Browsing by Subject "invasion"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    An evaluation of goat browsing and other restoration strategies in buckthorn-invaded woodlands and savannas
    (2022-12) Nelson, Sara
    Common buckthorn (Rhamnus cathartica) is a Eurasian shrub that has come to dominate many woodlands, grasslands, and urban areas throughout the Midwestern U.S. Buckthorn has several characteristics that make it harmful to native species assemblages and difficult to control. Because it is so problematic, buckthorn has been the subject of numerous studies investigating control methods. However, few studies to date have investigated the use of targeted goat browsing as a means of buckthorn control, and few studies have holistically assessed multi-component restoration strategies at buckthorn-invaded sites. My research addresses these gaps via: 1) an experimental study testing responses of deciduous-woodland understory plant communities to goat browsing, and 2) a retrospective study investigating multiple possible predictors of restoration outcomes in buckthorn invaded sites and identifying common factors that led to successful or unsuccessful restoration outcomes. I found that goat browsing resulted in immediate reductions in both R. cathartica abundance and native plant diversity and abundance; however, one year later, both R. cathartica and native plants had recovered. Among sites that used varied methods of restoration, I found that buckthorn density at the outset of the restoration and type of implementer predicted both buckthorn and biodiversity outcomes, the use of a diversity of management actions predicted better biodiversity outcomes, and the use of targeted goat browsing did not significantly predict restoration outcomes. Additionally, sites that were restored by long-term site stewards achieved better management outcomes than sites restored by other types of implementers, and longer periods of active management were associated with improved buckthorn outcomes, but not improved biodiversity outcomes. Overall, my findings suggest that targeted goat browsing is not sufficient, by itself, to restore buckthorn-invaded sites, but may be useful as a component of a restoration project, and that good restoration outcomes are more likely to be achieved by incorporating diverse management strategies, ensuring sufficient capacity and time to restore highly invaded sites, and incorporating some form of long-term site stewardship into restoration projects.
  • Thumbnail Image
    Item
    Land-use proxies for aquatic species introductions in the Laurentian Great Lakes
    (2015-05) O'Malia, Elon
    Many nonindigenous aquatic species (NAS) adversely impact ecosystems, human health, and the economy of the Laurentian Great Lakes region. Targeted prevention and eradication efforts in response to early detection of NAS can be both cost advantageous and effective at preventing further spread or establishment. To help inform the process of priority site selection for early detection monitoring, I developed and evaluated land-use metrics of three prominent anthropogenic introduction pathways (commercial maritime traffic, recreational maritime traffic, and live release from urban areas). Logistic and linear regression analyses were conducted between species presence or species richness and introduction pathway intensity for 23 NAS over a five-decade period (1970 - 2013) to explain the apparent spatio-temporal patterns of historic aquatic invasions. The probability of NAS sightings increased with increasing city size, commercial maritime trips, and marina size for all NAS, decade, and pathway combinations. Of the land-use metrics evaluated, city population size was the best model factor and potential proxy of both NAS presence and richness, even for NAS introduced through ballast water discharge. The importance of commercial maritime traffic to NAS presence and richness may have been underrepresented due to rapid secondary spread of planktonic NAS away from port locations prior to detection. Nonetheless, city population size, total commercial maritime trips, and marina size may be reasonable proxies for propagule pressure given the significant relationships between these specific pathway metrics and NAS sightings and richness, and as such, are applicable to the development of early detection monitoring programs in the Laurentian Great Lakes.
  • Thumbnail Image
    Item
    Mercury concentration and aquatic food web alterations associated with zebra mussel invasion in Minnesota lakes
    (2023-08) Link, Denver
    Zebra mussels (Dreissena polymorpha) are an invasive species documented in 299Minnesota lakes, with 231 of those lakes managed as walleye fisheries. Many ecological changes are associated with zebra mussel invasion, including increased water clarity, depleted pelagic energy resources, expanded littoral zones, deposition of benthic organic matter causing anoxic sediments, and increased benthic energy reliance of food webs. However, the effects of zebra mussels vary among lakes, and few opportunities exist to examine lake ecosystems and food webs pre- and post- zebra mussel invasion. Furthermore, recent evidence has suggested that Dreissenid mussels may impact contaminant bioaccumulation in higher trophic levels, but these effects have not been well-studied in inland lakes. I took two distinct approaches to understanding zebra mussel impacts on food webs and contaminants in fishes. I combined historic Minnesota statewide mercury monitoring data from 1997-2021 coupled with stable isotope data to provide insight into food web dynamics and mercury concentration alterations with zebra mussel invasion. I found the probability of exceeding the Minnesota safe threshold for safe eating of average sized northern pike (Esox Lucius) and walleye (Sander vitreus) ranged from 60%-70%, with zebra mussel lakes having an increased chance. Specifically, mercury concentrations analyzed using Before-After Control-Impact (BACI) study design increased by 8.2% in northern pike and 15.4% in walleye for invaded lakes, while uninvaded systems were stagnant or decreased. To quantify resource use and community structure, food webs in Leech Lake were analyzed pre- and post- invasion using stable isotope analysis of δ13C and δ15N. Fish community response to zebra mussel invasion varied spatially in Leech Lake. Bays on the western shoreline of Leech Lake with heterogeneous habitat increased niche size by 39.81% and fish in those areas relied more heavily on benthic resources following invasion. In contrast to the western bays and to hypothesized effects of zebra mussels, fish from the main basin of Leech Lake, containing mostly homogenous pelagic resources, decreased in niche size by 32.26% and relied more heavily on offshore resources. Taken together, high mercury concentrations in Minnesota northern pike and walleye are exacerbated in zebra mussel lakes. Food web dynamics are variable within the same lake, suggesting preexisting food web structure and access to benthic resources are important to community resilience with zebra mussel invasion.
