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Title: Evolution and Ecological Consequences of Diverse Traits in Tropical Pitcher Plants (Nepenthes)
Abstract: Plants play a major role in Earth’s terrestrial biodiversity, and by extension so do their interactions with other organisms. The role of plant traits in influencing their interaction partners is well-studied in the case of pollination and herbivory, but plant symbiosis is somewhat less well-understood. One example is the class of interactions occurring in phytotelmata, or plant-held water bodies, in which diverse communities of organisms live. These systems have often been utilized as models for community ecology, but less has been done on the direct interactions between the plants themselves and the phytotelm community. As phytotelmata are essentially aquatic islands surrounded by terrestrial habitat, the aquatic inhabitants are completely reliant upon their host plant, so plant traits have strong potential to mediate interactions with symbionts. This is true of tropical pitcher plants (Nepenthes) whose pitchers (modified leaves) have evolved morphological and physiological traits to aid in prey capture and digestion, however there may also be selective influence on the pitchers exerted by symbionts in the phytotelm—as well as by herbivores and external abiotic conditions. So, Nepenthes present an excellent system for investigating how multiple biotic and abiotic interactors can shape and be shaped by plant diversity.
In this dissertation, I take a broad view of the Nepenthessystem in order to examine potential adaptive explanations of trait variation for the autecology of the plants as well as how those traits influence symbionts and other interactors. In Chapter One, I combine field observations of a single polymorphic species (Nepenthes gracilis) with phylogenetic comparative analysis of 85 species across the genus to investigate correlations between color polymorphism and ecological factors including altitude, light environment, and herbivory. I find that pitcher traits largely lack phylogenetic signal. However, stochastic character mapping shows that the evolution of color polymorphism is biased towards more red-pigmented lower pitchers. In N. gracilis, color does not correlate to prey-capture and symbiont colonization, but red pitchers experience less herbivory. This work highlights ecological correlates of the vast phenotypic diversity of this group of tropical plants and points to a need for future work examining herbivores ofNepenthesand experimental investigations on color polymorphism.
For Chapters Two and Three of my thesis, I examine the influence that inter- and intra-specific pitcher trait variation has on the bacterial and eukaryotic phytotelm community through metabarcoding. Recent work has shown convergence in communities between the very distantly related pitcher plants Nepenthes and Sarracenia. This shows that pitcher plants have specific characteristic communities. What is less well known is whether different species of pitchers actively cultivate their own characteristic communities. In Chapter Two of my thesis, I demonstrate that Nepenthes pitchers have the ability to filter their communities in species-specific ways and that certain pitcher traits, especially pH, can explain community differences. I use a common garden experiment with 16 Nepenthesspecies in cultivation that were never exposed to their native environment, all reared in the same controlled greenhouse, and all filled with pH 6.5 water from a common source. The different Nepenthesspecies differentially altered the abiotic characteristics within their pitchers, including pH and viscosity. In Chapter Three, I examine how altitude and intraspecific variation influences communities in the pitchers of N. mindanaoensis collected from an altitudinal transect on Mount Hamiguitan, Mindanao, the Philippines. I show differences in patterns of community assembly for bacteria and eukaryotes, despite their living together in the same aquatic microhabitats. Community similarity of eukaryotes, but not bacteria, are significantly influenced by altitude. On the other hand, pitcher dimorphism has an effect on eukaryotes but not bacteria, while variation in pH levels strongly influences both taxa. Additionally, I show that arthropod abundance in this system follows the classical trend of decreasing with elevation, and I point to some differences in the patterns of abundance for living inquilines insects as opposed to insect prey, in relation to intraspecific plant trait variation.
Committee: Naomi Pierce, David Haig, N. Michele Holbrook