Mountain ecosystems in western North America are highly sensitive to climate change and are warming faster than the global average. Found at the tops of these mountains, the alpine tundra ecosystem is especially threatened due to its fragmented distribution (so called “sky islands”), limited area, and the impossibility of alpine species moving to higher elevations. As a result, alpine sky islands are considered a “sentinel system” for detecting the biological impacts of climate change, and rapid changes in alpine biodiversity are expected in the coming decades. In this dissertation, I explore how climate change is driving shifts in alpine plant biodiversity patterns (chapter 1), how species interactions structure current patterns of alpine biodiversity (chapter 2), and how variation in climatic conditions may affect the relationship between biodiversity and ecosystem function (chapter 3). To investigate these questions, I employ an observational study of regional plant biodiversity across a 12-year period, a neighbor removal experiment paired with spatial point pattern analysis, and structural equation modeling using data from the Niwot Ridge Long Term Ecological Research Program, respectively. I find evidence that: 1) alpine biodiversity patterns are shifting, notably towards species possessing traits which enable drought tolerance; 2) species interactions and species spatial patterns are largely governed by traits related to plant size like leaf area and height, though the manner in which these traits relate to species coexistence mechanisms changes across alpine community types; and 3) the alpine biodiversity-ecosystem function relationship is also best predicted size-related traits; however, the ability of these traits to predict ecosystem function varies strongly depending on the amount of winter precipitation. Overall, my dissertation highlights that alpine biodiversity change is detectable over relatively short time periods, and that these changes are likely to have important implications for species interactions and the biodiversity-ecosystem function relationship.
Global climate change is driving the rapid redistribution of plant species, and, in dryland ecosystems in particular, we lack an understanding of how species have and will continue to respond to novel abiotic conditions. This gap exists largely because, while the performance of plant species is influenced by abiotic conditions, interactions with co-occurring species at the community level are also key determinants of their persistence and success. Thus, to predict the novel plant assemblages of the future, we require a more mechanistic understanding of how plant species respond to variation in both abiotic and biotic conditions simultaneously. In this dissertation, I investigate how plant functional traits of populations can explain interspecific responses to long-term climate change (Chapter 1), how biotic interactions at the level of the community interact with abiotic drivers to structure the functional composition of plant assemblages (Chapter 2), and how the plant functional diversity of the surrounding neighborhood influences the interaction outcome of a focal species across drought conditions (Chapter 3). To address these questions, I utilized fine-scale observational plant community data from a steep elevational gradient in the desert mountains of southern California and a greenhouse experiment manipulating community trait diversity across contrasting abiotic conditions. In Chapter 1, I documented substantial range shifts among perennial species across a large aridity gradient and showed that functional traits related to resource use and biotic interactions are predictive of range dynamics spanning the last forty years. In Chapter 2, I discovered that both competition and facilitation are ubiquitous in plant communities and that their relative prevalence varies with abiotic conditions to structure the functional trait composition of plant communities. In Chapter 3, I show that both the community functional composition of neighboring species, the trait values of a focal species, and the distance between the two are all important determinants of the net interaction outcome of a focal species when growing in diverse assemblages. Overall, my dissertation highlights that plant functional traits sampled at the appropriate scale can lend predictability to species’ distributions and community-level interspecific interactions and that patterns of functional trait composition can be explained by accounting for diverse interactions and how they change across environmental gradients.
In general terms, the overall goal of my dissertation is to acquire a better understanding of the phylogenetic relationships within the most diverse group of snakes in the Neotropics (i.e., the family Dipsadidae) and then use this information to identify the drivers of diversity patterns of the group at multiple spatial scales. To reach this objective, I integrate methodologies, concepts, and hypotheses from phylogenetics, macroecology, and community ecology. For Chapter 1 of my dissertation, I explored the potential causes of phylogenetic uncertainty in one of the two main groups of the Dipsadidae (i.e., the Xenodontinae) using phylogenomic data and a careful examination of the observed gene tree discordance and its potential impact on species tree inference. Although I did not recover a strong resolution of several recalcitrant nodes, I identified the potential factors driving gene tree discordance in the group, apart from finding strong and consistent support for other dipsadid lineages. For Chapter 2, I evaluated the potential drivers of species richness of the Dipsadidae across the entire American continent. In this regard, I recovered that regional diversification had the most important role in determining species richness variation across the distribution of the group. Finally, for Chapter 3 of my dissertation, I investigated the influence of a large-scale biogeographic event (the Great American Biotic Interchange, GABI) on the current species and functional diversity of a series of communities of the Dipsadidae across the Neotropics. Despite the increased input of taxa following the GABI, I did not recover that the communities involved in this event ended up being more diverse at this current time. Such a result indicates that in these communities, an equilibrium point has been reached between the processes of colonization and biotic interactions. In general, my studies continue with and expand upon the increasing recognition of the importance of phylogenetics in different areas of ecology and vice versa. Further, it is based on methodologies that can easily be implemented in other lineages and thus contribute to identifying more generalizable patterns about the drivers of species richness at the local and regional scales.
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