Date of Award

7-28-2023

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

A major goal of community ecology is to elucidate the processes that create patterns of biodiversity. Specifically, the discipline focuses on the processes that influence community assembly, diversity gradients, and/or species coexistence. Coexistence mechanisms have been traditionally thought of in terms of resource competition, whereby species are able to coexist by having different niche requirements, both biotic and abiotic, that maximize their fitness. Much of the previous work in this arena focused on the abiotic factors that influence species coexistence. However, many of the seminal ideas about the forces that drive abundance and coexistence patterns hypothesized that it was differences in the nature of biotic interactions between those species that were responsible for the observed patterns. However, due to the variability and complexity of biotic interactions, competition was generally the only interaction to be extensively considered. Further research gave evidence to the idea that antagonistic interactions with natural enemies are a possible niche axis that can determine coexistence of the hosts. The vast number of natural enemies and the even greater number of defensive chemical metabolites provide a probable mechanism of niche differentiation, where a species niche is defined by those enemies that they avoid through their defenses. Advances in analytical chemical metabolomics techniques are used in this dissertation to examine the role that more complex interaction types play in structuring communities. More recent research utilizing these techniques have found evidence of the role that plant chemical defenses play in mediating interactions between plant hosts and their natural enemies. Theoretical and technical advances are combined here to investigate the factors that drive species diversity patterns and that maintain species coexistence in 3 principal ways: 1) by testing the relative importance of competitive interactions among tree species across an elevational-diversity gradient in the tropical Andes (Chapters 2, 3); 2) by testing the relative importance of enemy mediated interactions via chemically-mediated niche differences across the same elevational-diversity gradient (Chapters 3, 4); and 3) by exploring the effects of the biotic and abiotic environments on trait patterns and community assembly processes. Chapter 2 investigated whether competitive interactions among tropical tree species vary systematically across a large-scale biodiversity gradient. Using tree species data collected from a network of permanent plots in the Bolivian Andes, I quantified the taxonomic and functional differences of neighboring species of individuals of each species within a plot. The results showed that the taxonomic composition of tree neighborhoods becomes more stochastic with increasing species diversity. This suggests that competitive interactions appear to be more unpredictable among neighborhoods of the same species in higher-diversity tree communities. Interestingly, similar patterns were not detected in the functional trait composition of tree neighborhoods. Taken together, the two patterns suggest that species traits maybe influenced by the abiotic environment and that biotic interactions other than competition could be at play. Chapter 3 investigated how enemy-mediated interactions varied over a gradient of climate, diversity, and elevation using an ecological metabolomics approach. Defensive chemical traits were measured from leaf compounds and used to gauge the strength of enemy mediated interactions within communities. The results showed that chemical dissimilarity among tree species increased with increasing community diversity and toward more benign conditions and that evolution of chemical defenses is more rapid along these gradients. This implies that natural enemies impose a stronger selective pressure on plant chemical defenses in more diverse communities and in more productive climates, and that the defensive chemical compounds that hosts use are less phylogenetically conserved in higher diversity communities. The differential strength of biotic interactions and the greater evolutionary lability of defensive metabolites across communities implies that these enemy mediated interactions likely play a role in the maintenance and origin of biodiversity gradients. Chapter 4 tested how community assembly is affected by both biotic and the abiotic environments, by examining the relationships between two suites of plant functional traits (chemical and morphological), and abiotic site characteristics (climate and soils). Chemical traits were used to approximate the biotic environment while morphological traits approximated the abiotic. The results showed systematic variation in morphological trait patterns but no similar patterns among the chemical traits. The abiotic environment was found to have a stronger than expected influence on chemical trait patterns, where trees may utilize chemical defenses differently in wet and dry environments. A focus on individual chemical defensive compounds to gauge their influences on community assembly and dynamics would be a fruitful continuation of this work. Overall, this thesis sheds light on how local biotic interactions scale up to affect larger-scale biodiversity patterns in tropical montane ecosystems. Understanding the link between local scale interactions and large-scale patterns is key for finding ties between complementary research and for protecting and conserving biodiversity by accurately predicting future changes between species.

Language

English (en)

Chair and Committee

Jonathan Myers

Committee Members

Juan Tello

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