Date of Award

Spring 1-1-2018

Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Daniel F. Doak

Second Advisor

William Bowman

Third Advisor

Katherine Suding

Fourth Advisor

Christian Rixen

Fifth Advisor

Sonja Wipf


Delineating the abiotic and biotic processes that set species geographic distributions has been a central theme in ecological research for hundreds of years, yet we still do not understand many aspects of this broad question. Discerning what processes determine past and current range limits is particularly critical for predicting how species distributions will respond to climate change. In Chapters 2 and 3, I address how the interactions of climate, inter-species interactions such as competition, and human disturbances together influence range limits. I show that disturbance has positive effects on the alpine cushion plants Silene acaulis and Minuartia obtusiloba at the species’ lower elevational range limits, likely through a reduction in competitive interactions. In contrast, at upper elevational range limits, where biotic interactions are minimal, disturbance exerts neutral or negative effects. However, disturbance has negative effects at the S. acaulis population level, as evidenced by a reduction in population density and reproductive indices. While facilitation by cushion plants is thought to increase with abiotic stress, it is also left unanswered if disturbance influences these facilitative effects. I show that disturbance does not alter plant-plant interactions, and that, in fact, competitive interactions prevail in S. acaulis communities. In Chapter 4, I address a second major issue in the understanding and prediction of range limits by examining if local populations respond differently to climatic drivers limiting their distribution. By constructing Species Distribution Models (SDMs) based on S. acaulis’ global distribution and separately with subpopulations based on genetic and habitat differences, I show that potential local population adaptation to climate renders a global SDM inaccurate. Furthermore, the manner in which subpopulations are defined greatly affects habitat suitability predictions, which are poorly linked to measures of S. acaulis population performance and facilitative interaction strength. The final aspect of my work is outlined in Chapter 5, for which I developed a citizen science application for smart phones to aid in the identification and hence appreciation of alpine plants in the Colorado Rocky Mountains. Altogether, my work illustrates the need to carefully examine all factors important in both setting species range limits and determining distribution shifts.