provide a mechanistic basis for temporal niche overlap (Chapter 4). The competitive response of both invasive species' performance was dependent on plant trait measured, community age, and native neighbor phenology treatment. I found evidence for a negative effect of temporal niche overlap, but resource pre-emption by the early phenology neighbors was more prevalent than stronger competition due to a shared phenology. Despite this, field nitrogen levels were still correlated with species phenology. This body of work supports the need for more research on the factors associated with native communities' ability to resist invasion.Item COMPETITION BETWEEN HYDRILLA VERTICILLATA AND VALLISNERIA AMERICANA IN AN OBSERVATIONAL FIELD STUDY AND GREENHOUSE EXPERIMENT.(2010) McChesney, Lauren Dalton; Engelhardt, Katharina; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Invasive species continue to have a pervasive influence on biodiversity but it is often unclear how invasive species affect native species. In field observations and greenhouse experiments, I examined the effect of the non-native submersed aquatic plant Hydrilla verticillata on the native species Vallisneria americana. Field monitoring from 2002 to 2006 showed that coverage of species peaked in 2004 after initial invasion of the estuarine study system in 2002. Substrate characteristics did not limit species distribution. In contrast, substrate and planting density affected plant growth and the outcome of intra- and inter-specific competition in the greenhouse. Although other environmental variables, such as water depth and turbidity, appear to override the effect of substrate in the field, the greenhouse experiment suggests that substrate can be an important driver of submersed aquatic plant community dynamics. Sediment characteristics should therefore be a factor in restoration design and the management of invasive species.Item Evaluating Competition between the Non-native Slug Arion subfuscus and the Native Slug Philomycus carolinianus(2010) Paustian, Megan Elisabeth; Barbosa, Pedro; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The degree to which invasive species have altered the demography, ranges, and microhabitat occupation of native species is poorly known. Yet, the competition-mediated decline of native populations, in concert with other factors such as habitat degradation, can place native species at risk of extirpation. Understanding whether competition between native and non-native species can take place under ordinary environmental conditions can allow us to extrapolate whether native species are likely to have experienced harm in the past and/or if they are likely to do so in the future. The native slug Philomycus carolinianus is likely to compete for resources with the aggressive non-native slug Arion subfuscus in central Maryland forests. In order to establish whether competition occurs between these two species, I tested for the following criteria: the existence of competitive displacement in the field, overlap in the use of limited resources (shelter and food), a decline in the fitness of P. carolinianus in the presence of A. subfuscus, and the action of competition mechanisms (interference and exploitation) between them. Field surveys showed that displacement between A. subfuscus and P. carolinianus does not apparently occur within mixed natural populations. Resource use of the two slugs overlapped, with part of the diet (i.e. fungus) and a large proportion of the microhabitats occupied (i.e. coarse woody debris) in common. A lab experiment established that low natural levels of food (fungus) can limit the fitness of each slug species, while shelter (coarse woody debris) was not limiting. When sharing a low-resource lab cage with either A. subfuscus or conspecifics, P. carolinianus experienced a similar decline in fitness, suggesting that exploitative resource competition was no greater between heterospecifics than between conspecifics. No evidence of heterospecific interference (competition independent of resource levels) was found. Given the limited support for the criteria of competition, A. subfuscus was not shown to be an immediate threat to the persistence of P. carolinianus.Item Fish movement, habitat selection, and stream habitat complexity in small urban streams(2006-08-04) Cushman, Susan Flanders; Morgan II, Raymond P; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Urbanization impacts have become more evident in the last 30-50 years, due to human population increase and subsequent land use change. Many aspects of stream ecosystems are influenced including hydrology, geomorphology, water quality, ecosystem function, riparian vegetation, and stream biota. Effects of urbanization on ecosystem structure and function are discussed, and the urban stream syndrome is introduced in Chapter 1. Chapter 2 reports differences in stream fish assemblages in the eastern Piedmont and Coastal Plain of Maryland, USA due to urbanization, and establishes a foundation for hypotheses presented in subsequent chapters. Chapter 3 describes a physical habitat survey that attempts to understand what instream and channel habitat attributes change across the urban-rural gradient (0-81% urban land use; ULU). While changes in stream habitat appear at 30% ULU, significant impacts occurred once a watershed has >45% ULU, at which point stream channels can not accommodate the power and intensity of impervious surface runoff. Fish habitat patch selection is examined in Chapter 4, which involved instream habitat manipulation experiments. I tested fish selection response of instream habitat using three treatments (woody debris, shade, and both) in first order urban (>60% ULU), suburban (27-46% ULU), and rural (<15% ULU) eastern Piedmont streams in Maryland. Blacknose dace (BND) Rhinichthys atratulus and creek chub (CKB) Semotilus atromaculatus selected shade and woody debris combined significantly more than other treatments in rural and suburban streams. Urban fish selected the shade treatment the most of all enhancements. CKB who selected the enhancement were significantly larger than those found in the control. Urban fish prefer shaded habitat providing overhead protection due to the general lack of habitat complexity in urban channels. CKB behavior may indicate intraspecific competition, particularly between juvenile and adult individuals for prime habitat positions. Chapter 5 presents a fish movement study, comparing rural and urban fish population behaviors. Urban BND and CKB displayed significantly larger home ranges than rural fish. The rural fish movement distribution was more leptokurtic. Competitive interactions are suggested as the reason for greater movement in urban stream populations. Finally, conclusions are submitted with significant findings in Chapter 6.