Master of Science
Dr. Catherine Hulshof
Chapter 1: Native plant response to deer overbrowsing in a serpentine savannah
Plants are particularly vulnerable to physical disturbance in low productivity areas, due to a high cost of replacing lost plant tissue. In the eastern United States, serpentine grasslands are fragmented ecosystems with high concentrations of rare endemic plant species, low concentrations of soil nutrients, and uncontrolled deer overpopulation. This study assessed functional responses of native angiosperms in a rare eastern serpentine savannah to selective deer browsing. Plant count, flower count, floral area, vegetative area, and plant height of 10 serpentine plant species were compared inside and outside of deer exclusion structures throughout the growing season of 2019 (April-October). Throughout the growing season and across the plant community, deer presence consistently reduced values for all plant response traits measured. Species most impacted by deer herbivory included Oenothera biennis, Solidago rugosa, Sabatia angularis, Liatris spicata, and serpentine endemic Symphyotrichum depauperatum. Eastern serpentine grasslands could lose biodiversity and be permanently degraded by continued intense browsing pressure. We recommend that management programs should be implemented to monitor rare serpentine flowering plant species, and that hunting be considered to limit the deer population in areas of high plant conservation priority.
Chapter 2: Disturbance disrupts pollinator network stability in a low diversity grassland
Plants are the foundation of terrestrial foodwebs, so disturbances that degrade mutualist networks may threaten ecosystem functionality and ecosystem services. While mutualist networks are generally robust the loss of weakly interacting species, disturbances that impact strongly interacting keystone generalist species can decrease pollinator network stability. This study assessed how metrics of mutualist network stability were impacted when keystone pollinator groups were negatively affected by deer browsing disturbance. Deer exclusion plots were used to compare plant and pollinator response between disturbed and undisturbed habitat throughout a growing season. Pollinators were sampled with pan traps and visual surveys were used to document plant-pollinator interactions. Highly abundant flower species Liatris spicata was found to be a keystone plant species, while the pollinator group of Dipteran flies were found to be keystone pollinators. Although flies were equally as abundant between control and exclusion plots, the diversity and intensity of their interactions (species strength) was significantly decreased in exclusion plots. It was found that while community stability metrics of interaction strength asymmetry (ISA) and connectance stayed constant between disturbed and undisturbed habitats, nestedness, which reflects network redundancy, decreased in disturbed habitats. As a result of decreased species strength of a keystone generalist pollinator group, community nestedness and overall network stability also decreased.
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