Date of Award

Summer 8-15-2018

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type



Earthճ current biodiversity crisis is now considered a true mass extinction event, with species level extinction rates well above background rates and population level extinction rates orders of magnitude more common that species extinctions. There are many threats driving this loss of biodiversity. How each threat impacts the viability of a species is highly context dependent, but all are anthropogenic in origin and so as the human population continues to increase, so too will the pressure of these threats on our natural systems. Ultimately, how much a threat decreases the viability of a species depends on how that threat influences a speciesՠdemographic vital rates and population sizes. Therefore, in this dissertation, I use demographic modeling to quantify viability and the impact of threats on viability for many rare or at-risk plant species. Then, I use the results of those models to make management recommendations to reduce plant speciesՠrisk of extinction.

I had three goals for my dissertation. My first goal was to simultaneously quantify the effects of multiple threats on the viability of two rare plant species as case studies to determine whether incorporating the complexity of interacting threats would change management recommendations. In the first case study, I found an interaction between the increase in drought frequency expected with climate change the habitat management frequency of Cedar Glades on the viability of Astragulus bibullatus. Typical management for open habitats like Cedar Glades is frequent disturbance. But by detecting this interactive effect, I was able to recommend managing disturbance to maintain two types of environments across a landscape; those that promote high survivorship during climatically unfavorable (drought) years, and those that promote increased reproduction and recruitment during climatically favorable (non-drought) years. In my second case study, I found an interaction between local scale threats (woody species encroachment and browsing by White Tailed deer) and climate change on the viability of Eurybia furcata. This interaction was such that populations well managed for local threats are not expected to be vulnerable to climate change, but populations with high levels of local threats are expected to be driven extinct as the climate changes. This result suggests that management actions targeting local threats (mostly woody invasive species encroachment) could increase E. furcataճ viability in two ways; by reducing the direct negative effect of woody encroachment and indirectly by decreasing the impact of climate change. Therefore, in both systems, I showed that complex non-additive effects among threats influence what is the most optimal or most cost-efficient management strategy.

My second goal was to expand the use of count-based population monitoring data in plant conservation biology. To my knowledge, I am the first to use this type of data to compare the relative impacts of many threats and test for non-additive effects among them. The methods I developed in my dissertation use these data for viability and threat assessments and can be widely applied to count-based monitoring data already in existence, expanding the use of these data in rare plant species conservation globally.

My last goal was to determine if incorporating complex multi-threat assessments into conservation decision making could substantially improve conservation outcomes over our current methods, which are largely based on practitioner observation and experience. I had a unique opportunity through a long-term rare plant monitoring program to compare assessments of speciesՠviability and the impact of threats on viability between practitionerճ expert opinions and quantitative analyses. I found no significant correlations between them, indicating the two sources of information result in conflicting priorities for rare plant conservation. Then, I showed that this conflict could arise from the complexity of threats themselves. For example, I found a strong three-way interaction among threats showing that rare plant species are particularly vulnerable to the compounding effects of threats during the time immediately after a disturbance management event (e.g. prescribed fire). Thus, making an observational assessment of any one theat difficult as the impact of the one threat is different depending on the level of another. Overall, all aspects of my dissertation highlight the critical need for comprehensive, multi-threat assessments to better understand what is causing a species to have poor viability, to more effectively manage rare plant species to reduce their risk of extinction, and ultimately to better combat the global biodiversity crisis.


English (en)

Chair and Committee

Scott Tiffany A. Mangan Knight

Committee Members

Matthew A. Albrecht, Adam Smith, Jonathan Myers, Rachel Penczykowski,


Permanent URL: 2018-08-15