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Title: Convergent interactions among pitcher plant microcosms in North America and Southeast Asia
Abstract: Ecosystems are composed of diverse suites of organisms whose interactions are mediated by both the biotic and abiotic constraints of their environments. The complexity of ecosystems makes them both resilient and difficult to understand. Analyzing the patterns and constraints of biodiversity across different systems can provide insights about the processes governing the formation and maintenance of communities. One analytical tool is convergence, where similarities emerge from different origins.
In this dissertation, I combine conceptual theory with empirical data to explore how natural selection repeatedly favors particular associations among different interacting species. In Chapter 1, I develop the concept of convergent interactions—the independent emergence of multispecies interactions with similar physiological or ecological functions. A convergent interaction framework facilitates prediction of the ecological roles of organisms (including microbes) in multispecies interactions and the selective pressures acting in poorly understood or newly discovered multispecies systems.
The modified leaves of carnivorous pitcher plants are elegant natural systems for studying ecosystem dynamics, as they are discrete, naturally replicated microcosms that have evolved independently three times on different continents. Pitchers house entire communities of arthropods, protists, fungi and bacteria. In Chapter 2, I take advantage of the visibility and existing knowledge of pitcher plant insect inquilines to evaluate next-generation metabarcoding as a means of characterizing complete contained communities. Correspondence of phylogenetic trees and correlations of organism and sequence counts confirm the effectiveness of metabarcoding methods.
Chapter 3 then extends the concept of convergent interactions to pitcher ecosystems. I characterize and compare the eukaryotic and bacterial communities from over 400 samples of pitcher microcosms from Nepenthes species in Southeast Asia and Sarracenia species in North America. Data from field collections as well as a relocation experiment are used to investigate whether convergence in form and function of a host extends to associated communities. Pitcher communities contain fewer species than those of surrounding habitats, and phylogenetically related subsets of bacteria and eukaryotes tend to colonize each system. When in a common environment, Nepenthes and Sarracenia communities converge in composition. The evolved pitcher form appears to strongly affect fundamental aspects of biodiversity within the pitcher ecosystem, including species richness, phylogenetic diversity, and community composition.