Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

Thomas E. Martin

Commitee Members

John Maron, H. Arthur Woods, Jeffrey Good, Blair O. Wolf


University of Montana


Environmental gradients provide natural forums for understanding how and why species differ. The study of interspecific variation across gradients has provided foundations for our understanding of community ecology, species distributions, life-history evolution and physiological ecology (Janzen 1967; Macarthur 1972; Martin 2015). In this dissertation, I explore questions in these disciplines, using a comparative approach on elevational and latitudinal gradients. My primary focus is understanding the biotic and abiotic processes that limit species distributions and how those same forces scale up to influence the composition of communities on an elevational gradient. I also test a physiological hypothesis for the gradient of life history strategies, the pace-of-life hypothesis, by examining the relationship between metabolic rate and adult mortality probability across a global latitudinal gradient.

I examine these issues in songbird communities on Mt. Kinabalu in Malaysian Borneo (6°N). Kinabalu Park is a large protected area encompassing the largest intact elevational gradient in southeast Asia (400m – 4100m). It is a reserve with tremendous conservation importance; it is a stronghold for more than 90% of Borneo’s endemic bird species including several whose mountaintop ranges put them at risk of extinction assuming upward range shifts due to climate change (Colwell et al. 2008). Understanding the mechanisms that underpin elevational distributions in such megadiverse threatened areas is a central goal of ecology and vitally important to conservation locally and globally.

In chapters 1-3 I used experimental and descriptive approaches to assess the importance of interspecific competition and physiology in setting elevational range limits and structuring communities across elevations. We found evidence that interspecific competition may set range limits in some species, but that aggressive interactions with close relatives could not explain range boundaries in other cases. We also found that birds occupying different elevations had similar thermal physiology, suggesting range limits are not directly set by climatic variables like temperature. Using phylogenetic and trait-based approaches, we found evidence that interspecific competition plays a strong role in structuring bird communities at low elevations, while environmental filtering appears to be important at high elevations. Based on clustering of morphological traits, particularly bill shape, we speculate that the influence of climate on prey size and diversity may restrict many species from high elevation communities.

Finally, we tested the ability of metabolic rate to explain variation in average lifespan within and across latitudes. The pace-of-life hypothesis posits that damaging byproducts from cellular metabolism are the primary physiological driver of lifespan, such that longlived tropical species are expected to have low metabolic rates (Pearl 1928; Hulbert et al. 2007; Wiersma et al. 2007; Williams et al. 2010). We tested this hypothesis by measuring metabolic rates and estimating adult survival probability in songbirds at Kinabalu Park (6°N) and in Arizona, USA (34°N). We found that metabolic rate explained variation in annual adult survival probability within sites, but that it could not explain the longer lives of tropical birds compared with temperate zone relatives.



© Copyright 2018 Andrew James Boyce