Biology Theses and Dissertations

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    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.
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    Epifaunal Disturbance By Periodic Low Dissolved Oxygen: Native Versus Invasive Species Response
    (2005-07-22) Jewett, Elizabeth Bromley; Sebens, Kenneth P; Hines, Anson H; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Invasive species and low dissolved oxygen (DO) threaten the biodiversity and ecosystem health of estuaries worldwide. To test the hypothesis that exposure to low DO reduces resistance of epifaunal community to invasion in the Chesapeake Bay, we conducted experiments using standardized settling panels, including 1) controlled experiments exposing epifaunal communities to low DO; 2) measurement of the short term response of motile and sessile epifauna to low DO; 3) survey of multiple sites in which community structure was correlated with low DO and other environmental variables; and 4) evaluation of the biological and structural effects of an invasive hydroid and a cryptogenic tunicate, both with high tolerance for low DO, on recruitment and development of epifauna. Periodic hypoxia was correlated with an increased cover of the native serpulid polychaete, <i>Hydroides dianthus </i>. Cover of invasive and cryptogenic species increased with exposure to moderate low DO. Cover and incidence of bryozoans, sabellid polychaetes, and cnidarians differed among DO treatments. Nematodes, caprellids, and harpacticoid copepods vacated epifaunal communities in response to low DO. In the multi-site survey, > 50% cover of invasive and cryptogenic species was associated with exposure to chronic low DO. Six of eight sites in the survey experienced periodic low DO (< 4 mg/l), but only one experienced chronic low DO ( > 40% of days below 4 mg/l DO). Shifting cover of <i>Hydroides dianthus </i>, barnacles, and invasive species was correlated (> 50%) with percent of days experiencing low DO. Epifaunal heterogeneity reflected environmental differences among sites. Species richness and diversity at local sites declined with increasing abundance of certain taxa in higher salinity, higher diversity areas. Heightened cover of <i>Molgula manhattensis</i>, <i>Hydroides dianthus</i> or barnacles led to reduced local diversity but regional species diversity was maintained through environmental heterogeneity across sites. Conversely, in lower salinity, lower diversity zone, <i>Cordylophora caspia</i>, an invasive hydroid, had a positive effect on some species. Temporal and spatial shifts in cover of dominant species and in species diversity in response to low DO disturbance and other environmental factors may facilitate persistence of less competitive native or invasive species.
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    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.
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    EFFECTS OF PRE-EXISTING SUBMERSED MACROPHYTES ON THE INVASION SUCCESS OF HYDRILLA VERTICILLATA
    (2004-07-06) Chadwell, Todd Byron; Engelhardt, Katharina; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    I evaluated (1) the effects of a native submersed aquatic plant species, <i>Vallisneria americana<i>, on the colonization success of a non-native and highly invasive submersed aquatic species, <i>Hydrilla verticillata<i>, through field and greenhouse experiments; and (2) the effects of <i>H. verticillata<i> propagule density on its colonization success of patches dominated by the native species. Results of the field study, located in a tidal freshwater region of Chesapeake Bay, suggest that pre-existing vegetation did not have significant negative effects on <i>H. verticillata<i> colonization. However, pre-existing <i>H. verticillata<i> biomass and <i>H. verticillata<i> colonization success were strongly correlated. In contrast, results from the greenhouse study showed that <i>V. americana<i> had a strong inhibitory effect on <i>H. verticillata<i> colonization by fragments and increasing <i>H. verticillata<i> fragment density again increased colonization success. Reduced water column nutrient concentrations, resulting from <i>V. americana<i> growth, appeared to negatively affect successful rooting and subsequent colonization by <i>H. verticillata<i>.