Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Evolutionary Biology

First Advisor

Jetz, Walter

Abstract

The concept of the niche is one of the building blocks of ecology and evolution. However, various issues have plagued the measurement of the niche, such as the “curse of dimensionality” and the problem of spatial scale. The lack of agreement on niche measurement not only hampers the development of niche-based theories but also imposes serious challenges on the application of niche modeling to addressing the threats of climate change. In this dissertation, I provided a geometric framework based on Hutchinson’s n-dimensional hypervolume to understand how niche estimates change across spatial scales. In the first chapter, I developed a computationally efficient method to measure the size and dissimilarity of n-dimensional hypervolumes. The new method allows us to compare the relative contributions of univariate niche factors and niche dimensionality to total niche variations, thus providing a partial solution to the “curve of dimensionality” (i.e., the difficulty of quantifying the shape and overlap of hypervolumes increases with the number of dimensions). In the second chapter, I reviewed the issue of spatial scale (grain)-dependence in environmental niche measurement and elucidated how this scale-dependence arises from a geometric perspective. I hypothesized that the scale-dependence of environmental niches is closely associated with species’ geographic range size, habitat specialization and environmental heterogeneity of the underlying landscape. I discussed its impact on the interpretations of empirical niche studies including the range size-niche breadth relationship, the suitability-abundance relationship, niche tracking, niche evolution and climate change vulnerability assessment. In the third chapter, I tested hypotheses about the drivers of the scale-dependence of environmental niches using Western Hemisphere bird data, and found that temperature heterogeneity has a dominant impact on the scale-dependence of the geometry of the environmental niche. The results highlight the risk of misidentifying environmentally specialized species when there is a mismatch between analysis grain and process grain. In the final chapter, I investigated how much the scale-dependence of environmental niches impacts the climate change vulnerability assessment. I developed a novel climate change vulnerability metric that partitions total vulnerability into the contributions of different niche factors, and again confirmed that temperature vulnerability is the primary driver of total vulnerability in most bird species in the Western Hemisphere. Moreover, by mapping the scale-dependence of climate change vulnerability, I found that species in the tropical areas—in particular, the Amazonian basin, Atlantic forests, and the Caribbean islands—are especially prone to the issue of spatial scale and therefore requires more attention in sampling strategy and modeling methods. Altogether, this dissertation paves the way for a more synthetic understanding of niche variations across spatial scales and more accurate assessment of the impact of climate change.

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