Family Matters: Harnessing Big Data to Investigate How Phylogenetic Niche Conservatism Has Shaped the Distributions of Contemporary Angiosperms Across the Americas

Abstract

Even as early as Darwin, biologists recognized that closely related species were likely to be more similar to one another than to more distantly related organisms. Modern phylogenetic community ecology has extended this framework to suggest that closely related species should be similar in functional traits, occupied niche space, and even geographic distributions. The extent to which this type of phylogenetic niche conservatism has shaped modern biodiversity patterns remains a contentious issue, with extreme views arguing that niche conservatism is either the sole driver of modern distributions or that abiotic filtering alone is responsible for explaining where contemporary species are found. In this dissertation, I investigate the degree to which phylogenetic niche conservatism is observed among American angiosperms. Notably, I adopt the perspective that both niche conservatism and niche shift are important, scale-dependent factors contributing to modern biodiversity patterns, and attempt to analyze the phylogenetic scale over which we can observe niche conservatism for different aspects of these organisms’ niches.

In CHAPTER ONE, I investigate the phylogenetic scale over which variation in a multi-trait resource-acquisition strategy is best observed. I show that coordinated evolution of height, growth form, seed mass, and wood density is best observed among species from con-ordinal families. Outside this phylogenetic scale, quantitative, clade-specific, variation in trait values among different angiosperm orders, as well as convergent evolution to similar trait values obscure the ability to detect phylogenetic niche conservatism.

In CHAPTER TWO, I develop novel niche characterization methods in order to quantify the degree of climatic and elevational niche similarity across the ranges of American seed plants at different phylogenetic scales. I find strong statistical support for phylogenetic niche conservatism as an important contributor to plant diversity patterns, corroborating the hypothesis that plants may tend to track favorable climate. In particular, I observed greater conservatism in precipitation niches and niche breadth compared to temperature or elevation, and niche conservatism was greater at smaller phylogenetic scales. Nevertheless, even among closely related species, differences in climatic niche were significantly greater than expected by chance and variance in niche similarity among close relatives was large, such that within-clade variation equaled or exceeded between-clade differences.

In the remaining chapters, I focus on American alpine communities across different mountain systems. I demonstrate that contrasting patterns of phylogenetic diversity and degree of alpine specialization differentiate the alpine communities occurring at opposite ends of the American mountain range system, despite broad climatic similarities in these two regions, thus suggesting an important role for historical factors in the assembly of their floras (CHAPTER THREE). I also use continuous character niche reconstruction to show how alpine lineages arise primarily from montane species, indicating that a perspective which views niche conservatism and niche shift as occurring on a continuous spectrum may be a more productive framework for understanding how species enter novel niche space (CHAPTER FOUR).

I CONCLUDE by discussing (1) the generality of an appropriate phylogenetic scale for observing niche conservatism among angiosperm lineages; (2) the degree to which process such as abiotic filtering, biogeographic history, and ecological dynamics have shaped different assemblages; and (3) how the age of “big data” can be used and abused to investigate big picture questions of community assembly and evolutionary history. This dissertation underscores the complex interplay of niche conservatism, abiotic filtering, and historical processes in shaping modern angiosperm biodiversity.