Date of Award

Summer 2019

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Rebecca J Rowe

Second Advisor

William C Clyde

Third Advisor

Thomas D Lee

Abstract

Understanding how biodiversity is distributed, maintained, and altered is a fundamental goal of ecology and is especially important for predicting the effects of ongoing rapid environmental change. Traditionally, diversity has been described in taxonomic terms using the number and abundance of species (e.g., species richness). However, biodiversity is multi-faceted and includes functional (ecological traits) and phylogenetic (evolutionary relationships) dimensions that emphasize the similarities and differences among species. Functional diversity is particularly appealing because it quantifies the range and prevalence of traits in an assemblage and helps link patterns of diversity to the ecological processes that generate them. I used a multi-dimensional diversity approach to investigate elevation-diversity patterns, community assembly processes, and patterns and drivers of change in small mammal community structure over the last century in mountain ranges in the Great Basin of western North America.

In Chapter 1, I developed a novel trait-based approach for discriminating between environmental filtering and biotic interactions as possible drivers of species co-occurrence across environmentally heterogeneous sites. Expectations of environmental filtering were assessed using species similarity in the traits of habitat affinity and geographic range location whereas expectations of biotic interactions were based on similarity of diet and body size. When applying this hypothesis-testing framework to small mammal species pairs distributed among and within local sites distributed across three broad elevational gradients, most associations were consistent with environmental filtering. However, negative associations among four species pairs were consistent with expectations under biotic interactions, including two pairs for which competitive exclusion has previously been documented (two species of chipmunk of the genus Tamias and two species of pocket mice of the genus Perognathus). Discerning the mechanisms responsible for co-occurrence patterns was made possible by developing and testing explicit hypotheses based on trait similarity.

Although the appreciation and measurement of multiple dimensions of biodiversity has grown recently, refinement of trait data for mammals is much needed. Most studies rely on categorical rather than continuous traits. As a result, finer variation present among species is overlooked which may obscure patterns, particularly for studies on smaller species pools. In Chapter 2, I identified three continuous ecomorphological traits that have a demonstrable link to function and reflect traditionally used functional guilds. Specifically, I investigated the relative medullary thickness (RMT) of the kidney as a measure of habitat affinity (mesic-to-xeric spectrum), hair density as a measure of thermoregulatory ability, and an integrated suite of cranial and dental measurements as an indication of diet specialization. Each trait captured traditional functional group differences for 32 species of Great Basin small mammals while also illuminating meaningful within-group variation. Although each trait had a strong phylogenetic signal, phylogeny alone obscures informative ecological differences (similar to the use of categories). The greater resolution of continuous trait data will facilitate more refined assessments of functional diversity and improve efforts to test ecological theories and track responses to environmental change.

With an improved functional trait matrix, including the ecomorphological traits from Chapter 2, I revisited the classic elevation-diversity relationship in Chapter 3 by investigating patterns of functional and phylogenetic diversity in addition to species richness along three elevational gradients. Elevation-species richness relationships are one of the most widely studied biogeographic patterns, but there have been few investigations using other dimensions of diversity. In contrast to the well-established mid-elevation peak in species richness, functional and phylogenetic diversity generally increased with elevation. Deviations among dimensions reveal that species richness is a poor surrogate for these other dimensions of diversity for small mammals. Decomposing functional diversity into subsets of traits that reflect specific niche axes can provide insight into the drivers of community assembly over elevation. Specifically, clustering of traits associated with abiotic conditions and habitat affinities provides evidence for environmental filtering where overdispersion among traits corresponding to resource acquisition and use suggests biotic interactions (namely competition) are structuring assembly among community members. I found strong evidence for environmental filtering in both low and high-elevation communities. Evidence for competition as a driver was not consistent with theoretical expectations under the stress dominance hypothesis, guild assembly rules, or competitor limitation of range margins.

In Chapter 4, I used resurveys of sites in Great Basin National Park and vicinity to track functional diversity responses to climate and habitat change. Over the 86-year interval between surveys, functional diversity decreased even though species richness and total community abundance were stable at sites. In general, communities become less functionally even; species with more generalized traits became more dominant and climate and habitat specialists constituted smaller components of most communities. Larger species with lower reproductive potential also tended to fare worse over time. Functional evenness decreased more due to climate responses whereas functional divergence and dispersion were reduced more among habitat traits. In sum, this analysis indicates how the individual and interactive effects of change in abiotic conditions, cover types, and resource base are translated to change in community structure through species’ traits. My results emphasize the importance of using abundance-weighted functional diversity metrics to detect subtle or early-stage changes to community structure that may serve as an early warning of more dramatic diversity loss in the future.

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