The Geography, Ecology, and Evolution of Mammalian Endemism
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Shipley, Benjamin Ranier
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Abstract
We are in the midst of an unprecedented biodiversity crisis caused by humans. As a result, we will have to make tough decisions about how to allocate our finite resources as a bulwark against the rising tide of extinctions across our shared planet. To make these decisions as effective as possible, we need a strong foundational understanding of where imperiled species live, their ecology, and their evolution. In this dissertation, I focused on one such group of imperiled species: endemic mammals. Endemic species are unique to a given region and are found nowhere else on Earth. These species have small range sizes, making them vulnerable to extinction (especially by agricultural and urban expansion). I employ and innovate a wide variety of analytical tools to explore the spatial patterns of endemism, its drivers, and the ecology of endemic species from multiple perspectives in this dissertation.
Levels of endemism form the basis of many conservation prioritization schemes, including some of the most commonly used definitions of biodiversity hotspots. However, there are many different definitions of endemism, each with their own interpretations, assumptions and biases. In Chapter 1, I reviewed the eight commonly-used definitions of endemism, their use in the literature, and their advantages and limitations. I applied these definitions to a case study of Mesoamerican mammals, demonstrating that the definition of endemism and the resolution of spatial units used profoundly influence the interpretation of endemism hotspots, and I propose a new scheme that integrates multiple endemism definitions to add nuance into endemism-based conservation.
After reviewing how endemism is defined across discipline, in Chapter 2 I evaluated how patterns in global mammalian endemism are driven by landscape factors like topography and long-term climate stability, trends in species richness, and the climatic specialization (niche breadth) of the species. To explore how these factors influence spatial patterns in endemism, I used linear models and developed a novel null-model technique that employs spatial randomization to remove ecological factors. Overall, these methods demonstrated that mammalian endemism patterns broadly mirror those of species richness, but endemism is even more prevalent in topographically diverse regions that have had stable climates through time. These are often found in coastal mountain or desert regions. However, I found little evidence that climatic niche breadth is influential in structuring continental patterns of small-ranged species.
In Chapter 3, I explored the ecological and evolutionary distinctiveness of highly endemic mammalian communities, revealing the morphological and life-history traits that covary with endemism. I found that species that live in areas of high endemism tend to have small body sizes, short lifespans, and few offspring per year, indicating they may have been unable to expand their ranges after habitat fluctuations. Furthermore, highly-endemic mammalian communities have greater redundancy and lower diversity of traits than mammalian communities not located in endemism hotspots. The high clustering of these species in functional space indicates that abiotic trait filtering from stable climates is a stronger influence than biotic interactions in structuring endemic communities.
In Chapter 4 I shifted my focus from global and continental endemism to island-endemic mammals, using the controlled situation of island biogeography to ask whether the climatic niches of these species diverge from their closest relatives. Using metrics of niche overlap and divergence, I evaluated how phylogenetic relatedness, phenotypic evolution, and geography influence the evolution of climatic niches on islands. I found that geography was by far the most influential driver of niche evolution in island mammals, and that neither relatedness nor phenotype influenced niche shifts. Overall, my results clearly demonstrate that understanding geographical patterns is vital for preserving unique species, communities, and ecosystems.
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Date
2023-06-14
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Dissertation