  • Thumbnail Image
    Item
    Pathways in old-field succession to white pine: Seed rain, shade, and climate effects
    (2005) Dovčiak, Martin; Frelich, Lee E; Reich, Peter B
    Trees slowly colonize old fields on sandy outwash in the prairie–forest ecotone of the north-central United States, and in the absence of fire, succession is expected to proceed toward oak woodland. We analyzed whether a case of unusually rapid and spatially extensive invasion by white pine (Pinus strobus) could be explained by the presence of specific temporal or spatial opportunity windows suitable for such invasion. We tested whether the invasion was temporally restricted to the period immediately after abandonment or to periods of favorable climate, and whether it was spatially restricted to areas of high seed rain or high forest-edge shade. White pine invasion into the field occurred in two waves separated from each other by a 1987–1989 drought period. The first wave (1980– 1985) occurred during a period of average climate and led to the establishment of dense sapling patches in shade near forest edges. The second wave (1991–1994) occurred during a period of high precipitation and cooler than normal temperature, and resulted in colonization of the unshaded field center. In addition to the two temporal windows, white pine invasion occurred within two spatial windows: in areas highly sheltered by forest edge and in areas receiving high white pine seed rain. Overall these windows produced three different successional pathways: (1) a slow, creeping white pine invasion into highly shaded areas with low seed rain near forest edges; (2) a rapid, discrete-step invasion in areas where seed rain was abundant enough to overcome mortality in lower shade and where early arrivals facilitate filling in by later arrivals; and (3) a deferred invasion in the field center where low seed rain and lack of shade allowed the persistence of a grassland stage until favorable climate resulted in a white pine recruitment pulse. Temporal variation in climate can accelerate or decelerate any of the three successional pathways.
  • Thumbnail Image
    Item
    Susceptibility and suitability of northeastern North American pines for mountain pine beetle, Dendroctonus ponderosae Hopkins
    (2016-06) Rosenberger, Derek
    The mountain pine beetle (Dendroctonus ponderosae Hopkins) is the most aggressive bark beetle (Coleoptera: Curculionidae) in western North America. This insect is currently expanding its range eastward through the Canadian boreal forest. Continued range expansion, or introduction via infested material from western North America, could lead to novel associations between the mountain pine beetle and eastern pines. This research seeks to determine the susceptibility and suitability of pine species common to northeastern North America for mountain pine beetle. Studies were conducted in a common garden environment on logs of four common eastern pines, red (Pinus resinosa), jack (P. banksiana), eastern white (P. strobus) and Scots (P. sylvestris) pine, and two historic hosts, lodgepole (P. contorta) and ponderosa (P. ponderosa) pine. Constitutive defenses pose no significant barrier to colonization of eastern pines by mountain pine beetle. The proportion of female beetles that established fertile egg galleries differed among eastern pines, but were similar to historic hosts. Foraging beetles were generally no more attracted to logs of eastern pines containing boring beetles than to a control. Chemical analysis of the phloem suggests that host kairomones critical for secondary attraction were minimal among novel pines and likely resulting in less attraction than to historic hosts. Overwintering capacity of developing brood was affected by eastern pines. Rapid development in novel pines resulted in greater proportions of brood entering winter in less cold tolerant life stages than historic hosts. The strategy and degree of cold tolerance of fourth instars were affected by natal pine host. Field validation of logistic regression models demonstrated that models based on the insect freezing temperature underestimate cold mortality, while lower lethal temperature curves more accurately project mortality due to cold. Finally, we found that constitutive defenses pose no barrier to reproduction among novel pines. Indeed, novel pines can have up to twice the reproductive potential, and demonstrate greater brood fitness than historic hosts. However, variable rates of development among novel hosts could result in desynchronized or extended emergence periods in mixed stands. Matches between species specific development rates and climate will be crucial for outbreaks to occur in eastern forests.