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Large mammalian herbivores exhibit remarkable morphological and behavioral diversity. These axes of variation feed back on one another, shaping population dynamics, facilitating coexistence, and governing herbivore impacts on ecosystems. My dissertation explores both the links between their morphological and behavioral diversity (Chapters 1-8), and how this variation informs plant-herbivore interactions (Chapters 9-10).
Chapter 1 shows that herbivores’ typical seasonal strategies constrain their responses to drought. In Kruger National Park, mixed feeders increased their woody plant consumption during drought, while grazers and megaherbivores moved to less-affected areas. Chapter 2 identifies broader constraints on these behaviors: large tropical herbivores tend to graze; seasonal diet switching is largely confined to mixed feeders; and migration is most common among large, grass-dependent, and/or high-latitude species. Extratropical herbivores exhibit less morphological and behavioral diversity overall.
Chapter 3 traces the evolution of migration within ungulates, showing that it arose in response to increasing grass dependence and high-latitude living and then promoted the evolution of larger body sizes. Chapter 4 reconstructs migratory behavior in extinct ungulates and macroevolutionary rates across ungulates, revealing that migration increases extinction risk. Chapter 5 examines partial migration, showing that resident numbers, but not migrant numbers, are decoupled from total population size. I present a model in which such patterns emerge from a socially and environmentally mediated decision-making process.
Chapters 6-8 shift focus to diet variability. Chapter 6 demonstrates that, across savanna assemblages, grass consumption increases with body size, especially in aseasonal sites lacking elephants. Chapter 7 uses hybrid zebras to show that gut microbiomes resemble those of their maternal species and covary with diet. Chapter 8 compares stable isotope and DNA metabarcoding diet reconstructions, showing that the two methods yield comparable, though not identical, diet estimates and that metabarcoding is robust to sampling protocol differences.
Chapters 9-10 explore herbivore impacts: Chapter 9 shows that elephant selectivity predicts plant responses to exclusion, while Chapter 10 demonstrates that elephant impacts on trees are spatially decoupled from elephant densities.
Together, these chapters demonstrate the intricate links between herbivore morphology, behavior, and impacts, offering new insights into the causes and consequences of their remarkable diversity.
