Abstract

Montane ecosystems represent some of Earth’s most ecologically complex regions in the world, harboring a disproportionate number of endemic species. For example, they comprise 25% of all land area, yet 87% of all terrestrial biodiversity can be found in them. Temperature, precipitation, and other abiotic factors change dramatically across short geographic distances in montane ecosystems, providing unique opportunities to study how communities assemble across space and time. Understanding how biodiversity is structured in montane ecosystems is, in many ways, a window into understanding how biodiversity is structured more broadly — making it not only a question of theoretical importance, but also one of urgent conservation relevance in the face of accelerating global change. While decades of research have explored patterns of species richness (alpha-diversity) across elevations, much less is known about beta-diversity. Beta-diversity captures the extent of species turnover or community differentiation across space or time and can provide insights into the processes of community assembly. My dissertation aims to address this gap through three goals: 1) assess how community composition of small mammals in California’s Sierra Nevada mountain range has changed over the past century in response to climate change, using historical resurveys, 2) evaluate patterns of within-elevation beta-diversity in forest tree communities across 38 ecoregions in temperate North America, and to test how these patterns are shaped by regional species pools and local assembly processes, and 3) test the Ecotone Hypothesis by conducting a global synthesis of avian turnover across elevations, using a null model approach to identify where turnover is higher than expected under random assembly. In chapter 1, I test whether small mammal communities in California’s Sierra Nevada have shifted in community composition over time in favor of more warm- and dry-adapted species across three transects distributed across the northern, central, southern regions of the mountain range. I found that communities have shifted directionally toward more warm- and dry-adapted species in the southern region, but not the northern and central regions. Importantly, changes in community structure in all three regions lagged behind corresponding changes in climate. These widespread lags between community composition and shifting temperature and precipitation regimes indicate that many communities are already out of equilibrium with their environments, increasing their vulnerability to future warming and drying. In chapter 2, I synthesized within-elevation beta-diversity patterns of tree communities from 38 mountainous ecoregions spanning the coterminous United States and performed a null model analysis aimed at disentangling the relative roles of regional sampling effects and local assembly mechanisms on these patterns. I found that beta-diversity after controlling for regional species pool size – i.e., beta deviations – does not exhibit consistent elevational patterns across ecoregions. This suggests the effects of local community assembly mechanisms are region-specific. These results underscore the need for region-specific management strategies that account for the interplay between regional species availability and local assembly processes, rather than relying on one-size-fits-all expectations for community responses to environmental gradients. In chapter 3, I test the ecotone hypothesis in 40 montane regions around the world using a null model analysis that randomizes bird species ranges while preserving range structure. I show that on average, half of the elevational transitions within mountain ranges showed greater compositional turnover than expected under the null model. The proportion of elevational bands exceeding null expectations was positively associated with mountain range area, elevational extent, and species richness, suggesting that topographically and biologically complex systems are more likely to exhibit non-random community turnover. These findings suggest that ecotones play a key role in structuring bird communities worldwide, and that shifts in their position or permeability could precipitate rapid, spatially concentrated reorganizations of avian assemblages. Collectively, these findings advance our understanding of how beta-diversity is structured in montane systems, how it varies across space and time, and how it reflects underlying ecological and evolutionary processes. By integrating resurvey data, national forest inventories, and global biodiversity datasets, this dissertation contributes to a more nuanced understanding of community structure in mountains—one that is essential for predicting biodiversity responses to environmental change.

Committee Chair

Jonathan Myers

Committee Members

Adam Smith; J Tello; Michael Landis; Rachel Penczykowski

Degree

Doctor of Philosophy (PhD)

Author's Department

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

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-19-2025

Language

English (en)

Author's ORCID

https://orcid.org/0000-0002-3450-195X

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Available for download on Wednesday, August 18, 2027

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