Year of Award


Document Type

Dissertation - Campus Access Only

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

John L. Maron

Commitee Members

Ragan M. Callaway, Winsor H. Lowe, Thomas E. Martin, Rebecca E. Irwin


University of Montana


A fundamental goal of ecology is improving our understanding of mechanisms underlying species’ distributions. While current ecological theory predicts that both niche-based mechanisms and non-niche mechanisms such as dispersal limitation act in concert to constrain species’ ranges, very few empirical studies examine the causes of range limits through evaluation of this integrative framework. In these studies, I explored the determinants of Astragalus utahensis’ latitudinal distribution. I addressed the extent to which patterns of abundance, demography, and population performance across and beyond the range match those predicted by models that appeal to niche-based mechanisms, dispersal-based mechanisms, or a combination of both. I also elucidated the contributions of two biotic interactions to individual- and population-level performance from the center of the distribution to its northern edge. Finally, I evaluated the degree to which a species distribution model for the presence of A. utahensis can predict its density and performance across the latitudinal range.

The density and growth rate of populations declined from the center of the distribution to its northern edge in accordance with the prediction that niche constraints contribute to this range boundary (Chapter 1). However, transplanted individuals within and beyond the range displayed similar performance, indicating simultaneous dispersal limitation of the northern range edge (Chapter 1). These results support ‘metapopulation’ models for the determinants of range limits, which postulate that species’ distributions result from both declining performance and dispersal limitation. Pre-dispersal seed predation significantly decreased population growth rate in most populations at the range center and northern edge; this was also true of pollen limitation in some populations (Chapter 2). Despite the lack of a latitudinal signature in the strength of population-scale effects of pollen limitation and pre-dispersal seed predation, both biotic interactions are likely to contribute to low demographic performance at the northern range edge and consequently to the establishment and maintenance of this edge. Projected abiotic suitability for the presence of A. utahensis derived from a species distribution model accurately predicted density across the north-south range of A. utahensis, and showed a marginally significant positive relationship with projected population growth rate, indicating predictable abiotic control over aspects of abundance and demographic performance (Chapter 3).

This record is only available
to users affiliated with
the University of Montana.

Request Access



© Copyright 2017 Kathryn Carey Baer